Effect of Localized Muscle Fatigue on Postural Control Stability and Muscle Activation Strategies

Abstract

Optimal postural control involves the integration of sensory inputs from the visual, vestibular, and proprioceptive systems. It requires both individual muscle activation and intermuscular interactions that adapt balance strategies to task demands, while being shaped by sensory feedback. Neuromuscular fatigue (NMF) is thought to destabilize postural control by degrading proprioceptive reliability and altering muscle activation strategies. Yet, whether fatigue effects generalize across postural contexts remains uncertain. This thesis aimed to examine how NMF influences postural control stability and muscle activation strategies during two tasks of varying stability demands: quiet standing and forward lean. Specifically, we sought to determine how fatigue influences: 1) lower-limb muscle activation, co-contraction patterns and intermuscular coherence, 2) center of pressure (COP) behavior across different time scales, and 3) the relationship between EMG coherence and open- and closed-loop postural control mechanisms inferred from COP Discrete Wavelet Transform analysis. Fourteen healthy adults performed quiet standing and forward-lean tasks with eyes open or closed, before and after a standardized isometric plantarflexion/dorsiflexion fatigue protocol. We quantified neuromuscular activity and coordination (EMG amplitude, co-contraction, intermuscular coherence) as well as postural sway (center-of-pressure (COP) metrics and timescale-specific control (wavelet-based energy). The effects of fatigue and postural task were evaluated using repeated-measures ANOVA models. Pearson's correlations were also used to assess the relationship between changes in EMG and COP variables. Results showed that muscle activation patterns were primarily driven by postural task demands rather than fatigue. Intermuscular coherence analysis revealed task- and frequency-dependent fatigue effects, with agonist-agonist coherence increasing significantly in mid- and high-frequency bands during the most challenging condition (forward lean, eyes-closed) after fatigue (p < 0.01). Traditional center of pressure measures showed limited fatigue effects, and discrete wavelet analysis revealed no significant fatigue-induced changes in frequency band energy distributions. These findings demonstrate that NMF produces subtle, task-dependent alterations in postural control that are most evident during challenging postural demands.