Modulation of bilateral lower-limb muscle coordination when performing increasingly challenging balance exercises

Balance exercises have proven effective in enhancing and regaining neuromuscular control. However, how the bilateral homonymous muscles are coordinated to achieve bipedal equilibrium remains unclear. In terms of increasingly difficult balance tasks, the current study focused on two levels of muscle coordination: individual homonymous muscles and groups of homonymous muscles. In 25 physically active young adults, a cross-correlation between the bilateral electromyographic (EMG) signals of both legs (i.e., bilateral EMG-EMG correlation) was conducted on seven muscles measured when performing bipedal balancing on three different support surface instabilities. Then, the patterns of bilateral EMG-EMG cross-correlation coefficients were determined through a principal component analysis (PCA). It was hypothesized that modulations of bilateral lower-limb muscle coordination should be observed in the specific relevant muscles or in the patterns of bilateral muscle coordination. The results showed that only the first hypothesis was supported as changes in the strength of bilateral EMG-EMG correlation (p ≤ 0.005) and in the time delays (p < 0.001) were mostly restricted in the lower-leg muscles. The dorsiflexor and plantar flexor muscles showed opposite coordination behaviors. Larger bilateral EMG-EMG correlation and shorter time delays appeared only in the tibialis anterior muscle, suggesting that bilateral dorsiflexor muscle coordination is needed for exercising on multiaxial-unstable platforms.