Increased Mass-Specific Maximal Fat Oxidation Rate with Small versus Large Muscle Mass Exercise

Abstract

Introduction: Skeletal muscle perfusion and oxygen (O2) delivery are restricted during whole-body exercise due to a limited cardiac output (Q[Combining Dot Above]). This study investigated the role of reducing central limitations to exercise on the maximal fat oxidation rate (MFO) by comparing mass-specific MFO (per kg active lean mass) during one-legged (1 L) and two-legged (2 L) cycling. We hypothesized that the mass-specific MFO would be higher during 1 L- than 2 L cycling. Methods: Twelve male subjects (V[Combining Dot Above]O2peak: 59.3 ± 8.4 mL·kg-1·min-1; mean ± SD) performed step-incremental 2 L- (30-80% of V[Combining Dot Above]O2peak) and 1 L (50% of 2 L power output, i.e., equal power output per leg) cycling (counterbalanced) while steady-state pulmonary gas exchanges, Q[Combining Dot Above] (pulse-contour analysis), and skeletal muscle (vastus lateralis) oxygenation (near-infrared spectroscopy) were determined. MFO and the associated power output (FatMax) were calculated from pulmonary gas exchanges and stoichiometric equations. A counterweight (10.9 kg) was added to the contralateral pedal arm during 1 L cycling. Leg lean mass was determined by DEXA. Results: The absolute MFO was 24% lower (0.31 ± 0.12 vs 0.44 ± 0.20 g·min-1; P = 0.018) while mass-specific MFO was 52% higher (28 ± 11 vs 20 ± 10 mg·min-1·kg-1; P = 0.009) during 1 L- than 2 L cycling. FatMax was similar expressed as power output per leg (60 ± 28 vs 58 ± 22 W; P = 0.649). Q· increased more from rest to exercise during 1 L- than 2 L cycling when expressed per active leg (ANOVA main effect: P = 0.003). The tissue oxygenation index and Δ[deoxy(Hb + Mb)] were not different between exercise modes (ANOVA main effects: P ≥ 0.587), indicating similar skeletal muscle fractional O2 extraction. Conclusions: Mass-specific MFO is increased by exercising a small muscle mass, potentially explained by increased perfusion and more favorable conditions for O2 delivery than during whole-body exercise.

Increased Mass-Specific Maximal Fat Oxidation Rate with Small versus Large Muscle Mass Exercise