Moving quickly through your sets could also be a sign you're misusing momentum, executing the exercise incorrectly, or using too much or too little weight.
With any strength move, go at a slow, controlled pace in the lifting and lowering phases—two seconds up, two seconds down. "There should be no point at which you pick up speed," says California trainer Jay Blahnik, author of Full-Body Flexibility. With crunches, hold dumbbells or a medicine ball on your chest to slow you down. For lunges, hold a dumbbell in each hand as you perform your reps or try doing them with your front foot on a Bosu balance trainer.