What mechanisms explain the positive psychological benefits of exercise such as reducing anxiety and ...

Although the mechanisms are not empirically established, hypotheses fall into two camps, physiological and psychological. Physiological hypotheses mainly center on (1) facilitating synaptic transmission of brain monoamines or (2) boosting brain endorphin levels. The monoamines are the feel good neurotransmitters present in the brain such as serotonin, norepinephrine, and dopamine. If exercise facilitates monoamine action in the brain, then it should produce anti-depressant effects just as anti-depressant medications boost mood states. There is some suggestive evidence that serotonin metabolism increases after exercise due to an increase in the brain's supply of serotonin's precursor molecules, but more research is needed to confirm the monoamine hypothesis.