Update:
Leg curls are a useful bodybuilding exercise, stretching out and developing the short head of the biceps femoris while working the sartorius that runs along the whole length of the front inner thigh. The risks mentioned here were exaggerated. Nonetheless, it is difficult to reach a stretched position for the other far larger muscles of the hamstrings by using anything besides a standing leg curl machine, so a stiff-legged dead-lift or other hip extension exercise will be needed for stretch-mediated hypertrophy.
Some rightfully disdain leg extensions since they can harm the knees. They also work the muscles less well than a compound exercise. They then justify the leg curl, defending it as the best way to work the hamstrings, as if it possessed none of the issues associated with the leg extension. The evidence shows otherwise.
Leg curls, whether seated, lying, or standing, carry all the disadvantages of isolation and machines. The leg curl has further problems though.
Reasons
When things go wrong, don’t go with them.
– Elvis Presley
- It can allow active insufficiency.
A proper leg curl machine, the lesser of two evils, requires a bend in the bench where it rests against the hips. This flexes the hip to prevent overshortening of the hamstrings. This occurs because the hamstrings attaches to both the hip and the knee. If it shortens at both joints, this leads to active insufficiency. The contraction sites bunch up and overlap, allowing less connections. The gastrocnemius, the large muscle of the calf, can also enter active insufficiency if you point your toes.
Even with a machine that compensates for this, you tend not to get enough hip flexion. Trainees will feel this as a cramping sensation at the top of the movement. This also lessens the strengthening effect since it forms much less tension throughout the range of motion. Tension is the main stimulus for more size and strength.
- The long moment arm harms the knees.
The long distance between the weight placed at your ankles and the knees maximizes the shearing forces.
- It harms the quadriceps-to-hamstrings strength ratio.
A very uneven ratio may predispose you to injuries such as muscle tears. Many aim to balance the quads-to-hamstrings ratio with the leg curl. The leg curl, as an unnatural exercise, makes little sense as a solution. It instead imbalances the lower body through isolation. Someone with less knowledge of anatomy may not realize how hard the hamstrings work on a correct squat, which involves all the major muscles of the lower body equally.
- It harms knee stability.
During normal activities for the lower body, the hamstrings contract with the quadriceps. This stabilizes the knee, since forces from the front and back work to hold it in place. The ligaments and tendons provide medial and lateral support, though the muscles assist as well. The leg curl prevents force from the front, destabilizing the knee.
- It can hyperextend the spine.
The prone position of the lying leg curl, along with the same mechanics applying to all the variations, tends to hyperextend the spine as the torso, hips, or both rise off the bench. The spine must remain neutral to protect the lower back.
- Knee flexion acts as an unloaded function of the hamstrings.
The hamstrings contribute to hip extension best. We know this since the muscle resides further away from the attachment site on the hip. This grants a mechanical advantage for the muscle acting at this joint.
Try to think of a free weight movement that would involve knee flexion with resistance. You would have to attach something to your leg. If you performed this while standing, you would enter active insufficiency as your heel came closer to your rear.
Loading this motion would have served no role in the movements needed throughout our evolution. The body would have used the hamstrings for hip extension involved in running, jumping, and so on. Knee flexion only happens to alter the position of the leg. The body does not expect heavy loads for it. Therefore, the knee has not adapted to tolerate it well.
- You cannot isolate the muscles of the hamstrings.
The hamstrings are a group of muscles that all attach to the knee and the hip. When performing action at these joints with any heavy weight, all of the muscle fibers will come into play based on motor unit recruitment. Only with very light weights, which would have a negligible effect on muscle building, could you perhaps isolate certain muscles. The anatomy fails to support this concept though.
Though the short head of the hamstrings can work only through knee flexion, this muscle will not gain much size and strength. Training muscles meant for speed or for an unloaded action can harm you. It places forces on the joint your body did not come to expect. It still gets worked though by resisting knee flexion and external rotation of the hip.
Avoid Leg Curls
Those that admit the shortcomings of the leg curl may try to temper them by using more reps. They ignore that any load at all would carry the same risks due to the knee flexion. They try to make the best out of a bad yet unnecessary situation.
Perform hip extension to strengthen the hamstrings. Avoid overemphasizing hip extension though. Pick a lower body movement that balances knee extension and ankle plantarflexion with hip extension. This occurs during a squat with the right bar placement and good form. You should feel as if no muscle or joint takes over the exercise.
Ignore leg curls and focus on squats.