What Muscles Are Used in Leg Curls?

Woman doing leg lifts

Leg curls are commonly performed at a fitness gym on an exercise machine. There are a variety of leg curl machines, including ones that place you in seated and lying positions. When using one of the machines, adjust the machine so that your knee joints line up with its rotating axis. Place your calves against the leg pads and bend your knees as far as you can. Because there’s only movement at your knees, there are only a limited number of muscles involved in leg curls.

Knee Flexion

There’s only one joint involved in the leg curl exercise, which means that it’s considered an isolation strength-training exercise. During leg curl, your knee joints bend against resistance. Anatomically, this movement at the knees is referred to as flexion.


The major muscle group that performs knee flexion is the hamstrings. The hamstrings consist of three different muscles, including the biceps femoris, semitendinosus and semimembranosus. They originate at the back of your pelvic bone and the top of your femur and then run down the back of your upper thighs, where it then inserts at the back of the top of your tibia bone. The biceps femoris has both a short and long head. During a seated leg curl, the short head is ineffective at contributing because your hips are bent. However, during lying leg curls, when your hips are extended, both the short and long heads of the biceps femoris are active.


Also assisting with knee flexion is the gastrocnemius muscle in the calves. The gastrocnemius muscle is largest calf muscle that originates at the bottom of your femur and then runs down the back of your lower leg where it inserts at your heel. Because your gastrocnemius crosses your knee joint, it’s able to contribute to the bending of your knees.


If you’re interested in putting more emphasis on your hamstrings by limiting how much your gastrocnemius muscle can contribute, perform the exercise while your ankles are held in a plantar position. A plantar flexed ankle position means that you hold your foot in a position where you point your toes. If you were to hold your ankle in a dorsiflexed position, meaning your foot is pulled up toward your shin, your gastrocnemius muscle will be able to contribute more and you’ll be able to curl a greater amount of weight.