Study leader Kathleen Sluka of the University of Iowa found a protein involved in muscle pain works in conjunction with the male hormone testosterone to protect against muscle fatigue.

Chronic pain and fatigue often occur together -- as many as three out of four people with chronic, widespread musculoskeletal pain report having fatigue; and about 94 percent who have chronic fatigue syndromes report muscle pain.

The researchers compared exercise-induced muscle fatigue in male and female mice with and without ASIC3 -- an acid-activated ion channel protein shown to be involved in musculoskeletal pain. Male mice with ASIC3 were less fatigued than female mice, but male mice without the ASIC3 protein showed levels of fatigue similar to the female mice.

The study, published in the American Journal of Physiology -- Regulatory, Integrative and Comparative Physiology, found the difference in fatigue between males and females depends on both the presence of testosterone and the activation of ASIC3 channels, which suggests they are interacting somehow to protect against fatigue.