Behavior of cardiac output during progressive exercise tests: a preliminary report.

The behavior of cardiac output (Q) during progressive incremental exercise tests was studied in young healthy men. Q approached a plateau and leveled off at almost the same work rate at which oxygen uptake (VO2) attained its maxima, while heart rate (HR) still continued to rise. This suggests that the limiting factor for maximal aerobic capacity in healthy subjects is Q. The rate of increase in Q and HR accelerated from a work rate which is close to the ventilatory anaerobic threshold (AT). The prime cause of the progressive augmentation in cardiac activity is probably an accelerated release of plasma catecholamine and/or potassium. There is a possibility that these substances might also affect the AT.