I've seen studies like this before with similar conclusions, and I've never seen one that refutes this claim. One way to look at it is: Exercise beats depression, as if exercise were a kind of drug. Another way to look at it is that the lack of exercise is unnatural. In other words, depression is a side-effect of a severe exercise-deficiency.
If you assume human bodies need exercise to be healthy and happy (or at least undepressed), then exercise would function like any other essential. If you don't get enough vitamin C, your connective tissue begins to disintegrate (scurvy). If you don't get enough vitamin B-3, your skin peels and you start going insane (pellagra). And if you don't get enough physical exercise, you lose your ability to rise out of depression.
It's something to think about, anyway. At least it's something to try that won't hurt and has other positive benefits even if it doesn't completely lift one's depression. If you know someone who is depressed or discouraged, send them this article, and also the article, Undemoralize Yourself.
Exercise works for all of us. Even if you're not depressed, a little exercise usually raises your mood. It's an all-purpose mood raiser that just about anyone can use. If you haven't exercised in the last couple days and you're not feeling as good as you would like, try doing some exercise today and see if that helps. I'll bet it will.
Read more about how exercise relieves depression:Exercise, the 'best antidepressant,' also prevents heart disease
Adam Khan is the author of Principles For Personal Growth, Slotralogy, Antivirus For Your Mind, and co-author with Klassy Evans of How to Change the Way You Look at Things (in Plain English). Follow his podcast, The Adam Bomb.