The factors limiting maximal oxygen consumption (VO2max) in humans are analyzed according to a multifactorial model derived from the O2 conductance equation. The alveolar ventilation (VA) and lung O2 transfer (GL) are not considered to be limiting, at least at sea level in healthy subjects, because changes in VA and/or GL are not accompanied by changes in VO2max, due to the shape of the O2 dissociation curve. Thus, the limits to VO2max are shared between O2 transport by the circulation and a peripheral factor, including O2 transfer from capillaries to tissue and mitochondrial O2 utilization. In untrained healthy subjects at sea level, O2 transport by the circulation is responsible for about 70% of the overall limits, the rest depending on the peripheral factors.