How is Heart Rate Reserve Calculated

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Understanding your cardiovascular fitness requires more than just knowing your resting pulse. One of the most effective metrics for determining exercise intensity is the Heart Rate Reserve (HRR). Unlike simple maximum heart rate calculations, HRR takes into account your resting heart rate, providing a personalized range that reflects your actual fitness level.

Before diving into the comprehensive guide on the physiology and application of HRR, use the calculator below to determine your specific numbers and training zones.

Heart Rate Reserve Calculator

Your Maximum Heart Rate (MHR): 0 bpm

Your Heart Rate Reserve (HRR): bpm

Target Heart Rate Training Zones (Karvonen Formula)

Target HR = ((HRR × Intensity%) + Resting HR)

Zone	Intensity	Target Range (BPM)	Benefit

function calculateHRR() {
// 1. Get Elements
var ageInput = document.getElementById(‘calc-age’);
var rhrInput = document.getElementById(‘calc-rhr’);
var mhrInput = document.getElementById(‘calc-mhr’);
var errorDiv = document.getElementById(‘error-message’);
var resultArea = document.getElementById(‘result-area’);
var resMhrSpan = document.getElementById(‘res-mhr’);
var resHrrSpan = document.getElementById(‘res-hrr’);
var zonesBody = document.getElementById(‘zones-body’);
// 2. Parse Values
var age = parseFloat(ageInput.value);
var rhr = parseFloat(rhrInput.value);
var mhrCustom = parseFloat(mhrInput.value);
// 3. Reset UI
errorDiv.style.display = ‘none’;
resultArea.style.display = ‘none’;
zonesBody.innerHTML = ”;
// 4. Validation Logic
if (isNaN(rhr)) {
errorDiv.innerHTML = “Please enter your Resting Heart Rate.”;
errorDiv.style.display = ‘block’;
return;
}
var mhr = 0;
// Determine Max Heart Rate (MHR)
if (!isNaN(mhrCustom)) {
mhr = mhrCustom;
} else {
if (isNaN(age)) {
errorDiv.innerHTML = “Please enter your Age or a custom Maximum Heart Rate.”;
errorDiv.style.display = ‘block’;
return;
}
// Standard formula: 220 – age
mhr = 220 – age;
}
// Logic Check: RHR cannot be higher than MHR
if (rhr >= mhr) {
errorDiv.innerHTML = “Your Resting Heart Rate cannot be higher than or equal to your Maximum Heart Rate. Please check your numbers.”;
errorDiv.style.display = ‘block’;
return;
}
// 5. Calculate Heart Rate Reserve (HRR)
// Formula: HRR = MHR – RHR
var hrr = mhr – rhr;
// 6. Display Main Results
resMhrSpan.innerText = Math.round(mhr);
resHrrSpan.innerText = Math.round(hrr);
// 7. Calculate Karvonen Zones
// Formula: Target HR = (HRR * intensity) + RHR
var zones = [
{ name: “Zone 1 (Warm Up)”, min: 0.50, max: 0.60, benefit: “Recovery, basic health” },
{ name: “Zone 2 (Fat Burn)”, min: 0.60, max: 0.70, benefit: “Endurance, fat metabolism” },
{ name: “Zone 3 (Aerobic)”, min: 0.70, max: 0.80, benefit: “Aerobic capacity, blood circulation” },
{ name: “Zone 4 (Anaerobic)”, min: 0.80, max: 0.90, benefit: “Speed endurance, high intensity” },
{ name: “Zone 5 (VO2 Max)”, min: 0.90, max: 1.00, benefit: “Maximum effort, sprint speed” }
];
var htmlRows = “”;
for (var i = 0; i < zones.length; i++) {
var z = zones[i];
var minBpm = Math.round((hrr * z.min) + rhr);
var maxBpm = Math.round((hrr * z.max) + rhr);
htmlRows += "

“;
htmlRows += “

“;
htmlRows += “

“;
htmlRows += “

“;
htmlRows += “

“;
}
zonesBody.innerHTML = htmlRows;
resultArea.style.display = ‘block’;
}

The Heart Rate Reserve Formula

The calculation for Heart Rate Reserve is straightforward mathematically but significant physiologically. It is defined by the following formula:

HRR = Maximum Heart Rate (MHR) – Resting Heart Rate (RHR)

For example, if a 40-year-old athlete has a Maximum Heart Rate of 180 bpm and a Resting Heart Rate of 60 bpm, their Heart Rate Reserve is 120 bpm (180 – 60).

Why HRR is More Accurate than Standard MHR

Most basic fitness trackers use a simple percentage of your Maximum Heart Rate (e.g., 220 minus age) to calculate zones. However, this method assumes everyone has the same starting point (0 bpm), which is obviously incorrect.

Heart Rate Reserve is superior because it incorporates your Resting Heart Rate, which is a key indicator of cardiovascular efficiency. A lower RHR typically indicates a stronger heart and higher stroke volume. By factoring this in via the Karvonen Formula, training zones become personalized. Two people of the same age might have the same MHR, but if one has an RHR of 50 and the other 80, their training intensities should differ significantly.

How to Obtain Accurate Inputs

1. Determining Maximum Heart Rate (MHR)

• Age-Predicted: The most common formula is 220 - Age. While widely used, it can vary by +/- 10-15 beats per minute.
• Tanaka Formula: A slightly more accurate estimation for older adults is 208 - (0.7 × Age).
• Field Test: Under medical supervision, perform a graded exercise test (like running up a hill repeatedly) until failure to find your true max.

2. Measuring Resting Heart Rate (RHR)

To get an accurate RHR, take your pulse immediately after waking up in the morning, before sitting up or drinking coffee. Count the beats for 60 seconds. Do this for 3-5 days and calculate the average.

Using HRR for the Karvonen Method

Once you have calculated your Heart Rate Reserve, you apply it to find your specific training zones using the Karvonen Method:

Target HR = (HRR × Intensity %) + RHR

Using this method ensures that when you are training at “70% intensity,” you are training at 70% of the capacity available to you above your resting baseline, rather than 70% of an arbitrary maximum.

Interpreting Your Results

• Higher HRR: Generally indicates better fitness. As you get fitter, your RHR drops, increasing the gap (reserve) between your resting and maximum rates.
• Lower HRR: May indicate detraining, fatigue, or aging (since MHR drops with age).

Monitoring changes in your Heart Rate Reserve over time is an excellent way to track cardiovascular improvements without needing expensive laboratory equipment.