To compare lower limb joint angle variability between functional ankle instability (FAI) and healthy controls (CONs) at different running speeds using linear and nonlinear methods. Fifteen males with right-side FAI and fifteen matched CONs ran on a treadmill at self-selected, 20% faster, and 20% slower speeds. From 25 gait cycles, the mean coefficient of variation (CV), Sample Entropy (SampEn), and largest Lyapunov Exponent (LyE) of hip, knee, and ankle angles were computed. A two-way (two groups × three speeds) mixed-design ANOVA was applied (α = 0.05). No significant interaction effects were observed. No significant differences were observed in the CV. SampEn showed group effects: FAI had lower values in hip horizontal, knee sagittal/coronal, and ankle coronal planes, but higher in the hip sagittal plane. Speed effects showed greater SampEn in the ankle sagittal and lower in the hip coronal plane at slow speed. LyE was reduced in FAI for hip, knee, and ankle sagittal planes. Speed effects indicated higher LyE in the knee sagittal and lower in the hip coronal plane at slow speed. FAI showed reduced variability, particularly in the sagittal plane, reflecting rigid control. Slower speeds increased ankle and knee sagittal variability but decreased hip coronal variability.

The effects of foot abduction angle and stance width on muscle activation remain inconsistent in current research. We aim to investigate this issue from the perspective of muscle synergies, thereby providing evidence-based squatting guidelines for sports training and rehabilitation. Nineteen males performed squats under six conditions: three widths (wide, medium, narrow) × two foot abduction angles (0°, 30°). Surface electromyography was recorded from seven lower limb muscles. We employed principal component (PC) analysis to investigate the muscle synergies of seven lower-limb muscles. Repeated-measures two-way ANOVA was performed to statistically analyze both muscle peak activation and PC scores, with significance set at p < .05. In the descending PC3, which is primarily composed of the peroneus longus and vastus lateralis, 0° abduction scores were higher than 30° (p = .003); narrow stance scores were higher than wide stance (p = .017). In the ascending PC2 is composed of vastus lateralis, rectus femoris, and soleus, the score at 0° abduction was higher than that at 30° abduction (p = .046). The other PCs were statistically non-significant (p > .05). Increasing foot abduction angle and stance width appear to shift neuromuscular control from a strategy dominated by stabilizer muscles toward one dominated by the prime movers, indicating that in healthy adult men, simple adjustments to these stance parameters can predictably redistribute muscular load between stabilizer muscles and prime movers.