THE LIMITS OF EQUILIBRIUM IN YOUNG AND ELDERLY NORMAL SUBJECTS AND INPARKINSONIANS

Body sway was studied at various body inclinations, voluntarily mainta ined for about 1 min, in young and elderly normals and in idiopathic p arkinsonians. They stood on a dynamometric platform, whose output gave the instantaneous centre of foot pressure (CFP), its mean value and b ody sway area, with eyes open (EO) or closed (EC). Subjects held the n ormal upright stance, or the maximum possible inclined posture (body s traight, rotated at the ankle joints) in forward or backward direction , or intermediate postures. EMG was recorded from tibialis anterior (T A), soleus (Sol), extensor digitorum brevis (EDB) and flexor digitorum brevis (EDB). The cross-correlation function between the profile of t he EMG envelope and the profile of the shift of CFP along the sagittal plane was calculated. In young subjects standing with EO, the maximum extent of antero-posterior (A-P) displacement of CFP was about 60% of foot length. EC reduced this value to about 50%. In the elderly norma ls, the maximum A-P displacement was about 40% (EO) and 30% (EC). In b oth groups, sway area was minimal during normal stance with EO and inc reased progressively when the subjects leant forward or backward. With EC, sway area further increased during normal stance and the rate of increase in relation to inclination augmented markedly. Sol was tonica lly active during normal stance. Forward leaning increased Sol EMG and induced activity in FDB. TA and EDB were active during backward leani ng. The peak of the cross-correlation function between Sol EMG and ins tantaneous CFP was higher during normal stance than forward inclinatio n, while the reverse was true for FDB. This suggests a role of FDB in the fine-tuning of postural adjustment during forward leaning, and a w eight-supporting role of Sol. During backward inclination, TA but not EDB was cross-correlated with CFP. In the parkinsonians, maximum A-P d isplacement of CFP was just about 30% of foot length (EO; about 20% wi th EC); its extent was inversely correlated with the severity of the d isease. The relationship between sway area and A-P displacement was si milar to the elderly, both with EO and EC, within the common range of inclination. In the patients affected by the long-term syndrome, A-P d isplacement was further reduced while sway area increase at the critic al postures was often absent. In all patients, the relationship betwee n muscle activity and body inclination was comparable to normal. The f indings challenge the notion that the larger the sway the higher the r isk of failing, and suggest that large areas of sway, whilst holding c ritical postures, are the expression of a deliberate action aimed at r eaching further displacement, rather than a sign of instability. In pa rkinsonians, the limiting factor in body leaning need not be traced to a defective stabilizing feedback mechanism, or to a disordered patter n of muscle activation. Rather, the organization of the motor plan to achieve extreme inclinations seems to be impaired.