Muscles Worked in a Seated Leg Curl

The seated leg curl is an isolation exercise that strengthens your knee flexors, which are the muscles that bend your knee. It primarily targets the hamstrings, your strongest knee flexors, but also works several other thigh and calf muscles that help the hamstrings bend the knee, including the gracilis, sartorius and gastrocnemius.

Seated Leg Curl

To use a seated leg curl machine, sit on the seat. Adjust the seat so that the axis of the machine aligns with your knee joints and the padded lever rests against the back of your ankles. If the machine has a lap pad, lower it to rest against the top of your thighs. Holding the machine's handles, bend your knees to pull the lever toward the back of your thighs, then return to the starting position.

Hamstrings

The leg curl primarily works the hamstring muscles. The three hamstring muscles are located on the back of the thigh. Two are on the inner, or medial thigh, and one is on the outer, or lateral thigh. The two medial hamstrings are the semitendinosus and the semimembranosus. They originate on the pelvis at the sitting bone, or ischial tuberosity, and insert onto the upper shin bone, or tibia. The lateral hamstring is the biceps femoris. It also originates on the ischial tuberosity, and attaches to the long bone on the outside of the lower leg, the fibula.

Synergistic Muscles

Several other thigh muscles also get a workout in the seated leg curl. The gracilis is one the adductor muscles of your inner thigh. It runs from your pubic bone to your upper shin bone. Although its main job is to draw the thighs together, it also helps to bend the knee. The sartorius originates on your outer pelvis and wraps across the front of your thigh to attach to the upper shin bone, near where the gracilis attaches. Its primary task is to fold the legs into a cross-legged position, but it also aids the hamstrings in flexing the knee.

Calf Muscles

You can also target your calves with leg curls. The gastrocnemius, one of the larger muscles of the calf, originates from the lower part of the back of the thigh bone, or femur, and inserts through the Achilles tendon onto your heel bone. Its main function is to point your foot, but it also helps to bend the knee. To focus on the gastrocnemius when doing leg curls, keep your foot flexed throughout the movement.