1. A method of determining a heart rate of a user, the method comprising, by a device: generating, using a pulse sensor of the device, a sequence of heart rate data samples; determining, using the sequence of heart rate samples, a direct heart rate estimate; determining an activity context of the wearable device, the activity context including an activity and an intensity of the activity; generating, based in part on the activity context and in part on a user-specific parameter, a modeled heart rate estimate, the modeled heart rate estimate being generated independently of the sequence of heart rate samples; and determining a final heart rate estimate based on the direct heart rate estimate and the modeled heart rate estimate.