In a memo posted to the Marine Corps' fitness website, leathernecks were told to "suspend high-intensity workouts, endurance heavy training, and working out in large groups." The guidance, titled COVID-19 Fitness Tips, was issued by Col. Stephen Armes, director of the Marine Corps' Force Fitness Division.
"Human performance research shows that extended periods of high-intensity endurance exercise can make athletes more susceptible to illness for up to 72 hours after the exercise session," Armes wrote.
That's not to say Marines should cut out workouts, Armes told Military.com. But they should temporarily shorten the time they spend exercising.
"Limiting the duration of the workout to approximately 20 minutes of total time will limit the negative effects from doing prolonged high-intensity activity," Armes said.
To keep intensity levels at the appropriate level amid the coronavirus threat, Armes said workouts should be designed with rest ratios "every minute on the minute." If a Marine does 10 burpees every minute for 10 minutes, he said, they build in about 30 seconds of rest into each of those minutes.
"This keeps heart rate at a sustainable level, and moderates the intensity of the workout," Armes said.
Jeffrey Woods, a kinesiology professor at the University of Illinois, said research does show that heavy exercise can have an effect on the immune system. But that's mostly for those unaccustomed to it, he added.
"If you're highly fit, it's probably okay to maintain your fitness -- high intensities and durations," Woods said.
For recruits just showing up to boot camp who might not be as used to tough workouts, he said the Marine Corps might want to take new precautions.
"Especially in a setting where they're all together and there could be risk for infectious disease spread," Woods said. "If there was a recommendation to be made, I would say under those instances where people are not accustomed to doing high-intensity exercise training, this is probably not a good time to start."
In his guidance to Marines, Armes said exercise done near others should be stopped.
Military leaders have been pushing social-distancing measures amid the coronavirus outbreak. Base gyms have closed, and service members have been told to keep their distance whenever possible.
Those are important protocols to follow during the viral pandemic, Woods stressed. Exercising in group settings just can't happen anymore, he said.
The Marine Corps doesn't have a force-wide policy barring group physical training, Capt. Monica Witt, a spokeswoman at the Pentagon, said. But commanders have been directed to cut out large gatherings and implement social-distancing guidelines -- including during PT, she said.
"The current guidance is to observe 6 feet distance from others at any given time," she said.
Commandant Gen. David Berger said this week that it's up to commanders to make changes to training, grooming standards and other policies during the coronavirus threat.
In place of unit PT, Armes said Marines should use tools available to them to workout in other places, including the FitForce app, which includes a barracks-room fitness program, and a new Facebook weekly video series from the Semper Fit High Intensity Tactical Training staff.
He also recommended getting at least seven to nine hours of sleep per night, eating well, practicing good hygiene including handwashing, and taking up reading or other mental breaks.
Exercise can improve mental health, Woods said, but Armes is right to tell Marines to make sure they find ways to rest and unwind, too.
"In this viral pandemic, everybody's a little bit more mentally stressed, especially with social isolation," he said.