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Quad-Bulking Leg Extensions
This machine is found in every gym, says Matthews, despite the fact that it isn't that useful. You've heard it before, but it bears repeating: Unless you're using this under the supervision of a physical therapist (to recover from runner's knee, for example), it's a waste of your time. The weight is placed uncomfortably near the ankle, where it can put a high amount of torque on the knee joints when you lower it, says Matthews. Some people cause even more problems by starting with their feet tucked way under their seat. This exercise bulks up your quads, but women tend to have stronger quads than hamstrings, so unless you're doing a separate exercises for the back of the legs, you risk muscle imbalance and future strains.

Be safe: Matthews suggests sticking with basic squats, which work the front and back of the legs and can be done anywhere, anytime, without machinery.

Next: 10 common workout mistakes to avoid