From there you figure different heart zones to work
in. 70-80% they say is where you achieve good aerobic
fitness. I take medication to slow my heart, as I have been diagnosed with inappropriate sinus tachycardia. However, I am still able to achieve HR in 70-90% of my target heart rate. However, I only spend a little time in upper ranges. I base my workouts on how my BP is doing. If i do overexert, too much time at upper ends, it can drive my HR up.