The effects of foot position on balance displayed by classically trained ballerinas in passe
Lallathin, Jayma Rene
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The purpose of the study was to compare the effects of the demi and en pointe foot positions on stability outcomes and balance strategies displayed by ballet dancers during performance of the passé position. Ground reaction force (GRF) and electromyographic (EMG) signals (1200 Hz) of 11 muscles of the supporting limb were obtained, and 3-D spatial positions of reflective markers on the dancer’s body were captured using high-speed digital cameras (120Hz), during the dancer’s performance of 10 passé balance trials in each foot position. Outlines of the dancer’s foot were traced to calculate the size of the base of support (BoS) in each foot position. Stability outcomes of time to stabilization (TTS) and sway area (SA) were derived from GRF and center of pressure data. Balance strategies for each foot position were detected using EMG data to determine muscle synergies using non-negative matrix factorization (NMF); overall muscle contribution (OMC) for each muscle was obtained from the synergies, then the muscles were rank ordered by their OMC; and lower extremity joint moment magnitudes were examined. From paired-samples t-test outcomes (p < .05), dancers en pointe compared to demi pointe had decreased TTS (Cohen’s d = .07), BoS (d = 5.22), SA (d = .89), and mean ankle plantarflexor moments (d = 1.53). To maintain balance en demi pointe, the majority of dancers (~65%) exhibited an ankle strategy as classified by the rank order of the OMC. However, the muscle synergies reflected that hip musculature also contributed to balance. When maintaining balance en pointe, the majority of dancers (~71%) used a ‘whole-limb’ balance strategy, as the muscle synergies and OMC reflected that muscles affecting control of all lower extremity joints contributed to balance. Co-contraction of opposing muscle groups at all joints also was displayed, suggesting a muscle activation strategy of maintaining high joint stiffness to maintain balance during ballet when the body must appear to be motionless.