No pain, no gain. Believe it or not, that saying doesn't always apply at the gym. Because dropping a 30-lb dumbbell on your foot, dislocating a joint, or tearing a muscle offer only misery—and will force you to take a seat on the sidelines that can seriously trip up your progress. “Every New Year, hopeful exercisers make the resolution to train hard and transform their bodies," says Scott Weiss, DPT, ATC, CSCS, a New York physical therapist and athletic trainer. "Unfortunately, many of these people lack experience or guidance—and others will approach their goals with more enthusiasm than muscle. Injuries ensue.”

So MF got experts to talk about the scariest workout injuries they see—and how to prevent them. Study up. And if any of these happen to you, see a doc ASAP. (We mean it, tough guy.)

[See: Five Most Common Gym Injuries]

1. PECTORAL INJURY. Typically a tear of the chest muscle or of its tendon off the upper arm, this injury often results from losing control of the dumbbell or barbell while bench pressing, but it can also occur while attempting to perform dumbbell flies with a weight that’s heaver than you’re accustomed to lifting—and that you’re not quite strong enough to handle yet, says David Geier, MD, orthopedic surgeon and director of sports medicine at the Medical University of South Carolina (MUSC). "You'll feel a tearing sensation, and the chest and upper arm often turns black and blue," says Geier.

Prevent it: Only work with a load you can control—you'll know it's too heavy if it wobbles and feels as if it's going to drop—and use a spotter for heavy sets.

NEXT: 6 More Scary Workout Injuries