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You Recently Had a Concussion

Do not exercise or participate in any sport until your doctor says it is safe to do so -- even if you feel OK. A concussion is a traumatic brain injury, and your brain needs to heal properly. “If another head injury occurs due to exercise before the [concussion] has healed, the brain is put at an increased risk of swelling and potential catastrophic damage,” Rice says.

Your Old Sports Injury Is Bothering You

Skip the workout and see your doctor. This is usually not a good sign, especially if you have pain during activity, Rice says. Sudden pain requires immediate medical attention.

You Didn't Sleep Last Night and Now You're Too Tired to Exercise

Get out of bed and get moving. “A morning workout may be just what you need after a restless night to boost your energy level and get you ready for the day,” Rothstein says. But if you're always fatigued -- not just a bit tired, but to the point of not being able to function -- skip the workout and see your doctor. Extreme or persistent fatigue can be a sign of illness.

You Felt a Sharp Pain the Last Time You Worked Out

Do not work out until you see a doctor to rule out an injury. If you exercise anyway, you may make the injury worse. “Although it is expected to feel soreness after you work out, it is never OK to feel pain,” Rice says.

Your Back Hurts

Take it easy for a few days and see if your back feels better. Pay attention to what makes the pain better or worse: Does bending or twisting cause you to squirm or say ouch?  Try to avoid those movements to promote healing, Rothstein says. If your pain continues or interferes with your daily activities, see your doctor. 

Your Muscles Are Sore

You can go to the gym, but make it a light-intensity workout, such as walking instead of running. It's also OK to skip your workout and rest, if the soreness is too severe.  And if your muscles are super sore because you overdid it the last time you exercised, make a point of making your workouts more reasonable. You don't want your fitness habit to become  a flash in the pan.

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