Endurance training protects the heart during maximal exercise in long-distance runners

Abstract Introduction Cardiac function (cardiac output) and oxygen extraction (arterio-venous O2 difference) are key physiological determinants of maximal oxygen consumption and endurance performance. Purpose The aim of the study was to determine differences in maximal cardiac function represented by cardiac output and cardiac power output and oxygen extraction represented by arterio-venous O2 difference between long-distance runners and age- and gender-matched healthy individuals. Methods In a prospective observational study, 63 healthy males were recruited: 47 long-distance (half- and full-marathon) runners (aged 32.6 ± 8.6 years, body mass index 23.4 ± 2.2 kg/m²) and 16 healthy untrained individuals (control group, aged 30.9 ± 11.2 years, body mass index 24.8 ± 2.4 kg/m²). All participants underwent maximal graded cardiopulmonary exercise stress seting using an electromagnetic cycle ergometer with simultanous monitoring of metabolic (gas exchange) and hemodynamic (bioreactance) measurements. Cardiac power output, expressed in watts, as a measure of overall function and pumping capability of the heart, was determined as the product between cardiac output and mean arterial blood pressure. Oxygen extraction (arterio-venous O2 difference) was calculated as the ratio between O2 consumption and cardiac output. Results Maximal O2 consumption was significantly lower in the control group compared to runners (absolute values, 3.44 ± 1.12 vs. 3.99 ± 0.52 L/min, p < 0.01; and relative values, 45.3 ± 16.4 vs. 53.6 ± 6.9 ml/kg/min, p < 0.01). However, maximal cardiac output and cardiac power output were significantly higher in the control group compared to runners (21.4 ± 3.9 vs. 18.7 ± 3.0 L/min, p = 0.02; 5.94 ± 1.35 vs. 5.01 ± 1.00 watts, p = 0.01). Maximal stroke volume was significantly higher in control group compared to runners (126 ± 28 vs. 109 ± 21 ml/beat, p = 0.04), whereas maximal heart rate was not significantly different betwen the groups (174 ± 12 vs. 177 ± 22 beats/min, p = 0.70). Arterio-venous O2 difference was significantly higher in runners compared to control group (21.6 ± 3.7 vs. 14.5 ± 3.6 mlO2/100 ml blood, p < 0.01). Conclusion During maximal exercise stress testing long-distance runners demonstrate higher cardiorespiratory fitness and oxygen extraction but lower cardiac output and cardiac power output compared to untrained indivuduals. Ability to extract and utilise more oxygen at lower cardiac output during maximal physiogical stress suggests cardiac protective role of endurance training in long-distance runners.