Polarized Training (POL) has been recently suggested to be superior to other training intensity distributions (TIDs) regimens for improving endurance performance. The first aim of this thesis was to systematically review and meta analyze the evidence comparing POL to other TIDs on surrogates of cardiorespiratory function. PubMed, Scopus, and Web of Science were searched for studies with more than or equal to four weeks duration comparing POL with other TID exercise interventions assessing either VO2max/peak, time-trial, time to exhaustion or velocity or power at the second ventilatory or lactate threshold. Eighteen studies met the inclusion criteria (n = 449 subjects). The pooled effect estimates suggest that POL is superior to other TIDs for improving VO2max/peak. This superiority, however, only occurs in shorter duration interventions (<12 weeks) and for the case of moderately/well trained subjects. All the remaining cardiorespiratory function surrogates analyzed were similarly affected by POL compared to other TIDs. The second aim of this thesis was to perform an experimental study in Wistar Han rats to compare an eight-week POL vs Threshold training (LIM) protocol, regarding VO2max, endurance capacity, diaphragm muscle structure, mitochondrial respiration and markers of mitochondrial function and dynamics in the left heart ventricle, diaphragm, tibialis anterior and soleus muscles. At the end of the training protocol, differences were not observed in relative or absolute VO2max between groups and there were also no differences in endurance capacity between POL and LIM groups at the intermediate and final timepoint assessments. Differences were also not observed in mitochondrial respiration not in markers of mitochondrial dynamics between POL and LIM groups in any of the tissue samples analyzed. The results from our experimental study suggest that POL training induces similar, functional, structural and bioenergetic adaptations compared to LIM training.
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