Near-infrared spectroscopy (NIRS) is intensively used for measuring changes in muscle oxygenation during dynamic and static exercises, especially in the field of exercise physiology. In general, volumetric blood flow rate is associated with significant changes in the velocity of blood flow in muscles during dynamic exercise. However, it remains unclear how measurements of hemoglobin concentration by NIRS are influenced by changes in blood flow or velocity in the muscular beds. It is important to determine this relationship during muscular contractions, because measuring oxygen consumption in the tissue is of paramount importance for understanding the relationship between muscular activity and energy expenditure and between oxygen delivery and oxygen utilization. Here, we assessed the relationship between changes in hemoglobin concentration and blood flow velocity in an in vitro NIRS experiment, which the relationship should contribute to explain the oxygen dynamics in the vascular beds and the contracting muscular tissues. We found that the changes in hemoglobin concentration that measured in this experiment were independent of the increase in blood flow velocity. Our results support those of the several studies showing that changes in deoxyhemoglobin concentration measured by NIRS reflect the changes in arteriovenous oxygen difference in muscle tissues at rest and during exercise.
