Estimating maximum acceptable duty cycles (MADC) for overhead exertions

Overhead work poses a significant risk for shoulder musculoskeletal disorders due to elevated arm postures and sustained loading, yet current ergonomics tools provide limited guidance on the acceptable percentage of time such tasks can be performed. This concept, referred to as the Maximum Acceptable Duty Cycle (MADC), represents the proportion of time within a work cycle that an exertion can be sustained at a psychophysically acceptable level. This study addresses that gap by reconfiguring an existing ergonomics assessment tool for overhead work to estimate MADC, rather than maximum acceptable forces. MADC values across the overhead workspace were mapped using computational modeling of over 800,000 overhead hand positions under three superior-directed load conditions (5, 10, and 20 N). MADC varied substantially with hand position and force demand: at 5 N, values ranged from 0-40 %, while at 20 N, MADC never exceeded 14 %. A consistent ergonomic 'sweet spot' was identified slightly above and forward of the shoulder, where MADC is maximized, providing the largest design space for allowable task duty cycles. The reconfigured tool offers actionable, evidence-based guidance for overhead task design by informing duty cycle limits in industrial settings, where current one-size-fits-all thresholds lack empirical justification.