If you're a competitive athlete, you're probably intimately familiar with the benefits of visualization, in which you mentally picture yourself doing an activity—serving a tennis ball, skiing a slalom course, shooting a basket—before you physically do it. The theory is that this prepares your body for the task to come and thereby improves performance, which it does for many athletes.

So how does that help me, you ask, as you curl that dumbbell? Dave Smith, a sports psychology researcher at Manchester Metropolitan University in England, is trying to prove that mental practice--not just rehearsing the activity in your head, but also feeling the same sensations you would if you were really doing the activity—can also improve your muscle strength.

"Mental practice makes you stronger by engaging the same neurological mechanism that lifting a weight does. To simplify the process, there are certain parts of the brain that know the difference [between lifting a weight and only thinking about it] and other parts that don't know the difference." While not all of the signals to the muscles get through, some of them do, which causes strength gains.

In one study Smith conducted, researchers compared three groups of participants. One group performed 20 contractions of their pinky fingers twice a week for four weeks; another group did nothing and the last group only imagined doing the finger workout. At the end of the four weeks, the first group had increased their pinky strength by 33 percent, but the group that only imagined the contractions also increased their pinky strength—by 16 percent! (The group that did nothing saw no change.)

"You can definitely use mental practice to supplement your current strength training program," says Smith, who maintains that he's seen significant strength gains using mental practice twice a week himself. He recommends trying to do two "practice" sessions a week in which you mentally walk through your entire routine (or just focus on one muscle group) and try to feel the way you would feel--down to the sweat dripping off your brow, your muscles cramping and the smell of the barbells—during a workout. You don't have to go in slow motion, but you shouldn't rush through your workout either.

If possible, try imagining yourself using heavier weights than you would normally use to give yourself a "psychological boost." If you're doing it right, you might start to sweat, feel your heart rate increasing or even get tired.

Smith is currently in the process of studying whether mental practice—e.g., thinking about running, cycling or doing aerobics—can improve your cardiovascular health as well. Maybe it's time to put some thought into running that extra lap.