Increases in blood [H+] and lactic acid [La−] attend fatigue. We applied Stewart's physiological model of acid-base status and simple regressions to assess the importance of independent variables and [La−] on [H+] during repeated sprints. Eight well-conditioned Arabians performed 9 sprints. Plasma from jugular vein samples was analysed for pH, Pco2, Na+, K+ and Cl−. Plasma [La−] was calculated from blood [La−], plasma [H+] from pH, SID from Na+, K+, Cl− and La−, Atot from pH, Pco2 and SID.
Peaks for SID, Pco2 and [H+] were reached at sprint 1, -2 and -3, respectively. At sprint 3, the 5.7 nmol/l peak in [H+] was partitioned into 2.3, 2.7 and 0.7 nmol/l for Atot, Pco2 and SID, respectively. From sprint 3 to sprint 9, increases in Atot and decreases in SID tended to increase [H+] but were counteracted by a steady decrease in Pco2 that determined the progressive decrease in [H+]. Therefore Pco2 was the dominant determinant of [H+] during 9 repeated sprints, and the expected major effect of [La−] was moderated in the SID by opposing increases in [Na+] and [K+]. In the work-adapted phase (sprints 3–9), decreasing [H+] was correlated positively with Pco2 (r = 0.997, P<.001) but negatively with La−(r = -0.986, P<.001). Respiration was therefore completely compensating for the effects of metabolism on [H+]. During the transition from rest to sprint 3 (peak plasma [H+]), increasing [H+] was highly correlated (r=0.99, P = 0.011) with [La−] but no other variable. The empirical and physiological analyses were consistent with one another during the work-adapted phase, but emphasis was placed on [La−] by the regression analysis, in contrast to Pco2 by the Stewart analysis, during the rest-work transition.