Ice for sore muscles? Think again.

After a particularly vigorous workout or sports injury, many of us rely on ice packs to reduce pain and swelling in our torn muscles. But oh Precautionary new animal studies It turns out that icing alters the molecular environment inside injured muscles in harmful ways, slowing healing. The study involved rats, not people, but adds to the growing body of evidence that icing muscles after strenuous exercise isn’t just ineffective. It can be counterproductive.

Check inside the freezer or cooler in most gyms, locker rooms or athletes’ kitchens and you’ll find ice packs. Almost as common as water bottles, they are routinely bandaged to sore limbs after strenuous exercise or potential injuries. The rationale for cooling is obvious. Ice numbs the affected area, reduces pain, and keeps swelling and inflammation at bay, which many athletes believe helps their sore muscles heal more quickly. Is.

But, in recent years, exercise scientists have begun to throw cold water on the supposed benefits of icing. In a ___ A 2011 study, For example, people who Iced a torn calf muscle. Later, the legs felt as sore as those who had left their injured leg alone, and they were soon unable to return to work or other activities. Similarly, A The 2012 scientific review was concluded. Athletes who developed muscle soreness after intense exercise—or, for the narcissistically minded, immersed themselves in ice baths—regained muscle strength and power more slowly than their teammates. Got it. And A A 2015 serious study of weight training It found that men who regularly applied ice packs after exercise had less muscle strength, size and endurance than those who recovered without ice.

But little is known about how icing actually affects injured, damaged muscles at the micro level. What happens deep within these tissues when we ice them, and how do any molecular changes there affect and potentially hinder muscle recovery?

So, for the new study, published in March in the Journal of Applied Physiology, researchers at Japan’s Kobe University and other institutions long interested in muscle physiology recruited 40 young, healthy, male rats. Collected. Then, by using electrical stimulation of the animals’ lower legs to repeatedly contract their calf muscles, they simulated, in effect, a long, grueling and ultimately muscle-shredding leg day at the gym. .

Rat muscles, like ours, are made up of fibers that stretch and contract with any movement. Overload these fibers during unfamiliar or unusually strenuous activities and you can damage them. After healing, the affected muscles and their fibers should be stronger and better able to withstand the same forces the next time you exercise.

But it was the healing process that now interested the researchers, and whether icing would change that. So they collected muscle samples from some of the animals immediately after their simulated exercises and then strapped tiny ice packs to the legs of about half the mice, while leaving the rest cold. The scientists continued to collect muscle samples from members of both groups of mice every few hours and then for the next two weeks after their sham exercise.

Then they examined all the tissues under a microscope, looking specifically at what was happening to the inflammatory cells. As most of us know, inflammation is the body’s first response to any infection or injury, in which pro-inflammatory immune cells rush to the affected area, where they fight invading germs or tissue and cellular debris. Collect the broken pieces. Inflammatory cells then move in, quiet the inflammatory process, and stimulate the formation of healthy new tissue. But inflammation is often accompanied by pain and swelling, which many people understandably dislike and use to wet ice.

Looking at mouse leg muscles, the researchers saw clear evidence of damage to many of the muscle fibers. They also noted that tissue that was not iced had a faster accumulation of pro-inflammatory cells. Within hours, these cells began to remove cellular debris until, by the third day of contraction, most of the damaged fibers had cleared. At that point, anti-inflammatory cells appeared, along with specialized muscle cells that regenerate tissue, and by the end of two weeks, these muscles appeared to be completely healed.

This is not the case in iced muscle, where recovery is markedly delayed. It took seven days for these tissues to reach the same level of pro-inflammatory cells as three days in unchilled muscle, with both debris clearance and the influx of anti-inflammatory cells being similarly slowed. Even after two weeks, these muscles showed prolonged molecular signs of tissue damage and incomplete healing.

The conclusion of this data is that “in our experimental situation, icing inhibits a healthy inflammatory response,” says Takamitsu Arakawa, professor of medicine at Kobe University Graduate School of Health Sciences, who oversaw the new research.

But, as Dr. Arakawa explains, their experimental model simulates severe muscle damage, such as a strain or tear, not mild pain or fatigue. The study, of course, also included rats, which are not people, even if our muscles share the same makeup. In future studies, Dr. Arakawa and his colleagues plan to study mild muscle damage in animals and humans.

But for now, the results of his study suggest, he says, that damaged, sore muscles know how to repair themselves and our best response is to chill and leave the ice pack in the cooler.