Relationship between disturbances of CO₂ homeostasis and force output characteristics during isometric knee extension

To determine whether disturbances of CO₂ homeostasis alter force output characteristics of lower limb muscles, participants performed four isometric knee extension trials (MVC_{30 %}, 10 s each with 20-s rest intervals) in three CO₂ conditions (normocapnia [NORM], hypercapnia [HYPER], and hypocapnia [HYPO]). Respiratory frequency and tidal volume were matched between CO₂ conditions. In each MVC_{30 %}, the participants exerted a constant force (30 % of maximum voluntary contraction [MVC]). The force coefficient of variation (Fcv) during each MVC_{30 %} and MVC before and after the four MVC_{30 %} trials were measured. For the means of the four trials, Fcv was significantly lower in HYPER than in HYPO. However, within HYPER, a significant positive correlation was found between the increase in end-tidal CO₂ partial pressure and the increase in Fcv. MVCs in NORM and HYPO decreased significantly over the four trials, while no such reduction was observed in HYPER. These results suggest that perturbed CO₂ homeostasis influences the force output characteristics independently of breathing pattern variables.