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Introduction: The promotion of mechanical exposure variation through muscular load changes is suggested for preventing musculoskeletal disorders (MSD) related to industrial manual work. However, evidence of work design concepts like job rotation on preventing work-related MSD seems to be low. In order to develop more effective prevention measures in the workplace, a more detailed knowledge on the influence of different dimensions of mechanical exposures on physical strain is required. These dimensions comprise the load change ration (LCR: ratio between a higher and lower load), the cycle time (for cyclic work, duration until all work processes are completed) and the duty cycle (temporal proportion of higher vs. lower load). Therefore, we examined the relationship between LCR and muscular strain responses in a highly standardized laboratory experiment.

Methods: Six women and eight men performed in a within-subject design laboratory experiment isometric elbow extensions during five experimental conditions (exposures) of 40 min. The exposures differed in the LCR with two load levels alternating every 10 s. Cycle time and duty cycle were constant over all exposures. Each exposure was conducted at a mean load of 8% of the force generated during maximum voluntary isometric contractions of the elbow extensor muscles. Ratings of perceived muscle exertion using the CR10 Borg Scale and continuous recordings of surface electromyographic signals of the triceps brachii muscle were used to compare muscular strain responses between conditions.

Results: Subjective ratings and objective electromyographic measures showed a statistically significant effect of the LCR (all p<0.001). It was shown that the lower the LCR, the higher the corresponding muscular strain response. This relationship further appeared to be non-linear with similar strain responses for the two highest LCRs.

Discussion: The LCR level is crucial in order to reduce muscular strain effectively, because the effect of LCRs regarding a reduction of muscular strain appears to level off above a certain limit. However, these results have to be verified by studies that include larger and more diverse study samples.

Conclusion: In order to provide profound guidance for the development of work designs that contribute to the prevention of work-related MSDs, interactions with other dimensions (e.g. cycle time, duty cycle) of mechanical exposure must be taken into account and a threshold for the LCR level required for physiologically relevant strain reductions must be determined.