A wearable device to estimate an individual's maximum oxygen uptake (VO2max) during exercise. The wearable device includes a processor and a memory. The processor is configured to receive heart-rate measurement data and exercise workload data for an individual of the wearable device. The memory stores instructions that cause the processor to obtain the heart-rate and exercise workload of the individual, normalize the heart-rate with respect to the individual's maximum heart-rate to provide a data pair of normalized heart-rate (HRn) and exercise workload (w), and apply a probabilistic model that relates HRn to w and maximum oxygen uptake to provide an estimate of maximum oxygen uptake (VO2max) of the individual.