Level of muscle exertion and duty cycle percent synergistically interacted to markedly reduce muscle endurance and muscle strength in a laboratory study by Iridiastadi and Nussbaum. Cycle time was found to be influential at reducing muscle endurance (but not to a significant level) while having little impact on muscle strength. Reduced cycle time favorable impacted EMG measurements.
These findings suggest that evaluating both level of exertion and duty cycle is important in assessing risk of task muscle fatigue. Also, the authors state this data, when combined with other published reports, suggest that intermittent efforts at 21 percent or greater maximum voluntary contraction should be avoided. In tasks with moderate duty cycles and cycle times, maximum voluntary exertion of 10 percent or lower is reasonable in their opinion.
A resisted shoulder abduction motion (isometric contraction of shoulder abductors) was performed by 24 male and 24 female university students while the shoulder was positioned at 90 degrees of abduction and the subject in a supine position. The subjects generated a muscle contraction effort in one of six ways as per Table 1 (three independent variables performed in one of two ways) for a one hour duration or until the required exertion level could not be maintained.
Application to Subject
Isometric muscular exertion
12 percent of maximum contraction force
28 percent of maximum contraction force
Duty cycle (ratio of work duration to cycle time)
25 percent of cycle time
75 percent of cycle time
TABLE 1: Potential subject exertion performance.
Volunteers were randomly assigned to an exertion condition with an equal female/male distribution for each independent variable combination. Prior to the test, the subject’s maximum voluntary shoulder abduction exertion strength was measured in the test position through 4 five-second trials.
Perceived discomfort in the right arm/shoulder was reported every 10 minutes by the volunteer on a 1 to 10 scale. EMG data were collected from the middle deltoid muscle during the contraction phase of a work cycle. Maximum exertion capability was re-assessed every 10 minutes to evaluate fatigue-induced changes.
The average decline in strength over the test period was six times greater for the high (28 percent) maximum voluntary contraction than the low (12 percent) maximum voluntary contraction. The higher duty cycle (75 percent) reduced strength at twice the rate of the lower duty cycle (25 percent).
The rate of change in perceived discomfort (scale number/experiment duration) was significantly greater for high (28 percent) maximum voluntary contraction and high duty cycle (75 percent) compared to their respective category comparison.
High maximum voluntary contraction combined with high duty cycle led to an average endurance time of 30 minutes, strength reductions of nearly 20 percent and very high discomfort ratings (9-10)
This article originally appeared in The Ergonomics Report™ on 2007-03-08.