What Causes Delayed Muscle Soreness, Post-Workout

Every body (literally) feels the aftermath of exercise differently.

As fitness editors, we here at Shape go to a lot of workouts together. If we hit one before work Monday, the question on Tuesday is always, “I’m pretty sore today. Are you?” My answer is almost always no-until Wednesday when I’m groaning every time I stand up.

In fact, this happens so often-part of the staff is always sore 24 hours later, while the rest of us don’t feel it for two or three days-that the standard response around the office is now, “No, I’m a two-day-sore person.”

Here’s the thing: That’s not an actually accurate statement, I’ve learned.

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What Causes Soreness?

Muscle soreness isn’t a reflection of your individual physiological nuances, says Chris Jordan, a certified strength and conditioning specialist and the director of exercise physiology at the Johnson & Johnson Human Performance Institute. It’s much more complicated than that.

It’s important to understand what muscle soreness comes from. “When you work out, you’re damaging your muscles on a cellular level. Once you tear tissue, there’s an inflammatory response, which means swelling, pain, and soreness,” he explains. There are four main factors that impact how much damage is being done-and therefore how sore you will be:

1. Exercise intensity: The harder you work, the greater the likelihood of microtrauma on the cells of your muscle. Both lifting heavy weights and explosive-but-short sprinting-both too much, too soon-can cause your body to overdo it, Jordan explains.

2. Exercise duration: A comfortable, low-speed jog may be low intensity, but if you’re used to three miles and decide to tackle 15, the prolonged and unfamiliar duration that your muscles are expected to work for will cause similar microtrauma.

3. Exercise type: There are two types of motion: concentric training and eccentric training. With the first, muscle fibers are shortening-like the up of a bicep curl-and in the latter, they’re lengthening-like the down of a curl. Because your muscle fibers are lengthening under tension, though, eccentric tension is more likely to cause soreness. That means anytime you do an explosive up and slow, steady down, or when you run downhill, your chances of sore muscles go up.

4. Fitness level: If you’ve been out of the gym and just hit it hard, it’s fairly obvious you’re going to be sore. You’re asking a lot of your unconditioned body, Jordan says. But the same goes for trying a new workout. Almost every activity-from running to spinning to barre-uses completely different muscle groups. And working unconditioned muscles (oh hello, abs!) can shock your system.

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