After a particularly vigorous exercise or sports injury, many of us rely on ice packs to reduce pain and swelling in our muscles. However, a cautious new animal study found that icing deleteriously altered the molecular environment in injured muscles and slowed healing. The study included mice, not humans, but adds to the growing evidence that freezing muscles are not only ineffective after strenuous exercise. it could be counterproductive.
You can find ice packs in the freezers or refrigerators of most gyms, locker rooms, or exercise kitchens. Almost as common as water bottles, they are routinely strapped to aching limbs after strenuous exercise or potential injury. The reasons for the cooling are obvious. Ice numbs the affected area, relieves pain, and keeps swelling and inflammation at bay, which many athletes believe will help their sore muscles heal faster.
In recent years, sports scientists have begun to throw cold water at the supposed benefits of icing. For example, in a 2011 study, people who iced up a torn calf muscle later experienced just as much leg pain as those who left their aching leg and were previously unable to return to work or other activities. Similarly, a 2012 scientific report concluded that athletes who freeze sore muscles after strenuous exercise or immerse themselves in ice baths for masochists regained muscle strength and strength more slowly than their uncooled teammates. A sobering 2015 strength training study found that men who regularly donned ice packs after exercising developed less muscle strength, size, and endurance than men who recovered without ice.
Little is known, however, about how icing really affects sore, damaged muscles on a microscopic level. What happens deep in these tissues when we freeze them, and how do molecular changes there affect and potentially hinder muscle recovery?
For the new study, published in March in the Journal of Applied Physiology, researchers from Kobe University in Japan and other institutions with a long history of muscle physiology gathered 40 young, healthy male mice. They then used electrical stimulation to simulate the animals' lower legs to repeatedly contract their calf muscles, a long, strenuous and ultimately muscle-tearing leg day in the gym.
Rodent muscles, like ours, are made up of fibers that stretch and contract with every movement. Overload these fibers during unfamiliar or exceptionally strenuous activities and you will damage them. After healing, the affected muscles and their fibers should become stronger and better able to withstand these forces during the next workout.
But it was the healing process itself that interested researchers now, and whether the icing would change it. Immediately after their simulated exertion, they collected muscle samples from some animals and then strapped tiny ice packs onto the legs of about half of the mice, while the rest were left unrefrigerated. The scientists continued to collect muscle samples from members of both groups of mice every few hours for the next two weeks and then days after their pseudo-training.
Then they examined all tissues microscopically, with a special focus on what might happen to inflammatory cells. As most of us know, inflammation is the body's first reaction to infection or injury. Inflammatory immune cells penetrate the affected area, where they fight off invading germs or wipe up damaged tissue and cell debris. Anti-inflammatory cells then move in, calming the inflammatory destruction and promoting the formation of healthy new tissue. But inflammation is often accompanied by pain and swelling, which many people understandably do not like and dampen with ice.
When looking at the leg muscles of the mouse, the researchers found clear evidence of damage to many muscle fibers. They also noticed rapid accumulation of pro-inflammatory cells in the tissue that was not frozen. Within a few hours, these cells began busily removing debris until most of the damaged fibers were removed by the third day after the contractions. At this point, anti-inflammatory cells showed up along with specialized muscle cells that rebuild the tissues, and by the end of two weeks these muscles appeared to be completely healed.
Not so in frozen muscles, where recovery seemed to be significantly delayed. These tissues took seven days to reach the same levels of pro-inflammatory cells as the uncooled muscle on the third day, similarly slowing both the removal of debris and the arrival of anti-inflammatory cells. Even after two weeks, these muscles showed persistent molecular signs of tissue damage and incomplete healing.
The result of these data is that "in our experimental situation, icing delays healthy inflammatory responses," says Takamitsu Arakawa, professor of medicine at Kobe University's Graduate School of Health Sciences who oversaw the new study.
Dr. However, Arakawa points out that her experimental model simulates serious muscle damage, such as stress or tears, and not just pain or fatigue. The study apparently also included mice that are not humans, even if our muscles have similar makeup. In future studies, Dr. Arakawa and colleagues to study gentler muscle damage in animals and humans.
However, the results of his study suggest that damaged, sore muscles know how to heal themselves. Our best response is to rest and leave the ice packs in the cooler.