Exactly how exercise fights depression isn't clear. Tkachuk said it may stimulate pleasure-inducing brain chemicals such as serotonin, just like medications do. Whatever the explanation, Johnsgard swears by exercise. If you can get someone to keep it up for three to four months, he says, they'll do it for the rest of their lives. And to him, that sounds a lot better than a lifetime of medication.
Exercise is another tool they can use.