Three men’s lacrosse players at Tufts University remain hospitalized after a voluntary, supervised 45-minute workout with a Navy SEAL graduate left them with a dangerous condition called rhabdomyolysis, the university said. A spokesperson for the university previously said five remained hospitalized.

The September 16 workout was instructed by a Tufts alumnus who is a “recent graduate of the BUD/S Navy SEAL training program,” which stands for Basic Underwater Demolition/SEAL training, according to Patrick Collins, Executive Director of Media Relations for the university.

“Approximately 50 team members participated in the workout. All have been evaluated by medical professionals, with a total of nine requiring hospitalization for rhabdomyolysis (rhabdo),” Collins said in an updated statement Monday. “Three remain hospitalized.”

Rhabdomyolysis or rhabdo – a rare condition that can be life-threatening – happens when muscles break down, possibly even disintegrate, after a strenuous workout, according to the Cleveland Clinic.

The broken-down muscle fibers can contain large amounts of elements such as potassium and phosphate that can be released into the bloodstream, causing complications with the kidneys, the Cleveland Clinic says.

“The most common symptoms are feeling like prolonged kind of muscle stiffness and discomfort and weakness,” Dr. Shruti Gupta, an associate physician at Brigham and Women’s Hospital who specializes in kidney disease, told CNN affiliate WCVB in an interview while explaining more about the condition. “Usually we see these cases more in isolation and so it’s puzzling for it to happen in particularly … people who are athletes.”

Following the workout and hospitalizations, Tufts University has postponed team practice and appointed an external independent investigator to review what happened.

“The university continues to closely monitor the condition of the team, and some individual team members have been medically cleared to resume training,” Collins said. “The team is a tight-knit group of young men who have shown remarkable resilience, understanding and care for each other throughout this episode. We will continue to monitor and work with them closely, and we hope for a rapid return to good health for all involved.”

What causes rhabdomyolysis?

Dr. Robby Sikka, a sports physician doctor who serves as medical director for the Professional Tennis Players Association, said cases of rhabdomyolysis often occur in high school or collegiate athletes when they return from breaks in training.

“A typical time of year when we see it in athletes is in the off-season, or getting ramped back up,” Sikka, who also serves as a team physician for the New York Yankees, told CNN.

Sikka said certain nutritional supplements, including increased caffeine and creatine, can add to the risk of rhabdomyolysis, although it’s not clear those contributed to the cases at Tufts.

CNN’s Meg Tirrell contributed to this report.