Side differences in NIRS-derived muscle oxygen saturation during treadmill running

Abstract Purpose Near-infrared spectroscopy (NIRS) offers a promising non-invasive method to assess muscle oxygenation in real time. While asymmetries in muscle oxygen saturation (SmO 2 ) between dominant and non-dominant legs have been studied in cycling, data in running are limited. Such differences may affect NIRS use in training control. This study investigated SmO 2 differences between dominant and non-dominant vastus lateralis during treadmill running at three workloads using a wearable continuous-wave NIRS device. Methods Forty-eight participants (20 female, Age = 21.6 ± 1.5 years) performed two graded treadmill exercise tests, 14 days apart. The Moxy Monitor was placed bilaterally on the vastus lateralis to measure SmO 2 and analysed across three individualized workloads (LOW, MID, HIGH). Linear mixed models were used with participant as random, and side and workload as fixed effects. To assess adipose tissue thickness (ATT) effects, a 7mm cut-off defined two subgroups (ATT<7mm, ATT>7mm). Bland-Altman plots examined mean bias and limits of agreement. Results No significant side effects were found for either group (ALL p = .759, ATT<7mm p = .713 and ATT>7mm p = .890). Bland-Altman analysis revealed a small mean bias (ALL: -0.34%, ATT<7mm: − 0.48%, ATT>7mm: -0.22%) with wide limits of agreement between the legs. Conclusion No systematic differences between SmO 2 response between the legs were found. However, wide limits of agreement suggest that individual side differences may exist. A single NIRS device may sufficiently reflect bilateral SmO 2 responses at the group level, while individual side-to-side-comparisons should be interpreted with caution.