Decreased prefrontal oxygenation elicited by stimulation of limb mechanosensitive afferents during cycling exercise

Our laboratory reported using near-infrared spectroscopy that feedback from limb mechanoafferents may decrease prefrontal oxygenated-hemoglobin concentration (Oxy-Hb) during the late period of voluntary and passive cycling. To test the hypothesis that the decreased Oxy-Hb of the prefrontal cortex would be augmented depending on the extent of limb mechanoafferent input, the prefrontal Oxy-Hb response was measured during motor-driven one- and two-legged passive cycling for 1 min at various revolutions of pedal movement in 19 subjects. Furthermore, we examined whether calculated tissue oxygenation index (TOI) decreased during passive cycling as the Oxy-Hb did, simultaneously assessing blood flows of extracranial cutaneous tissue and the common and internal carotid arteries (CCA and ICA) with laser and ultrasound Doppler flowmetry. Minute ventilation and cardiac output increased and peripheral resistance decreased during passive cycling, depending on both revolutions of pedal movement and number of limbs, whereas mean arterial blood pressure did not change. Passive cycling did not change end-tidal CO₂, suggesting absence of a hypocapnic change. Prefrontal Oxy-Hb decreased during passive cycling, being in proportion to revolution of pedal movement but not number of cycling limbs. In addition, prefrontal TOI decreased during passive cycling as Oxy-Hb did, whereas blood flows of forehead cutaneous tissue, CCA, and ICA did not change significantly. Thus, a decrease in Oxy-Hb reflected a decrease in tissue blood flow of the intracerebral vasculature but not the extracerebral compartment. It is likely that feedback from mechanoafferents decreased regional cerebral blood flow of the prefrontal cortex in relation to the revolutions of pedal movement.