When performing certain isometric exercises, the endurance of women is almost twice that of men performing the same exercises, at the same percent of maximum strength, a new study has found.

The study by Dr. Sandra Hunter, a postdoctoral research associate in the University of Colorado (Boulder) Department of Kinesiology and Applied Physiology's Neural Control of Movement Laboratory, aimed to compare the endurance times — or fatigability — of men and women for two types of low-force exercises using the elbow flexor muscles.

Eight men and eight women were asked to perform two isometric fatiguing contractions on separate days. In one task, the participants were asked to hold their arm in a rigid position (within a restraint) for as long as possible. In the second task, a weight bag was added to the wrist, and the individuals once again were asked to hold their arm in the same position for as long as possible.

Women outlasted men by an average of 75 percent for both tasks.

Importantly, the study showed the reason the women had longer endurance times was not due to differences in the motivation levels between men and women, or within the nervous system, but due to differences within the muscle.

Because the weaker subjects had longer endurance times, and women were weaker than men, the specific reason for the gender difference in endurance time may be an interaction between the muscle strength and the blood flow within the muscle, Hunter suggested. Other possible reasons may include the type of muscle fibers of men and women, or even the differences between men and women in hormones, such as estrogen, she explained.

Hunter discussed the research findings during the American Physiological Society's intersociety meeting, "The Integrative Biology of Exercise," in Portland, Maine.