Variability in musculoskeletal fatigue responses associated with repeated exposure to an occupational overhead drilling task completed on successive days.

Emerging research suggests that muscular and kinematic responses to overhead work display a high degree of variability in fatigue-related muscular and kinematics changes, both between and within individuals when evaluated across separate days. This study examined whether electromyographic (EMG), kinematic, and kinetic responses to an overhead drilling task performed until volitional fatigue were comparable to those of a repeated identical exposure of the task completed 1 week later. Surface EMG and intramuscular EMG, sampled from 7 shoulder muscles, and right upper limb kinematics and kinetics were analyzed from 15 male and 14 female participants. No significant day-to-day changes in EMG mean power frequency (MPF) were observed, though serratus anterior displayed significantly less fatigue-related increase in EMG root-mean-squared (RMS) signal amplitude on day 2. Unfatigued upper kinematics on day 2 featured an increase in thoracohumeral elevation, elbow flexion, and decrease in wrist ulnar deviation compared to unfatigued state on day 1. Fatigue-related changes in shoulder joint flexion moment that were present on day 1 were reduced on day 2, suggesting that a more efficient overhead work strategy was learned and preserved across successive days. Day-to-day changes in upper limb joint angle variability, quantified by median absolute deviation (MdAD), were joint dependent. Despite yielding a variable fatigue-related kinetic strategy on both days, kinematic and kinetic fatigue-related changes on a second day of completing an overhead drilling task suggested a potential kinematic learning effect.