07-12-06

Study finds overweight women exercise harder, experience less pleasure

AMES, Iowa -- Regular physical activity is an essential component of effective weight loss programs, particularly during the first six months, and yet many who are overweight find it hard to stick with a fitness routine. According to a recent study by Iowa State University researchers, that may be because overweight people exercise at a higher intensity of their peak aerobic capacity than their normal-weight counterparts -- producing a gradual decrease in pleasure over time.

Panteleimon "Paddy" Ekkekakis, associate professor of health and human performance at Iowa State, joined with ISU doctoral student Erik Lind on the pilot study of 25 women (16 overweight, nine of normal weight). They authored a paper on their results titled "Exercise does not feel the same when you are overweight: the impact of self-selected and imposed intensity on affect and exertion," which appeared in a recent issue of the International Journal of Obesity.

The researchers defined the overweight subjects as those who had a Body Mass Index (BMI) of 25 or higher, with the normal-weight subjects having BMIs less than 25. The participants were all between 35 and 53 years of age and had all been physically inactive for at least one year prior to their participation in the study. After responding to a seven-day physical activity recall interview, baseline physical measurements were conducted on BMI, heart rate and oxygen uptake through a treadmill test. Subjects were given the results of their fitness assessment and an individualized physical activity prescription upon the completion of the study.

After the treadmill test to assess their fitness level, subjects underwent two 20-minute treadmill exercise sessions. The first was at a self-selected speed. The second was at a speed adjusted by the researchers to be 10 percent higher than the self-selected level. There was a minimum of at least 48 hours between sessions, which were all conducted at the same time of day. All the participants were able to complete the entire 20-minute session without stopping. The subjects' pleasure, displeasure and perceived exertion were assessed during the exercise.

The overweight women showed higher oxygen uptake and perceived exertion than their normal-weight counterparts during exercise, both when the intensity was self-selected and when it was imposed by the researchers.

"The higher physiological intensity -- in terms of oxygen uptake and heart rate -- is a byproduct of the extra weight they are carrying around," said Ekkekakis. "The treadmill speed that normal-weight and overweight women selected was not different. What is different is that the overweight women have a lower aerobic capacity when this is considered in relation to their body weight -- as milliliters of oxygen uptake per kilogram of body weight per minute. So, in order to walk at the same speed, they have to utilize a higher fraction of their maximal capacity."

The key finding of the study was that although both groups maintained steady and similar ratings of pleasure-displeasure during the exercise at self-selected intensity, the overweight group responded with a gradual decrease in pleasure over time during the imposed-intensity exercise, while the normal-weight subjects' remained stable.

"Given the fact that the intensity of physical activity is prescribed (i.e., imposed) in most studies that have reported lower adherence rates for overweight and obese participants, it is reasonable to speculate that the lower adherence might be attributed, in part, to the decline in pleasure found under such conditions in the present study," the researchers wrote in the paper.

"In a follow-up study we are conducting now, we are looking at other factors that might make the exercise more physiologically stressful -- and psychologically unpleasant -- for the overweight women," said Ekkekakis. "One strong possibility is their ability to properly regulate core body temperature during exercise. We speculate that the core body temperature of the overweight women during exercise will be higher than the normal-weight ones. We do know from the literature that a higher core temperature is one of the key factors that can perturb homeostasis and have an impact on how a person feels."

Ekkekakis reports that previous research literature on how overweight and obese individuals perceive exercise is "extremely limited." The researchers are planning a larger randomized clinical trial to refine their methodology and research results.

"In essence, we are only beginning to understand how the extra weight and adipose tissue impact physiological and psychological responses during exercise -- not to mention gait patterns, injury potential, and the like," Ekkekakis said. "Therefore, it's going to be a while before we are fully prepared for a large-scale clinical trial."

He believes the results support the use of exercise performed at a self-selected intensity for overweight women -- at least in terms of the average intensity being well within the appropriate range, while not being dangerously high. "I am emphasizing that this is the case 'on average' though," he said, "because there are certainly individuals who will deviate by either selecting an intensity that is too low or by selecting an intensity that is too high."

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