I didn’t know anything was wrong during my workout—my first back at the gym since the start of the pandemic. Adrenaline was flowing, and I felt relieved to have returned to a regular strength-training routine after 14 months. Nor did anything feel out of whack as I stopped to dig into a plate of chicken shawarma on the way home (have to get that protein, after all). Or even the next day, when I woke up with what felt like a run-of-the-mill case of DOMS—delayed-onset muscle soreness—that “good pain” you often feel a day or two after a decent session.

But it became clear three days after my workout that all was not right. I couldn’t straighten my arms beyond about 65 degrees because the pain was too severe; if I forced them, they’d bounce right back up. It was painful to type at a normal desk angle, and to carry a light lunch bag. I was walking around, arms limp in front, like a T. rex. After my symptoms led me down a Reddit rabbit hole, I decided to get checked out. Reddit was right: I had a condition called rhabdomyolysis.

Rhabdomyolysis, or “rhabdo,” is a condition stemming from putting too much exertion on your muscle fibers, which can cause them to give up and break down, leading their contents to try and rapidly flush themselves out of the body, leaking into the blood in hazardously high concentrations. Most dangerously, the rush of myoglobin, a protein that carries oxygen within the muscles, can clog the kidneys—and in the worst cases, cause long-lasting kidney damage and even death. It’s been reported in complete novices to exercise as well as pro athletes, but the common factor is that it often occurs when starting or restarting exercise, leading experts to predict that as people flock back to the benches and machines post-vaccine, there may be a surge in these cases. Their warning: Go slow.

“I’ve been predicting for some time that when the gyms open up again, we’re going to see an uptick in rhabdo,” says Joe Cannon, a personal-trainer certifier with a master’s degree in exercise science. Cannon has written a book about rhabdo and hosts a podcast on which he interviews rhabdo patients. He does it, he says, to build awareness around a “strange phenomenon” that we know perilously little about. In his interviews with the rhabdo-afflicted, he hears three common echoes: one, “I’d never even heard of this before”; two, “the pain was excruciating”; and three, “I’m terrified to exercise again.”

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