Leg curls help you add size and strength to your rear thigh muscles. The exercise is performed on a machine by curling your legs against a form of resistance. The machine can include a padded support or set of cables attached to ankle cuffs to help you perform the movement. Before starting a resistance training regimen, consult a physician.
Movement
Lie face down on the machine, adjusting the lever so the pad of the lever presses a few inches below your calves. Keep your torso flat on the bench and make sure your body is fully extended. Grab the side handle and curl your legs up by flexing your knees. Keep your upper legs in contact with the pad. Hold the fully contracted position for a moment and slowly lower your legs. Bodybuilding.com notes that using an angled leg curl machine targets the hamstrings more effectively than a machine with a flat bench. Avoid putting stress on your lower back by keeping your torso on the bench.
Hamstrings
Leg curls primarily target your hamstring muscles. Flexing your knee during the concentric phase of a leg curl brings your lower leg toward the back of your thigh. This places stress on the hamstrings, helping develop leg power for sports like sprinting, football, basketball and soccer. The hamstrings contain four different heads and are typically underdeveloped compared to the quadriceps, or muscles on your front thighs.
Assisting Muscles
ExRx.net notes that during leg curls, synergist muscles, or muscles assisting the hamstrings to complete the movement, include the gastrocnemius, sartorius, gracilis and popliteus muscles. The gastrocnemius, or calf muscle, is activated when your knee is flexed during a leg curl. The sartorius, which is the longest muscle in your body, and the gracilis both span the inner thigh. These muscles support the hamstring as your knee bends. The popliteus, or the knee flexor, also directly assists the hamstrings in knee flexion.
Stabilizing Muscles
Performing leg curls causes the rectus femoris and tibialis anterior to contract, helping to stabilize forces in your leg by counteracting the hamstrings. The rectus femoris is the primary head of the quadriceps muscle. The tibialis anterior, or shin, spans from below your knee to your foot.
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