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The good news: It’s not all in your head. “Older animals are more susceptible to damage, and they take longer to recover, even if the damage is about the same” as in younger animals, says Dr. Timothy Koh, an associate professor at the University of Illinois whose research focuses on tissue repair. Note: He said “animals.” To date, researchers have performed the majority of studies attempting to identify the biological reasons behind these observations on animals. (Because it would be hard to justify purposefully straining your grandma’s quad.) 

The bad news: Researchers still don’t know exactly why we’re more susceptible to injury as we age. But they do have some ideas about why we recover more slowly—and possibly less completely:

Idea #1: Elevated inflammatory response

Any time you have an injury, you have an inflammatory response. Older people typically have more inflammatory cells accumulate in damaged muscle, but those cells “may not be functioning as well” as in younger people, Koh says.  “Inflammation is an important part of the healing response, but it needs to be correctly regulated—there can’t be too much, and there can’t be too little.” Koh says. If you completely block the inflammatory response, healing will be significantly delayed. Too much inflammation will also lead to impaired recovery.

Idea #2: Cell exhaustion

“There are other cells in the muscles that are responsible for helping to repair the damage—a kind of muscle stem cells. The function of those cells may be compromised with age as well, so they may not be as good at their job,” Koh says. Those cells replicate themselves when you’re injured, and there may be a limit as to how many times they can do so. “If you’re an elite runner, you do that for long periods of time, and you sustain lots of injuries, you may exhaust your population of stem cells sooner than a sedentary person.”

Idea #3: Changing biochemical environment

“Hormonal levels will change over time with aging,” Koh says. Lowered testosterone levels, for example, could be involved in impaired muscular healing. “That’s why people try to supplement illegally. A main factor in how those anabolic steroids work is they allow people to recover quickly from really hard workouts.”

THE BOTTOM LINE: Recovery does take longer as we age. Scientists, however, are still figuring out the exact mechanisms behind the slowdown.