Influence of mental workload on muscle endurance, fatigue, and recovery during intermittent static work.

Most occupational tasks involve some level of mental/cognitive processing in addition to physical work; however, the etiology of work-related musculoskeletal disorders (WMSDs) due to these demands remains unclear. The aim of this study was to quantify the interactive effects of physical and mental workload on muscle endurance, fatigue, and recovery during intermittent work. Twelve participants, balanced by gender, performed intermittent static shoulder abductions to exhaustion at 15, 35, and 55% of individual maximal voluntary contraction (MVC), in the absence (control) and presence (concurrent) of a mental arithmetic task. Changes in muscular capacity were determined using endurance time, strength decline, electromyographic (EMG) fatigue indicators, muscle oxygenation, and heart rate measures. Muscular recovery was quantified through changes in strength and physiological responses. Mental workload was associated with shorter endurance times, specifically at 35% MVC, and greater strength decline. EMG and oxygenation measures showed similar changes during fatigue manifestation during concurrent conditions compared to the control, despite shorter endurance times. Moreover, decreased heart rate variability during concurrent demand conditions indicated increased mental stress. Although strength recovery was not influenced by mental workload, a slower heart rate recovery was observed after concurrent demand conditions. The findings from this study provide fundamental evidence that physical capacity (fatigability and recovery) is adversely affected by mental workload. Thus, it is critical to determine or evaluate occupational demands based on modified muscular capacity (due to mental workload) to reduce risk of WMSD development.