Background: The strength training industry has failed in designing a machine for exercising the pronators and supinators, despite their substantial medio-lateral bracing function.
Objective: The present study documents the muscle strength generation capabilities of the pronators and supinators within their functional anatomic movement plane, using an innovative strength training machine with an oblique axis.
Methods: By using two force transducers, the angle-torque relationship of the pronators and supinators of 18 healthy male subjects was identified during maximum voluntary isometric contractions at five anatomical joint angles. Surface EMG was recorded from anterior tibial (TA), peroneus longus (PL) and soleus (SOL) muscles.
Results: The pronator strength curve showed an inverted U-shaped characteristic, whereas the supinator curve descends from pronated to supinated position. Compared to the muscle activities for one-leg heel raise and toe raise, PL (108-131%) and TA (59-83%), respectively, showed highest activity during pronations. The most activated supinator is SOL (67% of a one-leg heel raise).
Conclusions: Differences in the shape of the pronator and supinator strength curves revealed that two different variable cams have to be implemented for matching the human torque capability. We anticipate our study to be a starting point for preventive machine-based training interventions.
Keywords: Machine-based strength training; ankle stability; muscle activity; strength curves; subtalar joint.