Oxygen Uptake Dynamics Conform to Acute Changes in Muscle Excitation and Total Hemoglobin Concentration during Constant-Work Rate Exercise

This study aimed to investigate whether muscle excitation during constant-work rate (WR) cycling is connected with total[Hb + Mb] and whether they interact with the oxygen uptake (V̇O 2 ) dynamics. In experiment 1, 10 participants performed a 21-min constant-WR (CWR) within the heavy-intensity domain (i.e., 75% of the difference between the gas exchange threshold and the maximal metabolic steady state) and a ramp-to-constant-WR (rCWR) to the same WR. CWR and rCWR were repeated twice and allocated in random order. In experiment 2, nine participants performed a double-constant-WR (dCWR) consisting of a 21-min exercise bout, a short 20-s break, and a second bout of 21 min within the heavy domain. V̇O 2 , EMG root-mean-square (EMG RMS ), total[Hb + Mb], and deoxygenated hemoglobin ([HHb]) were collected from the vastus lateralis. The EMG RMS /total[Hb + Mb] and the EMG RMS /[HHb] ratios were computed. The EMG RMS was lower at minutes 1 and 7 and total[Hb + Mb] higher at minute 1 during the rCWR compared with the CWR condition (all P < 0.05). EMG RMS displayed an overshoot at minute 1, which was different from minute 21 during the CWR condition ( P < 0.05). EMG RMS did not display an overshoot after the 20-s break during the dCWR condition. The EMG RMS /total[Hb + Mb] inverted ratio was not different from V̇O 2 (%). The EMG magnitude of frequencies ranging from ~30 to 90 Hz was initially higher and decreased over time ( P < 0.05). This study demonstrated that EMG RMS -derived muscle excitation can be reduced by exercise protocols that promote higher total[Hb + Mb]. Furthermore, the interaction between muscle excitation and total[Hb + Mb] matched with systemic V̇O 2 .