The influence of body unloading (to 75, 50, and 25% body weight) on lower leg muscle activation during treadmill walking was investigated in patients with Parkinson's disease, age-matched and young healthy subjects. The aim of the study was to test the hypothesis that patients suffer from impaired extensor load receptor function. Although lower leg flexor muscle electromyographic (EMG) activity was relatively independent of body load, lower leg extensor EMG activity was markedly load sensitive. The EMG amplitude of the latter decreased with body unloading during stepping in all three groups of subjects. The load sensitivity progressively increased from patients to age-matched to young healthy subjects. In addition, the absolute level of leg extensor EMG amplitude during the stance phase of stepping became progressively larger from patients to age-matched subjects to young healthy subjects. It is suggested that decreased sensitivity of extensor load reflex mechanisms contributes to impaired gait in elderly subjects and is more pronounced in parkinsonian patients.