The mental health-cortisol link has research behind it, but the muscle injury recovery claim is a different story.

Other research does back up the idea that mental health and cortisol are connected in humans. One study in teenagers found higher hair cortisol was linked to more anxiety or stressful life events, though not everyone showed that link. Another found people with higher morning cortisol were more likely to have anxiety. A third, in people with mouth conditions, found those with more anxiety and depression had higher cortisol. But these are specific groups, and the connections aren't universal. As for the muscle injury recovery part, we couldn't find studies in our databases that directly tested it.