Plantar intrinsic foot muscle activity and its relationship with postural sway during single-legged and bipedal tiptoe standing in ballet dancers

During tiptoe standing, especially with the single-legged support, the foot joints in ballet dancers are heavily loaded. Thus, the activity of the plantar intrinsic foot muscles (PIFMs), which stabilize the foot joints, may be important in reducing postural sway during tiptoe standing. We compared PIFM activity during single-legged and bipedal tiptoe standing and examined its relationship to postural sway in dancers. In 11 female ballet dancers, the electromyography (EMG) amplitudes of PIFMs and the center of pressure (COP) data were recorded during single-legged and bipedal tiptoe standing tasks. The EMG amplitudes were normalized to those during the maximal voluntary contraction, and PIFM activity level and its coefficient of variation over time (EMG-CV_time) during the task were assessed. From the COP data, standard deviations in the anteroposterior (COP-SD_AP) and mediolateral (COP-SD_ML) direction, velocity, and area were calculated. PIFM activity level and COP velocity were 2-2.5-fold higher in the single-legged than bipedal task (p ≤ 0.003). Significant correlations were found between PIFM activity level and COP velocity (r = 0.666, p = 0.025) and between EMG-CV_time and COP-SD_AP or COP-SD_ML (r ≥ 0.738, p ≤ 0.010) only in the single-legged task. These results suggest that PIFM activity is associated with postural sway, especially during single-legged tiptoe standing in dancers.