According to him, those clinically depressed experience mood-boosting benefits the most.
The researcher, however, adds that people who are already relaxed, and have low levels of depression or anxiety can still experience the feelings of calmness, lowered levels of anxiety and less fatigue.
As regards hard workouts, Raglin said that intense exercise could bring about elevated feelings of anxiety and other unpleasant feelings immediately following the activity, but within five to 10 minutes they were usually replaced with the longer-lasting positive feelings.
His study also revealed that intense exercise routines could cause depression in otherwise healthy individuals.
Overtraining or staleness syndrome is a particular concern for serious endurance athletes, such as swimmers or runners. About 10 percent of such athletes experience the syndrome over the course of one year of training, and this increases significantly after five years of training.
Raglin said that his study had shown increasing levels of staleness syndrome among middle school and high school athletes.
"Exercise is a complex stimulus," he said.
Though several studies have highlighted the mental health benefits of exercise, Raglin said that little was known about why it could have such effect.
He said that there was no evidence that endorphins were behind it, despite popular belief.