Systems, methods, and devices are provided for determining exertion as a measure of accumulated exercise intensity. One such system includes a wearable device comprising a biosensor that periodically measures biometrics (e.g. heart rate); a processor operatively coupled to the biosensor, the processor configured to process electronic signals periodically generated by the biosensor; and a non-transitory computer-readable medium operatively coupled to the processor and storing instructions that, when executed, cause the processor to execute specific functions. In particular, the instructions are executed to cause the processor to generate biometric data from the biometrics, to generate a plurality of exercise intensity values based on the biometric data (e.g. a plurality of heart rate values), to aggregate the plurality of exercise intensity values over a particular measuring period, to weight the individual exercise intensity values, and to generate one or more of an exertion value, an exertion index, and an exertion load based on the aggregation.