Fatigue doesn't have to be physical to slow you down.

RODALE NEWS, EMMAUS PA—We all have days when our usual workout, bike ride, or run seems tougher than normal. A new study suggests that sometimes it’s a tired mind, rather than a tired body, that saps your energy. British researchers tested this by having volunteers ride exercise bikes after passively watching videos, or after playing a mentally fatiguing computer game. They found that the subjects reached exhaustion about 2 minutes sooner if they were tired out mentally beforehand.

THE DETAILS: The computer task that the subjects performed required them to quckly identify and respond to a specific sequence of letters that flashed on the screen, and it was designed to be especially mentally draining, says study author Samule M. Marcora, PhD. “I’d say the level of mental fatigue induced by 90 minutes of our cognitive task is equivalent to an entire day of demanding intellectual work,” he explains. Interestingly, the mental fatigue didn’t seem to affect any measures of physical performance, like heart rate or oxygen consumption. Instead, the researchers say, this phenomenon seems to affect our perception of how much effort we’re exerting, causing us to feel as though we’ve reached our limit sooner than we otherwise might. “It does not matter whether [the exercisers] could, from a physiological point of view, keep going for longer. What matters is how they feel, their perception of effort,” says Marcora. He notes that while many of us have probably felt this effect, this is the first study to demonstrate the phenomenon in the lab.

WHAT IT MEANS: The impact that mental fatigue had on the exercisers in this study suggests that it’s as bad as muscle fatigue for physical performance, Marcora says.

To keep a weary mind from slowing down your workout, follow this advice:

• Give your mind a break. It’s not known how long it can take for mental fatigue to dissipate, though Marcora is planning a study to look into it. In this study, the subjects had a 15-minute break before exercising, so you’ll need more time than that to clear your head. If the stakes are high—you’re running your first half-marathon, your softball team’s made it to the semifinals—you may want to avoid furrowing your brow for a whole day before the big event.

• Rearrange your schedule. If you have to go directly from work to, say, your spinning class or pick-up basketball game, try to arrange your day so you’re doing the least mentally demanding tasks immediately before you grab your sneakers and gym bag. Plan your toughest workouts for mornings and weekends, or days when you expect your mental workload to be light.