Heat-up tilting manoeuvre from 0 to 60 degrees induces oxygenated and deoxygenated haemoglobin concentration changes in the human gastrocnemius. These changes, measured by NIRS, can only be partially explained by the blood volume displacement due to the gravitational force. In the present study it is demonstrated, by a dye dilution technique (indocyanine green), that a reduction in blood flow (venous-arteriolar and/or spinal reflex) is responsible of the limited oxyhaemoglobin concentration increase observed when going from 0 (2.54 +/- 0.48 blood flow in arbitrary units, a.u.) to 60 (1.46 +/- 0.55 a.u.) degrees. The proposed technique is potentially applicable to the detection of specific pathological aspects of microcirculation, such as arterial occlusion in the leg, diabetes mellitus, and congestive heart failure, where the venous-arteriolar reflex may be affected.