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

Purpose: to investigate whether muscle excitation during constant-work rate (WR) cycling is connected with total[Hb + Mb] and interact with oxygen uptake (V̇O2) dynamics.

Methods: Experiment 1: ten 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. Experiment 2: nine participants performed a double-constant-WR (dCWR) consisting of a 21-min exercise bout, a short 20-sec break, and a second bout of 21 min within the heavy domain. V̇O2, EMG root mean square (EMGRMS), total[Hb + Mb] and deoxygenated hemoglobin ([HHb]) were collected from the vastus lateralis. The EMGRMS/total[Hb + Mb] and EMGRMS/[HHb] ratios were computed.

Results: The EMGRMS was lower at min 1 and 7 and total[Hb + Mb] higher at min 1 during the rCWR compared to the CWR condition (all P < 0.05). EMGRMS displayed an overshoot at min 1 which was different from min 21 during the CWR condition (P < 0.05). EMGRMS did not display an overshoot following the 20 sec break during the dCWR condition. The EMGRMS/total[Hb + Mb] inverted ratio was not different from V̇O2 (%). The EMG magnitude of frequencies ranging from ~30-to-90 Hz was initially higher and decreased over time (P < 0.05).

Conclusions: This study demonstrated that EMGRMS-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 V̇O2.