Adjustments of Cardiac Function During Prolonged Exercise Relative to Lactate Threshold.

Alterations in cardiac function during prolonged submaximal exercise relative to lactate threshold (LT) were evaluated on 7 normal healthy males aged 30.3 +/- 5.7 years. Systolic time intervals were analyzed through simultaneous recordings of electrocardiogram, phonocardiogram, and impedance cardiogram at a paper speed of 50 mm/s. Determination of stroke volume (SV) was based upon the method described by Kubicek et al. ANOVA followed by the Scheffe post-hoc comparison revealed that SV and myocardial contractility indices (MSER and PEP/LVET) remained relatively unchanged throughout 40-min cycling exercise, although the changes in heart rate and oxygen uptake were statistically significant. In addition, systolic blood pressure remained almost unchanged during the exercise. These results may be interpreted as evidence of the "contractility reserve", i.e., the ability of maintaining heart muscle contractility during prolonged exercise at LT intensity.