Comparison of the Biceps and Triceps to Determine Metabolic Thresholds Using Muscle Oxygen Saturation in the Spinal Cord Injury Population: An Exploratory Study

This exploratory study aimed to compare the determination of metabolic thresholds using muscle oxygen saturation (SmO2) and gas exchange data during graded exercise testing in individuals with spinal cord injury. Nine participants (six males, three females) performed a graded exercise test on an arm-crank ergometer, with continuous measurements of breath-by-breath pulmonary gas exchange and SmO2 from the biceps brachii and triceps brachii. Thresholds were identified as gas exchange threshold (GET) and respiratory compensation point (RCP), and their SmO2 counterparts as MOT1 and MOT2. The results showed no differences between GET and MOT1 in either muscle (biceps: p = 0.14; triceps: p = 1.00), and similar results were observed between RCP and MOT2 for triceps brachii (p = 0.39) and biceps brachii (p = 0.12). Reliability analysis revealed good but non-significant agreement for the triceps brachii (ICC = 0.44–0.60), while the biceps brachii demonstrated very good agreement at GET (ICC = 0.78, p < 0.01) and excellent agreement at RCP (ICC = 0.81, p < 0.01), and Bland–Altman analyses confirmed no systematic bias between muscle sites. In conclusion, SmO2 may be a valid and promising variable for detection in individuals with spinal cord injury during ramp exercise testing. Both muscles showed agreement with pulmonary gas exchange, but the biceps brachii provided more consistent and reliable estimates, particularly for the second threshold. These preliminary findings suggest the use of near-infrared spectroscopy as a non-invasive technology for metabolic threshold detection in spinal cord injury populations.