The exercise in which you lie face down and curl your heels toward your glutes is the leg curl, also called the hamstring curl. In the lying version, the lower legs bend against resistance while the thighs remain supported on a bench or machine.
The movement primarily trains the hamstrings, a group of muscles on the back of the thigh that help flex the knee. Lying leg curls are usually performed on a dedicated machine with a padded roller positioned behind the ankles. The lifter bends the knees to bring the roller toward the buttocks, then lowers it under control.
Leg curls are often confused with leg extensions. A leg extension is performed seated and straightens the knee to emphasize the quadriceps on the front of the thigh. Squats and leg presses involve several joints and muscle groups, whereas the leg curl is an isolation exercise focused on knee flexion. Other versions include seated and standing leg curls, each changing the hip position and the hamstrings’ working length.