Quadriceps Muscles O 2 Extraction and EMG Breakpoints during a Ramp Incremental Test.

Muscle deoxygenated breakpoint ([HHb] BP ) has been found to be associated with other indices of exercise tolerance in the vastus lateralis (VL) muscle but not in the vastus medialis (VM) and rectus femoris (RF). Purpose: To investigate whether the [HHb] BP occurs also in the VM and RF muscles and whether or not it is associated with other physiological indices of exercise tolerance, such as the EMG threshold (EMGt ) and the respiratory compensation point (RCP). Methods: Twelve young endurance trained participants performed maximal ramp incremental (RI) cycling tests (25-30 W·min-1 increments). Muscle oxygen extraction and activity as well as ventilatory and gas exchange parameters were measured. After accounting for the mean response time, the oxygen uptake ([Formula: see text]O2 ) corresponding to the RCP, [HHb] BP , and the EMGt was determined. Results: Peak power output (POpeak ) was 359 ± 48 W. Maximal oxygen consumption ([Formula: see text]O2max ) was 3.87 ± 0.46 L·min-1 . The [Formula: see text]O2 at the RCP was 3.39 ± 0.41 L·min-1 . The [Formula: see text]O2 (L·min-1 ) corresponding to the [HHb] BP and EMGt were: 3.49 ± 0.46 and 3.40 ± 0.44; 3.44 ± 0.61 and 3.43 ± 0.49; 3.59 ± 0.52, and 3.48 ± 0.46 for VL, VM, and RF, respectively. Pearson's correlation between these thresholds ranged from 0.90 to 0.97 ( P < 0.05). No difference was found for the absolute [Formula: see text]O2 and the normalized PO (%) at which the thresholds occurred in all three muscles investigated ( P > 0.05). Although in eight out of 12 participants, the [HHb] BP in the RF led to a steeper increase instead of leading to a plateau-like response as observed in the VL and VM, the [Formula: see text]O2 at the breakpoints still coincided with that at the RCP. Conclusions: This study demonstrated that local indices of exercise tolerance derived from different portions of the quadriceps are not different to the systemic index of the RCP.