Skin blood flow influences cerebral oxygenation measured by near-infrared spectroscopy during dynamic exercise

Purpose Near-infrared spectroscopy (NIRS) is widely used to investigate cerebral oxygenation and/or neural activation during physiological conditions such as exercise. However, NIRS-determined cerebral oxygenated hemoglobin (O 2 Hb) may not necessarily correspond to intracranial blood flow during dynamic exercise. To determine the selectivity of NIRS to assess cerebral oxygenation and neural activation during exercise, we examined the influence of changes in forehead skin blood flow (SkBF head ) on NIRS signals during dynamic exercise. Methods In ten healthy men (age: 20 ± 1 years), middle cerebral artery blood flow velocity (MCA V mean , via transcranial Doppler ultrasonography), SkBF head (via laser Doppler flowmetry), and cerebral O 2 Hb (via NIRS) were continuously measured. Each subject performed 60 % maximum heart rate moderate-intensity steady-state cycling exercise. To manipulate SkBF head , facial cooling using a mist of cold water (~4 °C) was applied for 3 min during steady-state cycling. Results MCA V mean significantly increased during exercise and remained unchanged with facial cooling. O 2 Hb and SkBF head were also significantly increased during exercise; however, both of these signals were lowered with facial cooling and returned to pre-cooling values with the removal of facial cooling. The changes in O 2 Hb correlated significantly with the relative percent changes in SkBF head in each individual ( r = 0.71–0.99). Conclusions These findings suggest that during dynamic exercise NIRS-derived O 2 Hb signal can be influenced by thermoregulatory changes in SkBF head and therefore, may not be completely reflective of cerebral oxygenation or neural activation.