"Give me five more!" could result in five less. Training partners who are too encouraging could limit your workout, finds a study to be published in the Journal of Medical Internet Research.

While people who trained with a partner held a planking pose nearly 79 seconds longer than those who exercised alone, planking times dropped by more than half a minute when the workout partner called out encouragements.

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What gives? When it comes to vocal encouragement, you're likely to interpret shouts of support as condescending or patronizing, not inspiring, says Brandon Irwin, Ph.D., the study's author and an exercise researcher at Kansas State University. It's also possible that random calls to "Push it!" might sound like your partner just trying to pump himself up, Irwin explains.

So who's your ideal workout buddy? "Someone who is very similar to you in just about every way, except he's a little stronger or more skilled," Irwin says. If your workout partner is too superior (and you know you can't keep up), your motivation evaporates, he adds. And tell your buddy to zip it -- or at least to be more personal with his vocal support. That means saying "Let's go, John!" as opposed to just, "Let's go!" Irwin explains. Case in point: The Worst Workout Buddy.