The effects of exercise intensity and duration on the relationship between the slow component of V̇O₂ and peripheral fatigue

Aim: If the development of the oxygen uptake slow component (V̇O_2SC) and muscle fatigue are related, these variables should remain coupled in a time- and intensity-dependent manner. Methods: 16 participants (7 females) visited the laboratory on 7 separate occasions: (1) three 6-minutes moderate-intensity cycling exercise bouts proceeded by a ramp incremental test; (2-3) 30-minutes constant power output (PO) exercise bout to determine the maximal lactate steady state (MLSS); (4-7) constant-PO exercise bouts to task failure (TTF), pseudorandomized order, at (i) 15% below the PO at MLSS; (ii) 10 W below MLSS; (iii) MLSS; (iv) 10 W above MLSS (first intensity and randomized order thereafter). Neuromuscular fatigue was characterized by isometric maximal voluntary contractions and femoral nerve electrical stimulation of knee extensors to measure peripheral fatigue at baseline, at min 5, 10, 20, 30 and TTF. Pulmonary oxygen uptake (V̇O₂) was continuously recorded during the constant-PO bouts and V̇O_2SC was characterized based on each individual V̇O₂ kinetics during moderate transitions. Results: The development of V̇O_2SC and peripheral fatigue were correlated across time (r²adj range of 0.64-0.80) and amongst each exercise intensity (r²adj range of 0.26-0.30) (all P <.001). Also, TTF was correlated with V̇O_2SC and neuromuscular fatigue parameters (r²adj range of 0.52-0.82, all P <.001). Conclusion: The V̇O_2SC and peripheral fatigue development are correlated throughout the exercise in a time- and intensity-dependent manner, suggesting that the V̇O_2SC may depend on muscle fatigue even if the mechanisms of reduced contractile function are different amongst intensities.