workout when sick

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The Potentially Valid Excuse: You're Sick

You should absolutely skip your workout if: The problem is happening below the neck.
The reason: Body aches, fever, chest congestion, diarrhea or other digestive issues can all be made worse by exercise, says Lipi Roy, MD, a clinical instructor in medicine at Massachusetts General Hospital in Boston. These symptoms are often accompanied by dehydration, too, and sweating won't help the matter. (Issues above the neck, such as a runny nose, scratchy throat or mild cough, though, may actually be improved by moderately intense exercise.)
When you can exercise again: Once symptoms subside, but be sure to dial back the intensity on the first session. If you had a fever, wait until your temperature has been normal for at least two days before resuming your workout routine, says Michele Olson, PhD, a professor of exercise science at Auburn University.
workout when tired

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The Potentially Valid Excuse: You Didn't Sleep Well Last Night

You should absolutely skip your workout if: You'd planned to exercise first thing but got a truly terrible night's sleep (we're talking five or six hours of shut-eye, max).
The reason: In a perfect world, you'd get the recommended amounts of sleep and exercise every day, but if you're forced to choose between them, choose sleep. It's essential for a host of health functions (cognitive functioning, appetite regulation, etc.), and your workout may be subpar anyway, if you push it, as better sleep leads to better workouts. Exercising on inadequate sleep could also increase your odds of getting hurt. "If an early-morning client tells me they're really tired, we're not going to do anything that involves complex movement patterns, because I know they're not alert and focused enough to do them correctly or safely," says Jessica Matthews, an ACE senior advisor on health and fitness education and assistant professor of exercise science at Miramar College.
When you can exercise again: As soon as you're back to seven to nine hours of sleep per night.
skip workout when sore

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The Potentially Valid Excuse: You're Still Sore from Your Last Workout

You should absolutely skip your workout if: It's bad enough to make everyday movements difficult, such as sitting down or getting out of bed in the morning.
The reason: Exercising with incredibly sore muscles is uncomfortable, but that's not the main factor here. "That level of soreness limits your range of motion, changing the way you move and increasing your risk for injury," says Matthews.
When you can exercise again: Once your muscles reach the good kind of sore, where it actually feels a little better when you move, and basic movements are no longer painful.
skip workout when injured

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The Potentially Valid Excuse: You're Injured

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.
skip workout when you're exhausted

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The Potentially Valid Excuse: You Treat Every Gym Session as a Personal Challenge

You should absolutely skip your workout if: Any of these sound familiar: workouts that would normally be challenging feel impossible lately, your muscles are perpetually sore, you're experiencing sleep disturbances or your resting heart rate feels higher than normal. These are all symptoms of overtraining syndrome, says Dr. Olson.
The reason: Your body is waving the white flag. It's not just elite athletes who experience overtraining syndrome, says Dr. Olson. If you dramatically increase the intensity or length of your workouts and try to sustain or surpass that level at every subsequent session (instead of, say, pushing yourself on Monday's workout and taking it a little easier on Wednesday), it can happen to you, too.
When you can exercise again: Once your sleep returns to normal—that's a good indicator that your body is ready to work out.