The purpose of this study was to determine the neuromuscular activation patterns of the quadriceps femoris (QF) synergists including the vastus intermedius (VI) during squat movement. For 15 healthy men, surface electromyography (EMG) was recorded at VI, vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) during the repetitive squat movements. The squat movement started upright posture. This movement consisted of eccentric (ECC) phase, isometric (ISO) phase at the bottom, and concentric (CON) phase with knee joint angle between 0° to 90°. The root mean square (RMS) of the EMG signals during three phases was calculated for knee joint angles ranging from 15° to 90°. Each ECC and CON phase was further divided into three subcategories. The RMS during squat was normalized by that of 15° to 40° during the ECC phase for all muscles. During the squat movement, a significant muscle-by-angle interaction in normalized RMS was found (P < 0.05). The normalized RMS of VI was signifi cantly higher than that of the VL and VM at all subcategories during ECC phase and ISO phase (P < 0.05). These results suggest that the VI plays an important role in the fl exed knee position during squat movement. This uncovered finding may help establishment of future effective therapeutic programs for dysfunction of QF related knee joint disorders.