Movement detection at the human big toe.

1. To be detected, movements of the interphalangeal joint of the big toe must be greater than at other joints. This poor acuity may arise because the anatomy of the foot and ankle results in poor coupling between the toe and the muscles that operate it. To vary this coupling, the effect of ankle position on proprioceptive acuity at the toe was measured. 2. We measured proprioceptive acuity at the toe with the ankle in different positions and found that ankle plantarflexion did improve acuity. This implies that, with the ankle at mid-range or dorsiflexed, toe movement is inadequately transferred to muscle fascicles. 3. To determine actual changes in fascicle length of the toe extensor, movements of extensor hallucis longus near the toe and at the muscle-tendon junction were measured during surgical exposure in one subject. Ankle position greatly affected movement transfer from toe to muscle-tendon junction: no tendon movement was transferred with the ankle dorsiflexed, but all movement was transferred with the ankle plantarflexed. 4. When the relationship between joint rotation and muscle fascicle length measured in vivo was used to express the smallest detectable movements of the toe as proportional changes in muscle fascicle length, these detectable changes were similar to those at all other limb joints. This suggests that change in muscle fascicle length is of major interest to the nervous system.