REN and cancer: rMSSD suffers normally a decrease during and after cancer treatments [121, 122], which exercise may regain by a decrease of renin and angiotensin, affecting the renin–angiotensin–aldosterone system; a decline of oxidative stress and inflammation caused by the increase in catecholamines impacting, therefore, in tumour cell microenvironment by an exponential decrease of reactive oxidative species and an increment in antioxidants [61, 123–126].