Introduction: Near-infrared spectroscopy (NIRS) has been widely used in rock-climbing research to assess muscle oxygenation (StO2) in the flexor digitorum profundus (FDP). However, the deep anatomical location of the FDP, combined with the maximum penetration depth of ~20 mm for most NIRS devices, raises uncertainty regarding whether the currently used optode placement is adequate for studying the FDP. This study aimed to characterize forearm morphology in climbers and examine the effect of NIRS optode placement on haemodynamic outcomes. Methods: This study comprised two parts. In Part 1 (n = 28), adipose tissue thickness (ATT), flexor digitorum superficialis (FDS) thickness, and FDP thickness were measured at three forearm sites (proximal, reference, and distal; 20 mm spacing) in supination and pronation using ultrasound. The reference site was defined as 33% of the distance between the medial epicondyle and lunate. Part 2 (n = 22) consisted of a vascular occlusion and incremental finger flexor endurance test with NIRS optodes positioned either perpendicular or parallel to the FDP fibres. The outcomes included half-time recovery of reoxygenation (StO2 HTR) from the vascular occlusion test, reoxygenation between contractions during the first (StO2 relief—first) and final stages (StO2 relief—last), and mean oxygenation (StO2 mean) during the incremental test. Results: The FDS-FDP border was located 17.6 ± 4.0 mm deep, indicating that the FDS dominated the near-infrared signal. During the incremental test, StO2 relief—last was 2.7 ± 2.9% lower (p < 0.001, g = 0.87) and StO2 mean was 5.5 ± 3.4% higher (p < 0.001, g = 1.56) with perpendicular optode orientation than with parallel optode orientation. StO2 HTR and StO2 relief—first were not different between optode orientations (p = .145, g = 0.33; p = 0.799, rrb = 0.06). Discussion: In conclusion, only a small portion of the FDP lies within the NIRS penetration depth. StO2 reoxygenation between contractions was more pronounced when the optodes were aligned parallel to the muscle fibres, although this was observed only at moderate intensities. Based on these findings, we recommend positioning the NIRS optodes over the reference site, aligning them parallel to the muscle fibres, and using the term "flexores digitorum" when referring to the muscles under investigation.
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