1. To be detected, movements of the interphalangeal joint of the big toe mu
st 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 pos
ition 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 imp
lies that, with the ankle at mid-range or dorsiflexed, toe movement is inad
equately transferred to muscle fascicles.
3. To determine actual changes in fascicle length of the toe extensor, move
ments of extensor hallucis longus near the toe and at the muscle-tendon jun
ction 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 movemen
t 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 t
he 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
.