4. STERNUM FRACTURE. If you’re a butterfingers on the bench press and that bar comes crashing down on your chest, your sternum (that’s the bone right in the middle) pays the price. "Performing a bench press and being unable to lift the bar to the safety point is one of the most common gym nightmares," says Weiss. Even worse: a dropped bar could do further damage if it rolls up to your trachea and cuts off oxygen. (Yes, this happens—most often during unsupervised home workouts.)
Prevent it: Use only weight you can handle and always employ a seasoned spotter, who knows the proper power stance and hand position. Also be sure to tell him how many reps you think you can do on your own ahead of time, says Weiss, so that he knows when to jump in.
5. JOINT DISLOCATIONS. Not paying attention to your form or loading the bar with too much weight can lead to a dislocated hand, shoulder, elbow or knee, says Weiss. How? All that excess pressure on the ligaments forces the bones out of their normal positions, creating what Weiss calls a medical emergency. (Yes, that’s right. If this happens, seek professional help—not a buddy who claims he can pop it back into place.)
Prevent it: Train your muscles in multiple directions in each workout, or be sure to alternate exercises for any one muscle group from one workout to the next. For example, instead of doing only chest presses, add incline or decline presses or flies. And keep weight low enough to maintain proper form and technique, says Weiss. "You should be able to press or pull in a smooth and steady motion to complete the set.”