New Findings What is the central question of this study? The finding that pulmonary oxygen uptake (inline image) kinetics on transition to moderate exercise is invariant and exponential is consistent with a first-order reaction controlling inline image. However, slowed inline image kinetics when initiating exercise from raised baseline intensities challenges this notion. What is the main finding and its importance? Here, we demonstrate how a first-order system can respond with non-first-order response dynamics. Data suggest that progressive recruitment of muscle fibre populations having progressively lower mitochondrial density and slower microvascular blood flow kinetics can unify the seemingly contradictory evidence for the control of inline image on transition to exercise. We examined the relationship amongst baseline work rate (WR), phase II pulmonary oxygen uptake (inline image) time constant (inline image) and functional gain inline image during moderate-intensity exercise. Transitions were initiated from a constant or variable baseline WR. A validated circulatory model was used to examine the role of heterogeneity in muscle metabolism (inline image) and blood flow (inline image) in determining inline image kinetics. We hypothesized that inline image and GP would be invariant in the constant baseline condition but would increase linearly with increased baseline WR. Fourteen men completed three to five repetitions of ∆40 W step transitions initiated from 20, 40, 60, 80, 100 and 120 W on a cycle ergometer. The ∆40 W step transitions from 60, 80, 100 and 120 W were preceded by 6 min of 20 W cycling, from which the progressive ΔWR transitions (constant baseline condition) were examined. The inline image was measured breath by breath using mass spectrometry and a volume turbine. For a given ΔWR, both inline image (22–35 s) and GP (8.7–10.5 ml min−1 W−1) increased (P < 0.05) linearly as a function of baseline WR (20–120 W). The inline image was invariant (P < 0.05) in transitions initiated from 20 W, but GP increased with ΔWR (P < 0.05). Modelling the summed influence of multiple muscle compartments revealed that inline image could appear fast (24 s), and similar to in vivo measurements (22 ± 6 s), despite being derived from inline image values with a range of 15–40 s and inline image with a range of 20–45 s, suggesting that within the moderate-intensity domain phase II inline image kinetics are slowed dependent on the pretransition WR and are strongly influenced by muscle metabolic and circulatory heterogeneity.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)