Characteristics of postural instability induced by ischemic blocking of leg afferents.

After minimizing proprioceptive input from the legs by ischemia without degradation of muscle force and excluding visual stabilization by eye closure, a characteristic anterior-posterior postural sway around 1 Hz was observed in three normal subjects. This is similar to the instability seen in two tabes dorsalis patients. From the spectral analysis of head and hip movements, displacements of the center of force and of ankle angle as well as from EMG recordings of the anterior tibial and gastrocnemius muscle it is concluded that the oscillations around 1 Hz are due to the long latency and high threshold of vestibularly induced leg muscle discharges (200-300 ms) arriving in the counterbalancing phase of the trunk, which causes an overshoot in body sway.