Why do I have painful days after training?

You crush your morning workout by running further, lifting heavier or taking one more turn in this circuit. But this sweet satisfaction can quickly make you regret the next morning, when you have too much trouble swinging your legs out of bed.

Many of us have felt the burning sensation, the pain and the jelly that starts hours, even days after the effort. But where does it come from and why does it always appear a few days after some workouts?

According to the American College of Sports Medicine (ACMS), any muscle pain experienced within 24 to 72 hours after physical exercise is called late muscle pain. This pain does not manifest after all workouts – only when you are doing new exercises or intense exercises that your body is not used to. And a 2003 study in Sports Medicine revealed that this applies to both professionals and novices alike. [The Best Back Exercises for Preventing Injury and Reducing Pain]

"It does not mean you have done wrong," said Dr. Michael Jonesco, a sports medicine physician at the Wexner Medical Center at Ohio State University. "This indicates that you have stretched your body to the point of causing muscle changes."

It hurts so much?

These changes begin during exercise. According to ACMS, muscle contractions cause microscopic tears along the muscle and nearby connective tissues. These small tears do not directly cause pain. On the contrary, pain is a side effect of the muscle repair process.

"Suffering is a byproduct of healing," Jonesco told Live Science.

Once the muscle is damaged, an inflammation ensues and electrolytes, such as calcium, begin to accumulate. The immune system is also involved, according to a 2016 study by Frontiers in Physiology, which sends immune cells called T cells to infiltrate lesion sites. Scientists still do not know how these processes combine to cause pain and pain, said Jonesco, but it's likely that they come together to trigger both healing and pain.

Moreover, despite what you may have heard, lactic acid buildup is not a cause of DOMS. Lactic acid, which is produced during exercise while the muscle continues to break down glucose after the use of all available oxygen, does not remain in the body long enough after the Exercise to cause pain, according to a 1983 study published in the journal The Physician and SportsMedicine. About 45 minutes after a workout, the lactic acid levels of the subjects in the study were not high, but they continued to develop DOMS two days later. Although the subject remains controversial, most scientists, including the American College of Sports Medicine, consider that the theory of lactic acid has been debunked.

What is the limit?

Muscle pain is a good sign that you are making progress, so you can wrap yourself in pain with some satisfaction. But that does not mean it's time to go and repeat the same training, Jonesco said. Indeed, it could increase your risk of more serious injury. The best thing you can do, Jonesco noted, is a light exercise. "Very light – and I insist that the exercise is light, the blood can flow and help this muscle to relax," he said.

Extreme muscular pain is a different beast. Pain that lasts for more than a few days or pain that is so severe that it prevents you from lifting a limb may indicate a more serious type of muscle damage that can lead to kidney damage, Jonesco said. If your pain does not improve or if your urine takes a color similar to that of a black tea, it's a red flag that says you need to see a doctor, Jonesco said.

However, in most cases, DOMS is a sign that your body is adapting. The training that made you suffer so much the first time will not let you feel bad next time – but do not put yourself too much at ease.

"A little pain indicates progress," noted Jonesco. "If you do not feel any pain, maybe it's time to change gears."

Originally published on Science live.