Gabe Mirkin, M.D.
You don't need to take your pulse during exercise. You can tell how fast your heart is beating by paying attention to how you breathe.
The only heart rate that should concern you is your training heart rate that makes your heart stronger. To strengthen your heart, you have to exercise vigorously enough to increase your heart rate at least 20 beats a minute beyond your resting heart rate. You can tell when your heart rate is high enough to strengthen your heart because at that point, your body will require more oxygen than it does at rest, and you will just start to breathe deeply and more rapidly. You should feel comfortable and still be able to talk. If you exercise so slowly that you never breathe more deeply or rapidly, you are not strengthening your heart, but you don't have to exercise as hard as you can to become fit.