Altered activation pattern in synergistic ankle plantarflexor muscles in a reduced-gravity environment.

We investigated relative activations in the synergistic ankle plantarflexor muscles, the soleus (SOL) and medial gastrocnemius (MG), while subjects were walking in water. Ten healthy volunteers walked in water under three different load- and four different speed-conditions. The depth of water was to the axilla of each subject, which reduced about 80% of his or her body weight. The EMG activities of the SOL and MG were recorded with surface electrodes, and ground reaction forces (GRF) were measured by using a water-resistant force platform. Relations of EMG activity levels between the two muscles during the stance phase were analysed with respect to the load and walking speed. The results revealed that the SOL EMG activity was more dependent on the load than the walking speed in water, while the MG EMG activity was more dependent on the speed. Such dissociated activation of the synergistic ankle plantarflexors might be relevant to the different roles required of these muscles in functional motor tasks.