Greater muscle mass does not affect peripheral hypercapnic chemosensitivity during exercise in healthy adults

Peripheral hypercapnic chemosensitivity (PHC) is the ventilatory response to a brief rise in arterial carbon dioxide tension. PHC is known to increase at the onset of exercise, with no further increases with progressive exercise intensity. The mechanisms for this augmentation remain unclear but may involve central command. We examined whether PHC scales with active muscle mass by comparing small-muscle (rhythmic handgrip, RHG), intermediate-muscle (single-leg extension, SLE), and large/whole-body (cycling, BIKE) exercise. Twenty-two healthy adults (n = 11 females) performed RHG, SLE, and BIKE exercise. PHC was measured before and during exercise using four trials, each consisting of two inhalations of 10% carbon dioxide. All exercise modalities produced a significant increase in PHC from rest. During exercise, SLE and BIKE (0.895 ± 0.388 and 1.028 ± 0.460 L·min^-1·mmHg^-1, respectively) elicited significantly greater PHC than RHG (0.683 ± 0.247 L·min^-1·mmHg^-1) (p = 0.037 and 0.015, respectively), while SLE and BIKE were not different (p = 0.263). Exercise sensitizes PHC regardless of recruited muscle mass, but augmentation does not scale linearly with muscle mass. As PHC increased with recruitment up to SLE, with no further increase during BIKE. These findings suggest that the augmentation of PHC is more closely associated with the transition to exercise, although the present exploratory study cannot determine the mechanism(s) responsible.