Adaptation to cyclic stance perturbations in Parkinson's disease depends on postural demands

The postural reactions of 10 moderate PD patients and 10 age matched controls were studied during stance on a sinusoidally back and forth moving platform during 5 blocks of 10, 1-min trials. Free stance was followed by weight bearing, lowering or raising the center of gravity (COG) by 10%. Next, a book was balanced on the head and finally, forward inclination was constrained. Normal stance strategy, including predictive muscle responses was found during free stance. Modifying COG height caused little response changes. Stance strategy became abnormal when head trajectory or forward inclination was restricted, indicating that ability to generate a nonstandard multi-joint stance strategy, rather than production of adequate stabilizing forces, is impaired in moderate, medicated PD patients.