An injury can be the result of simple bad luck, but often your own missteps are to blame. "If you lift with bad form or use weights that are too heavy, you're bound to hurt yourself at some point," says Mike Reinold, P.T., C.S.C.S., a Boston-based physical therapist and performance enhancement specialist. Read on to discover if you're headed for three common gym injuries, and then follow our advice to stay off the disabled list for good. ACL assessment Sudden stops and pivots can lead to a torn anterior cruciate ligament (ACL), one of the knee's key connectors. But trouble can also arise from lower-body instability. "If your hips are weak, more weight shifts to your knees," says Nick Tumminello, C.P.T., owner of Performance University. If they can't handle the stress, pop goes your ACL. Assess your risk Stand in front of a mirror with your feet shoulder-width apart. Keeping your back naturally arched, push your hips back and squat as low as you can. If your knees roll inward, you're vulnerable. Prevent it "Push your knees apart when you squat or lunge," says Tumminello. That encodes the proper "knees out" pattern in your motor memory. Reinforce it by looping a mini-band above your knees and pressing against it as you do body-weight squats--three sets of 10 reps once a week. Stretch those hip flexors to prevent your ACL from tearing with these 7 Easy Stretches You Can Do At Work. Shoulder exam Excessive overhead pressing or pulling can lead to weight-lifter's shoulder, also known as shoulder impingement. So can playing sports--such as baseball, racquetball, and swimming--that involve repetitive overhead movements. "An impingement occurs when the bones in your shoulder pinch the surrounding tendons and muscles," says Reinold. Assess your risk Place your right hand on your left shoulder and raise your right elbow toward the ceiling. Repeat with your left arm. If you feel tightness or discomfort in either shoulder, you may be on your way to a full-blown injury. Prevent it Eliminate overhead work from your routine and strengthen your rotator cuffs with W raises: Grab a light dumbbell in each hand and lie facedown on an incline bench. Bend your elbows slightly past 90 degrees and hold your upper arms by your sides, palms facing each other. Squeeze your shoulders and raise your upper arms; your arms should form a W. Lower and repeat. Work your way up to three sets of 15 reps, but don't lift to failure. Build them like the pros with this Surprising NFL Shoulder Exercise. Achilles quiz If you pound the pavement, you're a prime candidate for Achilles tendinitis, a wear-and-tear injury to the tendon attaching the heel and calf muscle. It occurs in about 10 percent of runners. Tendinitis is painful, and Reinold says it can easily lead to something far worse: a rupture. "If that happens, you can't even walk," he says, and recovery takes months. Assess your risk Assume a staggered stance in front of a wall and lean into it with both hands. Bend only your front knee and keep both heels on the ground. Switch legs and repeat. "If you feel your calves stretch, you're good," says Reinold. If you feel pain in the back of either ankle, you may have some form of the injury. Fix it Achilles tendinitis often occurs when you do too much too soon, says Reinold. Halve your weekly mileage, and then start increasing it by no more than 10 percent a week. Also, stretch and foam-roll your calves before and after every run to keep them loose. No matter if you're a weekend warrior or an aspiring Olympian, our Injury Prevention Workout Plan will keep you pain-free and in the game.