Muscle oxygen saturation reflects individualized responses to exercise intensity during rowing ergometer exercise

Precise monitoring of training intensity in rowing requires indicators capturing both systemic and local physiological responses. Muscle oxygen saturation (SmO₂), measured by near-infrared spectroscopy (NIRS), reflects local muscular oxygen utilization and may complement traditional systemic indicators such as heart rate (HR), oxygen uptake (V̇O₂), and mechanical power output (P). This study aimed to investigate SmO₂ dynamics in upper- (biceps brachii, BB) and lower-limb (vastus lateralis, VL) muscles during incremental exercise, representative training sessions, and a 2000 m rowing ergometer race, and to evaluate its practical application for individualized monitoring.

Twenty-six trained male rowers (age: 19.96 ± 2.66 years; height: 189.12 ± 6.13 cm; body mass: 80.12 ± 9.21 kg) performed a graded incremental test, four representative training sessions, and a 2000 m race on a rowing ergometer. SmO₂ was measured continuously on biceps brachii and vastus lateralis using portable NIRS devices, along with HR, V̇O₂, and P. Linear mixed-effects models were used to examine within-subject associations between SmO₂ and systemic indicators, accounting for repeated measures across exercise stages.

During incremental exercise, SmO₂-VL decreased from 73.7 ± 6.76% at rest to 7.88 ± 4.12% at maximal power, whereas SmO₂-BB decreased from 68.18 ± 4.49% to 28.93 ± 6.29%. Within-subject associations between SmO₂-VL and V̇O₂ (β = -1.255, 95% CI - 1.315 to - 1.194, p < 0.001), HR (β = -0.647, 95% CI - 0.675 to - 0.618, p < 0.001), and P (β = -0.186, 95% CI - 0.194 to - 0.177, p < 0.001) were significantly stronger than corresponding SmO₂-BB associations. SmO₂-VL also exhibited robust negative associations with HR (r = - 0.857, p < 0.001) and P (r = - 0.841, p < 0.001) during the 2000 m race. Variability in SmO₂ responses across training sessions reflected differences in exercise intensity.

SmO₂ decreases progressively with increasing exercise intensity, with the vastus lateralis showing greater sensitivity than the biceps brachii. Within-subject SmO₂-VL measurements provide a reliable indicator of local muscular oxygen extraction and can complement systemic variables for individualized training monitoring in rowing.