Cardiac function time intervals are known to change with aerobic fitness. Recently, the Tei index of cardiac function [defined as the sum of the isovolumetric contraction (ICI) and isovolumetric relaxation intervals (IRI) divided by the left ventricular ejection time (LVET)] has been proposed to be a very sensitive determinant of cardiac function in patients with cardiomyopathy, i.e. the index is greater in patients with cardiomyopathy than it is in normal subjects. The purpose of this study was to determine the relationship between the Tei index and aerobic endurance in healthy volunteers.The relatively new noninvasive method of seismocardiography was used to measure following resting left ventricular (LV) cardiac function time intervals in 51 subjects (18 males and 33 females); Tei index, R-R interval, LV ICI, LV IRI, LVET, LV systole and LV diastole. Designated on the basis of peak treadmill time (Bruce protocol), the following three groups were assigned: Group 1; treadmill time > or = 900 seconds; Group 2: treadmill time ranging from 721 to 899 seconds; and Group 3; treadmill time < 720 seconds.The Tei index value was lower as exercise capacity increased (p < 0.05) primarily due to a reduction in LV IRI (p < 0.05). LVET tended to be longer with increased treadmill time (p < 0.05) but this effect appeared to be secondary to a greater R-R interval in more fit subjects. When adjusted for the R-R interval, a reduced Tei index and LV IRI were still observed in more fit subjects, despite a similar LVET between groups.Peak treadmill time is inversely related to the Tei index. A reduction of LV IRI appears to be the primary factor in establishing these differences. These data support the important role of LV diastolic function in aerobic fitness.