Middle-aged non-exercisers enjoy far lower levels of mood enhancement from aerobic exercise than do their counterparts who exercise regularly, according to a study by Martin Hoffman, a researcher from the Department of Veterans Affairs and UC Davis. The finding may explain why this population so often has difficulty initiating and continuing new training programs. The study, published in the Archives of Physical Medicine and Rehabilitation, found little changes in vigor and fatigue among the non-exercisers while ultra-marathon runners and regular moderate exercisers experienced greater vigor and less fatigue.