We know to check iron and vitamin D levels for athlete health, but what if there was a blood marker that actually shows how much load your muscles are under—and may explain why you’re so sore?

Good news: It exists, and it’s becoming easier to test for. If you go deep on endurance sport training, you might have heard casual references to creatine kinase (CK) levels. This metric has been around for a while, but it’s not until recently that it’s been discussed outside of the CrossFit community. For years, it was only mentioned in reference to rhabdomyolysis, the intensely painful muscle cramping and damage that some CrossFitters fall victim to after gruelling workouts.

But as more people are using third-party “athletic” blood testing to check overall health and fitness levels, measuring creatine kinase levels has become more common as a metric for muscle damage for any athlete. But what do we really know about CK levels, and when do they matter? We reached out to the experts to find out.

What is creatine kinase?

Creatine kinase isn’t just a “red flag” marker that means you’re going too hard: Creatine kinase is critical for exercise.

“Creatine kinase is an enzyme found within tissue in the body, and its main role is to serve as part of the quick energy producing system that we’re using during hard efforts,” Kyle Thompson, M.Sc., a researcher in the exercise science department at the University of Guelph, tells Bicycling. “Creatine kinase is the enzyme that allows you to remake energy really rapidly during that initial few seconds of exercise.”

So, what makes it a marker of muscle damage? This is where researchers have a general idea, but aren’t entirely sure of how it works.

“The simplest way of looking at it is when you physically damage a muscle, now the muscle cells are damaged. And then, creatine kinase begins to leak out of the muscle cells and into the bloodstream,” says Thompson.

It’s important to note that he’s not talking about large scale muscle damage, like a rip or tear. Yes, that will also cause elevated CK levels, but even micro-tears from a day of heavy lifting if you haven’t been to the gym in a year can cause damage.

“If you keep pushing long enough, the muscle’s individual cells will become enlarged because they’re stressed,” he says. “When a cell becomes stressed, it takes in a lot of water. Think of a cell as a balloon: When you stretch it with all this fluid, it stays together, but the space between the fibres increases a bit. Now things can pass through the cell wall that wouldn’t normally be able to pass through the wall, like creatine kinase.”

What are normal ranges for creatine kinase levels?

As with most medical research, this answer is a hard “it depends.” Creatine kinase is measured typically in IUs (international units) per litre, and the range is immense: A normal level can be as low as 29, but upward of 10 000 has been reported with no ill effects in some cases. And, of course, it generally goes up and down with exercise and recovery.

“When looking at creatine kinase, it’s going to peak after intense exercise and that can last for a few days,” Stevie Smith, M.S., R.D.N., consultant for InsideTracker, tells Bicycling. “[Fewer] than 200 units is what is considered normal at InsideTracker, and we’d expect to have an elevated number drop within a couple days of intense exercise.”

However, while a company like InsideTracker may consider above 200 IUs/litre to be high, a general practitioner may not be worried about a high number until it reaches into the 10,000+ arena. And even then, might attribute it to a recent hard workout and anticipate that the number will drop quickly. A study done on ultra runners at altitude showed that CK levels over 14 000 weren’t causing problems beyond the fatigue that comes from running an ultra.

In fact, researchers have been arguing about resetting limits for ‘good’ CK levels for years. A 2012 study notes: “Base levels of serum CK in general populations are variable 35–175 U/L with ranges from 20 to 16,000 U/L, and this wide range reflects the inconsistent occurrence of subclinical disorders and minor injury, genetic factors, physical activity status, and medication.” That’s right: from 20 to 16 000.

In rhabdomyolysis cases, the CK levels can range from 10 000 to 200 000 or even higher—the highest recorded CK level was over 1 000 000, and was reportedly resolved with rehydration. In fact, Thompson says that seeing higher numbers in a test isn’t concerning, assuming it was after some kind of exercise.

“There are a number of times where I’ve seen people with numbers higher than 10,000, and they don’t have any problems,” he says. “Context is so important: Right now, we see it being measured primarily in research where muscles are damaged. We don’t know much about it. In light exercise contexts. I’m not even sure what a light exercise session would do versus a moderate. I’m only aware of what the most intense exercise will do. This is a problem when it comes to saying what a ‘good’ number is.”

What causes creatine kinase levels to be elevated?

It’s important to note that certain diseases like multiple sclerosis (MS) can cause CK levels to be extremely high, as can certain medications. But when it comes to raising CK levels through exercise or impact, there are a few major ways, but as Thompson says, it all comes down to stress.

“Exercise can be that stress, or disease can be that stress,” he says. “With exercise, though, high CK normally happens from acute bouts of exercise, though as you get more adjusted to exercise, your body gets better at mitigating that stress. If you hadn’t exercised in a long time and you decide to go to the gym and go hard, it’s likely that for the next few days, you will feel awful. But then the next time you go, you feel better faster.”

This can also apply to super hard efforts on the bike, but lifting heavy things (or climbing really hard at low RPMs) is more likely to raise your levels than a long ride at a chill pace.

Things like hydration level can also impact your CK levels during and after exercise. Thompson notes that in military training where soldiers are wearing a lot of gear in hot conditions, CK levels tend to be high. The same is true for football camps held in hot conditions where players are wearing a lot of padding. Moral of the story: Make sure you’re drinking enough water and taking in enough electrolytes, or you can throw off more than just hydration status.

Your CK levels can also be elevated from crash-related impacts.

“A crash causes actual muscle injury,” he says. “We talked about a stressor causing the cell to expand, and that’s not just going to be exercise stress. If you punch yourself in the leg really hard or smash it off of the ground because you wiped out, the muscle and the individual cells will swell, which can cause CK to leak out into the bloodstream.”

Chronically high creatine kinase levels are a problem

There is a correlation between high CK levels and injury: If you have high levels of CK and continue to exercise, you will increase your risk of injury, cramps, and fatigue. It can also be one of the blood markers that warns you that you’re approaching overtraining territory: One study on markers of overtraining cited CK as well as hormones like cortisol.

“I see a lot of athletes who are training for a certain race and they don’t want to ever skip a workout that’s on their schedule, no matter what,” Smith says. “They’ll continue to train through and push, push, push, and it can lead them to that overtraining state. And we might see that in creatine kinase [levels] before they start to feel it, so it might help us course-correct.”

It’s hard to get your levels down if you never stop training. A landmark study done back in 1980 found that the CK levels of marathon runners stay elevated after endurance training and competition. In fact, it takes about 7 to 14 days post-marathon for their levels to return to baseline. That makes it both interesting and relevant to athletes, and also extremely difficult to use as workable data. Basically, if you trained hard this week, your levels may be elevated beyond what’s considered “normal.”

How do you know if your creatine kinase levels are elevated?

Having delayed onset muscle soreness (DOMS) constantly is a good indicator that your levels are likely on the high side, but it’s not a guarantee. There’s no way to know for sure that your CK levels are high without a blood test, but if you suspect you might be dealing with something more severe than garden variety “I went too hard in the gym” cramps on a regular basis, it’s worth asking your doctor about bloodwork.

There are a lot of negative downstream health effects to continuing to train with extremely high CK levels, although if your levels are high enough to cause damage, you’re likely in too much pain to push through your next interval set.

How can you stop your creatine kinase levels from getting too high?

While exercise causes higher CK levels because it’s a natural component of muscle damage (again, not a bad thing!), there are ways to keep it in healthier ranges—and ways it can get more out of control. Nutrition can play a major role: Dehydration before, during, and after exercise can exacerbate the issue, so making sure you’re hydrating throughout the day—and especially in your workout—is key.

Getting in enough calories is also a major way to increase recovery and improve your overall recovery markers, including CK levels.

“We want to look at adequate overall intake,” Smith says. “If we’re not eating enough overall calories to begin with, without getting too hyper-focused on macronutrients or micronutrients, our bodies are not going to recover. When you’re an athlete, you ask a lot of your body. If you’re not fuelling it enough, it’s not going to recover or repair those muscles. Look at what you are doing post workout: Are you that person who just gets off the bike and doesn’t eat for a couple hours? You’re not recovering.”

And, no surprise here, but adequate rest and recovery is really your silver bullet when talking about optimising pretty much any blood markers.

Sleep is key when thinking about recovery, according to Smith. Low-impact exercise—like cycling—can also help lower your levels. That means shifting away from hard workouts (and any higher impact cross-training like weight lifting or running) and leaning into gentle spins on the bike paired with things like yoga, she says. “You can still train, but every workout can’t be super hard,” she adds.

How can I test my creatine kinase levels?

“The fact is that you shouldn’t always feel sore,” says Smith. “Every ride shouldn’t always feel terrible and very hard to get through. That’s not a very scientific way to look at it, but if you’re training a lot and things are feeling off, you never feel recovered, you can’t hit peak powers, your performance is falling off … those are signs you might want to get bloodwork done to check on a few different markers, including CK.”

Other bloodwork she would recommend in addition to CK levels including testing alanine aminotransferase (ALT) levels for liver function, cortisol levels for overall stress, and ferritin or iron levels, especially if you’ve been feeling tired in addition to sore. When you do go in for bloodwork, though, make sure you’ve taken a couple of easy days in order to get the most “normal” reading possible, rather than going in right after a suffer-fest.

“In a perfect world, we’d be looking at bloodwork every three months as athletes, in order to see how the body is handling training and any changes, and so you can adjust your recovery protocols accordingly,” says Smith. “You may realise that one recovery day a week isn’t enough, and you need to bump it to two. And then you can do that for three months and see how those two days off are impacting markers like CK, and see if everything is back [to normal] or heading back into the normal range.”

She adds that sometimes it can be interesting to test right after a major event, just to see the impacts on the body, but in those cases, expect markers to be extremely elevated.

But before even worrying about your current number, it’s worth remembering that the best defense from high CK is a good offense. Smith says that high CK (and other blood markers) levels are what business textbooks would refer to as “lag metrics,” meaning they’re what happens after you’ve already made the mistakes in training and recovery. Instead, focus more on “lead metrics,” like feeling great after a workout and paying attention to sleep quality and optimal nutrition. You shouldn’t wait until your test results come back with elevated numbers to prioritise your recovery!

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