The Impact Of Fatigue On Coordination During An Upper Extremity Lifting Task

PURPOSE: Consensus is lacking in how muscular fatigue leads to musculoskeletal disorders. This study aimed to determine the impact of an upper extremity fatigue task on the coordination of upper extremity muscle activity. METHODS: An asymptomatic sample of 15 males and 15 females between the ages of 18 and 30 were recruited. The EMG activity of the upper trapezius (UT), anterior deltoid (AD), and long head of the biceps brachii (LHB) was sampled at 2148 Hz during the performance of an upper extremity lifting task. The task was performed bilaterally and consisted of lifting a weight equal to 20% of the subject’s maximum lift from a table to a shelf level with the subject’s acromion and back. This was done at a rate of 50 Hz until the subject reported a rating of 8 on the Borg CR10 Scale of perceived exertion. The normalized mutual information (NMI) from the AD-UT, AD-LHB, and LHB-UT muscle pairs were obtained for the first and last ten reps of the task. The NMI were grouped according to limb dominance and fatigue status. A linear mixed model was used to test for differences in NMI by fatigue status and limb dominance. This accounted for the correlation structure of the data since both limbs were measured from the same subject and from exposure to both fatigue states. Descriptive statistics included means and standard errors. Statistical significance was set at an alpha level of 0.05. RESULTS: Only the NMI of the AD-UT and LHB-UT pairs was significantly (p < 0.05) affected by fatigue. The mean non fatigue NMI of the AD-UT, AD-LHB, and LHP-UT muscle pairs was 0.679 ± 0.005, 0.680 ± 0.006, and 0.685 ± 0.006, respectively. The mean fatigue NMI of the AD-UT, AD-LHB, and LHP-UT muscle pairs was 0.694 ± 0.005, 0.683 ± 0.006, and 0.697 ± 0.006, respectively. CONCLUSION: The NMI of 2 of the 3 upper extremity muscle pairs appears to increase with fatigue, suggesting that as muscles fatigue their coordination in the performance of the task increases. Dominance appears to have no direct effect on information exchange and thus muscle coordination.