Unlike most other lower-body exercises the squat placed dramatic metabolic demands onthe intire body. From flexion to extention, squatting engages three very large muscle groups--the quadriceps, hamstrings and glutes. During the downward or eccentric phase of a rep, the hams and glutes contract eccentrically to control the forward movement of the torso as a result of hip flexion; during the upward or concentric phase, these muscle concentrically, with as assist from teh paraspinals, to extend teh hips and torso.
While your body is doing its best to stabalized each rep, a number of other muscles are working isometrically. The reaps and rhomboids help hold the bar while the abs increas intra-abdominal pressure as a means of protecting the spine. And do not forget the spinal stabilizers themselves, along with other smaller groups--up to 240 muscles in all-- contracing to help you squeeze out one eye-popping rep after another. Metabolic demands, indeed. Uncomfortable? You bet.
But look at the cost of temporary discomfort vs. the benefits. Performed corectly, squats provide significantly greater all round benefits than all other lower body exercises. The squat ultimately generates the most powerfully involving co-contraction between the queads and hamstrings--neither the hack nor the leg press can do as much for overall development. And it is the only exercise to nail all four muscle sections of the quadriceps with adequate intensity to maximize muscle gains. Period.
