Q: Is there any significant difference between the squat and leg press with respect to the muscles involved? Can I get by with doing one or the other, or do I need to perform both?
A: If you're after optimal leg mass and development, you must use a variety of leg exercises, including the squat and leg press. Although both exercises work essentially the same muscles, the emphasis of each is slightly different, which means muscle development is different as well.
The squat, known as the granddaddy of all bodybuilding exercises, simultaneously works more major muscles than any other resistance-training movement. The quadriceps (quads), hamstrings and gluteus maximus (glutes) are specifically targeted, while the hip and torso muscles are incorporated for stabilization and to assist the primary muscles. You can see why many experts in weight training consider the squat to be a whole-body exercise, even though you utilize it as a lower-body move.
Stabilizer muscles are smaller—and often less visible—muscles called upon during free-weight exercises to provide support and prevent unwanted movement. To perform the squat safely and effectively, the abdominal muscles (including the rectus abdominis, external and internal obliques and the transverse abdominis) and spinal erectors of the lower back contract isometrically to hold the torso in place as movement occurs at the hips and knees. Likewise, the hip muscles contract to hold the hip girdle stationary, allowing only the desired movement (hip flexion and extension).
The squat range of motion (ROM) about the knee and hip joints is relatively large. Increased ROM translates into greater muscle activation and ultimately into superior muscle development. In particular, the wide ROM at the hip is what notably differentiates the squat from the leg press.