Complex sentence: These theories assume that the syndrome is perpetuated by reversible physiological changes of deconditioning and avoidance of activity; these changes result in the deconditioning being maintained and an increased perception of effort during exertion and increased symptoms after unaccustomed activity, leading to further inactivity.
Split into simpler sentences: These theories assume that the syndrome is perpetuated by reversible physiological changes of deconditioning and avoidance of activity. These changes result in the deconditioning being maintained and an increased perception of effort, leading to further inactivity.