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

Background: 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 20-100% 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° and 115° during both the concentric and eccentric phases, and it was significantly lower between 140° and 165° during concentric and eccentric phases, compared with those of vastus lateralis at 60-100% 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.