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?

espanolStress 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. EnglishLas pruebas de esfuerzo realizadas a intensidad constante han sido objeto de interes de los fisiologos, analizando los factores responsables del abandono. El objetivo ha sido estudiar si variables de intercambio respiratorio y del estado acido-base podrian justificar la fatiga durante un esfuerzo constante realizado a una intensidad superior al maximo estado estable de lactato. Doce ciclistas realizaron una prueba en su bicicleta durante 30 min a una intensidad del 5% por encima del maximo estado estable de lactato (MLSS5%). Se analizaron determinados parametros de intercambio de gases respiratorios y del estado acido-base en reposo, a los 5, 10 y 15 min de la prueba. Para determinar el efecto del tiempo y del grupo se realizo un analisis de varianza repetido de dos factores. Para valorar las posibles diferencias estadisticas se considero un valor de alfa de 0,05. El grupo que no finalizo la prueba MLSS5% (N-MLSS5%) comenzo con un estado de acidosis metabolica mayor que el grupo que si completo la prueba (Y-MLSS5%) (44,6 versus 41,7 nm/l H+) (F1,9 = 9,43, p = 0,013; η2 = 0,51). Durante la prueba, el estado acido-base fue mayor en el grupo N-MLSS que en el Y-MLSS (a los 15 min, 44,3 para el grupo Y-MLSS y 49,2 para el grupo N-MLSS). Ninguno de los dos grupos mostro una alteracion de la relacion ventilacion/perfusion (relacion VD/VT ), aunque el comportamiento de la PET CO2 podria sugerirlo. El cambio en el modelo respiratorio (VT /Ti ) tampoco explica el abandono del ejercicio estable en el grupo N-MLSS. En conclusion, los resultados de este estudio no explican el abandono del ejercicio en un grupo de ciclistas (N-MLSS) durante el esfuerzo realizado. Este hallazgo refuerza la hipotesis relativa a que la fatiga sucede debido a la integracion de la retroalimentacion aferente de diversos sistemas fisiologicos.