Contrasts between the reflex responses to tibialis anterior and triceps surae to sudden ankle rotation in normal human subjects.

A detailed comparative study was made of the reflex responses of human ankle flexors (tibialis anterior, TA) and extensors (triceps surae, TS) to sudden ankle rotation. Subjects were instructed to maintain a tonic contraction of either TS or TA and subjected to sudden displacements of the ankle from its midposition in either the dorsiflexing or plantarflexing directions. Joint position, torque and smoothed rectified EMGs were recorded. The results revealed 5 principal differences in the behavior of the two muscles. (1) The TS response was typically a short, synchronous burst of activity starting at about 40 msec and lasting no more than 50 msec, termed the MSR. in contrast, the TA responses started at about the same latency but lasted much longer and frequently a second component (termed the PSR) could be distinguished. (2) The threshold of the TA reflex was much higher than that of TS. (3) Increasing displacement velocity increased the magnitude of the TS MSR as well as that of the TA MSR, but decreased the magnitude of the TA PSR. (4) Increasing the level of tonic activity augmented the magnitude of the TS activity initially but saturation soon occurred. In contrast, both components fo the TA response increased with increasing tonic TA activity. (5) Finally, shortening of the tonically active TS evoked no consistent response, where as shortening of TA evoked a significant pause in its activity. The contrasting behavior of the ankle flexors and extensors suggest that there are significant differences in their reflex organizations.