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 information relating to tissue-level energy conversion 
metabolism. Several alternative embodiments are described, including those that would be useful in 
medical care, athletics, and personal 
health maintenance applications.