Muscle Synergies of the Lower Extremities During Gait Initiation in Individuals With and Without Chronic Ankle Instability.

Background: Chronic ankle instability (CAI) disrupts postural stability after ankle sprains and inadequate treatment. Gait initiation (GI), governed by central nervous system (CNS) patterns, is used to evaluate stability. Muscle synergy, which reflects coordinated activations, reveals neuromuscular control. This study investigates lower limb muscle synergies during GI in individuals with and without CAI to understand their neuromuscular strategies.

Design: Cross-sectional study.

Setting: Laboratory.

Method: This study involved 20 participants, 10 healthy men and 10 patients with CAI. Six electrodes were applied per the SENIAM guidelines, and markers were set according to the cluster model. The participants initiated gait after an auditory cue was presented on a force plate. OpenSim simulated a musculoskeletal model using kinematic and muscle activity data. Muscle synergies were analyzed via HALS in MATLAB. Statistical tests, including Wilcoxon and one-way ANOVA, were conducted in SPSS with p < 0.05 as the significance threshold.

Results: The number of muscle synergies was not significantly different between the healthy and CAI groups (p > 0.05). However, muscle weight differed significantly between synergies 1 and 2 (p < 0.05). In synergy 1, the TA had greater weighting in the CAI group, whereas synergy 2 had higher RF and GM_L weightings in the CAI group. Synergy 3 revealed greater PL weight in the control group (p < 0.05).

Conclusion: In CAI, PL muscle weakness is offset by the TA, RF, and GM_L muscles resulting in altered ankle strategies during gait instability. This compensation disrupts motor chains, increases movement complexity, and involves the CNS, framing CAI as a global movement issue rather than a localized problem.