Working out in a group setting tends to bring out our competitive streak—especially if it's with someone we know. For a study published in 2012, researchers at Michigan State University divided 58 female participants into three groups. One group exercised alone, the second with a partner, and the third with what they were told was a "virtual partner"—although in reality, the "virtual partners" were actually just videos on loop of people exercising, so that it seemed like they were outpacing their real-life counterparts.
It worked: During each exercise, the third group outlasted the second group by an average of two minutes, and the first, partnerless group by 10. That means that if you really want to step up your gym game, it's worth recruiting someone who'll keep you on your toes—even if you're just sharing stats over text.