When Jennifer Love Hewitt broke her wrist in a boxing class, she took to the Twittersphere to spread the word. Ouch. It just shows: Although group fitness can be a great way to get exercise and stick to a weight-loss plan, classes aren't all super-fun sweat sessions -- sometimes, they involve blood and tears, too.

Whether you're a badass boxer like Hewitt or swear by your morning spin class, check out these tips from the fitness industry's top instructors for how to get fit without getting hurt: