Researchers at the University of Iowa have revealed a biological link between pain and fatigue.
The finding may help explain why more women than men are diagnosed with chronic pain and fatigue conditions like fibromyalgia and chronic fatigue syndrome.
Led by Kathleen Sluka, Ph.D., the research team conducted the study on mice and found that a protein involved in muscle pain works in conjunction with the male hormone testosterone to protect against muscle fatigue.
To find the association between pain and fatigue, and the influence of sex, the researchers compared exercise-induced muscle fatigue in male and female mice with and without ASIC3, an acid-activated ion channel protein that the team has shown to be involved in musculoskeletal pain.
A task involving three one-hour runs produced different levels of fatigue in the different groups of mice as measured by the temporary loss of muscle strength caused by the exercise.
Researchers found that male mice with ASIC3 were less fatigued by the task than female mice.
However, male mice without the ASIC3 protein showed levels of fatigue that were similar to the female mice and were greater than for the normal males.
Also, researchers found that when female mice with ASIC3 were given testosterone, their muscles became as resistant to fatigue as the normal male mice.
On contrary, the muscle strength of female mice without the protein was not boosted by testosterone.
"The differences in fatigue between males and females depends on both the presence of testosterone and the activation of ASIC3 channels, which suggests that they are interacting somehow to protect against fatigue," Sluka said.
"These differences may help explain some of the underlying differences we see in chronic pain conditions that include fatigue with respect to the predominance of women over men," Sluka added.
The study indicates that muscle pain and fatigue are not independent conditions and may share a common pathway that is disrupted in chronic muscle pain conditions.
The study was published in the Feb. 28 issue of the American Journal of Physiology -- Regulatory, Integrative and Comparative Physiology.