Contact us

Ergo reports




Env. Design




Ergo humor

Search site

About us

File readers


Humanics Ergonomics

Arm support & armrest design


About Ergo  |  Keyboards  |  Anthropometrics  |  Vision & lighting  |  Places  |  Humor

Office Ergonomics  |  Ergonomics  |  Child  |  Disabled  |  Medical  |  Science  |  Legal

More on sitting & seating   |   Sitting & seating ergonomics  |  ErgoExpo slides


Arm support & armrest design review

Citations  (Lueder & Allie)

Adams, M. A.; Green, T. P.; Dolan, P. (1994) The strength in anterior bending of lumbar intervertebral discs, Spine, 19, 19, 2197-203.

Abstract:Study Design. This was a motion segment experiment. Objectives. To determine the strength in bending of lumbar intervertebral discs. Methods. Eighteen "motion segments" consisting of two vertebrae and the intervening disc and ligaments were loaded to simulate forward bending movements in life. The range of flexion and the resistance to bending of each specimen was compared before and after removal of the posterior elements. Nine of the discs were tested after damaging the vertebral end plate in compression had reduced the intradiscal pressure.

Results. The posterior elements restricted the disc to 80% of its full range of flexion. The strength in anterior bending of isolated discs decreased with age and was 33 Nm on average. Reducing intradiscal pressure did not affect the discs’ strength in bending.

Conclusions. Disc failure in bending occurs through overstretching of the outer annulus in the vertical direction. In life, the posterior elements may not adequately protect the posterior annulus from fatigue damage.  15 lumbar spines of cadavers.  Isolated discs provided negligible resistance to the first few degrees of flexion, but then bending moment increased in approximately linear fashion with flexion angle until failure occurred at an angle of 18.3° ± 3.7°.  The posterior elements resisted 81% of the bending moment at the motion segment’s limit of flexion, and restricted the discs to 79% ± 9% of their full range of flexion. The motion segment limit was 3.9° ± 1.9° short of the flexion angle to damage the disc." Close inspection of the discs after testing usually revealed a ragged horizontal tear in the outer posterior annulus fibrosis, either at mid-disc height or close to one of the end plates.

At the point of failure, the tear would spread rapidly across the disc surface with audible "crackling" sound.  The posterior longitudinal ligament sometimes was damaged, but in most cases remained essentially intact, but with extensive "plastic" stretching that remained after the load removed.  Dissection revealed that the peripheral annulus fibers remained attached to the vertebral rim, but sections of the middle annulus usually pulled away from the underlying bone, taking the hyaline cartilage end plate with it." "No damage was observed in the anterior margins of the vertebral bodies, although the possibility of microscopic damage cannot be specifically ruled out".  After hyper-flexion injury, the damage to the inner annulus probably is minimal because it acts to constrain the nucleus pulposus and failure was not accompanied by a sudden fall in IDP.  Also, discs with and without lowered IDP had similar strengths.  The vertical tensile force in the posterior annulus must be responsible for most of the 1.32 Mpa intradiscal pressure at the point of failure.  "This comparison suggests that at the motion segment’s limit of flexion, the bending moment acting on the disk is about 44% of that required to injure it. Thus, the disc appears to be well protected from hyperflexion, because the posterior ligaments themselves would be safeguarded by the back muscles.  However, this does not necessarily imply that hyperflexion injuries to the disc must be rare, because the protecting ligaments and muscles may be injured."  "Flexion damage to the posterior annulus may also occur as a result of repetitive fatigue loading.  The apparently close association between a disc’s resistance to bending and the vertical tensile stress acting in the posterior annulus suggests that at the motion segment’s limit of flexion, this stress if 44% of the value required to cause failure, just as the bending moment is 44% of the value." Fatigue failure could occur if the applied tensile stress exceeding about 45% of that required to cause failure in a single loading cycle.

Adams, M. A.; McNally, D. S.; Dolan, P. (1996) "Stress" distributions inside intervertebral discs. The effects of age and degeneration. J Bone Joint Surg Br, 78, 6, 965-72.

Abstract: We investigated the distribution of compressive "stress" within cadaver intervertebral discs, using a pressure transducer mounted in a 1.3 mm diameter needle. The needle was pulled along the midsagittal diameter of a lumbar disc with the face of the transducer either vertical or horizontal while the disc was subjected to a constant compressive force. The resulting "stress profiles" were analyzed in order to characterize the distribution of vertical and horizontal compressive stress within each disc. A total of 87 discs from subjects aged between 16 and 87 years were examined. Our results showed that age-related degenerative changes reduced the diameter of the central hydrostatic region of each disc (the "functional nucleus") by approximately 50%, and the pressure within this region fell by 30%.

The width of the functional annulus increased by 80% and the height of compressive "stress peaks" within it by 160%. The effects of age and degeneration were greater at L4/L5 than at L2/L3, and the posterior annulus was affected more than the anterior. Age and degeneration were themselves closely related, but the stage of degeneration had the greater effect on stress distributions. We suggest that structural changes within the annulus and endplate lead to a transfer of load from the nucleus to the posterior annulus. High "stress" concentrations within the annulus may cause pain, and lead to further disruption.

Alexander, N. B.; Koester, D. J.; Grunawalt, J. A. (1996) Chair design affects how older adults rise from a chair, J Am Geriatr Soc., 44, 4, 356-62.

Abstract: To determine how modifications of key chair design aspects, such as seat height, posterior seat tilt, backrest recline, seat compressibility, and armrest placement, affect how older adults rise from a chair and the seating comfort they experience. Design: Cross-sectional comparison. Setting: Congregate housing facility and university laboratory. Subjects: Two groups of volunteers, Old (n = 29, mean age 84) and Young (n = 21, mean age 23). Measurements: Analysis of time to rise, body motion (determined by use of digitized videotaping), and self-reported difficulty when subjects rose from a variety of controlled chair settings thought to represent important chair design specifications encountered by older adults. Subjects also reported their comfort while being seated in these settings. Results: Lowered seat height, increased posterior seat tilt and backrest recline, and perhaps increased seat compressibility cause increased time to rise, increased body motion, and increased self-reported ratings of rise difficulty in both Young and Old groups. Under the most challenging conditions, the effect appears to be stronger in the Old than in the Young: a few Old were unable to rise, and the Old took disproportionately longer to rise and used disproportionately greater neck motion (p generally 0.001) compared with the Young. Armrest placement did not alter rise performance or ratings significantly. The conditions in which rise difficulty increases or decreases do not correspond exactly to conditions in which comfort increases or decreases. Some aspects that increase rise difficulty, such as tilt/recline and seat compressibility, may also increase comfort. Conclusions: Aspects of chair design such as lowered seat height, increased posterior seat tilt, increased back recline, and increased compressibility interfere with chair egress in older adults. While decreasing ease of egress, however, these same factors may increase seating comfort. Furniture designers and manufacturers must find a balance between degree of sitting comfort; ease of egress and the degree to which the seating device facilitates functional independence, particularly to meet the needs of disable older adults.

Alexander, N. B.; Schultz, A. B.; Warwick, D. N. (1991) Rising from a chair: effects of age and functional ability on performance biomechanics, J Gerontol, 46, 3, M91-8.

Abstract: Although difficulty in rising from a chair is common to elderly people, few studies have compared chair rise performance in young and elderly adults with differing functional abilities. Using an instrumented chair and a videotape analysis, controlled chair rise performances were quantified in three groups of volunteers: young adults (Young, n = 17, mean age 23 years), elderly adults able to rise without the use of armrests (Old Able, n = 23, mean age 72 years), and elderly adults unable to rise without the use of armrests (Old Unable, n = 11, mean age 84 years). Rises both with and without the use of hands were observed. The total time to rise and the percent of that time spent in the two distinct phases of the rise, the body segment rotations used, and the hand forces exerted were measured. Despite no apparent functional impairment, the Old Able compared to the Young spent a larger percent time in the first phase of the rise and rotated their body segments by different amounts. When rising with use of hands, the Old Unable compared to the Old Able group took more time and used different body segment rotations and larger ratios of hand force to body weight. These data quantify chair rise performance in young adults and in elderly adults with differing functional abilities and enable biomechanical analyses of the importance of joint torque strengths and postural stability in that performance.

Andersson, B. J.; Ortengren, R.; Nachemson, A. L.; Elfstrom, G.; Broman, H. (1975) The sitting posture: an electromyographic and discometric study, Orthop Clin North Am, 6, 1, 105-20.

Abstract: The disc pressure of the third lumbar disc and the myoelectric activity of several muscles of the back were measured. Three standing and nine unsupported sitting positions were studied as well as eight support parameters and six sedentary tasks. 1. Myoelectric activity is about the same in standing and in relaxed unsupported sitting. In the unsupported sitting positions the highest level of activity is found in anterior sitting and the lowest in posterior sitting. In the muscles of the cervical and lumbar regions the activity is always lower than in the muscles of the thoracic region. The disc pressure is considerably higher in unsupported sitting than in standing. In the unsupported sitting positions the highest disc pressure is found in anterior sitting and the lowest in sitting straight. 2. Both the myoelectric activity and the disc pressure decrease when the back is supported. Of the support parameters, the backrest inclination is the most important, myoelectric activity and disc pressure both decreasing with an increase in inclination. The disc pressure is considerably reduced also when the lumbar support is increased and when armrests are used. 3. Myoelectric activity and disc pressure are both comparatively low in writing, higher in typing, and still higher in lifting. In the car driver’s seat the disc pressure increases both when the gear is shifted and when the clutch pedal is depressed. When the gear is shifted, there is also an increase in myoelectric activity.

Arborelius, U. P.; Wretenberg, P.; Lindberg, F. (1992) The effects of armrests and high seat heights on lower-limb joint load and muscular activity during sitting and rising, Ergonomics, 35, 11, 1377-91.

Abstract: The loading moment of force on the hip, knee, and ankle joints of nine healthy men rising from four different types of stools were compared, together with the levels of myoelectrical activity (EMG) in four leg muscles. Two types of stool (stand stools) had higher seats than a normal chair. The other two were of ordinary seat height, but one also had armrests. The bodyweight carried by the different stools when sitting was also measured, and the subject estimated the effort required for each trial. The mean maximum knee moment was over 60% lower when rising from the high stool than from "ordinary" seat height. The difference between the high and low stand stool was also significant (p < 0.001). Using the high stool or help of the arms reduced the mean maximum hip moment by about 50%. The different stools only marginally influenced the mean maximum ankle moment. Knee moment was influenced more by seat height than was hip moment. Vastus lateralis activity was significantly higher when subjects rose from "ordinary" height than when rising from either stand stool (p < 0.001). The rectus femoris muscle was little activated and the semitendinosus muscle was activated earlier when rising from higher seat heights. All subjects estimated the effort of rising from the higher stand stool to be lower than from the lower stand stool or from "ordinary" height without arm rests. It was concluded that stand stools are good alternatives for workers who change frequently between sitting and standing work.

Banzett, R. B.; Topulos, G. P.; Leith, D. E.; Nations, C. S. (1988) Bracing arms increases the capacity for sustained hyperpnea, Am Rev Respir Dis, 138, 1, 106-9.

Abstract: Patients with severe chronic obstructive pulmonary disease (COPD) frequently lean forward, bracing their arms. We wondered whether the resulting shoulder girdle support improves the function of the ventilatory pump. We tested this possibility in 4 normal men by measuring the maximal ventilation that they could voluntarily sustain for 4 min while seated with their elbows braced firmly on a table and while seated with their elbows held just above the table. Bracing the arms increased ventilatory capacity significantly in all subjects, but the magnitude of the change was small (8%). We attribute the change to improved function of the accessory muscles that expand the rib cage. We speculate that this effect assumes greater importance in patients with COPD, whose diaphragms are flattened and ineffective, because such patients depend more on the inspiratory muscles of the rib cage.

Beck, F. M.; Weaver, J. M.; Blozis, G. G.; Unverferth, D. V. (1983) Effect of arm position and arm support on indirect blood pressure measurements made in a dental chair, J Am Dent Assoc., 106, 5, 645-7.

Besarab, A.; Frinak, S.; Sherman, R. A.; Goldman, J.; Dumler, F.; Devita, M. V.; Kapoian, T.; Al-Saghir, F.; Lubkowski, T. (1998) Simplified measurement of intra-access pressure, J Am Soc. Nephrol, 9, 2, 284-9.

Abstract: The measurement of intra-access pressure (P[IA]) normalized by mean arterial BP (MAP) helps detect venous outlet stenosis and correlates with access blood flow. However, general use of P(IA)/MAP is limited by time and special equipment costs. Bernoulli’s equation relates differences between P(IA) (recorded by an external transducer as PT) and the venous drip chamber pressure, PDC; at zero flow, the difference in height (deltaH) between the measuring sites and fluid density determines the pressure deltaPH = P(IA) – P(DC) Therefore, P(DC) and PT measurements were correlated at six different dialysis units, each using one of three different dialysis delivery systems machines. Both dynamic (i.e., with blood flow) and static pressures were measured. Changes in mean BP, zero calibration errors, and hydrostatic height between the transducer and drip chamber accounted for 90% of the variance in P(DC), with deltaPH = -1.6 + 0.74 deltaH (r = 0.88, P 0.5 and 1229 ± 112 ml/min for P(IA)/MAP 0.5. DeltaPH varied from 9.4 to 17.4 mmHg among the six centers and was related to deltaH between the drip chamber and the armrest of the dialysis chair. Concordance between values of P(IA)/MAP calculated from PT and from P(DC) + deltaPH was excellent. It is concluded that static P(DC) measurements corrected by an appropriate deltaPH can be used to prospectively monitor hemodialysis access grafts for stenosis.

Branton, P. (1969) Behaviour, body mechanics and discomfort, Ergonomics, 12, 2, 316-27.

Abstract: With technological advances and the continuing increase in the time for which people sit, the problem (between seats and body malfunctions) is unlikely to diminish in importance and, even if some may harbor the suspicion that there are no ideal solutions, the search must continue.   Perhaps this is the moment to pause and reconsider the conceptual framework of research on sitting on seats.   This paper will try to show that behavioral study reveals some gaps to exist in this framework and will suggest ways of bridging them.   In our view the gaps result from the fact that the problems are of an interdisciplinary nature and there are three areas for further researching in which our understanding could be considerably advanced by joint effort.

The first is the area between body mechanics and behavior, insofar as it lies between the biological and behavioral approaches to human action.  The second is the area between behavior and subjective feelings, where we might look for a theoretical basis for research into comfort and its measurement. In the third area, the technological, demands raised in the other two are to be translated into hardware and then tested systematically to secure validity as a precondition for acceptance by the public at large.

Dolan, P.; Adams, M. A.; Hutton, W. C. (1988) Commonly adopted postures and their effect on the lumbar spine, Spine, 13, 2, 197-201. Abstract: The activity of the erector spinae muscles and the changes in lumbar curvature were measured in 11 subjects in a range of commonly adopted postures to see if there were any consistent trends. Surface electrodes were used to measure back muscle activity and lumbar curvature was measured using electronic inclinometers. The results showed that many commonly adopted postures reduced the lumbar lordosis when compared with erect standing or sitting, even at the expense of increasing the back muscle activity.

Dufosse, M.; Hugon, M.; Massion, J. (1985) Postural forearm changes induced by predictable in time or voluntary triggered unloading in man. Exp Brain Res, 60-2, 330-4.

Abstract: Human subjects sitting in a chair were asked to maintain their right forearm in a horizontal position in half supination. The forearm was loaded with a constant weight of one kilogram. Vertical force at the wrist level, angular position of the elbow and EMG activity of biceps, brachio-radialis and triceps muscles was recorded. Unloading was tested under four different conditions, the first two having been used in a previous study (Hugon et al. 1982): Voluntary unloading by the subject’s other hand. An "anticipatory" deactivation of the load bearing forearm flexors is observed preventing the elbow rotation of that arm. Unpredictable passive unloading. This results in an upward forearm rotation, which provokes the classical "unloading reflex". Two new conditions were tested in the present paradigm: Imposed unloading predictable in time (tone signal preceding unloading by a fixed interval). Unloading being actively triggered when the subject presses a key. Under the two latter conditions, no anticipatory deactivation of the flexor supporting muscles preceding the onset of unloading as in situation A was observed. During the first 120 ms after the onset of unloading, the forearm rotation was the same as in situation B (unpredictable passive unloading). Thereafter, the rotation was smaller in some subjects, apparently due to an ameliorated reflex action. It is concluded that temporal information concerning the precise time of the unloading or the triggering of the load release by a voluntary movement (key press) was not by itself able to induce the anticipatory deactivation of the forearm flexors that was seen with a coordinated voluntary release of the load by the contralateral arm.

Ewing, M. B. (1968) Modifications of wheelchair footrest and armrest to improve function and convenience, Arch Phys Med Rehabil, 49, 8, 480-1.

Faucett, J.; Rempel, D. (1994) VDT-related musculoskeletal symptoms: interactions between work posture and psychosocial work factors, Am J Ind Med, 26, 5, 597-612.

Abstract: Video display terminal (VDT) operators (n = 150) in the editorial department of a large metropolitan newspaper participated in a study of day-to-day musculoskeletal symptoms. Work posture related to the VDT workstation and psychosocial work factors were also investigated for their contributions to the severity of upper body pain, numbness, and stiffness using a representative subsample (n = 70). Self-report measures included Karasek’s Job Content Instrument and the author-designed Work Interpersonal Relationships Inventory. Independent observations of work posture were performed using techniques similar to those reported by Sauter et al. [1991]. Pain during the last week was reported by 59% (n = 88) of the respondents, and 28% (n = 42) were categorized by symptom criteria potentially to have musculoskeletal disorders. More hours per day of VDT use and less decision latitude on the job were significant risk factors for potential musculoskeletal CTDs. Head rotation and relative keyboard height were significantly related to more severe pain and stiffness in the shoulders, neck, and upper back. Lower levels of co-worker support were associated with more severe hand and arm numbness. For both the region of the shoulders, neck, and upper back and the hand and arm region, however, psychological workload, decision latitude, and employee relationship modified the contributions of relative keyboard and seat back heights to symptom severity with the supervisor. Alternative explanations for these findings are discussed.

Gilsdorf, P., Patterson, R. and Fisher, S. (1991) Thirty-minute continuous sitting force measurements with different support surfaces in the spinal cord injured and able-bodied, J Rehabil Res Dev, 28, 4, 33-8.

Abstract: Able-bodied, paraplegic, and quadriplegic subjects sat for 30-minute intervals on various surfaces in a wheelchair with a forceplate mounted on the seat in order to determine factors that could contribute to the formation of decubitus ulcers. All three groups of subjects sat on ROHO and Jay cushions; in addition to sitting on the two cushions, the able-bodied subjects sat on a hard surface. Factors studied were: normal and shear seat forces, the location of the center of mass, and armrest force. The forceplate was under the cushions; therefore, the values reflect average forces over the buttocks and posterior thighs. These factors were compared between disability levels as well as between surface types. Larger, normal, and forward shear forces and a more anterior position of the center of mass were observed with the ROHO cushion. More frequent and larger lateral weight shifts occurred with the Jay cushion. The armrests tended to support from 5 percent of the body weight for quadriplegics to 8 or 9 percent for normals and paraplegics. The results suggest that armrests reduce seat forces by carrying some of the body weight.

Gonvers, M.; Zografos. (1981) A new mobile armrest, Am J Ophthalmol, 91, 3, 403-4.

Abstract: Developed a weight-and-pulley armrest for surgeons to use during ocular microsurgery. The armrest is mobile, sterilizable, and allows the weight stress on the forearm to be regulated.

Hasegawa, T.; Kumashiro, M. (1998) Effects of armrests on workload with ten-key operation [In Process Citation], Appl Human Sci, 17, 4, 123-9.

Abstract: The constrained posture used for work using a visual display terminal (VDT), such as data entry, can produce static muscular fatigue. Based on the application of ergonomic principles, we conducted an experiment using a prototype VDT chair designed with an armrest adjustable to heights from 22 to 28 cm. The experiment – conducted to assess the static muscular strain based on varying heights of the armrest and distance from the keyboard – was performed with male subjects. The subjects were asked to input five-figure numbers using the 10 keys arranged on the right side. Using a surface electromyogram, we measured the strain imposed on the arm and the shoulder. This was followed up with a performance, weight-loading onto the armrests and a questionnaire. Results show that use of armrests is effective for the alleviation of muscles in one-handed keyboard operation where operators work on a desk whose height is unadjustable according to their body height or where operators are unable to rest their wrist on the desk. A chair with height-adjustable armrests is considered desirable when used by several people.

Higgs, P. E.; Edwards, D. F.; Seaton, M. K.; Feely, C. A.; Young, V. L. (1993) Age-related differences in measures of upper extremity impairment, J Gerontol, 48, 4, M175-80.

Abstract: Upper extremity cumulative trauma disorders (CTDs) are among the most prevalent and costly occupational injuries. These disorders include nerve compression syndromes, tenosynovitis, epicondylitis, tendinitis, and arthritis. These have been related in the past to repetitive use of the upper extremity. The expected increase in the age of the American work force and the assumption that older workers are more susceptible to the disorders prompted this investigation of the relationship of age to signs and symptoms of upper extremity impairment. Methods. A battery measuring seven objective signs and four reported symptoms of upper extremity cumulative trauma disorders was administered to two stratified random samples of workers. One group (n = 157) processed cooked poultry and the second group (n = 118) performed data entry at VDT terminals. Workers were separated into three age groups (younger: 20 – 35 years, middle-aged: 36 – 50 years, and older: 51 – 71 years). Results. A series of analyses of variance were computed to determine whether the older workers were more impaired. No significant age differences were found for sign, symptom, or total scores in either sample, and no significant Age x Gender interaction was present. Older workers were more impaired for vibratory sensation, cutaneous pressure, and motor latency. The results support the hiring of older workers for general tasks in the workplace without significant worry of increased susceptibility to CTDs.

Hildebrandt, W.; Herrmann, J.; Stegemann, J. (1993) Vascular adjustment and fluid reabsorption in the human forearm during elevation, Eur J Appl Physiol, 66, 5, 397-404.

Abstract: Elevation of vascular hydrostatic pressure is known to increase capillary filtration causing, for example orthostatic plasma fluid losses. The present study investigated possible compensatory fluid intravasation in the human forearm during graded elevation that is during hydrostatic venous collapse. Recordings were made of forearm fluid volume (impedance-plethysmography), forearm blood flow (venous-occlusion-technique), and finger arterial pressure (Finapres). A group of 20 male subjects were seated upright and had their horizontal right forearm passively elevated to 0, 18, 36, and 54 cm above the heart (3rd intercostal space) after equilibration at a reference level 18 cm below the heart. All positions were maintained for 15 min and taken in random order. The vascular volume, which drained or refilled within 1.5 min after change of position was found to increase with height. The slow linear volume reduction representing the transcapillary reabsorption rate was found to be almost identical in the three positions above the heart (0.0382, 0.0372, and 0.0398 ml.100 ml-1.min-1). Forearm blood flow reached its highest values at heart level and decreased with height. Calculated total vascular resistance increased with a progressive slope up to about 200% of the value at heart level. As a main finding similar reabsorption rates suggested good maintenance of capillary pressure in positions up to 54 cm above the heart thus contrasting with findings on the calf. The coincidence with increasing total vascular resistance led us to the conclusion that graded venous collapse indicated by grading in venous volume makes for a considerable decrease in pre- to postcapillary resistance ratio with elevation.

Hildebrandt, W.; Herrmann, J.; Stegemann, J. (1994) Fluid balance versus blood flow autoregulation in the elevated human limb: the role of venous collapse, Eur J Appl Physiol, 69, 2, 127-31.

Abstract: This study evaluated the postural vascular adjustment in the human forearm which may be responsible for the recent observation that transcapillary fluid balance is maintained above the level of the heart while blood flow decreases in a linear fashion. In this study further evidence was provided that a posturally graded profile of collapsed veins holds for both an overall increase of resistance with height and compensation for hydrostatic effects on capillary pressure. This was achieved by manipulating peripheral venous profile/volume: a proximal outlet resistance (upper arm cuff) was used for re-opening of collapsed distal veins. In test (a), 12 healthy subjects underwent recordings of fluid reabsorption rate and blood flow in a 20-cm segment of their forearm horizontally placed at 36 cm above heart level (third intercostal space). Applying upper arm cuff pressures randomly between 0 and 25 mmHg (0-3.33 kPa) for 15 min led to maxima of blood flow and reabsorption rates at inflations of 5 or 10 mmHg (0.67 or 1.33 kPa). This was attributed to minima in postcapillary resistance facilitating flow and reducing capillary pressure. In test (b) the flow-maximizing outlet resistance found was studied for its effect in different forearm positions (-18, 0, 18, 36, 54 cm relative to heart level). Blood flow then showed a shift of its maximum from heart level to 36 cm above heart level, while the reabsorption rate increased above 18-cm height – in contrast to previous findings with a free circulation. It was therefore concluded that the venous profile in the forearm adjusts postcapillary resistance in such a way that local dehydration is confined at the cost of blood supply. Thicker and less collapsible veins may ensure better flow autoregulation during impaired fluid balance – as seen in the legs.

Hughes, C. J.; Weimar, W. H.; Sheth, P. N.; Brubaker, C. E. (1992) Biomechanics of wheelchair propulsion as a function of seat position and user-to-chair interface, Arch Phys Med Rehabil, 73, 3, 263-9.

Abstract: This study investigated the biomechanics of lever and hand-rim propulsion and the effects of seat position on propulsion mechanics. Nine able-bodied and six paraplegic spinal cord injured persons participated. Subjects performed hand-rim and lever propulsion on a wheelchair test simulator at a speed and load of 3km/hr and 7.5 watts/side, respectively. A 2 x 3 matrix of randomized seat positions was used. Three-dimensional motion measures of the trunk, shoulder, elbow, and wrist were collected over four-second sample periods for each seat position. Hub torque and stroke arc measurements were determined. Upper extremity motions were significantly different (p < .05) for the two methods of propulsion. Hand-rim propulsion required less elbow motion, greater shoulder extension, less shoulder rotation and less arm abduction than lever propulsion. Both methods of propulsion required a substantial amount of internal rotation at the shoulder. Seat position changes had a greater effect on joint motion ranges when hand-rim propulsion was performed. No significant differences (p > .05) were found for trunk motion for the treatments. The findings provide additional information for development of a model for the optimization of wheelchair propulsion.

Hugon, M.; Massion, J.; Wiesendanger, M. (1982) Anticipatory postural changes induced by active unloading and comparison with passive unloading in man, Pflugers Arch, 393, 4, 292-6.

Abstract: Normal human subjects, sitting in a chair, were required to maintain stable elbow flexion against loads of 0.5 kg or 1.0 kg. Unloading was affected either "passively" by the experimenter, or "actively" with the subject’s own contralateral arm. Elbow angle, force exerted by the load, and electromyographic activity (EMG) of biceps and triceps muscles of both arms were recorded and averaged. "Passive" unloading was followed by a reduction of biceps EMG activity, starting 50 – 80 ms after weight lift, and by an upward deflection of the forearm. With "active" unloading, however, a reduction of the biceps EMG activity slightly preceded the onset of unloading (0 – 30 ms). This reduction of the "actively" unloading arm occurred at about the same time as the activity of the contralateral unloading arm. In this experiment, the unloaded forearm maintained an almost stable position. Thus, the anticipatory adjustment of elbow posture, observed when unloading was performed by the subject, appears to optimize limb stability during the mechanical perturbation.

Maciel, M. H.; Marziale, M. H. (1997) [Postural problems and problems of equipment: an ergonomic study of users of microcomputers in a nursing school] [In Process Citation], Rev Esc Enferm USP, 31, 3, 368-86.

Abstract: The computer usage has become essential nowadays and the Nursing staff meeting this technological improvement has been assimilating it in practice, which is started in Graduation course. As users of the computer division from a Nursing School we have observed students’ complaints related to adequacy of the furniture used, then, we were interested in developing this research which has as object to analyze the furniture from the computer division according to the ergonomic recommendations and identify body posture adopted by the students in typing activity. The procedures used were performed in three stages, which were: free supervision of environment and furniture measure, surveillance of body posture and interviews. The results pointed out that the furniture is inappropriate referring to: fixed counter without foot support, without document case; chairs are not regulative, back support width out of recommended standards, there are no armrest and wheels, there are only four legs and the covering is slimy and stiff. Only in 18.6% of the observation period the students adopted to perfect posture in typing activity, that is, upright spinal column, elbows at the same level of the counter, deflected legs and supported feet. It is suggested that the workplace be planned for typing from known facts related to anthropometric dimension of the users and use of proper furniture in order to obtain a concordant set among furniture, milieu and user providing comfort and avoiding health problems.

McKay, W. P.; Gregson, P. H.; McKay, B. W.; Blanchet, T. (1998) Resting muscle sounds in anesthetized patients, Can J Physiol Pharmacol, 76, 4, 401-6.

Abstract: It is known that contracting muscle makes low frequency sound vibrations. Small vibrations of uncertain origin are found over resting muscle. These could be shown to be of muscle origin if they significantly diminish in response to agents expected to decrease muscle activity. Thiopental, propofol, and neuromuscular-junction blocking muscle relaxants have such properties. Twenty-one subjects slated for elective surgery for which they would routinely be anesthetized and paralyzed gave informed consent to having a small accelerometer taped upon their supine biceps (9 subjects), or volar forearm (12 subjects). Recordings were made in four stages while subjects: (i) lifted a 2-kg weight just off the sponge armrest on which their outstretched arm lay; (ii) relaxed their arm in the awake state prior to anesthesia; (iii) had anesthesia induced with intravenous thiopental (n = 11) or propofol (n = 10); and (iv) were paralyzed. Recordings were digitized at 172-Hz and 6-s segments fast Fourier transformed (FFT). Total signal power, as determined by the area under the power spectrum, was significantly different (p 0.05) in all stages for the biceps and in all but stages (iii) from (iv) in the forearm.

McMillan, D. W.; McNally, D. S.; Garbutt, G.; Adams, M. A. (1996) Stress distributions inside intervertebral discs: the validity of experimental "stress profilometry", Proc Inst Mech Eng [H], 210, 2, 81-7.

Abstract: This paper evaluates a technique for measuring the distribution of compressive stress within cadaveric intervertebral discs. A strain-gauged pressure transducer, side-mounted near the tip of a 1.3 mm diameter needle, was inserted into cubes of disc tissue and into intact discs. Regardless of the position and orientation of the transducer within the tissue or disc, its output was found to be proportional to the compressive force applied to the specimen. The distribution of compressive stress was measured by pulling the instrumented needle through the specimen and the resulting stress profiles were reproducible to within 20 per cent. Profiles obtained at different applied loads showed a similar distribution of stress within the disc, suggesting that the compressive stress at any location and direction increased in proportion to the applied load. Since transducer output was also proportional to applied load, it was reasoned that it must be proportional to compressive stress within the disc. The average vertical compressive stresses acting on various regions within a disc were calculated from the stress profiles and multiplied by the cross-sectional area of each region: the resulting force was then compared with the known applied force in order to assess the calibration coefficient of the transducer. Agreement between the two forces was good, indicating that the calibration coefficient established in a saline bath was applicable to disc tissues also. However, artifactual stress peaks could be generated if the transducer was pulled across a bony asperity. It is concluded that the transducer measures the mean compressive stress acting upon it within disc tissues. Errors associated with the technique are small compared to differences in stress distributions, which occur naturally, for example when interal discs are loaded to simulate different postures in a living person.

Moore, S.; Brunt, D. (1991) Effects of trunk support and target distance on postural adjustments prior to a rapid reaching task by seated subjects, Arch Phys Med Rehabil, 72, 9, 638-41.

Abstract: The purpose of this study was to determine the presence of postural adjustments before a reaching task by seated subjects. Nine seated subjects performed a rapid reaching task to a target placed at shoulder height, 45° to the right of midline. Reach time, timing and magnitude of forces produced by the movement, and onset of deltoid muscle activity were analyzed under supported versus unsupported, and near versus far reach conditions. Reach time was proportional to reach distance. For all reach conditions, initial movement of the center of body mass occurred in a direction toward the target, i.e., opposite in direction to reactive forces produced by the accelerating arm. Regardless of support, reactive forces were recorded approximately 30 to 50 msec before arm movement, and represented a greater percentage of body weight, for the far reach conditions. Peak acceleration to the right occurred at proportionately the same time into reach regardless of support or reach distance. Peak acceleration occurred earlier in the supported conditions. This paper provides kinetic evidence of postural adjustments before a rapid reaching task performed by seated subjects.

Moore, S.; Brunt, D.; Nesbitt, M. L.; Juarez, T. (1992) Investigation of evidence for anticipatory postural adjustments in seated subjects who performed a reaching task, Phys Ther, 72, 5, 335-43.

Abstract: The paradigm for study of anticipatory postural adjustments permits investigation of the coordination of postural and voluntary components of functional movement. The purpose of this study was to investigate whether there were anticipatory postural adjustments for voluntary movement in seated subjects under clinically relevant conditions. Eight neurologically normal subjects performed a reaching task to a target placed at shoulder height, 45° to the right of midline. Onsets and magnitudes of lateral and fore-aft reactive forces associated with the movement and of electromyographic (EMG) activity of the ipsilateral deltoid and external abdominal oblique and contralateral paraspinal muscles were monitored. Conditions of trunk support, reach speed, and distance reached were manipulated. Onsets of deltoid muscle EMG activity preceded onsets of postural muscle (external oblique and paraspinal) EMG activity in 70% of all trials for seated subjects in contrast to reports of EMG activity onset in the postural muscles in advance of the prime mover in standing subjects who performed a similar task. The role of the trunk musculature and the significance of reactive forces in advance of hand movement were equivocal. This study has implications for evaluation of postural instability in persons unable to stand for testing.

Munro, B. J.; Steele, J. R.; Bashford, G. M.; Ryan, M.; Britten, N. (1998) A kinematic and kinetic analysis of the sit-to-stand transfer using an ejector chair: implications for elderly rheumatoid arthritic patients, J Biomech, 31, 3, 263-71.

Abstract: Twelve elderly female rheumatoid arthritis patients (mean age = 65.5 ± 8.6 yr.) were assessed rising from an instrumented Eser Ejector chair under four conditions: high seat (540 mm), low seat (450 mm), with and without the ejector mechanism operating. Sagittal plane motion, ground reaction forces, and vertical chair armrest forces were recorded during each trial with the signals synchronized at initial subject head movement. When rising from a high seat, subjects displayed significantly (p 0.05) greater time to seat off; greater trunk, knee and ankle angles at seat off; increased ankle angular displacement; decreased knee angular displacement; and decreased total net and normalized arm rest forces compared to rising from a low seat. When rising using the ejector mechanism, time to seat off and trunk and knee angle at seat off significantly increased, whereas trunk and knee angular displacement, and total net and normalized arm rest forces significantly decreased compared to rising unassisted. Regardless of seat height or ejector mechanism use, there were no significant differences in the peak, or time to peak horizontal velocity of the subjects’ total body center of mass, or net knee and ankle moments. It was concluded that increased seat height and use of the ejector mechanism facilitated sit-to-stand transfers performed by elderly female rheumatoid arthritic patients. However, using the ejector chair may be preferred by these patients compared to merely raising seat height because it does not necessitate the use of a footstool, a possible obstacle contributing to falls.

Munton, J. S.; Ellis, M. I.; Wright, V. (1984) Use of electromyography to study leg muscle activity in patients with arthritis and in normal subjects during rising from a chair, Ann Rheum Dis, 43, 1, 63-5.

Abstract: A previous study indicated the need for patients with arthritis to have an armchair from which it is easy to rise. To determine criteria for such a chair a greater understanding of the rising activity and the objective assessment of chair design is required. The major muscle groups of the leg were monitored by electromyography (EMG) in normal subjects and for patients with arthritis during rising from a chair. The effects on EMG patterns of changes of seat height, foot position, and the use of armrests were studied. This paper outlines the practical difficulties that must be borne in mind when designing an EMG study on arthritic and elderly subjects. The results did not illustrate any differences in the pattern of muscle activity between arthritic and normal subjects, nor did they show any differences caused by changing the variables of chair design.

Murthy, G.; Kahan, N. J.; Hargens, A. R.; Rempel, D. M. (1997) Forearm muscle oxygenation decreases with low levels of voluntary contraction, J Orthop Res, 15, 4, 507-11.

Abstract: The purpose of our investigation was to determine if the near infrared spectroscopy technique was sensitive to changes in tissue oxygenation at low levels of isometric contraction in the extensor carpi radialis brevis muscle. Nine subjects were seated with the right arm abducted to 45°, elbow flexed to 85°, forearm pronated 45°, and wrist and forearm supported on an armrest throughout the protocol. Altered tissue oxygenation was measured noninvasively with near infrared spectroscopy. The near infrared spectroscopy probe was placed over the extensor carpi radialis brevis of the subject’s right forearm and secured with an elastic wrap. After 1 minute of baseline measurements taken with the muscle relaxed, four different loads were applied just proximal to the metacarpophalangeal joint such that the subjects isometrically contracted the extensor carpi radialis brevis at 5, 10, 15, and 50% of the maximum voluntary contraction for 1 minute each. A 3-minute recovery period followed each level of contraction. At the end of the protocol, with the probe still in place, a value for ischemic tissue oxygenation was obtained for each subject. This value was considered the physiological zero and hence 0% tissue oxygenation. Mean tissue oxygenation (±SE) decreased from resting baseline (100% tissue oxygenation) to 89 ± 4, 81 ± 8, 78 ± 8, and 47 ± 8% at 5, 10, 15, and 50% of the maximum voluntary contraction, respectively. Tissue oxygenation levels at 10, 15, and 50% of the maximum voluntary contraction were significantly lower (p 0.05) than the baseline value. Our results indicate that tissue oxygenation significantly decreases during brief, low levels of static muscle contraction and that near infrared spectroscopy is a sensitive technique for detecting deoxygenation noninvasively at low levels of forearm muscle contraction. Our findings have important implications in occupational medicine because oxygen depletion induced by low levels of muscle contraction may be directly linked to muscle fatigue.

Myhr, U.; von Wendt, L. (1991) Improvement of functional sitting position for children with cerebral palsy [see comments], Dev Med Child Neurol, 33, 3, 246-56.

Abstract: Twenty-three children with cerebral palsy were photographed and video-filmed in six different sitting positions – including a hypothetical functional position – and the video-films and photographs were analyzed. It was found that pathological movements were minimized and postural control and arm and hand function best when the child was sitting in a forward-tipped seat, with a firm backrest supporting the pelvis, arms supported against a table and feet permitted to move backward.

Nelson, N. A.; Silverstein, B. A. (1998) Workplace changes associated with a reduction in musculoskeletal symptoms in office workers. Hum Factors, 40, 2, 337-50.

Abstract: The purpose of this study was to identify factors associated with reductions observed in musculoskeletal symptoms when office workers were moved to a new building. A questionnaire including items regarding symptoms and aspects of the work environment was administered to 577 office workers before and after they were moved from nine buildings to a single new facility in 1992. Employees working in two reference buildings, where they remained throughout the study period, were also surveyed. Two musculoskeletal outcomes, hand/arm and neck/shoulder/back, were selected for study. In matched multivariate analyses, the reduction in hand/arm symptoms from 1992 to 1993 was associated with improved satisfaction with the physical workstation (odds ratio [OR] = 2.0); the reduction in neck/shoulder/back symptoms was associated with improved chair comfort (OR = 1.8), fewer housekeeping responsibilities (OR = 3.6), female gender (OR = 1.8), and low pay range (OR = 1.7). Longitudinal results suggested that changes in workstations resulted in decreased symptoms. Results of this investigation might be used to develop workplace changes that result in reductions of musculoskeletal disorders.

Netea, R. T.; Bijlstra, P. J.; Lenders, J. W.; Smits, P.; Thien, T. (1998) Influence of the arm position on intra-arterial blood pressure measurement, J Hum Hypertens, 12, 3, 157-60.

Abstract: The reference level for the measurement of blood pressure (BP) is the level of the right atrium. In practice this is regularly disregarded, as the patient’s arm is usually placed lower than the right atrial level. The aim of the study was to determine the influence of first, different arm positions and second, different transducer positions on the intra-arterially (i.a.) recorded BP. In 16 healthy men (age 28.1 ± 8.0 (s.d.) years), i.a. BP was recorded at the left arm in supine position, using a 5-7 cm long cannula. The baseline position was with the tip of the cannula placed precisely at the level of the right atrium. Subsequently, the following changes were made: 5, 10, 15 and 20 cm above and 5, 10, 15, and 20 cm below the baseline position. A 2-min rest period was allowed in each position before the BP was measured. The whole procedure was done either with the transducer connected to the arm at the place of the cannula (n = 7), or with the transducer placed next to the subject and continuously kept at the right atrial level during the BP measurement (n = 9). Simultaneously, baseline BP was measured indirectly, with a standard mercury sphygmomanometer, in the opposite arm maintained with the Cubital fossa at the right atrial level during the whole procedure. This resulted in the first group of seven volunteers for both the i.a. systolic (SBP) and diastolic BP (DBP) values to significantly decrease (P 0.001) when the arm together with the transducer were elevated above the level of the right atrium, and returned to the initial value when the arm and the transducer were placed back at the right atrial level. Intra-arterial SBP and DBP significantly (P 0.001) increased as the arm, together with the transducer, were lowered below the right atrial level and returned to the initial value when the arm and the transducer were placed back at the right atrial level. In both directions, each 5-cm change in the arm level was accompanied by a 3-4 mm Hg change in the i.a. BP value. The baseline BP, measured sphygmomanometrically at the contralateral arm, remained constant during the whole duration of the procedure. The changes in the i.a. BP were minimal in the second group of nine subjects in which only the arm but not the transducer was placed at different levels. We conclude that small deviations in arm position above or below the "gold standard", i.e., the fossa cubiti at the right atrial level, will result in largely erroneous BP values. The correct positioning of the arm during BP measurement is therefore mandatory for the diagnosis and follow-up of hypertensive subjects.

Oberg, T. (1993) Ergonomic evaluation and construction of a reference workplace in dental hygiene: a case study, J Dent Hyg, 67, 5, 262-7.

Abstract: Purpose. In an earlier survey study, a high frequency of complaints from the neck and shoulder were found in dental hygienists employed by the National Dental Service in Jonkoping, Sweden. As a consequence of these survey findings, a new in-depth study of the workplace of one Swedish dental hygienist was performed to identify critical factors causing work-related pain in the neck and shoulders. This also served as a pilot study for testing simple ergonomic methods suitable for field studies in dental hygiene. Methods. Several workplace evaluation methods were used: time distribution study, posture targeting diagrams, biomechanical computations, serial photography, and video recording. Results. All methods showed the same general picture – fixed working postures, a sparse movement pattern, work within a very limited work space, and long-standing static load on the neck and shoulder muscles. Conclusions. As a result of these findings, a reference workplace was designed to provide an ergonomically desirable environment. The load was reduced by means of a specific horseshoe-shaped support for the patient chair and a special armrest for the operator chair.

Palanisami, Prabhu, Narasimhan, T.M. and Fernandez, J. E. (1994) The effect of sitting on peak pinch strength. Aghazadeh, F., Advances in Industrial Ergonomics and Safety VI, San Antonio, Taylor and Francis, 587-594.

Abstract: This study compares the effect of various forearm and elbow postures on peak chuck pinch strength in sitting with armrest, sitting without armrest, and standing postures. Twenty able-bodied male subjects volunteered to participate in the study. Results indicate that elbow, forearm and body postures had significant effects on pinch strength. The effect of sitting on pinch strength was significantly lower than standing. A reduction in pinch strength in these postures is documented and needs to be considered in the ergonomic design of the workplace and tools.

Paul, R., Lueder, R., Selner, A., Limaye, J. (1996) Impact of New Input Technology on Design of Chair Armrests: Investigation on Keyboard and Mouse, Human Centered Technology – Key to the Future. Proceedings of the Human Factors & Ergonomics Society 40th Annual Meeting, Philadelphia, Pennsylvania, September 2-6, 1996., The Human Factors & Ergonomics Society, Santa Monica, California, 1, 380-384.

Abstract:  This study investigated the effects of addition of a mouse-input device on the design of chair armrests. Eleven subjects performed a VDT task for four hours sitting in a chair under three armrest conditions: 1) no armrests, 2) height adjustable armrests, and 3) height and rotation adjustable armrests. The VDT task consisted of 90 minutes of graphics work using a mouse and 60 minutes of keying. The three experimental conditions were performed on three separate days in a random sequence. Muscle fatigue in the forearm (flexor and extensor) and neck-shoulder (trapezius) muscles were measured using surface electromyography. Subjects’ working posture was measured using a SVHS camera and postural analysis was conducted using the Ariel Digitizing System. Discomfort and subjective preferences were recorded using the Corlett Scale and other questionnaires. The results suggest that height-adjustable armrests did not provide effective forearm support during mouse use; while the height and rotation adjustable armrests provided superior arm support. For keyboard work, both armrests reduced neck and shoulder fatigue measured in terms of the frequency shift. These results highlight the need for proactive research to tailor workplace design to match the demands imposed by new office technologies.

Pifer, R. G. (1978) Armrest modification for support of paretic or paralyzed upper extremity, Phys Ther, 58, 2, 182-3.

Pritz, M. B.; Hopkins, J. W. (1980) Armrest for STA-MCA bypass surgery. Surg Neurol. 14, 5, 370.

Raschke, G. (1979) [The severely disabled: a new way of driving a car (author’s trans.)], Rehabilitation (Stuttg), 18, 4, 218-23.

Abstract: In many cases, the provision of an automobile equipped with a special control system is a necessary part of the rehabilitation of the severely disabled. The car is so important to the disabled as it enables him to overcome daily the distance between home and place of work, hence increasing his mobility and life quality. The presented care control system was developed to meet the individual needs of a particular disability. It is, however, possible to use this system for, or adapt it to, similar types of disabilities. This problem complex has been overcome by following innovations: 1. The electronically controlled accelerator which can be operated with minimal finger pressure. 2. The LIDA shoulder device for tetraplegics, a connecting piece between the shoulder and the manually operated brake lever. 3. The removable armrest on the right hand side of the driver’s seat to improve the sitting stability. The car can be easily be converted for "normal driving". The author received financial assistance.

Rehkopf, P. G.; Lobes, L. A., Jr.; Grand, G. (1978) Armrest support for the slit-lamp delivery argon laser system, Am J Ophthalmol, 85, 5 Pt 1, 717.

Abstract: We designed a new armrest for use at the argon laser. The arm support can be used on either the right- or left-hand side of slit-lamp tables of varying thicknesses. The continuous screw adjustment allows for easy height adjustment of the arm support. The construction of the armrest facilitates the adjustment and manipulation of a fundus contact lens, which is so necessary to the successful performance of argon laser photocoagulation.

Riley, P. O.; Schenkman, M. L.; Mann, R. W.; Hodge, W. A. (1991) Mechanics of a constrained chair-rise, J Biomech, 24, 1, 77-85.

Abstract: A sit-to-stand task is analyzed by a method which estimates the segmental and whole body center of mass (CoM) kinematics and kinetics using bilateral whole body kinematic data from nine healthy young female subjects. The sit-to-stand, or chair-rise, task is constrained with regard to chair height, pace, initial lower limb position and arm use. The chair-rise maneuver is divided into four phases; (1) the flexion momentum phase; (2) the momentum transfer phase; (3) the vertical extension phase; and (4) the stabilization phase; the first three are examined in detail here. The momentum transfer phase, which immediately follows lift-off from the seat of the chair, is the most dynamic portion of the event, demanding a high degree of coordination. This maneuver is analyzed in order to determine if trunk movement is used only to position the body center of gravity or if the trunk motion generates momentum which is important during the brief but critical period of dynamic equilibrium immediately following lift-off from the chair. Our evidence points to the latter case and indicates that inter-segmental momentum transfer is possible during this period.

Rodosky, M. W.; Andriacchi, T. P.; Andersson, G. B. (1989) The influence of chair height on lower limb mechanics during rising. J Orthop Res. 7, 2, 266-71.

Abstract: The mechanics of the lower limb were analyzed in young, adult normal subjects when rising from a seated position. Limb mechanics were described in terms of flexion-extension motion and moments at the hip, knee and ankle while rising from four seat heights corresponding to 65, 80, 100, and 115% of the subject’s knee joint height. The results indicate that the maximum moment tending to flex the hip joint was higher than that occurring at the knee or ankle. The magnitude of the maximum flexion moment at the hip was not substantially influenced by chair height, changing by less than 12% between the highest and lowest chair heights. Conversely, the maximum knee flexion moments were found to be highly dependent on chair height and nearly doubled from the highest to the lowest position. The magnitude of the moments at the ankle did not change with chair height and were significantly lower than the magnitude of the moments found during normal walking. The magnitude of motion and moments at the hip were greater during chair-rising than during stair-climbing or walking. The range of motion required at the knee for the lower chair heights was also greater than was reported during stair-climbing studies. Thus, the combination of moments in joint angles during chair-rising are unique among common activities of daily living and should be considered in chair selection as well as in the guidelines for prosthetic devices.

Salo, R. E. (1978) A hammock wheelchair armrest, Am J Occup Ther, 32, 8, 525.

Schatz, H. (1982) Three-way adjustable armrest for fluorescein angiography, Am J Ophthalmol, 93, 2, 244.

Schneck, D. J. (1978) Aerodynamic forces exerted on an articulated human body subjected to windblast, Aviat Space Environ Med, 49, 1 Pt. 2, 183-90.

Abstract: A potential flow solution is presented for estimating the pressure distribution around the forearm of a human body subjected to windblast. The forearm is examined in three positions: resting and pressing against an armrest; resting, but not pressing against and armrest; and not resting at all against any surface. Results show that a high-speed wind stream approaching the limb at some finite angle of attack has a tendency to dislodge the forearm from a surface with which it is in contact. This is due to the generation of stagnation points in the flow, which lead to adverse pressure gradients as high as six times the free-stream dynamic pressure. When the inviscid analysis is corrected for the effects of flow separation, it is possible to predict the presence of a pressure drag, which acts to throw the forearm outward, away from the thorax. Both of these effects increase with angle of attack and they are mildly dependent on the taper of the forearm geometry.

Schultz, A. B.; Alexander, N. B.; Ashton-Miller, J. A. (1992) Biomechanical analyses of rising from a chair, J Biomech, 25, 12, 1383-91.

Abstract: Quantification of the biomechanical factors that underlie the inability to rise from a chair can help explain why this disability occurs and can aid in the design of chairs and of therapeutic intervention programs. Experimental data collected earlier from 17 young adult and two groups of elderly subjects, 23 healthy and 11 impaired, rising from a standard chair under controlled conditions was analyzed using a planar biomechanical model. The joint torque strength requirements and the location of the floor reaction force at liftoff from the seat in the different groups and under several conditions were calculated. Analyses were also made of how body configurations and the use of hand force affect these joint torques and reaction locations. In all three groups, the required torques at liftoff were modest compared to literature data on voluntary strengths. Among the three groups rising with the use of hands, at the time of liftoff from the seat, the impaired old subjects, on an average, placed the reaction force the most anterior, the healthy old subjects placed it intermediately and the young subjects placed it the least anterior, within the foot support area. Moreover, the results suggest that, at liftoff, all subjects placed more importance on locating the floor reaction force to achieve acceptable postural stability than on diminishing the magnitudes of the needed joint muscle strengths.

Schulze, R. R. (1979) Armrest for microsurgery, Am J Ophthalmol, 88, 6, 1097.

Seeger, B. R.; Sutherland, A. D. (1981) Modular seating for paralytic scoliosis: design and initial experience, Prosthet Orthot Int, 5, 3, 121-8.

Abstract: The conventional wheelchair sling seat provides little or no support to the spine of a child with myopathy or neurogenic muscular weakness. As the spinal muscles become weaker scoliosis may develop with associated deformity, pain and restriction of cardio-respiratory function. If muscle weakness is severe, the resultant fully developed deformity is virtually impossible to treat. Slowing the rate of increase of the deformity is, therefore, the most hopeful avenue of attack. This work addresses the hypothesis that custom moulded seating can increase sitting comfort and slow the rate of progression of spinal curvature in children with paralytic scoliosis, and further, that a range of standard or modular seats can achieve these goals at less cost. Previous work on this problem has ranged from simply padding the armrest, in order to distribute force over the rib cage, through to custom moulded seating. Our initial experience with custom moulding, using the beanbag evacuation and consolidation technique, produced several comfortable seats although the technique was labour intensive and therefore costly. This led us to attempt to develop a method of providing comfortable seating that would help control spinal deformity at reasonable cost. This paper describes the design of a standardized seating system for school age children with myopathy or neurogenic muscular weakness. Preliminary results indicate that this technique may have advantages over alternative methods of treatment. The radiological study is continuing.

Snijders, C. J.; Slagter, A. H.; van Strik, R.; Vleeming, A.; Stoeckart, R.; Stam, H. J. (1995) Why leg crossing? The influence of common postures on abdominal muscle activity, Spine, 20, 18, 1989-93.

Abstract: Study Design: Abdominal muscle activity is recorded in the supine position, unconstrained standing, and in the sitting position on an office chair with the use of backrest and armrests, with and without crossed legs. Objectives: To assess the role of oblique abdominal muscles in relation to the stability of lumbar spine and pelvis in commonly adopted unconstrained postures. Summary of background data: Cross-legged sitting is very common for men and women. No solid evidence exists for either a beneficial or a detrimental effect of this posture. No electromyographic study deals with the activity of abdominal muscles in this commonly adopted unconstrained posture. Methods: In healthy subjects, electromyographic activity of the rectus abdomini and external and internal oblique abdominals was recorded bilaterally during commonly adopted unconstrained postures. Results: The activity of the internal oblique muscle was significantly higher in the sitting position than in supine position. For the external and internal oblique abdominals, the activity was significantly higher in the standing position than in the sitting position. When sitting, the activity of the oblique abdominals is significantly lowered by crossing the legs in the preferred way (either upper legs cross or ankle on knee). In contrast, leg crossing does not significantly alter the activity of the rectus abdominis. Conclusions: From these remarkable findings, we conclude that leg crossing is physiologically valuable. It should be studied whether leg crossing can be implemented in the design of the workplace.

Stokes, M.; Blythe, M. (1995) Muscle sounds rediscovered [letter; comment]. Lancet. 346, 8977, 779.

Swan, P. D.; Spitler, D. L.; Todd, M. K.; Maupin, J. L.; Lewis, C. L.; Darragh, P. M. (1989) Effects of posture on upper and lower limb peripheral resistance following submaximal cycling, Arch Phys Med Rehabil, 70, 9, 678-80.

Abstract: The purpose of this study was to determine postural effects on upper and lower limb peripheral resistance (PR) after submaximal exercise. Twelve subjects (six men and six women) completed submaximal cycle ergometer tests (60% age-predicted maximum heart rate) in the supine and upright seated positions. Each test included 20 minutes of rest, 20 minutes of cycling, and 15 minutes of recovery. Stroke volume and heart rate were determined by impedance cardiography, and blood pressure was measured by auscultation during rest, immediately after exercise, and at minutes 1-5, 7.5, 10, 12.5, and 15 of recovery. Peripheral resistance was calculated from values of mean arterial pressure and cardiac output. No significant (p < 0.05) postural differences in PR were noted during rest for either limb. Immediately after exercise, PR decreased (55% to 61%) from resting levels in both limbs, independent of posture. Recovery ankle PR values were significantly different between postures. Upright ankle PR returned to 92% of the resting level within four minutes of recovery, compared to 76% of the resting level after 15 minutes in the supine posture. Peripheral resistance values in the supine and upright arm were not affected by posture and demonstrated a gradual pattern of recovery similar to the supine ankle recovery response (85% to 88% of rest within 15 minutes). The accelerated recovery rate of PR after upright exercise may result from local vasoconstriction mediated by a central regulatory response to stimulation from gravitational pressure on lower body circulation.

Tyler, A. E.; Hasan, Z. (1995) Qualitative discrepancies between trunk muscle activity and dynamic postural requirements at the initiation of reaching movements performed while sitting, Exp Brain Res, 107, 1, 87-95.

Abstract: Reaching movements are associated with widespread, nonfocal muscle activity. That activity is often assumed to play a postural role. We tested this assumption for the trunk muscles at the initiation of reaching movements with the following question. Does initial trunk muscle activity play a dynamic postural role by resisting the segmental interactive effects of the arm movement on the trunk? Seated subjects performed bilateral reaching movements while target direction was systematically varied. Muscle activity was recorded from flexors and extensors of the trunk and shoulder. Trunk muscle activity was compared with trunk torques calculated from simulations of reaching movements in which the trunk was modeled to stay still. Recorded trunk muscle activity was in qualitative agreement with torque predictions for only some target directions, suggesting that the target directions to counteract postural disturbances at the initiation of reaching movements.

Usaj, A. (1996) The increase of duration of isometric contraction may not relate to change of relative oxygenation of forearm muscle, Pflugers Arch, 431, 6, R265-6.

Abstract: The aim of this study is to ascertain, how an increase in duration of isometric contraction influences tissue oxygenation of forearm muscles, undergoing physical training. Four subjects underwent 6 weeks of training of the left forearm muscles by performing isometric contractions. Subjects repeated 10 s contractions, 10-15 times in the first, and 20 s contractions in the second 3-week period. Relative oxygenation of forearm muscles was measured by using. Near Infrared Spectroscopy (CWS2000, NIM Incorporated, Philadelphia). The training increased the duration of isometric contraction at 20 kp of experimental forearm muscles by 41 ± 25 s, which was more (P 0.05) than in control forearm (8 ± 4 s). This increase was not reflected in changes of maximum relative deoxygenation of experimental muscles, which decreased by only -6.9 ± 14.4%. The results show that an increase in duration of isometric contraction may not depend on the oxygenation of muscle tissue at fixed force of 20 kp.

Viano, D. C. (1994) Comparison of arm up and down in side impacts with BioSID and different armrests, J Biomech Eng, 116, 3, 270-7.

Abstract: BioSID dummy tests were run with the arm down at the side during loading of different armrests in simulated side impact crashes. The Hyge sled tests duplicated previous studies of BioSID with the arm up, SID, and animals. When the BioSID arm is against the side, the arm extends from the shoulder to the bottom of the third rib and has a steel shank covered by foam and vinyl. Loading through the arm transfers force to the three chest ribs and shoulder. In comparison, direct armrest loading of the chest or abdomen primarily involves a single rib and substantial rib deflection, when the armrest crush-force exceeds the strength of the rib. The Viscous response in BioSID showed the greatest difference of all criteria for the arm up or down. The response of the third rib correlated with injury risks determined from animal tests using the different armrest designs in a simulated high position. While injury data are not available for the arm at the side or for the armrest in the low position, the STIFF armrest may cause injury when the arm is not at the side and the armrest loads the liver and spleen. Rib deflection in BioSID showed the protrusion of the STIFF armrest into the abdominal region in both arm positions, because the loading was below the arm even in the down position. However, the arm extends laterally so it involves the upper ribs earlier than in the arm-up condition where more space is available.

\ Viano, D. C.; Andrzejak, D. V. (1993) Biomechanics of abdominal injuries by armrest loading, J Trauma, 34, 1, 105-15.

Abstract: Anesthetized pigs were exposed to impact against a padded side interior that included an armrest with either a SOFT or STIFF crush characteristic. The purpose was to assess liver and spleen injury under specific impact conditions. The STIFF armrest resulted in severe abdominal and thoracic injury in five side-impact animal tests. Injuries of the liver and spleen included deep lacerations, tears of major hepatic arteries and veins, and serious hemoperitoneum. The injuries averaged AIS = 4. In contrast, five animals exposed to the SOFT armrest design experienced injuries of lower severity than any animal in the STIFF armrest exposures (p 0.005). The average injury was AIS = 2. The STIFF armrest protruded into the abdomen and showed little sign of deformation with abdominal loading. This situation is consistent with the occurrence of lacerations at hepatic junctions and between lobes, and displaced rib fractures in several cases. The SOFT armrests crushed fully in each test, indicating that abdominal compression was lower and was limited by armrest deformation.

Viitasalo, J. T.; Gajewski, J.; Wit, A. (1994) Forearm tremor during three different isometric loadings, Electromyogr Clin Neurophysiol, 34, 3, 131-6.

Abstract: Forearm tremor was studied during a spring (stiffness 1090 N.m-1), a rigid isometric and a "dynamic" isometric (carrying a freely hanging mass) loading at the level of 50% of maximal isometric voluntary contraction. Thirteen physical education students ranging in age between 20 and 28 years flexed their dominant forearm isometrically towards the vertical direction (90° elbow angle) against the three different loads on three test occasions seated on a dynamometer which measured the force at the wrist together with vertical tremor (accelerometer). A power spectrum density function was established for the tremor (acceleration) between 1.0 and 19.9 Hz. A bandwidth of 6.9 – 19.9 Hz was subsequently analyzed in more detail including the determination of peak power (PMAX), peak power frequency (FMAX), mean power frequency (MPF), and average power (PAVER) as well as proposition (%) of the whole spectrum occupied by the selected bandwidth. The FMAX, MPF and band percentage variables had the best reproducibility (Cronbach’s Alpha 0.85-0.95), while for the PMAX and PAVER the coefficients were lower but still satisfactory (0.69-0.89). The coefficients were rather similar for all three loading conditions. In the spring loading the spectrum components inside the analyzed frequency band occupied almost 90% of the whole spectrum, FMAX was more clearly distinguished from the rest of the spectrum, and tremor amplitude was higher and tremor frequency lower than in the rigid isometric and "dynamic" loadings. The respective tremor amplitude and frequency characteristics showed statistically significant correlations between the rigid isometric and "dynamic" loading conditions.

Viitasalo, J. T.; Mikkonen, S.; Salonen, M.; Aura, O.; Gajewski, J.; Wit, A. (1994) Forearm tremor in relation to selected physiological and body dimensional variables, J Sports Med Phys Fitness, 34, 3, 228-34.

Abstract: Interrelationships between forearm tremor and a number of body dimensional, muscle structure, muscular strength and training background variables were studied among 13 male students with athletic backgrounds. The subjects performed isometric dominant upper extremity elbow flexions with a 90° joint angle and with the forearm held in a horizontal position. A freely hanging mass was attached via a strain gauge transducer, a metal chain and cuff to the forearm. An accelerometer attached to the cuff measured the vertical component of tremor. The power spectrum density function was calculated for a tremor acceleration signal and a bandwidth of 7-20 Hz was analyzed in more detail. The right M. vastus lateralis was biopsed in order to determine the muscle fiber composition. Arm mass and muscle fiber compositions were found to correlate statistically significantly with the tremor frequency characteristics. In the further analyses arm mass was found to be the only variable explaining the tremor frequency characteristics; the effects of the muscle structure variables were minor when the effects of arm mass was controlled in partial correlation analyses. Interindividual differences in motor unit firing properties were presumed to explain the dependence found between arm mass and tremor frequency.

Wheeler, J.; Woodward, C.; Ucovich, R. L.; Perry, J.; Walker, J. M. (1985) Rising from a chair. Influence of age and chair design, Phys Ther, 65, 1, 22-6.

Abstract: We studied the effect of age on the act of rising from a standard armchair in a younger (means = 24 years) and an older (means = 75 years) group of healthy adult women. Rising from a standard armchair and an armchair specially designed for comfort in sitting of the elderly was studied in the older group to determine the influence of the special chair. We used electrogoniometry, EMG, and videotape analysis to record the activity for both groups. The older group placed their feet farther back and showed greater vastus lateralis muscle activity than did the younger group to rise from the standard chair. These results suggest that rising from the standard chair was more difficult for the older than for the younger group. In the special chair, the older subjects showed even more vastus lateralis muscle activity, greater knee flexion, and greater trunk forward lean. Rising from the special chair, therefore, appeared to be more difficult than rising from a standard chair; this finding suggests that both comfort and function must be considered in chair selection for certain groups.

Wee, A. S.; Ashley, R. A. (1989) Vibrations and sounds produced during sustained voluntary muscle contraction, Electromyogr Clin Neurophysiol, 29, 6, 333-7.

Abstract: In 20 normal subjects, sounds or vibration signals were recorded from the biceps brachii muscle during voluntary isometric contraction. Fourier analysis showed presence of predominantly low frequency components with little contributions beyond 60 or 70 Hz. Relatively high amplitude peaks occurring below 20 Hz were seen in the frequency spectrum with the most prominent of these peaks occurring at a mean frequency of 11.3 Hz. A majority of subjects had one or two additional peaks, each appearing on either side of the major peak with mean frequencies of 7.3 and 16.2 Hz. In general, subjects who could not sustain a very steady contraction had more peaks compared to those who were able to maintain a smoother contraction. Between 20 and 50 Hz, several well-defined but much smaller peaks were also seen. The frequencies of some of these small peaks can be expressed as exact harmonics of the previously described.

Wise, A. K.; Gregory, J. E.; Proske, U. (1996) The effects of muscle conditioning on movement detection thresholds at the human forearm, Brain Res, 735, 1, 125-30.

Abstract: We have used the muscle history dependence of the sensitivity of muscle spindles to stretch, to provide evidence for their contribution to kinesthesia, the sense of position and movement. Stretch sensitivity is altered depending on whether or not slack has been introduced in intrafusal fibres [13]. At the human elbow joint detection threshold was measured to passive movements applied at different speeds to the forearm after a conditioning co-contraction of muscles of the upper arm, with the arm held either flexed (’hold short’) or extended (’hold long’). Test measurements were made with the elbow joint at 90°. For the three speeds of movement, 2° s-1, 0.2° s-1 and 0.02° s-1, after "hold short" conditioning thresholds were lower for movements into extension, after "hold long" conditioning they were lower for movements into flexion. It is concluded that when muscle conditioning introduces slack in the intrafusal fibres of muscle spindles, the test movements must take this up before the subject can detect them. It means that whenever detection thresholds to passive movements are measured at a joint, the contraction history of the muscles acting at that joint must be taken into account.

Wise, A. K.; Gregory, J. E.; Proske, U. (1998) Detection of movements of the human forearm during and after co-contractions of muscles acting at the elbow joint, J Physiol (Lond), 508, Pt 1, 325-30.

Abstract: 1. We report here observations on the effects of muscle contraction history on thresholds for the detection of movements at the elbow joint of human subjects. Detection thresholds were measured in the direction of flexion or extension to movements of the relaxed forearm at a speed of 0.2° s-1 with the elbow at 90°. 2. As reported previously, thresholds for movements in the direction of extension were lower than in the direction of flexion after a conditioning co-contraction of elbow muscles with the arm flexed by 30° from its mid-position (hold-short). After a co-contraction with the arm held extended by 30° (hold-long), thresholds were lower in the direction of flexion. 3. Here we have made two additional observations. Thresholds for movements of the passive forearm after a co-contraction at the 90° test position (hold-test) were low, both in the direction of flexion and extension. Secondly, when thresholds were measured while subjects were carrying out a co-contraction of forearm muscles (15-20% maximum voluntary contraction), thresholds were much higher. 4. It is concluded that muscle contraction history is an important factor to consider when making measurements of movement thresholds at the relaxed elbow joint. It is speculated that during an active contraction increases in muscle spindle discharges evoked by fusimotor activity lead to the rise in movement detection threshold.

Nottingham sit-stand seating  |    More sitting postures & chair design

Sitting & seating ergonomics  |  ErgoExpo slides NECE


Certificant, Board of Certification in Professional Ergonomics

Reach us Privacy & Copyright Valid Ergonomics XHTML 1.0 Transitional
©Humanics Ergonomics