The higher intensity tends to produce greater endocrine responses, including endorphins, brain-derived neurotrophic factor, norepinephrine, dopamine, testosterone and growth hormone, all of which have a mechanistic influence to improve depression and anxiety.34, 36 Despite not measuring endocrine response specifically in their study, Santos et al23 demonstrated similar psychological improvements using HIIT exercise protocol. Here, BDNF is linked to depressive symptom measurement.