"The HRR method is thought to be more accurate than the percentage of maximal heart rate method because it takes the individual’s resting heart rate into account. The ACSM recommends that to improve aerobic fitness, exercise intensity should be set at either 40–85% of heart rate reserve (HRR) or 55–90% of maximum heart rate (HR max). These ranges are deliberately broad to reflect different levels of fitness. Someone with relatively low levels of fitness who has just started an exercise programme may need to work on the lower end of the scale, whereas someone who has a higher level of fitness may need to work at an intensity towards the upper end of the scale. This demonstrates the importance of progression in an exercise programme. In Mariella’s case, her estimated maximum heart rate is 220 − 30 = 190 bpm. Her resting heart rate is measured at 70 bpm, ideally first thing in the morning, using a heart rate monitor or manually. Her heart rate reserve is 190 − 70 = 120 bpm. The lower target is (120 × 0.40) + 70 = 118 bpm, and the upper target is (120 × 0.85) + 70 = 172 bpm. Using this method, according to ACSM guidelines, Mariella should exercise somewhere between 118 and 172 bpm."