Blood oxygen monitors or Pulse Oximetry are features that help runners know the level of oxygen their blood receives. Many running watches companies have claimed that their watches with these features are not intended for medical use, however, this could be helpful for the overall fitness of a runner or an athlete.  As stated before, trail running sometimes requires you to run slower or yield more physical effort to surpass hills or uneven terrains. Areas with higher elevations have a lower concentration of oxygen. When there is a decrease in the oxygen level, your heart rate will increase, resulting in a running out of breath feeling. Pulse Oximeters in trail running watches can alert runners when they are getting lower values and guide them into going back to the normal values.