A method of determining an anaerobic threshold and an oxygen consumption rate of a human or animal subject, the method comprising: exposing a tissue of the subject to illumination radiation; collecting emitted radiation from the tissue, by means of a spectrometer the emitted radiation comprising a portion of the illumination radiation reflected or transmitted from the tissue; processing the emitted radiation by means of the spectrometer to form spectra of the tissue; and determining, by means of an electronic processor, based on the spectra of the tissue, the anaerobic threshold, and the oxygen consumption rate of the subject, wherein determining the anaerobic threshold is based on a plurality of pH values of the tissue determined at least twice during a period of physical exercise, wherein the pH values are determined at a frequency of 0.1 Hz or more, wherein each of the plurality of pH values is obtained by measuring a spectrum of the tissue and determining a pH value from the spectrum, wherein determining the anaerobic threshold based on the plurality of pH values comprises determining a plurality of hydrogen ion concentration values of the tissue from the plurality of pH values, fitting more than one of the plurality of hydrogen ion concentration values to a mathematical equation to determine parameters of the equation, and determining the anaerobic threshold from the mathematical equation, characterised in that the mathematical equation is of the form y = y 1 + s 1 x − x 0, x < x 0 y 2 + s 2 x − x 0, x > x 0 where x is the rate of oxygen consumption, x 0 is an adjustable parameter that corresponds to the anaerobic threshold, y is the hydrogen ion concentration, and y 1, y 2, s 1, and s 2 are adjustable parameters; and wherein the method further comprises determining the heart rate of the subject, the arterial oxygen saturation of the subject, the tissue oxygen saturation in the tissue and the hemoglobin concentration in the tissue based on the spectra of the tissue, and wherein determining the rate of oxygen consumption based on the spectra of the tissue comprises determining a first difference between arterial oxygen saturation and tissue oxygen saturation in the tissue, determining a difference between arterial and venous oxygen content using the hemoglobin concentration and the first difference, and determining a stroke volume of the subject based on the heart rate of the subject, wherein the stroke volume is determined less frequently than the rate of oxygen consumption is determined. exposing a tissue of the subject to illumination radiation; collecting emitted radiation from the tissue, by means of a spectrometer the emitted radiation comprising a portion of the illumination radiation reflected or transmitted from the tissue; processing the emitted radiation by means of the spectrometer to form spectra of the tissue; and determining, by means of an electronic processor, based on the spectra of the tissue, the anaerobic threshold, and the oxygen consumption rate of the subject, wherein determining the anaerobic threshold is based on a plurality of pH values of the tissue determined at least twice during a period of physical exercise, wherein the pH values are determined at a frequency of 0.1 Hz or more, wherein each of the plurality of pH values is obtained by measuring a spectrum of the tissue and determining a pH value from the spectrum, wherein determining the anaerobic threshold based on the plurality of pH values comprises determining a plurality of hydrogen ion concentration values of the tissue from the plurality of pH values, fitting more than one of the plurality of hydrogen ion concentration values to a mathematical equation to determine parameters of the equation, and determining the anaerobic threshold from the mathematical equation, characterised in that the mathematical equation is of the form y = y 1 + s 1 x − x 0, x < x 0 y 2 + s 2 x − x 0, x > x 0 determining a plurality of hydrogen ion concentration values of the tissue from the plurality of pH values, fitting more than one of the plurality of hydrogen ion concentration values to a mathematical equation to determine parameters of the equation, and determining the anaerobic threshold from the mathematical equation, characterised in that the mathematical equation is of the form y = y 1 + s 1 x − x 0, x < x 0 y 2 + s 2 x − x 0, x > x 0 where x is the rate of oxygen consumption, x 0 is an adjustable parameter that corresponds to the anaerobic threshold, y is the hydrogen ion concentration, and y 1, y 2, s 1, and s 2 are adjustable parameters; and wherein the method further comprises determining the heart rate of the subject, the arterial oxygen saturation of the subject, the tissue oxygen saturation in the tissue and the hemoglobin concentration in the tissue based on the spectra of the tissue, and wherein determining the rate of oxygen consumption based on the spectra of the tissue comprises determining a first difference between arterial oxygen saturation and tissue oxygen saturation in the tissue, determining a difference between arterial and venous oxygen content using the hemoglobin concentration and the first difference, and determining a stroke volume of the subject based on the heart rate of the subject, wherein the stroke volume is determined less frequently than the rate of oxygen consumption is determined.