Two new studies warn about the dangers of over-exercising.

When it comes to having too much of a good thing, exercise is a prime example.

Two recent studies have shed light on the impact of over-exercising, encouraging seasoned athletes and casual enthusiasts alike to take it easy. The first, from the University of Utah in Salt Lake City, has identified the biochemical reactions that cause muscle pain and fatigue when you work out.

And the second, from Loyola University Medical Center in Maywood, Ill., shows that wealthier, young athletes with who play specialized sports can experience more overuse injuries than their lower-income peers.

While the mantra of “no pain, no gain,” is a motivator for some, it can be dangerous when taken to the extreme.

Learn More: Know the Signs of Exercise Addiction »

Lactic acid build-up has been blamed for producing muscle pain, but three culprits—lactate, certain acids, and adenosine triphosphate (ATP)—are actually at fault. These substances are released during muscle contraction, causing pain and discomfort, scientists at the University of Utah found.

In the study, researchers isolated the substances and injected them into mouse nerve cells. There was no response initially, but when all three substances were injected at the same time, a many of the nerve cells responded. The neurons responded differently depending on how much of the substances were injected.

The results were similar in human subjects, who experienced little reaction when injected with the substances separately in their thumbs, but reported pain, swelling, and fatigue once the chemicals were combined. The accumulation of these substances is likely what causes your muscles to hit a wall, so to speak, when you exercise too much.

Read More: How To Set Exercise Goals »

Wealth has its advantages, especially in the world of fitness. But researchers from Loyola University examined health insurance data, which showed that children whose families had private insurance suffered severe overuse injuries far more often than children covered by public Medicaid programs. The more strain put on a particular body part—from practicing one sport exclusively—the more likely injuries are to arise in that area.

“Early sports specialization is quite common now in youth athletics,” said Dr. James Winger, a sports medicine specialist and assistant professor at Loyola who has done similar research on exercise. “It is likely that the increased media coverage of college athletics and the possibility of college scholarships or of playing at higher levels motivates families towards sports specialization.”

Many exercise-related injuries can be avoided with proper training, said Dr. Marci Goolsby, a primary care sports medicine physician at the Hospital for Special Surgery in New York City.

“The most common reason people get stress fractures is usually a common error,” Goolsby said. This could mean forgoing strength training or increasing the intensity of a workout too quickly. Athletes often also don’t account for the amount of calories and water they’ll need to supplement their workouts, she said. Being prepared is an easy but effective defense against exercise burnout.

Not allowing your body to reboot after exercise can affect future workout performance as well. Without enough recovery time, Goolsby explained, “the body starts to rebel,” with a drop-off in performance and difficulty maintaining a regular training schedule. In extreme cases, athletes can suffer from over-training syndrome, a disorder in which the body experiences increasing difficulty bouncing back.

There are other ways to get in shape that don’t involve vigorous training. On days off from heavy exercise, relax with some low-impact activities like yoga. “When you do a strenuous workout, the next day you don’t want to be sedentary and get muscle stiffness from sitting in a chair all day, but you also want to allow your body to recover,” Goolsby said.

Every body is different and every person reacts to muscle strain in a unique way. When in doubt, talk to a doctor before taking on a new exercise regimen.

“If there are exercises you haven’t done, it may be helpful to work initially with a physical therapist, certified athletic trainer, or personal trainer. They can show you exercises, make sure you are doing them safely and effectively, and give you a basic regimen to start with,” said Jonathan S. Kirschner, an assistant professor of rehabilitation medicine at the Icahn School of Medicine at Mount Sinai Hospital in New York City.

Muscle “burn” is to be expected during a challenging workout, but pain signals that something is not working properly.

If you think you might be binging on exercise, allow yourself to slow down so that you can start again another day when you are feeling refreshed and reinvigorated.