Changes in acid-base status of marathon runners during an incremental field test. Relationship to mean competitive marathon velocity.

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 (PO2 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 fc which was 50 beats.min-1 below their own maximal fc. Every subsequent bout, each of which lasted 6 min, was performed with an increment of 10 beats.min-1 as the target fc. Thus the last, the fifth run, was planned to be performed with fc amounting to 10 beats.min-1 less than their maximal fc. 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 (nu m). However, once nu m was exceeded, there were marked changes in acid-base status. In the bouts performed at a velocity above the nu m there was a marked increase in [la-]b and a significant decrease in pH, [HCO3-], BE and pCO2. The average marathon velocity (nu m) was 18.46 (SD 0.32) km.h-1.(ABSTRACT TRUNCATED AT 250 WORDS)