Fat Oxidation Kinetics Is Related to Muscle Deoxygenation Kinetics During Exercise

PURPOSE: The present study aimed to determine whether whole-body fat oxidation and muscle deoxygenation kinetics parameters during exercise were related in individuals with different aerobic fitness levels. METHODS: Eleven cyclists [peak oxygen uptake ([Formula: see text]): 64.9 ± 3.9 mL⋅kg(–1)⋅min(–1)] and 11 active individuals ([Formula: see text]: 49.1 ± 7.4 mL⋅kg(–1)⋅min(–1)) performed a maximal incremental cycling test to determine [Formula: see text] and a submaximal incremental cycling test to assess whole-body fat oxidation using indirect calorimetry and muscle deoxygenation kinetics of the vastus lateralis (VL) using near-infrared spectroscopy (NIRS). A sinusoidal (SIN) model was used to characterize fat oxidation kinetics and to determine the intensity (Fat(max)) eliciting maximal fat oxidation (MFO). The muscle deoxygenation response was fitted with a double linear model. The slope of the first parts of the kinetics (a(1)) and the breakpoint ([HHb](BP)) were determined. RESULTS: MFO (p = 0.01) and absolute fat oxidation rates between 20 and 65% [Formula: see text] were higher in cyclists than in active participants (p < 0.05), while Fat(max) occurred at a higher absolute exercise intensity (p = 0.01). a(1) was lower in cyclists (p = 0.02) and [HHb](BP) occurred at a higher absolute intensity (p < 0.001) than in active individuals. [Formula: see text] was strongly correlated with MFO, Fat(max), and [HHb](BP) (r = 0.65–0.88, p ≤ 0.001). MFO and Fat(max) were both correlated with [HHb](BP) (r = 0.66, p = 0.01 and r = 0.68, p < 0.001, respectively) and tended to be negatively correlated with a(1) (r = -0.41, p = 0.06 for both). CONCLUSION: This study showed that whole-body fat oxidation and muscle deoxygenation kinetics were both related to aerobic fitness and that a relationship between the two kinetics exists. Individuals with greater aerobic fitness may have a delayed reliance on glycolytic metabolism at higher exercise intensities because of a longer maintained balance between O(2) delivery and consumption supporting higher fat oxidation rates.