Phase-dependent preferential activation of the soleus and gastrocnemius muscles during hopping in humans

The electromyographic (EMG) activation patterns of the medial gastrocnemius (MG) and soleus (SOL) muscles were studied in 10 normal male subjects during hopping on a force-plate with special reference to three different phases of the movement: (a) precontact phase (PRE), (b) eccentric or stretching phase (ECC), and (c) concentric or shortening phase (CON). In a randomized order, each subject performed hopping on two legs with a minimal ground contact time, either with maximal frequency (FAST), maximal height (MAX) or at a constant frequency of 2 Hz (2HZ). The simultaneously digitized MG and SOL EMG activities were full-wave rectified and subjected to phase-dependent averaging that allowed repeated bursts of EMG signals during a hopping trial to be aligned in time and integrated with respect to mechanical events. Such analyses demonstrated that both these muscles were activated to a similar extent and in advance ( approximately 45 ms) of onset of ground contact during FAST hopping. Similarly, no significant differences in the integrated EMG activities (IEMGs) were observed between the MG and SOL during the CON phases of 2HZ and MAX conditions. During the PRE and ECC phases of MAX and 2HZ hopping, however, a significantly higher IEMG was noted in MG than in SOL (e.g., for the PRE: 13.3 +/- 1.4 vs. 4.7 +/- 1.0 muV . s, p < 0.01 for 2HZ and 12.6 +/- 1.3 vs. 4.7 +/- 1.2 muV . s, p < 0.01 for MAX). These results provide EMG evidence that preferential and movement phase-dependent neuromuscular activation exists within the ankle extensor synergy in humans.