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Despite the recommendations that an important design criterion is not to exceed 15% of an operator's maximum voluntary contraction (MVC) with any muscle that must be used for a long period of time, many tasks involve the exertion of much higher forces coupled with long-term contractions. Many studies have investigated the force-time relationship of isometric muscle contractions to determine the endurance time of a given relative force. To date, however, direct studies of muscle performance throughout fatiguing tasks have not been conducted to the same degree. This research was concerned with studying the effects of different muscle groups (biceps vs quadriceps) of subjects with different age groups (20-29 vs 50-59 years of age) on long-term muscular isometric contractions at different levels of %MVC (20%, 40%, 60%, 80%, and 100% MVC), and modelling the functional data to describe the time course of strength decrement. The data revealed that the time course of strength decrement was best modelled by the function: [formula: see text] An experiment, using 20 subjects with each subject performing 10 conditions (two muscle groups x five levels of %MVC), showed that this function accounted for over 95% of the variance of strength decrement. All parameter estimates were statistically significant.

To study the physiological and psychophysical costs of symmetric and asymmetric manual materials handling, two tasks were performed by 30 industrial subjects. In both tasks, box weight and handle position were varied. The symmetric task, lifting and lowering between floor and conveyor, showed handles to be beneficial. The asymmetric task was palletizing and depalletizing 36 boxes between a pallet and a conveyor. Both palletizing and depalletizing proved strenuous for females with heart rates exceeding 140b/min. All handle positions were better than No Handles, but the best handle position changed from asymmetric for 9 kg boxes to symmetric for 13 kg boxes. The effect of handles was equivalent to a weight change of 1-2 kg for Heart Rate and Rated Perceived Exertion, but much higher (2-14 kg) for Body Part Discomfort measures.

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