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Is there such a thing as too much exercise?

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We hear all the time about the benefits of exercise: it keeps weight down and energy up, improves mood, and staves off some diseases like diabetes. But can a person exercise too much? Can working out become too much of a good thing? In a way, yes. It all depends on how you're working out.

The science on excessive exercise is finding conflicting results

One study looking at longtime marathon runners found a higher prevalence of heart damage than their peers (Shutterstock)

Scientists don't yet know exactly how much exercise can hurt you in the long run. What the evidence shows for certain is that if you exercise a normal amount and give your body enough time to rest, it should generally be good for you. For example, one large-scale study of 650,000 US and Swedish individuals getting the CDC's recommended amount of exercise (30 minutes five times a week) found they lived an average of 3.5 to 4.5 years longer than people not getting any at all.

But what about people who go above and beyond, like Biggest Loser contestants trying to drop a lot of weight quickly or runners training for a marathon? The research is still up in the air about the effects of working out too much.

One study published in 2003 that suggested a positive impact from excessive exercise looked at almost 50,000 long-distance skiers in Sweden and found that people who completed more races were likely to live longer. In addition, a 2012 study of nearly 11 million long-distance runners over ten years found that only 59 people had gone into cardiac arrest during a race — and the majority of them had preexisting cardiovascular disease, which was presumably not caused by overexercising during training.

However, there is also some research that shows that training too hard can lead to heart damage. One animal trial found that intensive exercise led to cardiac arrhythmia, which is when a heart beats too fast, too slow, or with an irregular rhythm. The results haven't been repeated in humans, but scientists think it could offer clues of what might happen in us, too.

In 2008, researchers studied over 100 middle-aged German marathon runners who'd been racing for long periods of time. Compared to non-runners also studied, the marathoners had more heart problems and in some instances, a buildup of calcium in the arteries in their hearts.

And there's also the new jogging study published earlier this month in the Journal of the American College of Cardiology that said strenuous jogging leads to higher mortality rates than light jogging. It led to headlines like "Working out too much is as dangerous as never working out" and "Fast running is as deadly as sitting on couch, scientists find."

Researchers followed about 1,000 healthy joggers and 4,000 healthy non-joggers in Copenhagen for over a decade. They found a U-shaped relationship between death and the amount and level of jogging: Light and moderate joggers had lower mortality rates than the non-joggers, but strenuous joggers had similar mortality rates to the inactive people.

The conflicting research tells us there isn't yet a strong consensus on whether overexercising harms the heart, and it's unclear what future research will turn up. However, there are important steps you should take to protect yourself from well-known injuries from exercising, like overusing muscles.

Giving muscles enough rest is important

The time off matters as much as the workout, experts say (Shutterstock)

There is such a thing as excessive exercise, but it has less to do with how much you're doing and more to do with how you're pacing yourself. Muscles need the chance to rest.

When you work out, you damage your muscle fibers. This is normal. And then, when you're resting, your body repairs them by fusing those fibers together. This creates more (and thicker) muscle fibers, which is what leads to gains in strength. But your body needs that time off to make those adjustments.

But people who are doing nearly round-the-clock training (like athletes doing two-a-day practices or individuals trying to drop a lot of weight quickly) might not stop to let their muscles rest. "When you put so much wear and tear on the body, you're not allowing it to recover," says Jim White, a fitness specialist with the Academy of Nutrition and Dietetics.

And that can lead to a handful of sport-specific injuries. For example, players practicing the perfect serve might get tennis elbow when the muscles in the forearm get used too much. There's also swimmer's shoulder, a generic term for problems like tendinitis — swelling of the tendons that connect muscles and bones.

Runners who overdo it by going too fast or too far might get stress fractures, a partial hairline break in a bone which causes pain and can leave an athlete on crutches. Stress fractures are particularly likely to develop in runners who've escalated their workouts too quickly. Conditioning a muscle takes time, and muscles that aren't conditioned properly can't cushion bones as well as they should be able to.

Taking time off from working out isn't just important for serious athletes. Even if you exercise a moderate amount every day, not taking time off can become detrimental.

"Rest is equally as important as the actual exercise part," White says, no matter how much exercise you do. Not resting can cause fatigue, pulled muscles, and other injuries. If you're wearing out your muscles, you have to let them rest.

And that's not to mention the psychological impact. White says people who are afraid to take days off can harm themselves in the long run by feeling exhausted, quitting, and ending up back at square one.

How much exercise is too much

Knowing your limits is the best way to avoid an overuse injury like swimmer's shoulder (Shutterstock)

If you're getting the CDC's recommended 2.5 hours a week of moderate exercise, overdoing it shouldn't be a big concern. Put in effort, but know your limits.

Sore muscles are good when you're done working out because it means you've worn them out and they'll become stronger in recovery. But if your muscles are sore for more than a few days, you might be pushing it too hard, says White. To end up in a healthy place, he recommends making each workout a minimum of 20 minutes long and a max of an hour.

And be sure not to make your workouts too strenuous too quickly. That can definitely lead to injury. "You see a lot of people going from zero to 100 when they begin an [exercise] program, and that's what's gonna be damaging," he says. Ideally, a workout should increase 5 to 10 percent each week, whether that's in the length of a run or the amount of weight you're lifting.

WATCH: 'Why do people run the marathon?'

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