Leg curls are one of the best exercises for building size in the hamstrings -- the muscle group along the backs of your thighs. However, loading up the leg curl machine and pumping away won't do your knees any favors.
Many experts advise avoiding the leg curl machine and instead doing compound exercises that target the quadriceps as well as the other muscles of the thighs. This builds functional strength and size in a way that won't compromise knee health.
1. Stiff-legged Deadlifts
This move builds strength in the hamstrings, as well as in the glutes, abs and lower back. Performing it correctly takes some practice, so start out with a light weight. It's easiest to do these with a barbell, but you can use dumbbells or kettlebells as well.
HOW TO DO IT: Load a barbell with large plates or prop an empty bar up on parallette bars or exercise steps so the bar is at shin height. Step up to the bar with your shins almost touching. Contract your thigh, glute and core muscles and bend the knees slightly. Bend at the hips -- the knees remain stationary -- and with a flat back reach down and grasp the bar with an overhand grip a little wider than your shins. Keeping your legs locked, stand up with the bar until your hips are fully extended. With control, bend at the hips to return the bar to the ground.
If you're using dumbbells or kettlebells, hold one in each hand and only bend forward enough that the weights come to your mid-shins before standing back up.
2. Single-Leg Stiff-Leg Deadlifts
Isolate each leg and work on your balance and stabilizing muscles with this single-legged version of the deadlift.
HOW TO DO IT: Hold a dumbbell in each hand. Slightly bend your right knee and keep it steady. Contract your hamstrings, glutes and core. Lift your left foot off the ground and hinge at the hips so your torso comes down as your left leg lifts up. Keep hips and shoulders squared with the ground. Lower down until you feel a stretch in your right hamstrings, or until the dumbbells touch the ground, then hinge back up to starting position. Repeat all desired sets on each side.
3. Nordic Hamstring Curls
While this move also places more than ideal stress on the knee joint, it has been shown to actually prevent hamstring injury. You'll need a partner to hold your feet or a low sturdy object under which to tuck your ankles.
HOW TO DO IT: Get in a kneeling position on a mat to protect your knees. Cross your hands over your chest, contract your glutes, hamstrings and core muscles and begin to slowly lean forward. Keep your hips and shoulders in one line and do not break at the hips. Lower down as far as you can without your hips bending, then rise back up.
4. Glute-Ham Raises
The glute-ham developer is a simple machine that confuses a lot of people who aren't quite sure what to do with it. If your gym has one, it's a great tool for building powerful hamstrings.
HOW TO DO IT: Get into the machine and position the pad as close to your knees as possible. Lower your upper body down so it's in line with your lower body. Contract your core and hamstrings and press your toes against the foot pad as you bend at the knees and raise the upper body to vertical. Keep the back completely straight throughout the movement. Lower back down with control.