Is the correct hamstring curl done to a 90 degree angle only? To go beyond would be hard on the knees, right? Thanks.
One of the all-too-often bogus conclusions heard throughout the fitness industry is that one should exercise a joint through less than a full range of motion (ROM). The usual rationale for this advice is to avoid stress to a particular joint. The joint often in question is the knee, although we see the same advice often provided for the elbow.
Minus some medically related problem that severely limits range of motion, one should work a joint from full extension to full flexion. The stated reason for this is to enhance a joint’s natural range of motion, thus working muscles and connective tissues as they are designed to work. It also avoids muscular development that does not include the entire range of motion. To train through less than a full ROM runs the risk of creating hypertrophy (muscular growth) without complete joint flexibility.
Gaining muscular strength and/or growth while inhibiting the ability to move through a full ROM reinforces the notion of one becoming “muscle bound,” the old wives’ tale that resistance training has had to vigorously fight for decades as inaccurate.
So, perform exercises through a full range of motion, period. A full ROM should not provide any negative stress to a healthy joint. If someone has joint problems with a particular exercise, don’t use that move. Find a more suitable alternative.
There is nothing inherently dangerous in complete knee flexion and/or extension. It sounds like you are talking about a prone leg (knee) or hamstring curl. Full range of motion in this exercise, while recommended, may not provide adequate resistance as the machine’s arm is raised past perpendicular to the floor. In other words, momentum may contribute to knee flexion much beyond 90 degrees, but this depends on the machine involved.
You may find a standing leg curl, performed through a full ROM, to be more beneficial. Any form of leg curl is a pretty artificial means of training the hamstrings. We don’t function in this position in our day-to-day or athletic lives. The leg curl exercise is pretty much a bodybuilding specific attempt at targeting specific muscle hypertrophy through a single joint movement.
With proper coaching and technique, athletes find performing exercises such as the good morning and the stiff legged deadlift to be much more beneficial in terms of functional movement, greater muscle activation and improved hypertrophy. Both exercises are ground based, thus providing bone density benefits well beyond anything possible through a lying, single joint movement.