The body does not always achieve a steady state and is more often kept in a nonsteady state. It is important not only in terms of physiological anthropology but also in its application to rehabilitation and training to clarify whether oxygen transport or utilization is the limiting factor in oxygen uptake in the nonsteady state. Hypotheses concerning the limiting factor in oxygen uptake have their own rationales, and it is still controversial as to which is actually responsible. The limiting factor in oxygen uptake may vary according to exercise conditions. Under certain conditions, oxygen transport would be responsible for limiting the oxygen uptake, and under others, oxygen utilization or both oxygen transport and utilization would be responsible. These conditions differ according to the type of exercise (e.g. step exercise vs. cycling, leg vs. arm exercise, and dynamic vs. static exercise), posture, recruited muscle fibers, and other experimental conditions. Considering that oxygen uptake is regulated by coupling among respiration, circulation, and metabolism, it may be possible that interactions among these functions vary in a complicated manner according to exercise conditions, which may require a different limiting factor in regulating oxygen uptake.