The aims of the present study are twofold: (1) to compare the postural control mechanisms of young and elderly people as well as in Parkinson's disease (PD) patients during quiet standing and (2) to assess the impact of a stooped posture on these mechanisms. All subjects were required to maintain both a side-by-side and a 45 degrees foot position. Elderly subjects performed a third condition where they were requested to mimic the stooped posture as adopted by PD subjects. The net centre of pressure (COP(net)) and centre of mass (COM) profiles in the anterior/posterior (A/P) and medial/lateral (M/L) planes were analyzed. The COP(net) signal was recorded from two force plates and was categorized in two mechanisms: an ankle mechanism (COP(c)) and a load/unload hip mechanism (COP(v)). The results showed similar postural control mechanisms in young, elderly and PD subjects. When the feet were side-by-side, the COP(net) was controlled by the ankle plantar/dorsiflexors (COP(c)) in the A/P direction, while by the hip abductor/adductors (COP(v)) controlled in the M/L direction. When the feet were in the 45 degrees position, both the ankle and hip mechanisms contributed to the COP(net). However, the PD subjects showed significant smaller RMS amplitudes compared to the elderly people in the 45 degrees foot position and in the stooped posture. These findings suggest that PD subjects resort to a stiffening strategy to control their balance in postural tasks that imply a mixed control (ankle and hip mechanisms) but have adapted to their stooped posture.
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