Anticipatory postural reflexes in Parkinson's disease and other akinetic-rigid syndromes and in cerebellar ataxia.

We have examined anticipatory postural responses in calf muscles (triceps surae) in response to a small pull to the arm in standing subjects. This stimulus normally evokes a brisk automatic compensatory contraction of the calf muscles. This is not due to calf muscle stretch, for leg movement occurs after the response has appeared. It appears to be driven centrally by input produced by the pull to the arm. Such anticipatory postural responses were present in every one of the 50 normal subjects studied, but were absent or greatly reduced in many patients with Parkinson's disease when compared to age-matched normal subjects, particularly in those exhibiting postural instability. However, a normal anticipatory postural response was elicited in four patients with progressive supranuclear palsy and in a single patient with an akinetic rigid syndrome due to cerebrovascular disease. Cerebellar truncal ataxia only depressed the response in severe cases. We conclude that although these responses may not be necessary to maintain balance, they may be employed in fine adjustment of posture.