Large skeletal muscles, critical to athletic performance in both endurance and speed-and-power sports, require adequate oxygenation otherwise they are forced limited in performance by the reduced energy production efficiency and undesired metabolites (e.g., lactate).
Until recently athletic monitoring had been limited to tracking heart rate . Heart rate is useful in determining the total oxygen delivery ability of the circulatory system of an individual, but is unable to determine the oxygen consumption and, more importantly, the balance between oxygen delivery and consumption in localized muscles that are of critical importance for a given athletic activity. That is, good aerobic performance alone is no guarantee of adequate oxygen delivery nor of good conditioning of the muscle that is being exercised in a given activity, and SmO2 monitoring allows athletes to bridge that information gap and monitor the oxygen delivery/consumption balance of a given muscle in real-time.
Hemoglobin is the main oxygen-carrying protein in blood and it exists in two states: oxygenated hemoglobin (HbO2), in which each hemoglobin molecule carries up to four O2 molecules, and deoxygenated hemoglobin (HHb), in which state the hemoglobin molecule has been depleted of all of its oxygen molecules. SmO2 is an absolute measure of oxygen concentration in hemoglobin and it is given by
Where [HbO2] denotes the concentration of oxygenated hemoglobin and [HHb] denotes the concentration of deoxyhemoglobin. That is, SmO2 measures the oxygen-carrying hemoglobin as a fraction of total hemoglobin present in tissue.