Maximal-effort knee-extension exercise impairs skeletal muscle oxidative capacity and V_ O2 recovery in vivo

Abstract

In the present study, we examined how fatiguing exercise affects O2-based measures of skeletal muscle oxidative capacity in vivo by measuring changes in the rate constant of muscle V_ O2 recovery (kVO _ 2). Healthy young adults completed isokinetic (120̊·s-1), maximal voluntary dynamic contractions (MVDCs) lasting 24 (baseline kVO _ 2) and 240 s (postfatiguing exercise kVO _ 2). Vastus lateralis kVO _ 2 was measured using near-infrared diffuse correlation spectroscopy (NIRS-DCS) via the conventional repeated arterial occlusion method (part A, n = 14) or a novel NIRS-DCS “free-flow” method (part B, n = 13). Pulmonary V_ O2 (pV_ O2), muscle V_ O2 (mV_ O2), and surface electromyography (sEMG) measures of muscle activation were also measured throughout the 240-s trial. Compared with the 24-s trial, kVO _ 2 following 240 s of MVDCs was impaired by ∼25% (part A; P = 0.005) and ∼16% (part B; P = 0.017). Moreover, both pV_ O2 and mV_ O2 rapidly increased to maximal levels, where they remained for the duration of the 240-s trial, despite sEMG activity and peak MVDC power declining. These results demonstrate that fatiguing exercise not only impairs O2-based measures of skeletal muscle oxidative capacity, but also that mitochondrial O2 consumption is uncoupled from power output and ATP demand during fatiguing exercise.