CHANGES IN ACID-BASE STATUS OF MARATHON RUNNERS DURING AN INCREMENTALFIELD-TEST - RELATIONSHIP TO MEAN COMPETITIVE MARATHON VELOCITY

Four top-class runners who regularly performed marathon and long-dista nce races participated in this study. They performed a graded field te st on an artificial running track within a few weeks of a competitive marathon. The test consisted of five separate bouts of running. Each p eriod lasted 6 min with an intervening 2-min rest bout during which ar terialized capillary blood samples were taken. Blood was analysed for pH, partial pressure of oxygen and carbon dioxide (PO2 and PCO2) and l actate concentration ([la-]b). The values of base excess (BE) and bica rbonate concentration ([HCO3-]) were calculated. The exercise intensit y during the test was regulated by the runners themselves. The subject s 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). Ev ery 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 f(c) amounting to 10 beat s . min -1 less than their maximal f(c). The results from these runner s showed that the blood pH ranged very little in the bouts performed a t a running speed below 100% of mean marathon velocity (v(m)BAR). Howe ver, once v(m)BAR was exceeded, there were marked changes in acid-base status. In the bouts performed at a velocity above the v(m)BAR there was a marked increase in [la-]b and a significant decrease in pH, [HCO 3-], BE and PCO2. The average marathon velocity (v(m)BAR) was 18.46 (S D 0.32) km . h -1. The [la -]b at a mean running velocity of 97.1 (SD 0.8) % of v(m)BAR was 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 h igher. However, when running velocity exceeded the v(m) by only 3.6 (S D 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 velo city 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 incr ease significantly above the value at rest is a sensitive indicator of capacity for marathon running.