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The goal of this study was to collect anthropometric data that describe the geometry of the surface of the hand and to model this anthropometry as a function of gross external hand measurements. The depth and breadth of each segment of the hand were measured at points that were spaced at approximately equal distances between the joints of the hand. Linear models using hand breadth as the independent variable explained from 12% to 47% of the variation in segment breadths and from 6% to 74% in segment depths. Ellipsoids are used in biomechanical models as an efficient mathematical description of the shape of kinematic segments for use in the determination of contact with other objects. Therefore the primary objectives of this study were to approximate the semiaxis dimensions for these hand segment contact bodies using a linear model of the gross anthropometry and to evaluate the accuracy with which ellipsoids describe the geometry of the hand segments. Graphical comparisons showed that differences between the ellipsoidal approximations and the breadth and depth measurements were largest near the joints. Data collected in this study could be used to create a set of overlapping ellipsoids that would provide a more accurate representation of hand geometry, and this representation could be adapted to biomechanical models that use ellipsoids to define segment geometry.

Objective: To quantify the physical demands of hose insertion tasks in automotive assembly operations and how they are affected by method and the mechanical interference between the hose and the flange. Background: Insertion tasks were identified by workers as physically demanding and can often lead to fatigue or losses in production attributable to pain or injury. Methods: Six male and 6 female participants pushed a 25.4-mm flexible rubber hose onto a stationary flange during simulated insertions. Three insertion methods — rock, straight, and twist — were examined in the study. Muscle activity of the finger flexors was recorded to estimate grip effort during the simulated insertions. Results: The twist method (114.8 N) resulted in a 26% reduction in axial force compared with the straight method (155.7 N). Average muscle activity ranged from a low of 14% maximum voluntary contraction (MVC; men, straight method) to a high of 67% MVC (women, twist method). Hose resultant forces ranged from a low of 52.2 N to a high of 461.1 N for all participants. Men exerted 6% higher resultant forces with 37% less muscle activity than women. Conclusion: There are situations when the 26% reduction in the axial force attributable to twisting may be helpful during an insertion, despite the fact that forearm muscle activity was highest for both male and female participants during twisting insertions. Application: The results of this study can be applied to the future design of tasks that involve the joining of two parts such as a hose and flange.

Objective: The aim of this work was to quantify the interrater reliability of a set of scales that assess repetition, posture, and force as used on site when examining industrial work. Background: Interrater reliability of observational assessment methods can vary depending on the definition of the methods and situations in which they are used. Method: In several industries, 846 jobs were assessed using pairs of analysts to rate the repetition, force, and posture of the upper limbs. Twelve analysts with varying experience levels participated. Results: Using an interclass correlation coefficient (ICC), force and repetition had reliability values of .60 and .71 before and .82 and .87 after discussion, respectively. After discussion, peak posture ratings had ICCs of .60 to .83. ICCs for average posture ratings ranged from .31 to .51 initially to .55 to .67 in final ratings. Less experienced analysts changed their initial ratings more than did the senior investigators. Conclusion: The high interrater reliability of the repetition and force metrics indicates that a single analyst is appropriate for basic job assessment. Posture ratings benefit greatly from a two-analyst system. Average postures should be assessed across a full range of the scale for interrater reliability assessment. Analyst pairs should be rotated to avoid forming biases. Application: For basic assessments of forceful exertions and repetitive motions, a single analyst can be used, reducing the resource requirements for both industry and large epidemiological studies.

The objectives of this research were to investigate the accuracy and precision with which trained and untrained participants estimate the magnitude of forceful exertion and to evaluate the mathematical relationship between actual and estimated exertion. Three groups of participants estimated, as a percentage of maximum voluntary contraction (%MVC), the magnitude of submaximal exertion for 12 simulated tasks. In addition to the control group, one group was exposed to one physical benchmark (100% MVC) and another to three benchmarks (25%, 75%, and 100% MVC) prior to force estimation. Error (estimated minus actual) significantly decreased (p < .0001) from 14% MVC to 4% MVC with one benchmark and to -3% MVC with three benchmarks, as compared with the control group. Furthermore, the standard deviation decreased significantly (p < .0001) from the control group (16.6% MVC) to the one-benchmark group (13.8% MVC) to the three-benchmark group (11.6% MVC), indicating improved precision. Significant interaction effects were observed, but their impact on main effects was negligible. Also, linear, power, and logarithmic regression models described the relationship between perceived and actual exertion equally well (R2 = .64-.81). Applications of this research include improving the accuracy and precision of field-based psychophysical estimates of forceful exertion for epidemiological research and other field-based analyses.

Objective: The aim was to determine the maximum force that can be exerted on an object before it is pulled or slips from the grasp of the hand ("breakaway strength") for fixed overhead handholds of varying orientation, shape, and friction. Background: Many studies have quantified hand strength by having participants squeeze, pull on, or create torque on an object or handle, but few studies have measured breakaway strength directly. Method: In two experiments, hand strength was measured as both overhead breakaway strength for handholds typical of fixed industrial ladders and as maximum isometric grip strength measured using a common Jamar grip dynamometer. Results: Breakaway strength was greatest for a fixed horizontal cylinder ("high friction"; 668 ± 40 N and 691 ± 132 N for Experiments 1 and 2, respectively), then for a horizontal cylinder that simulated low surface friction ("low friction"; 552 ± 104 N), then for a vertical cylinder (435 ± 27 N), and finally, for a vertical rectangular-shaped rail (337 ± 24 N). Participants are capable of supporting only their own body weight with one hand when grasping the fixed horizontal cylinder. Breakaway strength for both the high- and low-friction horizontal cylinders was significantly greater than isometric grip strength (1.58 ± 0.25 and 1.26 ± 0.19 times, respectively). Conclusion: Results support the hypothesis that hand-handhold coupling is composed of active (isometric or eccentric finger flexion) and passive (frictional) components. Traditional isometric grip strength alone does not predict the strength of a couple between a hand and a handhold well. Application: This research shows that handhold shape, orientation, and friction are important in the safe design of grab rails or ladders.

A laboratory study was conducted to determine the effects of work pace on typing force, electromyographic (EMG) activity, and subjective discomfort. We found that as participants typed faster, their typing force and finger flexor and extensor EMG activity increased linearly. There was also an increase in subjective discomfort, with a sharp threshold between participants' self-selected pace and their maximum typing speed. The results suggest that participants self-select a typing pace that maximizes typing speed and minimizes discomfort. The fastest typists did not produce significantly more finger flexor EMG activity but did produce proportionately less finger extensor EMG activity compared with the slower typists. We hypothesize that fast typists may use different muscle recruitment patterns that allow them to be more efficient than slower typists at striking the keys. In addition, faster typists do not experience more discomfort than slow typists. These findings show that the relative pace of typing is more important than actual typing speed with regard to discomfort and muscle activity. These results suggest that typists may benefit from skill training to increase maximum typing speed. Potential applications of this research includes skill training for typists.

Objective: The aim of this study was to quantify the effect of handhold orientation, size (diameter), and wearing a glove on the maximum breakaway strength between a hand and handhold. Background: Manual breakaway strength is known to be greatly reduced for vertical compared with horizontal handholds, but oblique orientations have yet to be studied. Method: For this study, 12 young adults (6 female) attempted to hold on to fixed overhead cylindrical handholds with one hand in low-speed simulated falls as forces on the handhold were recorded in two experimental designs. Breakaway strength was measured for (a) three different-sized cylinders in four orientations while the participants were using the dominant hand and (b) a single-sized cylinder in four orientations while the participants were bare-handed or wearing a glove on the nondominant hand. Results: Handhold orientation ( p < .001), handhold diameter ( p < .001), and wearing gloves ( p < .001) significantly affected breakaway strength. Breakaway strength increased 75% to 94% as the orientation of the handhold was moved from vertical to horizontal. Breakaway strength decreased 8% to 13% for large-diameter (51-mm) handholds as compared with smaller diameters (22 mm to 32 mm), depending on orientation. Gloves may increase or decrease the ability to hang on depending on interface friction; greater friction increased breakaway force. Conclusion: Handles oriented perpendicular to the pull direction and high-friction gloves provide the greatest breakaway strength. Smaller handhold diameters than predicted by grip strength afford greater capability in these orientations. Application: These insights can be used to design handholds that increase the ability to support one's body weight and reduce the effort needed to pull or lift heavy items.

Objective: To investigate the relationship among friction, applied torque, and axial push force on cylindrical handles. Background: We have earlier demonstrated that participants can exert greater contact force and torque in an "inward" movement of the hand about the long axis of a gripped cylinder (wrist flexion/forearm supination) than they can in an "outward" hand movement. Method: Twelve healthy participants exerted anteriorly directed maximum push forces along the long axis of aluminum and rubber handles while applying deliberate inward or outward torques, no torque (straight), and an unspecified (preferred) torque. Results: Axial push force was 12% greater for the rubber handle than for the aluminum handle. Participants exerted mean torques of 1.1, 0.3, 2.5, and —2.0 Nm and axial push forces of 94, 85, 75, and 65 N for the preferred, straight, inward, and outward trials, respectively. Left to decide for themselves, participants tended to apply inward torques, which were associated with increased axial push forces. Conclusion: Axial push force was limited by hand-handle coupling — not the whole body's push strength. Participants appeared to intuitively know that the application of an inward torque would improve their maximum axial push force. Axial push forces were least when a deliberate torque was requested, probably because high levels of torque exertions interfered with the push. Application: A low-friction handle decreases maximum axial push force. It should be anticipated that people will apply inward torque during maximum axial push.

Objective: The effects of handle friction and torque direction on muscle activity and torque are empirically investigated using cylindrical handles. Background: A torque biomechanical model that considers contact force, friction, and torque direction was evaluated using different friction handles. Methods: Twelve adults exerted hand torque in opposite directions about the long axis of a cylinder covered with aluminum or rubber while grip force, torque, and finger flexor electromyography (EMG) were recorded. In addition, participants performed grip exertions without torque, in which they matched the EMG level obtained during previous maximum torque exertions, to allow us to determine how grip force was affected by the absence of torque. Results: (a) Maximum torque was 52% greater for the high-friction rubber handle than for the low-friction aluminum handle. (b) Total normal force increased 33% with inward torque (torque applied in the direction fingertips point) and decreased 14% with outward torque (torque in the direction the thumb points), compared with that with no torque. Consequently, maximum inward torque was 45% greater than maximum outward torque. (c) The effect of torque direction was greater for the high-friction rubber handle than for the low-friction aluminum handle. Conclusion: The results support the proposed model, which predicts a large effect of torque direction when high-friction handles are gripped. Application: Designing tasks with high friction and inward rotations can increase the torque capability of workers of a given strength, or reduce required muscle activities for given torque exertions, thus reducing the risk of fatigue and musculoskeletal disorders.

Repetitive trauma disorders of the upper extremity are a major cause of lost work in many hand-intensive industries. Reported risk factors include repetitive and forceful exertions, certain postures, mechanical stress, low temperatures, gloves, and vibration. Risk factors can be identified with job analysis procedures based on traditional work-methods analysis. Risk factors can be controlled through reallocation of work, balancing of tools, selection of alternative tool designs, work relocation, selection of suitable hand protection, and elimination of hand exposure to low temperatures and vibration. Drawing-board manikins are used with computer-aided design systems to estimate the best work location for a given task.

Objective: The aim of this study was to determine how hand space for manual insertion of flexible hoses is affected by insertion method and force. Background: Adequate space is needed during assembly tasks in which workers join parts together with their hands. Hose installations are an example of such a task. Hand clearance envelopes for insertion tasks that involve cylindrical objects, such as a hose, are currently unavailable in the literature. Methods: Participants inserted a flexible 25-mm rubber hose onto a stationary flange using simulated methods similar to those observed in field studies of automotive assembly tasks. Markers placed on the back of the hand and wrists were used to measure postures during the insertion task. Results: Hand clearance envelopes for high-force insertions were significantly larger across methods by an average of 15% for both male ( p < .05) and female ( p < .01) participants. Rocking insertions resulted in the largest hand clearance envelopes compared with other insertion methods. Rocking and twisting the hose resulted in mean increases in the cross-sectional area of the hand clearance envelopes of 35% and 24%, respectively, compared with the straight push. Differences were significant ( p < .05) for male and female participants for both bead height conditions. Conclusion: Both required insertion force and method affect hand clearance envelopes during simulated insertions. Application: These methods can be used by engineers to determine if there is adequate clearance for the hand to grip selected objects.

This study examines the relationship between forearm EMGs and keyboard reaction forces in 10 people during keyboard tasks performed at a comfortable speed. A linear fit of EMG force data for each person and finger was calculated during static fingertip loading. An average r2 of .71 was observed for forces below 50% of the maximal voluntary contraction (MVC). These regressions were used to characterize EMG data in force units during the typing task. Averaged peak reaction forces measured during typing ranged from 3.33 N (thumb) to 1.84 N (little finger), with an overall average of 2.54 N, which represents about 10% MVC and 5.4 times the key switch make force (0.47 N). Individual peak or mean finger forces obtained from EMG were greater (1.2 to 3.2 times) than force measurements; hence the range of r2 for EMG force was .10 to .46. A closer correspondence between EMG and peak force was obtained using EMG averaged across all fingers. For 5 of the participants the force computed from EMG was within ±20% of the reaction force. For the other 5 participants forces were overestimated. For 9 participants the difference between EMG estimated force and the reaction force was less than 13% MVC. It is suggested that the difference between EMG and finger force partly results from the amount of muscle load not captured by the measured applied force.

Objective: To quantify the effect of handhold size (diameter) on the maximum breakaway strength between a hand and handhold for children. Background: Falls from playground equipment are a major cause of childhood injury and death. It is unclear if recommendations for handholds on playground equipment are too broad. Methods: Breakaway strength was defined as the maximum quasistatic force that can be exerted on a grasped object before the object is forcibly pulled from the grasp. Hand anthropometry, grip, and breakaway strengths were measured for 397 children between the ages of 6 and 11 years. Three cylindrical handhold diameters were tested. Results: Breakaway strength was significantly affected by handhold size, gender, and hand dominance. Significant covariate predictors for breakaway strength included grip strength, age, and hand breadth. Breakaway strength was reduced for the largest diameter (3.81 cm) for children of all ages. Conclusion: Handhold design factors significantly affect the breakaway strength of children. Application: The results can be used as a basis for design recommendations for hand rungs used by children to reliably support their bodyweight.