Physiological basis of brief, intense interval training to enhance maximal oxygen uptake: a mini-review

Brief, intense interval training describes a style of exercise characterized by short bouts of strenuous effort interspersed with recovery periods. The method increases whole body maximal oxygen uptake (V̇o_2max), but the underlying physiological basis is unclear. V̇o_2max represents the functional limit of the integrative oxygen cascade, which refers to the physiological steps involved in oxygen transport and utilization from atmospheric air to mitochondrial metabolism. There is insufficient evidence to definitively state which steps in the oxygen cascade are responsible for the improvement in V̇o_2max after brief, intense interval training. Studies typically focus on specific physiological variables that are often characterized as "central" or "peripheral" based in part on their location in the body. Recent work suggests that training for ≥6 wk improves V̇o_2max in part by increasing maximal cardiac output and expanding blood volume, responses that are expected to augment central oxygen delivery. Other responses to brief, intense interval training, including increased capillary and mitochondrial density, may contribute to increases in V̇o_2max via enhanced skeletal muscle oxygen extraction and/or increased muscle diffusing capacity. This is especially evident after relatively short-term training and despite no change in central oxygen delivery factors. Mechanistic investigations, particularly employing contemporary technologies, are needed to advance our understanding of the early time course of the V̇o_2max response to brief, intense interval training and the extent to which changes in specific oxygen cascade processes compare with traditional endurance training.