BSXInsight measures the balance between oxygen delivery and consumption in a given muscle or tissue directly underneath the sensor location – see the FAQ page What is muscle oxygenation? for a more detailed explanation on muscle oxygenation (SmO2). SmO2 is an absolute measure of the fraction of oxygenated hemoglobin [HbO2] present in muscle, and it is given by
Where [HHb] denotes the concentration of deoxygenated hemoglobin. SmO2 differs from arterial oxygen concentration (SpO2), which measures the concentration of oxygenated hemoglobin delivered to the whole body, not specifically oxygen concentration of a given muscle immediately below the device, and therein lies an important distinction between the two: while SpO2 measures a systemic parameter that is not expected to vary by much, no matter where in the body of a user it is measured, SmO2 is highly specific not only to the muscle in which it is applied but also to the location within that muscle. Moreover, the depth and volume of the muscle being monitored is dependent on the device geometry.
Figure 1 explains conceptually the operation of the BSXInsight, which uses a non-invasive tissue monitoring technique called spatially resolved near infrared spectroscopy [1,2] to measure tissue oxygenation. In the BSXInsight light propagates into tissue from light emitting diodes LED1 and LED2 located in two different geometric locations separated by distances d1 and d2 from a single photodetector (PD).
Figure 1: Diagram showing light propagation from BSXInsight device into tissue being monitored. PD – photodetector. LED1 – light emitting diode 1. d1 – distance from LED1 to PD.
Multiple LED/PD pairs allow the BSXInsight to use BSX’s proprietary algorithms to monitor tissue oxygenation accurately and in real-time. The depth of tissue being monitored depends on the composition of the layers of tissue lying below the device, the wavelengths of light used, and the PD/LED distances. Nevertheless, for near infrared light propagating through typical layers of tissue that depth usually ranges from one half to one third the distance between LEDs and the photodetector and, in the BSXInsight, it ranges from approximately 5mm to 13.5mm.
BSXInsight algorithms have been optimized for highest accuracy when used over the calf muscle, using its included sleeve. Nevertheless, it can be effectively applied to other tissue as long as proper care is taken to assure that the device is firmly affixed and ambient light is effectively blocked. See the FAQ page Ensuring SmO2 reading consistency for more details.
If you're looking for accuracy testing, go to: BSXinsight muscle oxygen monitoring accuracy
 M. Ferrari, L. Mottola, and V. Quaresima, “Principles, techniques, and limitations of near infrared spectroscopy.,” Can. J. Appl. Physiol., vol. 29, no. 4, pp. 463–487, 2004.
 P. Rolfe, “In vivo near-infrared spectroscopy.,” Annu. Rev. Biomed. Eng., vol. 2, pp. 715–754, 2000.