Fatigue-related reductions in movement smoothness during a lateral shuffle and side-cutting movement

Although movement smoothness is considered a hallmark of well-coordinated motor tasks, it is unknown whether markers of movement smoothness can be used to assess the quality of neuromuscular control in athletes. As a first step into this direction, the purpose of this study was to test whether movement smoothness is reduced following a fatiguing lateral shuffle protocol due to fatigue-related adaptations in neuromuscular control. Sixteen healthy adults (7 females) completed a submaximal 3-minute lateral shuffle trial and a maximal fatiguing shuffle trial until task failure. The movement of the pelvis and feet was determined using 3D motion capture at 250 Hz. Movement smoothness was quantified using the log-dimensionless jerk, which represents how often and abruptly the segment acceleration changes over time. Pelvis and foot movement smoothness along with spatiotemporal variables characterizing the shuffling pattern were compared between the beginning and end of the fatiguing shuffle trial using Wilcoxon signed-rank tests (alpha = 0.05). Pelvis movement smoothness was significantly reduced in all movement directions (p <= 0.005, strong effects) while foot movement smoothness was predominantly reduced in the shuffle direction (p <= 0.018, moderate to strong effects). Reductions in movement smoothness coincided with a lower peak pelvis speed in shuffle direction (p = 0.007, strong effect) and shorter contact times when changing direction (p = 0.049, moderate effect). These reductions in movement smoothness may be explained by a change in the overall shuffle pattern characterized by "harder" side-cuts as well as by a muscle fatigue-related decline in the ability to precisely plan and execute foot trajectories.