Ice, or more specifically, the cold, is usually thought of as the go-to remedy to reduce swelling and relieve pain after exercise or following an injury. But ice isn’t a cure-all, and the timing of its application and the type of injury sustained inform whether ice is appropriate. We’ve asked Dr. Melvin Rosenwasser, MD, of Columbia University Medical Center and NewYork-Presbyterian Hospital, and the official hand consultant for the New York Yankees, to clarify the differences between treating an acute muscle injury and treating post-exercise muscle soreness with ice. And as it turns out, the two aren’t the same — and sometimes the solution isn’t ice, but heat.

“Ice is a fantastic initial treatment following any musculoskeletal injury,” says Rosenwasser, who specifies that acute soft tissue injuries are types of musculoskeletal injuries, as are heavy weight lifting exercises, which micro-tear muscles. With a more serious injury, like a fracture or dislocation, Rosenwasser emphasizes that it’s important to get the extremity lined up and splinted first. “But beyond that,” he says, “with the usual lumps, bumps, strains and over-exertions — ice is fantastic.”

“Ice is a fantastic initial treatment following any musculoskeletal injury.”

Understanding how the body reacts to injury is key to assigning the appropriate treatment. When muscles and ligaments are injured, the body’s inflammatory response is to increase the blood supply to the injured area. This results in increased warmth and redness. The damaged tissues also release cytokines (chemical messengers) which cause blood vessels to leak fluid into the surrounding tissues resulting in increased swelling. The swelling accumulates underneath a muscle’s inelastic membrane. As the membrane’s pressure increases, so does the pain. Acute soft tissue injuries can be identified by these four inflammatory signs (in the order that they occur): heat, redness, swelling and pain. Icing can minimize this inflammatory response, as well as the pain.

After a person injures themselves — say, by spraining an ankle or lifting too much weight — the injured muscle swells, making it difficult for the person to move. Ice can reduce this swelling and, in turn, may help that muscle recuperate faster. “Years ago when people would sprain their ankle, they thought it was better to put it in a cast to stop it from moving,” Rosenwasser says. “It turns out that immobilization is actually worse for it.” After a few days it’s helpful to start gently moving the muscle, which helps mobilize joints and strengthen the muscle. This also increases blood supply to the area. After 48 to 72 hours this increased blood supply — which will flush inflammatory substances like cytokines — will help the healing process.

Ice works for healing because it constricts the blood vessels that carry these harmful chemicals (the cytokines) to the injury, which in turn slows down the inflammatory process. “You want to slow down the increasing concentration of irritating substances,” says Rosenwasser. “The inflammatory mediators that are released after injury can be harmful. So ice just slows down that process. If you can slow down the swelling, it’s easier to come back up to a functional level.”

Rosenwasser instructs people to apply ice immediately after an acute soft tissue injury occurs; if this is not possible, then do so in the first 24 to 48 hours. “After about 48 hours, swelling is not amenable to icing.” To reduce the swelling at that point a person is going to have use either medication or a compression wrap. Ice stops swelling at the beginning, but after the injury is already swollen, ice isn’t going to help. That’s when heat comes into play.

After approximately 72 hours (not a hard rule), heat can help people recover from a muscle injury. Heat helps muscles regain flexibility if they’re swollen with tendonitis, sprains and strains, but not in the first 48 hours. Heat will accelerate and make worse all the processes that ice initially slows down. After the initial inflammation, heat helps you stretch and move your muscles better. “It’s usually heat before exercise, and ice to minimize the swelling after,” Rosenwasser says.

“After about 48 hours, swelling is not amenable to icing.”

There are other times when acute injuries shouldn’t be iced. According to Rosenwasser, if there’s a sensory problem — meaning there’s no feeling, sensation or circulation in an extremity — don’t use ice. No feeling can mean decreased blood flow, and icing an area without blood flow can only make it worse. “So I would never put an icepack on somebody who’s had a local anesthetic, because they wouldn’t sense when it was too cold, and they could actually injure themselves.” The same is true for an athlete who’s taking an ice bath: if they’re in for too long, they could actually get frostbite. But as long as the patient has sensation, ice is a great technique to reduce swelling.

Ice is thought to help endurance athletes recover as well — here’s how. When we run long distances our muscles require more energy. Normally, our body uses oxygen to break down glucose and produce more energy (this is called an aerobic metabolic process). However when we exercise intensely, our body demands more oxygen than we’re able to deliver. To compensate, we produce organic compounds that help break down glucose without oxygen (this is called an anaerobic metabolic process). Of these organic compounds, which linger in the bloodstream after intense exercise, lactic acid is widely believed to be the cause of muscle pain — however, this is controversial among physicians and certified fitness professionals. They believe that it’s leftover hydrogen that causes fatigue and soreness. Although there’s no definitive scientific study that proves one or the other, one of the byproducts is believed to cause post-exercise pain — but not necessarily swelling.

Ice is a vasoconstrictor. So when tired athletes ice their muscles, they’re simply attempting to slow these leftover byproducts from entering their bloodstream. It’s thought that icing sore muscles will constrict the surrounding blood vessels, allowing more time to metabolize the leftover lactic acid and hydrogen. But even though it might help people feel better, it hasn’t been proven that ice speeds up muscle recovery.

It’s worth noting that endurance athletes are experimenting with a variety of treatments to recover faster — compression socks, kinesiology tape, powders that help absorb fluid, and even devices, like the Marc Pro, that force muscles to contract via electrical impulses. Electric devices like these attempt to improve the targeted muscle’s circulation while also removing waste, such as lactic acid. Similar to ice, these practices can help reduce inflammation but haven’t been scientifically proven to help muscles recover more quickly.

The truth is, unless you’re a world-class athlete or extreme weightlifter, it’s probably overkill to ice after a workout — but icing after an injury is essential to the healing process if it’s done in the proper window of time. While the best solution would be to avoid injury altogether, knowing when to apply ice and heat is a simple (not to mention affordable) path to getting back to good health, fast.

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Sequels tend to suck (Caddyshack II, I’m looking at you), and when they’ve got 26.2 miles of pavement in them, the suck-potential goes exponentially up. After my second marathon, I came up with some advice to my former self, who was still prepping for his first. You can listen in. Read this story.