BACKGROUND It is widely accepted that aerobic exercise is beneficial for human health. However, recently, the concept of "Exercise Induced Cardiac Fatigue(EICF)," a transient decline of cardiac function after prolonged exercise, has been proposed. It is still unknown how left ventricular function would be changed during long-term aerobic exercise. PURPOSE: The purpose of the present study was to examine the effects of prolonged exercise on left ventricular (LV) function with sophisticated 3-dimensional (3D) echocardiography. METHODS: Six healthy males (37 ± 5 years old) with regular exercise habits were recruited. Maximal oxygen consumption (VO2max) was measured with the upright cycle ergometry for each subject at the first visit to our laboratory. At least one week later, each subject visited our laboratory again to exercise on the cycle ergometer with the 60 degree head up tile (ergoselect12, ergoline) at the intensity of 60-70% VO2max for 3 hours. Heart rate (HR) was continuously monitored with 5-lead ECG during the exercise. LV function was evaluated with 3D echocardiography with speckle tracking (Epiq7, Philips) before exercise (rest), 5, 30, and 60 minutes after the start of exercise, and every 30 minutes thereafter until 180 minutes. HR and LV function were also measured 15 minutes after exercise (recovery). RESULTS: HR was higher in rest than in recovery (rest:60 ± 10 bpm, recovery:85 ± 14 bpm, p < 0.05, paired t-test). Global longitudinal strain (GLS, rest: -24.1 ± 4.2%, recovery: -29.1 ± 6.5%, p = 0.10), global circumferential strain (GCS, rest:-34.2 ± 4.3%, recovery:-33.4 ± 5.8%, p = 0.40), and systolic twist (twist, rest:12.1 ± 4.1, recovery:11.1 ± 2.9,p = 0.38) at rest were not different from those at recovery. Neither GLS nor GCS changed during the exercise. On the other hand, systolic twist showed a gradual decrease during the exercise (p < 0.05, one-way repeated ANOVA). CONCLUSIONS: LV systolic twist gradually decreased during long-term moderate intensity aerobic exercise for 3 hours although LV strains were unchanged. This finding indicates that the EICF may be detected during exercise by using the index of LV systolic twist. Moreover, the EICF may potentially affect physical performance when exercise duration is very prolonged.