The squat is a classic, multi-joint exercise that targets a variety of muscle groups throughout your body, emphasizing those that extend your hip and knee joints. There are many variations, but the most popular may be the parallel back squat, which involves balancing a barbell across your shoulders and upper back, and squatting until your thighs are parallel to the floor. Learning the specific muscles and muscle groups that fire when you perform the squat will help you decide whether or not to include it in your personal exercise program.
Although you may not think of the squat as a back exercise, the erector spinae muscle group contracts isometrically -- without shortening and lengthening -- during both the downward-movement and upward-movement phases of the exercise. The erector spinae group includes the iliocostalis, longissimus and spinalis muscles that span the length of your back on both sides of your spine, attaching to various structures therein. Although the muscles don't contract concentrically or eccentrically when you perform the squat, they are the primary spine extensors and also assist with lateral, or sideways, spine flexion and torso rotation ranges of motion.
The gluteus maximus muscle attaches to the posterior, or back, portions of the coccyx, iliac crest, ilium and sacrum on the inside of the pelvis on one end, and to the back of the thigh bone and the iliotibial band, or IT band, on the other. It is the largest of the gluteal muscle groups, which also includes the gluteus medius and gluteus minimus muscles. It facilitates hip extension along with the hamstrings on the back of your thigh. The gluteus maximus contracts eccentrically -- while lengthening -- to control the speed of the downward-movement phase of the squat, and concentrically -- while shortening -- to extend your thighs during the upward-movement phase.
The three muscles that make up the hamstrings muscle group -- the biceps femoris, semimembranosus and semitendinosus -- are also active when you perform squats. All three muscles attach to the ischial tuberosity on the back of your pelvis on top, and to the tibia bone of your lower leg, just below the knee joint, at the bottom. The biceps femoris also attaches to the head of the fibula bone of your lower leg. The hamstrings help the gluteus maximus muscle with hip extension, so they function in the same way when you perform squats -- eccentrically during the downward-movement phase and concentrically as you return to the starting position.
The rectus femoris, vastus intermedius, vastus lateralis and vastus medialis muscles are referred to collectively as the quadriceps muscle group. The rectus femoris attaches to the anterior inferior iliac spine, just above the hip socket, and the others attach to the thigh bone, just below the hip socket, on top; all four muscles combine at the bottom, attaching to the patella on the front of your knee joint. The quadriceps muscles serve as the primary knee extensors, increasing the angle between your lower and upper legs. Like the hip-extensor muscles, they contract eccentrically during the downward-movement phase of the squat and concentrically during the upward-movement phase.