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Why Are My Arms Swelling Up After a Hard Workout?

I took a really hard class and now my arms are starting to get puffy. What’s going on?

If, after a workout, you’re the most sore you’ve ever been, you might congratulate yourself on your hard work. But if your muscles seem puffy, swollen, or hard, and your pee is reddish or brown, then what you’re experiencing probably isn’t just the side effects of Beast Mode. In fact, you should go to the emergency room, says Angela Smith, M.D., a sports-medicine specialist and orthopedic surgeon at Nemours Alfred I. duPont Hospital for Children in Wilmington, Delaware.

These are all symptoms of a condition called rhabdomyolysis, which can happen after severe dehydration, long surgeries, car accidents or earthquakes where people get crushed, or, believe it or not, going too hard at CrossFit. In rhabdo, as it’s known, muscle gets damaged and releases a protein called myoglobin into the bloodstream. The kidneys filter out the myoglobin where it breaks down, and the byproducts of that process can damage them, leading to kidney failure if not treated. (Myoglobin can make urine dark, but not everyone has that symptom, she says.) Rhabdo can happen to people starting or restarting a workout routine as well as extremely fit folks: All you have to do is overexert yourself, says Dr. Smith, who’s also a past president of the American College of Sports Medicine.

Of course, some post-workout soreness is normal since the stressing and repairing of muscles is how you get fitter in the first place, but rhabdo is the muscular equivalent of a stress fracture, she says. A refresher on how those work: When you start running, your bones adapt to the stress and get stronger. But if you do too much too soon, you exceed the amount of damage your body can repair overnight, so the bones start to break down. Hello, fracture.

Some people’s bodies can tolerate more myoglobin than others, so they can put themselves through seriously punishing workouts and never get rhabdo. Others face a higher risk of developing it because they either have a viral or bacterial infection, take cholesterol-lowering drugs called statins, or have a sickle-cell trait, that is, having one copy of the gene that causes sickle-cell anemia, says Dr. Smith, who’s also a clinical professor of orthopedic surgery and pediatrics at the Sidney Kimmel Medical College of Thomas Jefferson University. These factors can make you more likely to get rhabdomyolysis instead of regular muscle soreness.

You could safeguard yourself from rhabdo by never working out again, but experts don’t recommend that route. “It’s much better to go do some exercise than to sit around and be sedentary for millions of reasons,” Dr. Smith says. How can you avoid rhabdo? By following a legitimate workout plan. “You want to do things in moderation and then gradually increase the skill level, speed, intensity, endurance … whatever the parameters are for your particular activity so that your body has time to adapt.”

That’s why working with a certified trainer, or getting a workout plan from one online, is key. These plans won’t ask you to use the same muscles during consecutive workouts. They should also take into account your current activity level (so definitely don’t fib about that part). One-size-fits all workouts created by YouTube or Instagram trainers may not be up to snuff, so look for credentials before you try that trendy, high-intensity workout.

What about if you’re trying a workout you’ve never done before, like your first bootcamp or cycling class? Even if it’s a familiar workout you just haven’t done in a while, you may not know how much you can push yourself without hurting the next day (or the day after that — thanks, delayed-onset muscle soreness). A tip: don’t try to go all out the entire time. A well-designed workout includes a warmup, a cooldown, and recovery breaks between periods of hard effort. Serious soreness isn’t a badge of honor — it actually just prevents you from working out consistently.

You can also take care of your body by following best practices for sports nutrition, including eating something beforehand, staying hydrated, and refueling with a snack or meal that has both protein and carbs to help build and preserve lean muscle, Dr. Smith says. If your workout is especially hard, you should chug extra water afterward to help dilute any myoglobin buildup in the kidneys. If you’re really struggling, wait for the soreness to subside before pushing yourself again, though you could take it very easy on the bike or the elliptical if you’re dying to do something. But if you know you have one of the risk factors for rhabdo mentioned above, it’s best to wait until you’re fully recovered, she says.

“Like [with] everything, the key in this is moderation and allowing time for adaptation,” she says. “This is a situation where truly the tortoise wins the race, not the hare.”

Why Are My Arms Swelling Up After a Hard Workout?