Twenty-nine men and women in their 20s had casts put on their non-dominant arms that extended from below their elbows to past their fingers, leaving them unable to move or contract themuscles in their wrists or arms. Before the casts went on, researchers measured arm strength by recording the force the wrist flexors produced. For the next month, half the test subjects were put through a mental imagery exercise 5 times a week: They were told to imagine themselves contracting their forearm muscles and flexing their wrist for 5 seconds—though they didn’t actually move their muscles at all—then to envision their muscles relaxing for 5 seconds. They mentally contracted their muscles 52 times during each session. After 4 weeks researchers re-measured the strength of participants’ wrist flexors. Those who performed the mental imagery exercises lost about 24% of their strength; those who didn’t lost 45%. MORE: This Article On Your Aging Muscles Will Terrify You. But It Just May Change Your Life. “We expected that the imagery exercises would help with the loss of strength, but we were really surprised by what a huge effect that small amount of imagery had,” says lead researcher Brian C. Clark, PhD, a professor of physiology at Ohio University. So how does it work? “Imagining your muscles contracting keeps up normal levels of cortical excitability, which refers to the way neurons that control your muscles are able to react to stimulation,” he explains. “The easier your neurons are to excite, the more they’re able to activate your muscles.” And even though the study only looked at strengthening the wrist flexors, in theory, the technique would do the same for other parts of your body, too. The key: Make sure that you’re mentally contracting your muscles from a first-person perspective—that is, imagining yourself doing the task—rather than from a third-person perspective (thinking about watching yourself do the task), Clark says. While research suggests that this type of imaging can retain muscle strength in people who aren’t injured, don’t use it as an excuse to skip the gym. Alas, there’s no replacement for the benefits you’ll get from actually working out. MORE: This Is How A Dumbbell Can Make You Smarter