Relative Timing of Changes in Muscle Fatigue and Movement Coordination During a Repetitive One-Hand Lifting Task

Altered or improper biomechanics can increase the risk of repetitive stress injuries. Muscle fatigue resulting from repetitive movements can induce changes in motor coordination. However, while associations between muscle fatigue and coordination changes have been documented, no study has examined the time course over which these processes occur. The purpose of this study was to examine the temporal sequence of kinematic and electromyographic events that takes place during a fatiguing repetitive task. One male subject performed dumbbell rows with a 3 Ib dumbbell at the rate of 0.5Hz until volitional exhaustion (approximately 15 minutes). EMC data were recorded from the biceps, mid-deltoid, mid-trapezius, and post-deltoid muscles. Kinematic data were obtained from markers on the shoulder, elbow, and hand. Measures of both muscle function (EMG median frequencies and linear envelopes) and joint kinematics exhibited non-monotonic changes across the course of the experiment. Cross-correlation analysis between EMG median frequencies and cycle-to-cycle maximum joint angle excursions indicated that muscle fatigue preceded coordination changes by approximately 10 to 20 lifts. These results indicate that for complex multi-joint tasks, muscle fatigue is a non-monotonic process wherein localized muscle fatigue induces specific changes in motor coordination intended to forestall subsequent muscle fatigue.