How to Execute a Leg Curl

The leg curl is a basic isolation exercise the targets two primary muscle groups: the calf muscles (gastrocnemius and soleus muscles) and the hamstrings (the biceps femoris, semitendinosus, and semimembranosus muscles).

Bodybuilders typically turn to the motion of the hamstring curl to increase muscle mass. However, fitness pros don't normally favor this exercise, particularly for certain sports, as it can cause the foreshortening (or tightening) of the hamstrings, as the muscles simultaneously contract and grow in size from repetition. As a result, this move can impair flexibility in the hamstrings when worked excessively without focusing on stretching techniques. 

Learn how to execute the motion of the curl properly.

The leg curl (also known as a hamstring curl) is executed on a gym bench with a levered lifting bar at the foot end of the machine. The exercise is performed lying flat on your stomach with your ankles tucked under a padded roller.

When you pull your heels toward your buttocks, the cables connected to a system of pulleys smoothly lift the selected weight. As you lower your feet back down, the resistance shifts slightly, making your buttocks (gluteus muscles), thighs (quadriceps), and front of the shins (tibialis anterior) take on more work.

Newer ergonomic machines now place your hips in a bent position to take pressure off the low back throughout the exercise. There are also seated leg curl machines, which operate under a similar principle.

To start the leg curl, you need to get yourself in the correct position and adjust the roller pad on the lifting bar.

Lying flat on your stomach, adjust the roller pad so that it rests comfortably on the back of your ankles, just above the heels. You don't want the pads placed too high up the calves as this can place undue pressure on the Achilles tendon and reduce the range of motion.

To properly execute a leg curl, always start with a lighter weight. You don't want to force your body to overcompensate for the heavy weight by lifting the hips and flexing your lower back. This not only fails to isolate the calves and hamstrings, but it can also cause injury to your back.

Select a weight that allows you to do eight to 12 repetitions will reasonable effort and good form.

If you want to build muscle mass, gradually increase the weight as you get stronger, week after week. To keep your hamstrings limber, always do stretches after a workout.

If your goal is to increase power and performance, never overload the weights and be sure to counter leg curls with leg extensions to strengthen a more balanced body. A smart goal is to aim for strength workouts twice a week.

The motion of the hamstring curl itself is pretty straightforward. However, it does requires you to focus on form throughout the entire move. To do a proper leg curl:

1. Lightly grasp the support handles on the sides of the machine, and lift your feet smoothly as you exhale, keeping your hips firmly on the bench.
2. Flex at the knee joint and pull the ankles as close to the buttocks as you can.
3. Hold for a second before releasing the weight with control. Do not let the weight drop quickly back down. Instead, inhale fully as you gradually return your feet to the starting position.
4. Repeat the movement until you complete eight to 12 reps.

To a certain extent, you can target the hamstrings and the calf muscles by the movement of your toes. The hamstring can be targeted by dorsiflexing the toes (curling them toward the shin), while the calf muscle (gastrocnemius) can be isolated by pointing your toes (plantar flexion).