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A tissue-optical model is presented in which changes in the blood volume fraction, fv, and tissue saturation, SO2, are calculated from non-invasively measured intensity changes at two wavelengths during ischemia. Measurements were performed during occlusion and during muscle contraction at the human forearm with a sensor containing two LEDs, (lambda) equals 660 nm and (lambda) equals 940 nm, and photodiodes at 7.0 mm, 9.5 mm, and 20 mm from the LEDs. We used diffusion theory for a homogeneous semi-infinite medium to obtain registrations of (Delta) fv and (Delta) SO2 from measured changes in the photon fraction, (Delta) I, during the experiment for each detector separately. As expected, fv stays nearly constant during occlusion, whereas SO2 decreases, for each detector. During muscle contraction we observed that the intensity changes at each detector are much smaller than during occlusion. As expected, both fv and SO2 decrease at the beginning of the contraction period, but increase before the end of the contraction period. (Delta) SO2 depends more strongly than (Delta) fv on the assumed myoglobin concentration, the scattering coefficients, the blood volume fraction and the saturation at the beginning of the session. The success of the homogeneous model in the occlusion experiment is probably caused by simultaneous deoxygenation in the muscle and in the skin. However, during muscle contraction the changes in SO2 and fv were different at each detector. The failure of the homogeneous model in that case may be explained by the deoxygenation which is expected to be larger in muscle tissue than in skin tissue.
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