Co-contraction about the ankle increases with the threat of a walking perturbation.

The ability to prevent a fall after a walking perturbation is dependent upon the initial, modifiable conditions related to stability. Individuals may proactively modify their gait to maintain stability by increasing co-contraction about the ankle, increasing the resistance to a perturbing force. The purpose of this study was to investigate whether such proactive ankle co-contraction occurs when walking under the threat of large perturbations. Participants walked on a computer-controlled treadmill under nine randomly ordered task conditions with combinations of three walking speeds and three perturbation conditions. Bilateral EMG of the tibialis anterior and medial gastrocnemius were recorded. Co-contraction indices (CCI) of these muscles during stance prior to a perturbation were calculated. The effects of walking perturbation condition and speed on CCI were assessed using a 3x3 repeated-measures ANOVA. There were no significant interactions of perturbation and speed. There was a significant main effect of perturbation condition, with higher CCI in anterior and posterior perturbations compared to no perturbation. These results suggest that co-contraction occurs in a manner that may increase resistance to perturbations. Future studies will determine the extent to which populations with a high risk of falling are able to proactively modify their gait in anticipation of a perturbation.