DISCHARGE OF MUSCLE AFFERENTS DURING VOLUNTARY COCONTRACTION OF ANTAGONISTIC ANKLE MUSCLES IN MALL

The discharge of 38 tibialis anterior (TA) muscle spindle endings was recorded at rest and during isometric voluntary contraction of the ank le joint in eight healthy human subjects. With the ankle joint in 110 degrees plantarflexion, 24 endings (61%) were tonically active in the resting subject. During weak voluntary dorsiflexion, seven additional endings were activated, so that a total of 31 endings were active (82% ). 24 of these were either newly recruited or discharged at a faster r ate than at rest (average discharge rate 6.6 Hz at rest, 9.7 Hz during contraction). At matched levels of TA EMG, one ending was newly recru ited and 10 were more active during co-contraction of dorsi and planta r flexors than during isolated dorsiflexion. 26 endings were equally a ctive during the two tasks and one ending decreased its firing rate wi th co-contraction. Four of the 11 endings, which had a higher discharg e rate during co-contraction than during dorsiflexion, discharged fast er during plantarflexion than at rest although slower than during coco ntraction. Plantarflexion had no effect on the discharge of three endi ngs. The remaining four endings were not investigated during plantarfl exion. It is suggested that the increased discharge rate of muscle end ings during co-contraction is caused either by small changes in the le ngth of the TA muscle or by a disproportionately high fusimotor drive during co-contraction.