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

Abstract

To determine whether disturbances of CO2 homeostasis alter force output characteristics of lower limb muscles, participants performed four isometric knee extension trials (MVC30 %, 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 MVC30 %, the participants exerted a constant force (30 % of maximum voluntary contraction [MVC]). The force coefficient of variation (Fcv) during each MVC30 % and MVC before and after the four MVC30 % 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.