Objective: The aim of this study was to analyze muscle activation of the lower extremities as a function of changes of the center of pressure (CoP) of the foot during squats in order to provide quantitative information to trainers who would like to teach correct movements for developing muscles. Method: Ten men with over three years of weight training experience participated in this study (age: 26.1 ± 0.8 yrs, height: 171.2 ± 3.9 cm, body mass: 71.1 ± 5.7 kg, 60%RM: 84 ± 9 kg, career: 4.0 ± 0.7 yrs). The participants were instructed to perform a squat in each of 3 conditions, with different CoP's (the front, middle, and rear of the foot). Results: The position of the CoP showed significant differences according to instructions in both the eccentric and concentric contraction phases (p < .05). The range of movement of the hip and ankle joints showed significant differences corresponding to changes of the CoP position (p < .05). The rectus femoris and gluteus maximus muscle showed significant differences for different CoP positions only in the concentric contraction phase, while the gastrocnemius and anterior tibialis were significantly different in both the concentric and eccentric contraction phase (p < .05). Conclusion: When the target muscle of squat training is the gastrocnemius, the CoP should be located in the front of the foot for effective muscle training. When the target muscles of squat training are the gluteus maximus and quadriceps femoris, the CoP should be located on the rear of the foot.