Background: Injury prevalence data commonly indicate trends of higher rates of work-related musculoskeletal
disorders in older workers over their younger counterparts, and for females more than males. The purpose of this
study was to investigate age and sex-related differences in manifestations of shoulder muscle fatigue in a cohort of
young and older working age males and females, in a single experiment design allowing for direct comparison of
the fatigue effects between the target groups.
Methods: We report upper trapezius muscle fibre Conduction Velocity (CV) as an indicative measure of muscle
fatigability, and isometric endurance time, at three levels of shoulder abduction lifting force set relative to
participants’ maximal strength.
Results: Upper trapezius conduction velocity was significantly different between the young and old groups (p = 0.
002) as well as between males and females (p = 0.016). Shoulder abduction endurance time was affected by age
(P = 0.024) but not sex (p = 0.170).
Conclusions: The study identified age-related improvement in muscle fatigue resistance and increased resistance
for females over males, contrary to injury prevalence trends. The muscle fatigue effects are most likely explained by
muscle fibre type composition. Experimental fatigue treatments of the upper trapezius were tested at exposures
relative to the participants’ strength. Absolute strength is higher when young and is generally higher for males. The
findings of this study point towards age and sex-related differences in strength rather than in muscle fatigue
resistance as a primary cause for the differences in the injury trends.