such as olympic rowers in their 20s, maximum heart rates have been reported as varying from 160 to 220. such a variation would equate to a 60 or 90 year age gap in the linear equations above, and would seem to indicate the extreme variation about these average figures. figures are generally considered averages, and depend greatly on individual physiology and fitness. for example, an endurance runner's rates will typically be lower due to the increased size of the heart required to support the exercise, while a sprinter's rates will be higher due to the improved response time and short duration. while each may have predicted heart rates of 180 (= 220 − age), these two people could have actual hrmax 20 beats apart (e.g., 170–190). further, note that individuals of the same age, the same training, in the same sport, on the same team, can have actual hrmax 60 bpm apart (160–220): the range is extremely broad, and some say "the heart rate is probably the least important variable in comparing athletes." heart rate reserve heart rate reserve (hrreserve) is the difference between a person's measured or predicted maximum heart rate and resting heart rate. some methods of measurement of exercise intensity measure percentage of heart rate reserve. additionally, as a person increases their cardiovascular fitness, their hrrest will drop, and the heart rate reserve will increase. percentage of hrreserve is statistically indistinguishable from percentage of vo2 reserve. hrreserve = hrmax − hrrest this is often used to gauge exercise intensity (first used in 1957 by karvonen). karvonen's study findings have been questioned, due to the following: the study did not use vo2 data to develop the equation. only six subjects were used. karvonen incorrectly reported that the percentages of hrreserve and vo2 max correspond to each other, but newer evidence shows that it correlated much better with vo2 reserve as described above. target heart rate for healthy people, the target heart rate (thr) or training heart rate range (thrr) is a desired range of heart rate reached during aerobic exercise which enables one's heart and lungs to receive the most benefit from a workout. this theoretical range varies based mostly on age; however, a person's physical condition, sex, and previous training also are used in the calculation. by percent, fox–haskell-based the thr can be calculated as a range of 65–85% intensity, with intensity defined simply as percentage of hrmax. however, it is crucial to derive an accurate hrmax to ensure these calculations are meaningful. example for someone with a hrmax of 180 (age 40, estimating hrmax as 220 − age): 65% intensity: (220 − (age = 40)) × 0.65 → 117 bpm 85% intensity: (220 − (age = 40)) × 0.85 → 154 bpm karvonen method the karvonen method factors in resting heart rate (hrrest) to calculate target heart rate (thr), using a range of 50–85% intensity: thr = ((hrmax − hrrest) × % intensity) + hrrest equivalently, thr = (hrreserve × % intensity) + hrrest example for someone with a hrmax of 180 and a hrrest of 70 (and therefore a hrreserve of 110): 50% intensity: ((180 − 70) × 0.50) + 70 = 125 bpm 85% intensity: ((180 − 70) × 0.85) + 70 = 163 bpm zoladz method an alternative to the karvonen method is the zoladz method, which is used to test an athlete's capabilities at specific heart rates. these are not intended to be used as exercise zones, although they are often used as such. the zoladz test zones are derived by subtracting values from hrmax: thr = hrmax − adjuster ± 5 bpm zone 1 adjuster = 50 bpm zone 2 adjuster = 40 bpm zone 3 adjuster = 30 bpm zone 4 adjuster = 20 bpm zone 5 adjuster = 10 bpm example for someone with a hrmax of 180: zone 1(easy exercise): 180 − 50 ± 5 → 125 − 135 bpm zone 4(tough exercise): 180 − 20 ± 5 → 155 − 165 bpm heart rate recovery heart rate recovery (hrr) is the reduction in heart rate at peak exercise and the rate as measured after a cool-down period of fixed duration. a greater reduction in heart rate after exercise during the reference period is associated with a higher level of cardiac fitness. heart rates assessed during treadmill stress test that do not drop by more than 12 bpm one minute after stopping exercise (if cool-down period after exercise) or by more than 18 bpm one minute after stopping exercise (if no cool-down period and supine position as soon as possible) are associated with an increased risk of death. people with an abnormal hrr defined as a decrease of 42 beats per minutes or less at two minutes post-exercise had a mortality rate 2.5 times greater than patients with a normal recovery. another study reported a four-fold increase in mortality in subjects with an abnormal hrr defined as ≤12 bpm reduction one minute after the cessation of exercise. a study reported that a hrr of ≤22 bpm after two minutes "best identified high-risk patients". they also found that while hrr had significant prognostic value it had no diagnostic value. development