First of all, leg muscles account for most of the whole body muscles. When exercising, the legs need to consume a lot of oxygen, while the heart is far away from the thighs. When exercising thigh muscles, blood will basically circulate rapidly throughout the body, so exercising thighs is basically equal to exercising the whole body. Moreover, when doing squats and other actions, the barbell is pressed above the heart, which causes certain pressure on blood circulation, so it is particularly hard to do leg exercises, but the effect is also very good.