"ACSM guidelines recommend exercise intensity at 55–90% of maximum heart rate. For Mariella, whose estimated maximum heart rate is 190 bpm, the lower target is 190 × 0.55 = 104.5 bpm, rounded up to 105 bpm. The upper target is 190 × 0.90 = 171 bpm. This formula gives us an idea of maximum heart rate, but we must remember that it is just an estimate and not completely accurate. Therefore, according to ACSM guidelines, Mariella should exercise at a heart rate somewhere between 105 and 171 bpm. This is quite a wide range, so depending on her fitness levels, you would need to decide whether to prescribe exercise towards the upper or lower end of this scale. Online calculators are available to calculate this information."