Do the changes in acid-base status and respiratory gas exchange explain the voluntary termination of a test performed above the maximum lactate steady state? = ¿Pueden los cambios del estado ácido-base e intercambio de gases respiratorios explicar el abandono de una prueba realizada por encima del máximo estado estable de lactato?

Abstract

Stress tests at a constant load have been a great subject of interest for physiologists to analyze the factors which lead to voluntary termination. Several factors are responsible for voluntary termination in such efforts. The objective of this work was to study if any of the respiratory gas exchange and acid-base status variables could justify fatigue during a constant load test performed above maximum lactate steady state. Twelve amateur road cyclists performed a 30 min test on a road bicycle at an intensity of 5% above maximal lactate steady state (MLSS5%). Gas exchange and acid-base data were analyzed at rest and at 5, 10 and 15 min during the test. A two-way analysis of variance for repeated measures was conducted to test the effect of time and group (An alpha of 0.05 was used as the level of statistical significance for all analyses). The group that did not finished the MLSS5% (N-MLSS5%) started from a more pronounced state of metabolic acidosis than the group that ended the test (Y-MLSS5%) (44,6 versus 41,7 nm/l H+) (F1,9 = 9.43, P = .013; η2 = 0.51). During the test, the acid-base status was greater in the N-MLSS group than the Y-MLSS group (at 15 min, 44,3 for the Y-MLSS group and 49,2 for the N-MLSS group). Neither of the two groups showed an altered ventilation perfusion ratio, estimated by the VD/VT relationship, although the behaviour of PET CO2 could suggest this outcome. A change in the breathing pattern (VT/Ti) does not explain the termination of steady exercise in the N-MLSS group. In conclusion the results of this study do not explain the voluntary termination of exercise in a group of cyclists (N-MLSS) that made a steady effort over the maximal lactate steady state. This finding reinforces the hypothesis that fatigue occurs due to an integration of the afferent feedback of various physiological systems.