Anyone who has ever been coached by me knows, I like the weight room as a cross training day. They also know, I have a particular disdain for two particular “machines” in the weight room. My two least favorite: Leg extension, and leg curl. I find them both to have have little effect on functional strength improvement. In fact they may even contribute to injury in an athlete, especially runners. It also seems counter intuitive to me that you would want to sit down to work your legs.
The seated leg-curl machine: This mechanism has nearly zero effect on the functional strength of the hamstrings. It works your hamstrings because it requires you to flex your knees against a resistance. However, the main function of your hamstrings is actually to extend your hips, not flex your knee. The other way your hamstrings can work is as a decelerator, they slow you down when you squat, if they didn’t your knee would buckle and you would hit the floor.
The seated leg extension machine: The only functional strength this apparatus creates is the ability to kick something from a seated position. Leg extensions also place a significant amount of stress on your knees because the resistance is placed near your ankles, this creates high amounts of torque being applied to your knee joint when you lower the weight.
The other issue is that people use these machines with a total disregard for joint positioning and that creates even more misappropriated torque. If you feel you must use these machines, do yourself a favor and align the pivot points of the knee, with the pivot point of the machine. In the photo above the user has his knee joint far behind the pivot point of the machine. Not good.
The alternatives: Free weight squats, split squats, and lunges—performed with perfect form—are all better choices for working your quads and hamstrings while protecting your knees. What’s more, Auburn University scientists found that people who squat long-term have tighter, stronger knee ligaments than those who don’t squat at all.