Measures of excess CO₂ and recovery CO₂ as indices of performance fatigability during exercise: a pilot study

The severity of performance fatigability and the capacity to recover from activity are profoundly influenced by skeletal muscle energetics, specifically the ability to buffer fatigue-inducing ions produced from anaerobic metabolism. Mechanisms responsible for buffering these ions result in the production of excess carbon dioxide (CO₂) that can be measured as expired CO₂ ($$CO₂) during cardiopulmonary exercise testing (CPET). The primary objective of this study was to assess the feasibility of select assessment procedures for use in planning and carrying out interventional studies, which are larger interventional studies investigating the relationships between CO₂ expiration, measured during and after both CPET and submaximal exercise testing, and performance fatigability.

Cross-sectional, pilot study design. Seven healthy subjects (30.7±5.1 years; 5 females) completed a peak CPET and constant work-rate test (CWRT) on separate days, each followed by a 10-min recovery then 10-min walk test. Oxygen consumption ($$O₂) and $$CO₂ on- and off-kinetics (transition constant and oxidative response index), excess-$$CO₂, and performance fatigability severity scores (PFSS) were measured. Data were analyzed using regression analyses.

All subjects that met the inclusion/exclusion criteria and consented to participate in the study completed all exercise testing sessions with no adverse events. All testing procedures were carried out successfully and outcome measures were obtained, as intended, without adverse events. Excess-$$CO₂ accounted for 61% of the variability in performance fatigability as measured by $$O₂ on-kinetic ORI (ml/s) (R²=0.614; y = 8.474x - 4.379, 95% CI [0.748, 16.200]) and 62% of the variability as measured by PFSS (R²=0.619; y =  - 0.096x + 1.267, 95% CI [-0.183, -0.009]). During CPET, $$CO₂ -off ORI accounted for 70% (R²=0.695; y = 1.390x - 11.984, 95% CI [0.331, 2.449]) and $$CO₂ -off Kt for 73% of the variability in performance fatigability measured by $$O₂ on-kinetic ORI (ml/s) (R²=0.730; y = 1.818x - 13.639, 95% CI [0.548, 3.087]).

The findings of this study suggest that utilizing $$CO₂ measures may be a viable and useful addition or alternative to $$O₂ measures, warranting further study. While the current protocol appeared to be satisfactory, for obtaining select cardiopulmonary and performance fatigability measures as intended, modifications to the current protocol to consider in subsequent, larger studies may include use of an alternate mode or measure to enable control of work rate constancy during performance fatigability testing following initial CPET.