In this short review, the frame of current understandings concerning what determines the upper limit of oxygen flow from the ambient air to the muscular mitochondria during maximal dynamic exercise in humans (i.e., VO2max), was summarized mainly from its modeling perspective. Several models and experimental evidences which appeared repeatedly in the recent debates regarding the factors limiting VO2max, were adopted and criticized. In conclusion, VO2max is determined in particular by the integrated interaction between the diffusive and convective factors both to lung O2-loading and muscular O2-unloading in the pathway for O2 flow, although all experimental observations cannot be satisfactorily explained at the present time.