"Cognitive tasks, as well as emotional tasks such as regulating your emotions, can deplete your self-regulatory capacity to exercise," says Kathleen Martin Ginis, associate professor of kinesiology at McMaster University, and lead author of the study.

Have you ever sat down to work on a crossword puzzle only to find that afterwards you haven't the energy to exercise? Or have you come home from a rough day at the office with no energy to go for a run?

A new study, published today in Psychology and Health, reveals that if you use your willpower to do one task, it depletes you of the willpower to do an entirely different task.

"Cognitive tasks, as well as emotional tasks such as regulating your emotions, can deplete your self-regulatory capacity to exercise," says Kathleen Martin Ginis, associate professor of kinesiology at McMaster University, and lead author of the study.

Martin Ginis and her colleague Steven Bray used a Stroop test to deplete the self-regulatory capacity of volunteers in the study. (A Stroop test consists of words associated with colours but printed in a different colour. For example, "red" is printed in blue ink.) Subjects were asked to say the colour on the screen, trying to resist the temptation to blurt out the printed word instead of the colour itself.

"After we used this cognitive task to deplete participants' self-regulatory capacity, they didn't exercise as hard as participants who had not performed the task. The more people "dogged it" after the cognitive task, the more likely they were to skip their exercise sessions over the next 8 weeks. "You only have so much willpower."

Still, she doesn't see that as an excuse to let people loaf on the sofa.

"There are strategies to help people rejuvenate after their self-regulation is depleted," she says. "Listening to music can help; and we also found that if you make specific plans to exercise—in other words, making a commitment to go for a walk at 7 p.m. every evening—then that had a high rate of success."

She says that by constantly challenging yourself to resist a piece of chocolate cake, or to force yourself to study an extra half-hour each night, then you can actually increase your self-regulatory capacity.

"Willpower is like a muscle: it needs to be challenged to build itself," she says.

The study was made possible through funding by the Social Sciences and Humanities Research Council of Canada.

RELATED NEWS

The ball is in your courtSeptember 11, 2009

Doctor danceSeptember 3, 2009

Excessive exercise can be addictive, new study saysAugust 19, 2009