A study was undertaken to elucidate the mechanism responsible for the specific changes in systolic time intervals (STIs), diastolic time (DT) and the ratio of total electromechanical systole to DT (QS₂/DT), which were observed during prolonged exercise^{17, 19)} Sixteen healthy male students performed short-term incremental maximal exercise and 40-min submaximal exercise with a work load requiring 65% of maximal oxygen consumption on a bicycle ergo-meter, Heart rate (HR), stroke volume (SV), blood pressure (BP), STIs and DT were calculated from electrocardiogram, phonocardiogram, derivative of ear densitogram, impedance cardiogram and finger arterial pressure wave.
1) During the short-term exercise, STIs, DT and QS₂/DT changed rectilinearly in accordance with increased HR, whereas they changed in a specific zigzag pattern during the prolonged exercise.
2) During the prolonged exercise, SV and BP were lower than those during the short-term exercise, except for SV between 1 and 2 min after the start of the exercise. From 2 min onwards, left ventricular ejection time (LVET), QS₂ and QS₂/DT became smaller than those during the short-term exercise.
3) Differences between the measured values of LVET, pre-ejection period (PEP) and PEP/LVET and those predicted by multiple regression equations during the prolonged exercise were smaller than those during the short-term exercise.
From these findings, it was concluded that the specific changes observed in STIs, DT and QS₂/DT during prolonged exercise are produced by decrease of SV and BP in the early stage, and probably influenced by a decrease in myocardial contractility in the late stage.