Shortening reaction of human tibialis anterior.

The shortening reaction of tibialis anterior was observed in 6 of 25 normal subjects, in 6 of 40 patients with upper motor neuron syndromes, and in 11 of 17 patients with Parkinson's disease. The latency of the shortening reaction was comparable with that of the latter part of the long-latency stretch reflexes. The magnitude of the shortening reaction increased with the velocity of the movement that produced it and increased with background voluntary force of plantar flexion in all but the patients with Parkinson's disease. It was not affected by vibration in the patients with Parkinson's disease. The presence of the shortening reaction was not correlated with the clinical impression of increased tone.