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

¹Departments of Kinesiology and Anatomy and Physiology, Kansas State University, Manhattan, Kansas; and ²ISS Inc., Champaign, Illinois

Submitted 28 July 2005 ; accepted in final form 25 September 2006

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 [deoxy(Hb+Mb)] (15.0 ± 7.8 µM), [oxy(Hb+Mb)] (–4.8 ± 5.8 µM), Total[Hb+Mb] (10.9 ± 8.4 µM), and Sti(–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 [deoxy(Hb+Mb)] = 24.5 ± 15.6 µM, [oxy(Hb+Mb)] = –9.7 ± 8.2 µM, Total[Hb+Mb] = 14.8 ± 8.7 µM, and Sti= –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.

tissue oxygen saturation; incremental exercise; kinetics; recovery

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