BDNF and Anxiety: At present, there is no detailed research to explain how exercise relieves anxiety and depression, but it is hypothesized that the mechanism of action is similar to that of antidepressants, with effects including increased expression of brain-derived neurotrophic factor (BDNF), increased availability of serotonin and norepinephrine, regulation of hypothalamic–pituitary–adrenal axis activity, and reduced systemic inflammatory signaling [42].