What is cryotherapy? The trendy recovery tool used by athletes and Hailey Bieber, explained by experts.

Hailey Bieber is sharing a cool part of her wellness journey: ice baths. 

Earlier this month the model and media personality, 26, took to TikTok to share a video of herself "cold plunging."

"Back with another cold plunge series," she captioned the clip. "Cold plunging has helped me a lot with anxiety and overall mood."

Ice baths, one of several types of cryotherapy, are a recovery tool gaining traction in the athletic world and beyond.

To better understand cryotherapy, we asked experts about what you should know.

What is cryotherapy?

Susan Kwiecien, clinical research manager at Manhattan Eye, Ear and Throat Hospital's Nicholas Institute of Sports Medicine and Athletic Trauma, or NISMAT, defines cryotherapy as the "reduction of tissue temperature by the withdrawal of heat from the body."

To withdraw that heat, you use something cold — whether it's an ice bath or a whole body cryotherapy chamber.

"Ice itself is a type of cryotherapy if you put it on your skin," she says. 

What happens next is a reaction at your skin, where the temperature starts to decrease, Kwiecien explains. Then the temperature penetrates through your skin, and depending on how much body fat someone has, continues through your body fat and into your superficial muscles. 

"Then eventually if it's a stimulus that's strong enough, it will reach your deep tissue, where it will ideally decrease inflammation and metabolism," she says, adding this has mostly been studied in animal models. "It has not been shown in humans, unfortunately. It's possible that in the future, the research will get there, but it's just not there yet."

In quick cold plunges like Bieber posted about, something called "divers reflex" occurs, Kwiecien explains.

"This phenomenon is brought about when superficial (skin) temperature drops rapidly and there is an initial redistribution of warm blood from the periphery to the core in order to enable the preservation of core temperature. ... It's essentially a subconscious survival mechanism."

The rise in core temperature corresponds with an elevated heart rate, which is evident immediately after the plunge.

"This reaction is consistent with the cardiovascular cold pressor response; same thing happens when you splash cold water on your face and might audibly gasp," Kwiecien says. 

What are the benefits of ice baths?

If you think of an ice bag you might put on an ankle you sprained at the gym, this is the most basic form type of cyrotherapy, explains Malachy McHugh, director of research at NISMAT. The reason people do this is to reduce inflammation or the swelling that happens after an injury.

"The swelling is to immobilize the joint so that you don't further injure the ligaments, but it's an exaggerated response, and clinically, we want to blunt that response. You're not stopping it, you're blunting it. And if we apply ice rapidly, as soon as possible from the time of injury, then you can blunt that response so that there's less swelling of the tissue and the recovery then the next few days might theoretically be better."

After the first 12 to 24 hours, the only benefit to icing an injury is to make it less painful.

If you're looking at quick icy plunges like Bieber, Kwiecien says research does show cold water immersion can "trigger cardiac parasympathetic activity, lower sympathetic tone, and restore cardiac autonomic modulation — which are all good things." 

The parasympathetic nervous system is a "network of nerves that relaxes your body after periods of stress or danger," according to the Cleveland Clinic. These reactions can play a role in your heart rate, blood pressure and other functions.

Kwiecien says research also shows that people struggling with stress, anxiety, lack of sleep or overtraining from exercise can have an imbalance in their parasympathetic cycle, meaning cold water immersions can act like "pressing a reset button to help restore cardiovascular homeostasis."

What do ice baths do for athletes?

In their research focused on cryotherapy used for recovery from exercise, Kwiecien and McHugh find that applying ice as soon as possible after a game or workout is key for athletes. 

The same is true for people who are getting back into fitness. If your body isn't accustomed to exercise, getting back to the gym can leave you feeling sore the following days.

"You feel great (at the gym), and then the day after a workout you're sore and you can't move around. And then the next day you're really sore. Well, you can minimize that response somewhat by doing some type of intervention immediately after the exercise," McHugh says. "If you do an ice bath or (another form of) cryotherapy immediately after the exercise, you can mediate that response and reduce that soreness that you're going to get the next day."

This is because about four to six hours after exercise, there's a process that takes place where tissues starts to break down, McHugh says, explaining that's why timing is critical. Because if you wait too long after your workout, the ice may help with pain, but it will be too late to blunt the bad inflammation response in order to help you recover faster.

How cold should you go?

The goal isn't to withstand the coldest temperature — which people often misunderstand, according to the experts. Instead, their research shows milder temperatures, around 59 degrees Fahrenheit, for longer is actually better.

"If you're looking to use cryotherapy for recovery from exercise, we do know that 'cool' temperatures' are more effective than 'cold' temperatures since you can withstand them for longer durations and thus elicit superior reductions in temperature at the tissue that you want to recover," Kwiecien adds. 

Whole body cryotherapy chambers, for example, are set at colder temperatures, meaning people are exposed for a much shorter period of time.

"They don't have the same effect," McHugh says, adding they haven't been shown to help with recovery. 

When not to use cryotherapy

Although it can benefit you in certain situations, for at-home athletes, it's not something to be used all the time, Kwiecien explains.

"There are detrimental effects that have been shown in the research from using cryotherapy chronically ... when you're trying to build muscle. It actually inhibits that response," she explains.

So if you're someone getting back into fitness looking to build muscle, Kwiecien wouldn't advise using an ice bath, because although it may help with soreness, it could also hinder that muscle growth.

If you can build recovery time into your schedule instead, opt for that. 

If your goal is to reduce soreness, however, cryotherapy could be useful. For example, it could help if you're going skiing for the weekend and don't want to be sore after your first day.

"In that situation, yeah, you should do the cryotherapy so that you decrease the soreness so that you're not in pain when you're trying to get the most out of your weekend," she explains. "But if you are a skier that's trying to become competitive, then perhaps reconsider."

Cold plunges and anxiety

While anecdotal evidence suggests a cold plunge can make people "feel better," Kwiecien says, it might be doing "more harm than good in anxious individuals."

This is because evidence shows anxiety may diminish the "cold shock response/divers reflex," meaning that "magnitude and duration of the cardiac component of these naturally occurring phenomena are significantly increased in individuals in an anxious state," she explains. "Unfortunately there is no direct research available investigating the direct impact of cold plunge on anxiety or other mood states, so the aforementioned perspectives are speculative."

For those who do want to explore the technique at home, Kwiecien says you don't need a special plunge pool like Bieber — "you can just give yourself a quick cold rinse at the end of your shower," she suggests.