Inverse activation between the deeper vastus intermedius and superficial muscles in the quadriceps during dynamic knee extensions

Introduction: We sought to determine the neuromuscular activation patterns of 4 synergistic muscles of the quadriceps femoris (QF), including the vastus intermedius (VI) muscle, during dynamic knee extensions. Methods: Nine healthy men performed dynamic knee extensions in the range of 20100% of the one-repetition maximum (1RM) load. Surface electromyography (EMG) was recorded from the 4 muscles of the QF. Results: The normalized EMG amplitude of the VI was significantly higher at knee joint angles between 90 degrees and 115 degrees during both the concentric and eccentric phases, and it was significantly lower between 140 degrees and 165 degrees during concentric and eccentric phases, compared with those of vastus lateralis at 60100% of 1RM load (P < 0.05). Conclusions: These results suggest that the VI plays a key role during dynamic knee extension with flexed joint angles and specifically contributes to the initial concentric and final eccentric phases of knee joint movements. Muscle Nerve 47: 682690, 2013