Effects of assuming constant optical scattering on measurements of muscle oxygenation by near-infrared spectroscopy during exercise

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Abstract
The aim of this study was to examine the effects of assuming constant reduced scattering coefficient (μ ) on the muscle oxygenation response to incremental exercise and its recovery kinetics. Fifteen subjects (age: 24 ± 5 yr) underwent incremental cycling exercise. Frequency domain near-infrared spectroscopy (NIRS) was used to estimate deoxyhemoglobin concentration {[deoxy(Hb+Mb)]} (where Mb is myoglobin), oxyhemoglobin concentration {[oxy(Hb+Mb)]}, total Hb concentration (Total[Hb+Mb]), and tissue O2 saturation (Sti ), incorporating both continuous measurements of μ and assuming constant μ . When measuring μ , we observed significant changes in NIRS variables at peak work rate Δ[deoxy(Hb+Mb)] (15.0 ± 7.8 μM), Δ[oxy(Hb+Mb)] (−4.8 ± 5.8 μM), ΔTotal[Hb+Mb] (10.9 ± 8.4 μM), and ΔSti (−11.8 ± 4.1%). Assuming constant μ resulted in greater (P < 0.01 vs. measured μ ) changes in the NIRS variables at peak work rate, where Δ[deoxy(Hb+Mb)] = 24.5 ± 15.6 μM, Δ[oxy(Hb+Mb)] = −9.7 ± 8.2 μM, ΔTotal[Hb+Mb] = 14.8 ± 8.7 μM, and ΔSti = −18.7 ± 8.4%. Regarding the recovery kinetics, the large 95% confidence intervals (CI) for the difference between those determine measuring μ and assuming constant μ suggested poor agreement between methods. For the mean response time (MRT), which describes the overall kinetics, the 95% confidence intervals were MRT − [deoxy(Hb+Mb)] = 26.7 s; MRT − [oxy(Hb+Mb)] = 11.8 s, and MRT − Sti = 11.8 s. In conclusion, μ changed from light to peak exercise. Furthermore, assuming a constant μ led to an overestimation of the changes in NIRS variables during exercise and distortion of the recovery kinetics.