PERONEUS LONGUS AND TIBIALIS ANTERIOR MUSCLE-ACTIVITY IN THE STANCE PHASE - A QUANTIFIED ELECTROMYOGRAPHIC STUDY OF IN CONTROLS AND 25 PATIENTS WITH CHRONIC ANKLE INSTABILITY

The electromyographic activity of the peroneus longus and anterior tib ial muscles of 25 patients with chronic ankle instability (18 patients with bilateral symptoms and 7 patients with unilateral complaints) an d 10 controls was registered during the stance phase under different w alking conditions. With balance secured by external support, there was a variable amount of peroneal activity, most of which was found in th e third quarter of stance. A high increase in peroneus longus activity starting after foot-flat was found when subjects had to maintain bala nce in a natural way. No difference in peroneal activity was found in relation to instability complaints. It is thought that the peroneus lo ngus serves to maintain balance, that this function decreases with inc rease of speed and that one cannot rely on this muscle to prevent an i nversion injury during normal walking. The anterior tibial muscle was predominantly active in the first quarter after heel contact. An incre ase in activity in the second quarter as an effect of loss of secured balance suggests that this muscle plays some part in balance control, but this is not its main function. A significant increase in tibialis anterior activity was found in patients with bilateral instability. No significant difference was found between the symptomatic and asymptom atic leg of patients with unilateral instability under the same walkin g conditions. These findings suggest changes in central control.