Role of active plantar-flexion and/or passive dorsi-flexion of ankle joints as the countermeasure for unloading-related effects in human soleus

Abstract Exercises in space are not always effective for prevention of atrophy in soleus, which is most susceptible to gravitational unloading. Therefore, this study was performed to investigate the optimum exercise prescription for prevention of the gravitational unloading-related atrophy in human soleus. Effects of active plantar-flexion of ankle joints or front sole, or passive dorsi-flexion of ankle joints during various exercises on the mobilization of soleus muscles were studied. Electromyogram (EMG) activities in soleus were recorded in 7 healthy male subjects during 1) cycling at 70, 130, and/or 170 W, by pedalling using arch or front sole, 2) weight lifting (isometric and/or squat push-up) with or without active plantar-flexion of the ankle joints, and 3) knee extension/flexion exercise in a supine position with or without forced passive dorsi-flexion of ankle joints. Greater EMG levels were observed, when the cycling load was increased and the subjects pedaled by using front sole. Mobilization of soleus was increased, when the subjects tried to plantar-flex the ankle joints during weight lifting. Soleus muscles were not mobilized during knee extension/flexion exercise with passively plantar-flexed ankle joint. But EMGs of soleus were elevated ∼22 times in response to forced ankle-joint dorsi-flexion, not knee-joint extension/flexion exercise. The data indicated that active plantar-flexion or passive dorsi-flexion of ankle joints is essential for stimulation of soleus muscle mobilization.