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This study represents a continuation of a series of psychophysical studies on repetitive motions of the wrist and hand conducted at the Liberty Mutual Research Center for Safety and Health. The purpose of the study was to quantify maximum acceptable forces of six motions performed on separate days but within the context of the same experiment. The six motions were wrist flexion with a power grip, wrist extension with a power grip, wrist flexion with a pinch grip, wrist extension with a pinch grip, ulnar deviation with a power grip, and a handgrip task (with a power grip). A psychophysical methodology was used in which the subject adjusted the resistance on the handle and the experimenter manipulated or controlled all other variables. Thirty-one subjects performed the six tasks at repetition rates of 15, 20 and 25 motions/min. Subjects performed the tasks for 7 h per day, 5 days per week, for 4 weeks. The subjects were instructed to work as if they were on an incentive basis, getting paid for the amount of work performed. Symptoms were recorded by the subjects during the last 5 min of each hour. The results revealed that maximum acceptable torques ranged from 11 to 19% of maximum isometric torque depending on frequency and motion. Maximum acceptable torques for the tasks that could be compared with previous studies showed the same patterns of response. However, the selected forces were substantially lower using the mixed protocol. A table of maximum acceptable torques and forces is presented for application in the field.

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The purpose of this experiment was to investigate maximum acceptable initial and sustained forces while performing a push of 7.6 m, performed at a frequency of 1 push min-1 on a magnetic particle brake treadmill and a high-inertia push-cart. Eight male industrial workers performed a 40 min treadmill pushing task in the context of a larger experiment and two, 2 h push-cart tasks with a unique water loading system. A psychophysical methodology was employed, whereby the subjects were asked to select a workload they could sustain for 8 h without straining themselves or without becoming unusually tired, weakened, overheated or out of breath. The results revealed that maximum acceptable initial and sustained forces of pushing on the high inertia cart were significantly higher (28 and 23%, respectively) than pushing forces on the magnetic particle brake treadmill. It was concluded that adjustments to the pushing and pulling data bank by Snook and Ciriello (1991) may be appropriate if verification of this experiment yields similar results.

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The purpose of this experiment was to quantify maximum acceptable forces for ulnar deviation motions of the wrist at various repetition rates. Subjects grasped a handle with a power grip and moved it through a 1.40 rad (80 degrees) ulnar deviation wrist motion (similar to a knife cutting task). A psychophysical methodology was used in which the subject adjusted the resistance on the handle and the experiment manipulated or controlled all other variables. Two series of experiments were conducted. Thirteen subjects completed the first series, which investigated repetition rates of 15 and 20 motions per minute. Eleven subjects completed the second series, which investigated 15, 20, and 25 motions per minute. Subjects performed for 7 hours per day, 5 days per week, for 4 weeks in the first series and 5 weeks in the second series. The subjects were instructed to work as if they were on an incentive basis, getting paid for the amount of work they performed. Symptoms were recorded by the subjects during the last 5 minutes of each hour. The results are presented and compared with maximum acceptable forces for wrist flexion and extension.

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The purpose of this experiment was to investigate the feasibility of using psychophysical methods to determine maximum acceptable forces for various types and frequencies of repetitive wrist motion. Four adjustable work stations were built to simulate repetitive wrist flexion with a power grip, wrist flexion with a pinch grip, and wrist extension with a power grip. The study consisted of two separate experiments. Subjects worked for two days per week during the first experiment, and five days per week during the second experiment. Fifteen women completed the first experiment, working seven hours each day, two days per week, for 20 days. Repetition rates of 2, 5, 10, 15 and 20 motions per minute were used with each flexion and extension task. Maximum acceptable torques were determined for the various motions, grips, and repetition rates without dramatic changes in wrist strength, tactile sensitivity, or number of symptoms. Fourteen different women completed the second experiment, performing a wrist flexion motion (power grip) fifteen times per minute, seven hours per day, five days per week, for 23 days. There were no significant differences in maximum acceptable torque from day to day. However, the average maximum acceptable torque for a five days per week exposure was 36.3% lower than for the same task performed two days per week. Assuming that maximum acceptable torques decrease 36.3% for other repetition rates and motions, tables of maximum acceptable force were developed for female wrist flexion (power grip), female wrist flexion (pinch grip), and female wrist extension (power grip).

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STUDY DESIGN: An experiment was conducted to investigate the effect of wearing a weight lifting belt on the endurance and fatigue characteristics of the extensor muscles of the spine. OBJECTIVE: To evaluate the effectiveness of back belts in preserving the endurance characteristics of the spinal musculature. SUMMARY OF BACKGROUND DATA: Previous studies have investigated the use of back belts in increasing intra-abdominal pressure and thus increasing the stabilization of the spine. Other studies have looked at the association of low back disability and wearing back belts. No study has examined the relationship between wearing back belts and measures of spinal muscle fatigue and endurance. METHODS: Thirteen male industrial workers performed a lifting and lowering task from the floor to a 76.2 cm height at 4.3 min-1 2 days a week for 4 weeks. The load was psychophysically determined and averaged 28.1 kg for a total of 28.9 metric tons moved in 4 hours. Subjects lifted for 1, 2, 3, and 4 hours, respectively, for the first 4 days of the experiment. These were considered training days. During the last 4 days, subjects lifted for 4 hours per day; 2 days with a back belt, and 2 days without a back belt. Maximum isokinetic endurance of the extensors was measured for 50 repetitions (5 sets of 10 repetitions) at the end of 4 hours of lifting. Median frequencies of the electromyographic signal from six locations over the back extensors were measured for 30 seconds at 80% of maximum isometric voluntary contraction after 4 hours of lifting. RESULTS: There were no significant differences in maximum isokinetic endurance, or in the slope of median frequency, between lifting with a back belt and lifting without a back belt. Similarly, a modified Borg scale and a postexperiment survey question were unable to distinguish between wearing a back belt and not wearing a back belt. CONCLUSION: There were no significant differences in maximum isokinetic endurance and electromyographic spectral parameters of the back extensor muscles as a result of wearing a back belt during heavy lifting and lowering tasks.

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The effects of lifting boxes without handles, pulling long distances, carrying different size boxes, combining three handling tasks, and lifting with extended horizontal reach were studied using a psychophysical methodology. Six male industrial workers performed 42 variations of lifting, lowering, pushing, pulling, and carrying tasks. Lifting boxes without handles produced consistent decreases (median, 16%) in maximum acceptable weights when compared with lifting boxes with handles. Lifting with extended horizontal reach (approximately 48 cm) produced consistent decreases (median, 48%) in maximum acceptable weights when compared with lifting close to the body (approximately 17 cm). No significant heart rate or oxygen consumption differences occurred in either of these variables. No significant differences in maximum acceptable weight of carry were observed among box sizes; however, the maximum acceptable force of pulling was lower for longer (15.2 m) pulling tasks. The maximum acceptable weight for combination tasks was similar to that of the limiting component.

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Four new manual handling experiments are reviewed. The experiment used male and female subjects to study lifting, lowering, pushing, pulling, and carrying tasks. Each experiment used a psychophysical methodology with measurements of oxygen consumption, heart rate, and anthropometric characteristics. Independent variables included task frequency, distance, height and duration; object size and handles; extended horizontal reach; and combination tasks. The results of the four experiments were integrated with the results of seven similar experiments published previously by this laboratory. The integrated data were used to revise maximum acceptable weights and forces originally published in 1978. The revised tables are presented and compared with the original tables.

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The purpose of this experiment was to investigate maximum acceptable weights and forces when performing manual handling tasks continuously for four hours at frequencies of 4.3 min-1 or slower. Twelve female and ten male second shift industrial workers performed 18 varieties of lifting, lowering, pushing, pulling, and carrying. A psychophysical methodology was employed, whereby the subjects were asked to select a workload they could sustain for 8 h 'without straining themselves or without becoming unusually tired, weakened, overheated or out of breath'. Measurements of heart rate, oxygen consumption, dynamic and static strengths were also taken. The weights selected after 40 min were not significantly different from the weights selected after four hours. The average oxygen consumption for the fast tasks was 28% VO2 max, within physiological guidelines for eight hours. The results also revealed that the maximum acceptable weights for the combination task of lifting, carrying, and lowering were limited by the lifting and lowering components. It is concluded from the results of this study that the psychophysical methodology is appropriate for determining maximum acceptable weights for task frequencies of 4.3 min-1 or slower. It is also concluded that the maximum acceptable weight for a combination task is limited by the lowest acceptable weight of any of the components.

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This study investigated the relationship between the Botsball (BB) and WBGT in varied environmental conditions. Two levels of radiant heat, two levels of air movement, three levels of relative humidity, and dry bulb temperatures ranging between 17.2 and 42.8 degrees C (63 and 109 degrees F) yielded 210 different environmental conditions. The results indicate that if good estimates of wind speed, humidity and radiant heat are available, BB can predict WBGT with +/-0.4 degrees C (90% confidence level). Only when there is limited knowledge of the type of environment does the predictability fall to +/-2.9 degrees C (90% confidence level).

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