Physiologic sensors and methods of application are described. These sensors function by detecting recently discovered variations in the spectral optical density at two or more wavelengths of light diffused through the skin. These variations in spectral optical density have been found to consistently and uniquely relate to changes in the availability of oxygen in the skin tissue, relative to the skin tissue's current need for oxygen, which we have termed Physiology Index (PI). Current use of blood gas analysis and pulse oximetry provides physiologic insight only to blood oxygen content and cannot detect the status of energy conversion metabolism at the tissue level. By contrast, the PI signal uniquely portrays when the skin tissue is receiving ‘；less than enough oxygen,’； just the right amount of oxygen,′； or ‘；more than enough oxygen’； to enable aerobic energy conversion metabolism. The PI sensor detects one pattern of photonic response to insufficient skin tissue oxygen, or tissue hypoxia, (producing negative PI values) and a directly opposite photonic response to excess tissue oxygen, or tissue hyperoxia, (producing positive PI values), with a neutral zone in between (centered at PI zero). Additionally, unique patterns of PI signal response have been observed relative to the level of physical exertion, typically with a secondary positive-going response trend in the PI values that appears to correspond with increasing fatigue. The PI sensor illuminates the skin with alternating pulses of selected wavelengths of red and infrared LED light, then detects the respective amount of light that has diffused through the skin to an aperture located a lateral distance from the light source aperture. Additional structural features include means of internally excluding light from directly traveling from the light emitters to the photodetector within the sensor. This physiology sensor and methods of use offer continuous, previously unavailable infor
