Ankle eversion torque response to sudden ankle inversion Torque response in unbraced, braced, and pre-activated situations.

In 13 young ankle stable subjects, ankle eversion torque and peroneal EMG were simultaneously recorded in response to sudden ankle inversion. The eversion torque response was bi-phasic. The initial development of torque, which was responsible for 30% of the maximal eversion torque response, was observed 135 ms after the start of platform rotation and correlated well with the onset of the automatic postural peroneal EMG response. The remaining eversion torque response commenced after 305 ms, strongly correlating with the onset of the peroneal long latency voluntary EMG activity. With the ankle unbraced, 66% of the maximal torque level was reached in 326 ms. While braced, the same torque magnitude was reached using 230 ms (p<0.02), and pre-activation of the peroneal muscles allowed the subjects to reach the same level of torque in 89 ms (p<0.0005). Prior to the study, a common reaction pattern to sudden inversion was expected in an ankle stable population, but review of the eversion torque and EMG data from the 13 subjects revealed three different voluntary reaction patterns: 10 subjects showed an efficient activation of evertor muscles; two subjects stiffened their ankles with activation of both in- and evertor muscles; and one subject showed a marginal voluntary activation of the ankle evertors. The results of the study indicate that the reaction to sudden ankle inversion is not solely automatic. The main part of the torque response is voluntarily mediated and inter-individual differences in strategy seem to exist in healthy subjects.