Ever tried to run with one of your less experienced friends and found that the slower-than-your-usual pace kinda felt like a waste of exercise time? (Bonding time notwithstanding, of course.)

That's not at all unusual—and may actually be normal, says Janet Hamilton, exercise physiologist, and founder and head coach of Running Strong in Atlanta. “If you have a marathoner who can run a five-minute-mile pace and you send him out on a nine-minute-mile training run, it won’t stimulate any physiological changes that will make him a better runner,” she says. “Is it bad? Probably not—but he won’t get anything out of it, as it’s not appropriate for him.”

But some people actually find that running slowly actually kind of hurts.

If you find running slowly is physically uncomfortable—cramping in your calves or shin splints, for example—it may be a sign that you're not running as you should be. More specifically, you may be changing your cadence (or steps taken per minute) to match a slower pace, Hamilton says. Changing your cadence alters your body’s mechanics, thereby placing stress on muscles where there ordinarily shouldn’t or wouldn’t be stress.

“You may be slowing your pace down by taking fewer steps per minute, which increases the contact time on the ground, causing your legs to absorb more shock,” she says.

That isn’t to say you should never run with less-speedy pals. (How do you think your fleet-footed girlfriend feels when she has to drag you along on her marathon training runs?) If you can keep your cadence or rhythm the same as it always is, but reduce the force you get out of the ground with each step (which will naturally shorten your stride length, or distance covered per footfall), then voila: You'll run at a slower pace with no leg funkiness. In other words: Think about running lighter, not slower.

“With most accomplished runners—ones who have been running for years—the rhythm is already established in their brains,” says Hamilton. “If they run with a slower partner, they will push off with less emphasis, covering less ground in each stride, but keeping the same rhythm.”

That said, even with focus it can be tough to maintain your cadence at a much slower pace over many miles, so you could end up shuffling or looping back into those bad habits on a longer run. Therefore, you should keep your buddy pacing to a manageably short distance—after all, those three miles will take longer than you’re used to.

But maybe more important, Hamilton points out that many runners do their “easy” training runs at far too fast a pace and should slow them down, in the name of improving endurance or aiding recovery. “Most runners tend to settle in at a moderately hard pace when they run,” she says. "Your easy pace—for building up endurance—is actually a lot slower and should really feel like it’s no big deal, and you could go on forever.” Recovery runs can be even more leisurely, as a means to increase blood flow and stimulate enzymes for muscle healing.

“Runners will tell me they’re not comfortable with running slower,” she says. “But it’s not that they really feel physically uncomfortable—they’re just uncomfortable with the concept.”