Methods and systems are disclosed for determining an anaerobic threshold and/or an oxygen consumption rate in a human or animal subject. The methods include exposing a tissue of the subject to illumination radiation, collecting emitted radiation from the tissue, the emitted radiation including a portion of the illumination radiation reflected or transmitted from the tissue, processing the emitted radiation to form a spectrum of the tissue, and determining, based on the spectrum of the tissue, the anaerobic threshold and/or the oxygen consumption rate of the subject.
