ACL tear
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Every year about 200,000 people tear their anterior cruciate ligament, leading to some 100,000 reconstructive surgeries. This injury will take you out for about six months or more and, after surgery, as many as 1 in 20 people suffer another rupture. People with repaired ACLs are also at greater risk of injuring their other knee.

The Fix: Hamstring, quad, and calf exercises and stretches have long been advised but may not be enough. "Now we're realizing it's much more than that, in terms of the control of the knee coming from the hip and the core," says McAdams. He specifically advises hamstring curls, leg presses, and hip abductor strengthening.