The response of the peroneal muscles to sudden inversion of the ankle during standing was investigated. The variation of the inversion angle with time was measured by means of a potentiometer attached to a specially designed test apparatus. During the tests, volunteers were subjected to sudden and unexpected inversion of their ankle, during which the surface EMG of the peroneal muscles was also recorded. Two groups were tested, one of normal subjects and one consisting of subjects with recurrent ankle sprains. There were 8 females and 3 males in each group. The subjects in the second group, who suffered from recurrent ankle sprains, had been asymptomatic during the last 2 months prior to the tests. For each subject in the two groups, both ankles were tested. The results indicated a latency time ranging from 60 to 80 ms for both groups. It was concluded that the reflex contraction of the peroneal muscles due to a sudden stretch inversion motion has no role in protecting the ankle joint during sprain and that this protection is mainly provided by the passive tissues.

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The characteristics of impact forces on the legs during vertical landing of human vertical free fall in different falling conditions were studied to reveal the parameters which take part in the attenuation of these impact forces. The following parameters were investigated: body position during landing, range of flexion of the joints of the legs at impact, usage of ground-roll immediately after impact and softness of the ground.The results indicate that joint movements and muscle action play a major role in reducing peak forces during landing. This emphasizes the importance of adequate training to improve the pre-programmed non-reflex muscle action, necessary in the early phase of impact.