Execution Velocity as a Task Constraint Modulates Motor Variability and Movement Complexity in Dynamic Resistance Exercise

Execution velocity is a key constraint in resistance exercise, yet its influence on motor variability and movement complexity remains poorly understood. This study examined how different execution velocities modulate the magnitude and temporal structure of motor variability during a dynamic resistance task. Seventy-two resistance-trained participants performed parallel back squats at 50% of one-repetition maximum under three velocity conditions: preferred velocity, slow velocity, and maximal intended velocity. Acceleration signals were recorded using an inertial measurement unit placed at the lumbar region, while ground reaction force was obtained from a force platform. The magnitude of variability was quantified using standard deviation, whereas movement complexity was assessed through Fuzzy Entropy and Detrended Fluctuation Analysis. Repeated-measures ANOVAs revealed significant effects of execution velocity on standard deviation and Fuzzy Entropy in both measurement systems, with effect sizes of η2p = 0.937 and 0.738, respectively, for the IMU, and η2p = 0.882 and 0.712, respectively, for the force platform. Maximal velocity increased the magnitude of variability, whereas slow velocity produced higher complexity values, particularly for Fuzzy Entropy. Detrended Fluctuation Analysis showed a significant effect only in force-platform data (η2p = 0.182). These findings indicate that execution velocity modulates motor variability in distinct ways: faster executions amplify mechanical fluctuations, whereas slower executions increase the temporal complexity of the movement pattern. Therefore, velocity should be considered not only as a mechanical or physiological training variable, but also as a task constraint capable of shaping the organization of motor behavior during resistance exercise.