Impact of Hypertrophy-Focused Resistance Training on Mitochondrial Capacity: A New Perspective Through Near-Infrared Spectroscopy.

This study aimed to compare the oxidative capacity of resting skeletal muscle between experienced natural bodybuilders and untrained individuals, using near-infrared spectroscopy (NIRS). Thirty-four male subjects participated voluntarily and were classified into two groups: 1) bodybuilding (n = 17; age: 24.3 ± 3.6 years, height: 180.1 ± 7.1 cm, body mass: 90.5 ± 9.3 kg), and 2) control (n = 17; age: 21.8 ± 2.7 years, height: 176.7 ± 5.7 cm, body mass: 71.4 ± 10.3 kg). The muscle oxygen consumption (mVO₂) of the vastus lateralis was measured using continuous-wave NIRS with transient arterial occlusions. The recovery rate of mVO₂ was fitted to a mono-exponential curve with the time constant (TC) used as an index of muscle oxidative capacity. The TC obtained from the mono-exponential fit was significantly lower in the bodybuilding group (TC: 23.5 ± 7.1 s) compared to the control group (TC: 40.7 ± 8.3 s), indicating enhanced muscle oxidative capacity (p < .001; effect size: 2.24). Additionally, initial end-exercise mVO₂ values were significantly different between the bodybuilding group (3.5 ± 1.3% · s^-1) and control group (2.5 ± 1.0% · s^-1). The reduced TC observed in the natural bodybuilders group demonstrates that RT focused on enhancing muscle hypertrophy improves the muscle oxidative capacity. These findings provide valuable insights into metabolic adaptations induced by RT and underscore the utility of NIRS for assessing muscle oxidative capacity.