Heart Rate Reserve (HRR) represents the functional range of your heart, measured as the difference between your highest and lowest achievable heart rates. This measurement is considered the most accurate method for establishing personalized exercise intensity because it accounts for your unique fitness level. Unlike simple age-based estimates, HRR recognizes that two people of the same age can have very different training capacities. Understanding and applying this specific number allows you to optimize your workouts for maximum efficiency and safety.
Determining Maximum and Resting Heart Rates
The first step is establishing your Maximum Heart Rate (HRmax), the highest number of beats your heart can achieve per minute during strenuous physical activity. The most precise way to find this number is through a medically supervised exercise stress test. For the general public, the most common estimation formula is subtracting your age from 220 (HRmax = 220 – Age). Note that this age-predicted formula is only an estimate and can have a margin of error of 10 to 12 beats per minute.
The second number needed is your Resting Heart Rate (RHR), which reflects the number of times your heart beats per minute when your body is completely at rest. The most accurate way to measure RHR manually is to take the measurement first thing in the morning, immediately after waking up and before getting out of bed. Place your index and middle fingers on the radial artery in your wrist or the carotid artery on the side of your neck. Count the number of beats felt over 15 seconds, and then multiply that number by four to calculate your beats per minute.
To establish a reliable baseline, repeat this measurement over several consecutive mornings and calculate the average. Since stress, caffeine, or recent exercise can elevate this number, ensure you are in a fully rested state for an accurate reading.
Calculating Heart Rate Reserve
Calculating the Heart Rate Reserve (HRR) involves a straightforward subtraction once both the Maximum Heart Rate (HRmax) and Resting Heart Rate (RHR) have been determined. The formula is simply HRmax minus RHR (HRR = HRmax – RHR). This result is the range of heartbeats available for your body to use between a state of rest and maximum effort.
For example, a 40-year-old individual would have an estimated HRmax of 180 beats per minute (220 – 40). If this person’s accurately measured RHR is 60 beats per minute, their HRR would be 120 beats per minute (180 – 60).
Using HRR to Define Training Zones
HRR is applied using the Karvonen formula, a method that incorporates the individual’s fitness level via the RHR. The Karvonen formula calculates a specific Target Heart Rate (THR) for any given intensity percentage. The calculation is: THR = ((HRmax – RHR) \(\times\) Intensity %) + RHR.
Training zones are typically broken down by intensity percentage, each corresponding to a different physiological adaptation. A moderate-intensity aerobic zone, for instance, often falls between 60% and 70% of the HRR, improving endurance. Training at a higher intensity, such as 80% to 90%, targets the anaerobic zone, which helps improve speed and lactate buffering capacity.
To find the target heart rate for a 70% intensity workout using the previous example (HRR of 120 and RHR of 60), the calculation is performed in two steps. First, multiply the HRR by the intensity percentage: 120 beats per minute \(\times\) 0.70 equals 84 beats per minute. Second, add the RHR back to this result: 84 beats per minute + 60 beats per minute equals a THR of 144 beats per minute. This final number, 144 beats per minute, is the precise heart rate the individual should aim for to achieve a 70% intensity level.