The study, published in the Archives of Physical Medicine and Rehabilitation, focused on middle-age non-exercisers rather than the young and fit, who are most often studied in exercise research.

"The lack of an increase in exercise-induced vigor or related mood enhancement factors among non-exercisers may contribute to the difficulties experienced by these people in maintaining a regular exercise program," study lead author Martin Hoffman of the University of California, Davis said in a statement. "Trainers would be wise to share this information with those who are initiating an exercise program so that they may be more likely to persist through the initial phases of the program."

Hoffman analyzed pre- and post-exercise responses to mood-related questions among non-exercisers, regular exercisers and marathon runners. Each group had eight men and eight women with a mean age of over 40.

Hoffman found aerobic exercise sessions resulted in an increase in vigor and a decrease in fatigue among marathon runners and regular exercisers but not among non-exercisers.