If you ask most fitness experts, they'll tell you that people simply aren't exercising hard enough. Generally speaking, they're probably right: After all, it's no secret that Americans aren't nearly as fit as we should be.

But don't tell that to the my colleague who ran the New York City Marathon last Sunday. He may have actually been working out too much. One giveaway: He called in sick the day after the race. Yes, it was after the longest, hardest run of his life -- and no doubt well-deserved. But ask yourself this: Should exercise ever leave you feeling so bad that you can't sit in front of a computer the next day? Probably not.

So when is your fitness plan too hard? Let your body answer that question. "If you have nagging injuries, aren't seeing results, or are actually losing weight when you're trying to bulk up, you're probably overtraining," says Craig Ballantyne, C.S.C.S., a Toronto-based strength and conditioning coach. This can happen to anyone: You may not be training for a marathon, but you might still be going to hard for your body. Here's how to make sure you don't exercise to excess. (And to make sure you don't eat to excess, avoid the 20 Habits That Make You Fat.)

Avoid Failure
Conventional wisdom says that the best way to build muscle is to work to failure—that is, until you absolutely can’t do another rep. Common sense offers a different conclusion: "That just increases your risk of injury," says Ballantyne.
Try this: Stop 1 rep short of failure. "You achieve full muscular activation by that point, so there’s no need to go beyond it," says Ballantyne.

Take It Easier
When it comes to cardio, what doesn’t kill you can make you weaker. If you notice a decrease in performance, constant soreness, elevated resting heart rate, irritability, headaches, and/or a pervasive sense of fatigue, you’re probably training beyond your capacity to recover. (To find the right fitness plan for your goals and lifestyle, check out The Men's Health Workout Center.)
Try this: Cut your mileage by half, and lay off the hard workouts for 2 weeks. Then shift more focus to recovery: Go really easy on easy days, and include at least one dedicated rest day a week.

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Listen to Your Body
A "no pain, no gain" mentality is ultimately self-defeating. "A sharp pain means something is wrong," says Ballantyne. "It seems like a no-brainer, but many people try to work through it." In the process, they worsen muscle tears, pulled or impinged tendons, or stress fractures.
Try this: Lift more slowly, focusing on controlled movements. If you feel pain, move to a different exercise. If it persists, see a doctor.

Do Fewer Sets
Occasional muscle soreness isn’t a problem. It's typically results from microscopic muscle tears caused by a new or tough workout. But if it’s chronic, you’re lifting more than you can handle, says Ballantyne. “You don’t need to rip a muscle apart in order to make it stronger.”
Try this: Cut back on your number of sets by 25 to 50 percent, or lift less weight. And fuel your recovery: "Each day, aim for 1 gram of protein per pound of body weight, plus 2 grams of vitamin C and at least 3 liters of water," says Ballantyne. "Protein repairs muscle, vitamin C repairs connective tissue, and water supports both." (Make sure to read The Truth About Protein to learn all the facts about this key nutrient.)

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