Final answer:
Leg curls target the hamstrings, which are the muscles at the back of the thighs and work in tandem with the quadriceps to allow leg movement.
Step-by-step explanation:
Leg curls primarily focus on the hamstrings, which are the muscles located on the back of your thighs. While performing a leg curl, you flex your knees, engaging the hamstrings to a great extent. The hamstrings are one of the muscle pairs in our body, working in conjunction with the quadriceps, which are on the front of the thighs. The quadriceps extend the knee and are the opposing muscle group to the hamstrings. Together, these muscle groups allow for the movement of the legs back and forth, especially during activities like running.