Firing properties of muscle spindles supplying the intrinsic foot muscles of humans in unloaded and free-standing conditions

We recently developed an approach to record from muscle spindles in the intrinsic muscles of the foot in free-standing humans by inserting a tungsten microelectrode into the posterior tibial nerve behind the medial malleolus of the ankle. Here, we characterize the behaviour of muscle spindles in the small muscles of the foot in (i) seated subjects with the leg horizontal and foot naturally plantarflexed and (ii) in standing subjects. In the first study recordings were made from 26 muscle spindle afferents located within flexor digiti minimi brevis (n=4), abductor digiti minimi (n=3), quadratus plantae (n=3), plantar interossei (n=4), flexor digitorum brevis (n=3), dorsal interossei (n=2) and lumbricals (n=2), with one each supplying abductor hallucis, adductor hallucis and flexor hallucis brevis. The identity of another two muscle afferents was unknown. The majority of the units were silent at rest; only 7 (27%) being spontaneously active. Because of the anatomic constraints of the foot, some spindles supplying muscles acting on the toes responded to movements of one or more digits. In the second study 12 muscle spindle afferents were examined during standing. The ongoing discharge of 8 spindle afferents covaried with changes in the centre of pressure during postural sway. We conclude that the majority of spindle endings in the small muscles of the foot are silent at rest, which may allow them to encode changes in conformation of the foot when it is loaded during standing. Moreover, these muscle spindle afferents can provide useful proprioceptive information during standing and postural sway.