A method of determining a fitness level of user with a physiological sensor. The method includes measuring a physiological value of the user with the physiological sensor, correlating the measured physiological value into a measurement of the user's respiratory rate and tidal volume, calculating a second respiratory rate value using the measured tidal volume, calculating a breathing efficiency (BE) ratio based on a comparison of the user's measured respiratory rate and the calculated second respiratory rate value, correlating the calculated BE ratio to a predetermined threshold, and assigning a classification to the user based on the calculated BE ratio. The classification is indicative of the user's respiratory function performance.