Some embodiments described herein relate to receiving lactate concentration data from a non-invasive sensor. A ratio of aerobic to anaerobic exertion can be determined based on the concentration of lactate, and real-time feedback can be provided based on the ratio of aerobic to anaerobic exertion. For example, the user can be encouraged to maintain a level of intensity in which he or she approaches (but does not exceed) maximal oxygen consumption.
