Four workouts a week may be better than six
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To determine if either of these possibilities held true among their volunteers, the researchers in the current study tracked the women's blood levels of cytokines, a substance related to stress that is thought to be one of the signals the nervous system uses to determine if someone is overdoing things physically. They also measured the women's changing aerobic capacities, muscle strength, body fat, moods and, using sophisticated calorimetry techniques, energy expenditure over the course of each week.
By the end of the four-month experiment, all of the women had gained endurance and strength and shed body fat, although weight loss was not the point of the study. The scientists had not asked the women to change their eating habits.
There were, remarkably, almost no differences in fitness gains among the groups. The women working out twice a week had become as powerful and aerobically fit as those who had worked out six times a week. There were no discernible differences in cytokine levels among the groups, either.
However, the women exercising four times per week were now expending far more energy, over all, than the women in either of the other two groups. They were burning about 225 additional calories each day, beyond what they expended while exercising, compared to their calorie burning at the start of the experiment.
The twice-a-week exercisers also were using more energy each day than they had been at first, burning almost 100 calories more daily, in addition to the calories used during workouts.
But the women who had been assigned to exercise six times per week were now expending considerably less daily energy than they had been at the experiment's start, the equivalent of almost 200 fewer calories each day, even though they were exercising so assiduously.
"We think that the women in the twice-a-week and four-times-a-week groups felt more energized and physically capable" after several months of training than they had at the start of the study, says Gary Hunter, a UAB professor who led the experiment. Based on conversations with the women, he says he thinks they began opting for stairs over escalators and walking for pleasure.