A Muscle Oxygen Monitor uses light to measure how much oxygen is in your muscles. It uses a method called near-infrared spectroscopy (NIRS). The sensor sends safe light through the skin into the muscle. Based on the light that comes back, the device calculates how much oxygen is being used. When you exercise, your muscles use more oxygen. The monitor shows a percentage called SmO2 (muscle oxygen saturation). A higher percentage means your muscles are getting more oxygen, while a lower one means they are working harder.