Background Chronic lateral ankle instability afflicts many athletes. The peroneal muscles are the most effective lateral stabilizers whose strength increase might brace the ankle joint complex laterally. Purpose of this study was to identify the morphological and biomechanical effects of functional pronator and supinator strength training compared to traditional plantar and dorsiflexor resistance exercises.

Methods A functional anatomic strength training machine for the pronators and supinators of the foot (FSTM) was constructed. 22 healthy male sport students performed ten weeks of single-set high resistance strength training with 3 training sessions per week. The subjects conducted functional pronator/supinator training (FPST) at the FSTM with the right leg, the left leg was trained with plantar and dorsiflexions (PD) at traditional training machines. Intraindividual changes in foot behaviour during a sudden ankle turn were recorded by using a custom made supination platform, which was mounted on a force plate. Muscle reaction times (EMG) of m. tibialis anterior (TA) and m. peronaeus longus (PL) as well as supination angle (electrogoniometer) were measured. Additionally, the muscle volumes of both lower legs were quantified by MRI (3 Tesla) of 9 randomly chosen subjects. The results were analyzed by repeated measures analyses of variances (ANOVA).

Results The intraindividual comparison (FPST vs. PD) revealed similar biomechanical effects despite specific strength increase. Compared to PD, FPST resulted in significantly higher pronator (14% vs. 8%, p<0.01) and supinator MVC (25% vs. 12%, p<0.01). During sudden inversions both, FPST and PD, resulted in reduced (p<0.05) and delayed (p=0.06) first peak vertical impact as well as reduced supination velocity (p<0.01). Muscular reaction was faster after the training in PL (p<0.01) and TA (p<0.05) with an interaction trend in favour of FPST in PL (p=0.06).

Muscle volume increased specifically in m. tibialis posterior (+10%; p>0.01) and m. flexor hallucis longus (+6%; p>0.01) after FPST. PL, TA and m. gastrocnemius showed significant muscle volume increase without interaction effects.

Discussion/Conclusions After both, FPST and PD, the ankle joint complex is more mechanically stabilized against sudden supinations due to muscle volume increase. According to the typical ankle sprain mechanism (plantarflexed and supinated foot in a landing maneuver), a combination of dorsiflexor and specific pronator strength training might show even higher functional ankle stability. Future clinical studies might show the preventive potential of shank muscle training.