Author(s): MARINARI, G., MARINARI, G., TRAMA, R., ZAGATTO, A.M., IANNETTA, D., MURIAS, J.M., Institution: UNIVERSITY OF CALGARY, Country: CANADA, Abstract-ID: 1384

INTRODUCTION:
Patterns of muscle activation and near-infrared spectroscopy (NIRS)-derived total hemoglobin concentration ([THC]; a proxy for the dynamics in red blood cell volume), may have an important role for changes in oxygen uptake (VO2) during exercise. However, the interplay between electromyography (EMG)-derived muscle activation and [THC] when taken as a single variable, and its potential association to V̇O2 during constant-work rate (WR) exercise are yet to be explored. This study investigated the EMG/[THC] ratio and its potential correspondence to the dynamics of VO2 during heavy-intensity exercise.
METHODS:
Eleven and nine participants were recruited in part one and part two of the study, respectively. The heavy-intensity domain was identified through the step-ramp-step protocol. Part one: participants randomly performed: i) a 21-min square-wave transition from 20 W to a WR corresponding to 75% of the heavy-intensity domain amplitude (CWR); ii) a ramp (20 W·min-1) to the same constant-WR, which was then sustained for 21 min (r-CWR). The CWR and r-CWR trials were repeated to increase the signal-to-noise ratio. Part two: participants performed a 42-min square-wave transition to the same WR, interrupted by a 20-sec period at 20 W between min 21 and 22 (CWRx2). Pulmonary VO2, as well as the root mean square derived from the EMG signal and [THC] from the vastus lateralis, were measured and expressed as a percent of baseline (0%) to end exercise (100%). The EMG/[THC] ratio was calculated and expressed as a percent of its peak value and its end value. The EMG signal was further decomposed using wavelet transform. Significance was set at P<0.05
RESULTS:
Part 1: No significant main effect of condition existed for both CWR and r-CWR between the dynamics of VO2 and the dynamics of the EMG/[THC] ratio (P>0.05), which was consistent for all the 21 min pairwise comparisons (all P>0.05). The EMG signal during the CWR condition decreased across time from min 1 (P<0.05), while no differences were observed during the constant-WR portion of the r-CWR condition (P>0.05).
No significant main effect of condition for the median power frequency (MPF; Hz) was observed (P>0.05), with a significant time effect (P<0.05) for the CWR condition where MPF was increasing over time, with stable responses during the r-CWR condition (P>0.05).
Part 2: No differences in EMG were observed between min 21 and min 22 (i.e., pre and post the 20-sec interruption at 20 W) for the CWRx2 condition (P>0.05).
CONCLUSION:
This study indicated that the VO2 dynamics during heavy-intensity exercise at constant-WR conform to the changes in EMG/[THC] ratio. This suggests that changes in muscle activation and peripheral oxygen delivery concurrently adjust to support oxidative phosphorylation over time. Further, although muscle activation decreased over time during the CWR condition, MPF increased, suggesting an overall reduced number of recruited muscle fibers, but with a shift to higher-order muscle fibers.