Too low, your exercise is not intensive enough to stimulate your heart or tone up other parts of your body. Too high, that's not good to your heart either. According to the study published in a July 2010 issue of Circulation, on average, women's peak heart rate in exercise is 206 - (0.88 x age). Dr. Weil's recount of the study is more readable to non-medical-professionals. He also noted that the formula "that works for men is 220 minus age".