After our 2 July Kickstarter update, in which we announced that the first version of Stryd will support heart rate in addition to power, a few people asked whether the power number is based on heart rate. The answer is "no". Stryd determines power independently of heart rate, based on your body's motion and the slope of the ground. That means the power number responds immediately to changes in training intensity and doesn't depend on temperature or humidity. We will be sharing more details on this in upcoming posts.