5 Times It's Completely Okay to Skip Your Workout

You should absolutely skip your workout if: No caveats here—if you know you're injured, or even think you might be, take a break.

The reason: You might think you can work out around an injury, by, say, focusing on upper-body muscles if you've got a sprained ankle, but it's more complicated than that. "Every part of the body is connected," says Matthews. "So even if the injury is just in the shoulder, the motion your body makes when it walks slowly on a treadmill, for example, could make whatever's wrong with your shoulder worse." As the injury heals, you'll probably be able to ease back into the gym, but that advice needs to come from a medical professional. Consult your doctor if you think you have an injury—he or she can evaluate how serious it is and refer you to a physical therapist or sports-medicine trainer for more specific exercise recommendations.

When you can exercise again: When your doctor or specialist says so.