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

Area of Science:

• Exercise Physiology
• Metabolic Adaptations
• Skeletal Muscle Physiology

Background:

• Skeletal muscle perfusion and oxygen delivery are limited during whole-body exercise due to restricted cardiac output.
• Understanding factors influencing maximal fat oxidation rate (MFO) is crucial for optimizing exercise strategies.
• Central exercise limitations can impact substrate utilization during physical activity.

Purpose of the Study:

• To investigate the effect of reduced central exercise limitations on mass-specific MFO.
• To compare mass-specific MFO during one-legged (1L) versus two-legged (2L) cycling.
• To test the hypothesis that mass-specific MFO is higher during 1L cycling.

Main Methods:

• Twelve male subjects performed incremental 2L and 1L cycling protocols.
• Measurements included pulmonary gas exchange, cardiac output (Q̇), and vastus lateralis muscle oxygenation.
• Mass-specific MFO and FatMax were calculated, with leg lean mass determined by DEXA.

Main Results:

• Absolute MFO was 24% lower in 1L vs 2L cycling, but mass-specific MFO was 52% higher.
• FatMax was similar when expressed as power output per leg.
• Cardiac output increased more per active leg during 1L cycling, with similar muscle oxygenation between modes.

Conclusions:

• Mass-specific MFO increases when exercising a smaller muscle mass.
• This enhancement may be attributed to increased muscle perfusion and improved oxygen delivery conditions.
• Reducing central exercise limitations optimizes conditions for fat oxidation in active muscle.