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Study Shows Self-Control Comes in Limited Doses

Sept. 25, 2009 -- You want to exercise. You know you should be exercising more. But even though you up wake every morning committed to hitting the gym or taking a long walk after work, your resolve is gone by the end of a long day.

Sound familiar?

For some lucky people, exercise is second nature. For the rest of us it takes willpower.

Now new research suggests that one big reason people fail to follow through on their exercise plans is that they have used up their willpower on other tasks.

The study examined the theory that people have limited stores of self-control, or willpower, in any given day.

Just like the money in your wallet, the theory goes, willpower is a finite resource that can't be used on one thing if it is already spent on another.

"When you use self-control for other things -- like meeting a deadline at work or resisting the temptation to eat a doughnut -- you deplete your pool of self-control," exercise scientist Kathleen Martin Ginis, PhD, of McMaster University tells WebMD. "We wanted to see how that impacted exercise."

Ginis and colleague Steven R. Bray, PhD, designed a laboratory experiment to do just that and recruited 61 university students who were not regular exercisers to take part.

The students were initially asked to work out on lab-based exercise machines. Half the participants were then asked to perform a task, known as the Stroop test, designed to deplete their willpower stores.

The test involved showing the students the words for colors printed in a different color. For example, the word red might be printed in green ink and so on.

The students were told to say the color they saw, and resist the temptation to say the color they were reading.

"It sounds pretty innocuous, but it definitely takes self-control to ignore the written word," Martin Ginis says.

The students were then subjected to a second round of exercise, and, as the researchers had suspected, those whose willpower had been challenged did not work out with the same intensity as those who had not taken the test.

The willpower-challenged students also worked out less over the next eight weeks.

The study was published this week in the journal Psychology and Health.

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