The squat exercise is a fundamental movement in athletic training and rehabilitation. In this study, we measured muscle activities in a normal squat posture (NSP) and a squat posture with the center of foot pressure (COP) intentionally shifted forward as far as possible (FSP). Ten healthy men performed double-limb squats, adopting the NSP and FSP, with three knee flexion angles (30, 60, and 90 degrees). The muscle activities of the vastus medialis (VM), semitendinosus (ST), tibialis anterior (TA), and gastrocnemius muscle lateral head (GL) were measured using surface electromyography, and activity patterns were analyzed. Compared to that for the NSP, the COP was significantly shifted forward in the FSP by at least 30% of the foot length for all knee flexion angles (p < 0.05). At all knee flexion angles, VM muscle activity significantly decreased, while GL muscle activity increased, in the FSP compared to that for the NSP (p < 0.05). In addition, ST muscle activity increased significantly in the FSP compared to that for the NSP at knee flexion angles of 30 and 60 degrees (p < 0.05). TA muscle activity significantly decreased in the FSP compared to that for the NSP at only 90 degrees of knee flexion (p < 0.05). These results demonstrate that muscle activity patterns vary significantly according to squat posture. Thus, the active control of the COP position during the squat can be a new training approach in targeting specific muscle groups.