Changes in acid-base status of marathon runners during an incremental field test

Summary Four top-class runners who regularly performed marathon and long-distance races participated in this study. They performed a graded field test on an artificial running track within a few weeks of a competitive marathon. The test consisted of five separate bouts of running. Each period lasted 6 min with an intervening 2-min rest bout during which arterialized capillary blood samples were taken. Blood was analysed for pH, partial pressure of oxygen and carbon dioxide (P0₂ and PCO₂) and lactate concentration ([la^–]_b). The values of base excess (BE) and bicarbonate concentration ([HCO₃^–]) were calculated. The exercise intensity during the test was regulated by the runners themselves. The subjects were asked to perform the first bout of running at a constant heart rate f_c which was 50 beats · min^–1 below their own maximal f_c. Every subsequent bout, each of which lasted 6 min, was performed with an increment of 10 beats · min^–1 as the target f_c. Thus the last, the fifth run, was planned to be performed with fc amounting to 10 beats · min^–1 less than their maximal f_c. The results from these runners showed that the blood pH changed very little in the bouts performed at a running speed below 100% of mean marathon velocity (_m). However, once _mwas exceeded, there were marked changes in acid-base status. In the bouts performed at a velocity above the _mthere was a marked increase in [la^–]_b and a significant decrease in pH, [HCO₃^–], BE and PCO₂. The average marathon velocity (_m) was 18.46 (SD 0.32) km·h^–1. The [la^–]_b at a mean running velocity of 97.1 (SD 0.8) % of _mwas 2.33 (SD 1.33) mmol ·l^–1 which, compared with a value at rest of 1.50 (SD 0.60) mmol·l^–1, was not significantly higher. However, when running velocity exceeded the vm by only 3.6 (SD 1.9) %, the [la^–]b increased to 6.94 (SD 2.48) mmol·l-1 (P<0.05 vs rest). We concluded from our study that the highest running velocity at which the blood pH still remained constant in relation to the value at rest and the speed of the run at which [la^–]_b began to increase significantly above the value at rest is a sensitive indicator of capacity for marathon running.