The exercise tracking is a bit weird. I just got my Watch on Thursday. Friday I took it out for a 40 minute walk using the Workout app, because it told me I should do at least 20 minutes with the Watch and the iPhone, presumably to calibrate something. My heart rate was as high as the 140s and at least 100 for almost all that time. I got credit for only a few minutes of exercise. Sunday I wore it at the gym lifting weights for 35 minutes, using the Workout app set on Other. HR was lower, but the whole workout counted as exercise. I was happy that it did, but still not sure why the walk didn't count.