Muscle Oxygenation During Incremental Exercise And Recovery

Abstract

Near infrared spectroscopy (NIRS) has been used to assess muscle oxygenation during incremental exercise and recovery. However, modifications in the tissue optical properties (e.g. light scattering) have not been accounted for (i.e. scattering assumed constant), which could lead to erroneous results. PURPOSE To evaluate the effects of assuming constant reduced scattering coefficient (μ's) on the muscle oxygenation response to incremental exercise and its recovery kinetics. METHODS Nine subjects (age: 24.7 ± 6.3 yrs) underwent incremental (ramp) cycling exercise test. A frequency domain NIRS (OxiPlexTS, ISS) was used to measure deoxyhemoglobin (HHb), oxyhemoglobin (HbO2), total hemoglobin concentration (THb) and tissue O2 saturation (StO2) a) incorporating dynamic measurements of μ's, and b) assuming constant μ's (based on the mean baseline value of the population studied). The changes (Δ) in NIRS variables were calculated as peak exercise minus basel ine. The recovery kinetics of HHb, HbO2 and StO2 were determined by nonlinear regression. RESULTS Small changes were observed for μ's at wavelength (λ) 830 nm (μ's = −0.01 ± 0.25 cm−1). For λ = 690 nm μ's remained relatively constant throughout exercise and recovery in 4 subjects (μ's = −0. 02 ± 0.19 cm−1), but changed significantly in 5 subjects (μ's= 1.0 ± 0.14 cm−1). For these 5 subjects, assuming constant μ's resulted in significantly greater changes (P <0.05) in the NIRS variables with exercise compared to the responses when μ's was measured (ΔHHb = 18.3 ± 1.1 vs 28.0 ± 2.5 μM; ΔHbO2 = −5.0 ± 9.1 vs −10.6 ± 9.9 μM; ΔTHb = 13.3 ± 8.8 vs 17.4 ± 9.5 μM; ΔStO2 = −12.6 ± 2.9 vs −19.8 ± 3.6 %; for measured μ's vs constant μ's, respectively). For all 9 subjects, the results of recovery kinetics were more complex. For HHb, the time delay (TD) and time constant (τ) of the initial (first ∼ 60 s) response were not modified appreciably by assuming constant μ's; however, the overall kinetics (360 s) described by the mean response time (MRT) were remarkably different in 6 subjects. Regarding HbO2 and StO2, the initial TD and τ were systematically different but the MRT was relatively similar for measured and constant μ's, although widely variable when individual data were considered. CONCLUSION In 5 out of 9 subjects μ's of λ = 690 nm changed appreciably during incremental exercise and recovery. The changes in HHb, HbO2 and StO2 with exercise were overestimated, and their recovery kinetics distorted when μ's was assumed constant. These results show that the assumption of constant μ's during exercise is not valid and can lead to erroneous conclusions regarding dynamic changes in muscle oxygenation. Supported in part by AHA Grant-in-Aid 0151183Z