Quantitative EMG analysis to investigate synergistic coactivation of ankle and knee muscles during isokinetic ankle movement. Part 2: time frequency analysis.

Fundamental to intralimb coordination in the lower extremity, ankle-knee synergy induced by motor irradiation has long been employed to secure facilitation of paralyzed muscles. This study, a companion research subsequent to the time amplitude analysis of surface electromyography in part 1, was to investigate the recruitment strategy of irradiated muscles and prime movers during ankle isokinetic contraction at different contraction speeds (30, 60, 120 and 240 degrees/s) with time frequency analysis. The results indicated the recruitment strategies of the major irradiated muscles (ipsilateral rectus femoris/ipsilateral biceps femoris) and prime movers (anterior tibialis/gastrocnemius) were time-dependent and significantly different in terms of the instantaneous median frequency. In general, the prime movers for ankle isokinetic concentric contraction demonstrated a similar recruitment strategy, irrespective of different contraction speeds. This finding is consistent with the idea of generalized motor programs that speed is one of the constraint parameters supplied to motor programs. Nevertheless, the recruitment strategies of the irradiated muscles were highly inconsistent, varying across trials at different contraction speeds, and were not relevant to those of the prime movers. In addition, the recruitment in the irradiated muscles seemly limited to motor units of low threshold, in spite of maximal voluntary contraction of the prime movers.