function calculateHRR() {
var age = parseFloat(document.getElementById('userAge').value);
var rhr = parseFloat(document.getElementById('restingHR').value);
var intensity = parseFloat(document.getElementById('intensityLevel').value) / 100;
if (isNaN(age) || isNaN(rhr) || age <= 0 || rhr <= 0) {
alert("Please enter valid positive numbers for Age and Resting Heart Rate.");
return;
}
// Standard Max HR formula: 220 – Age
var mhr = 220 – age;
// Heart Rate Reserve (HRR) = Max HR – Resting HR
var hrr = mhr – rhr;
// Karvonen Formula: Target HR = (HRR * Intensity) + Resting HR
var thr = Math.round((hrr * intensity) + rhr);
document.getElementById('resMaxHR').innerText = mhr;
document.getElementById('resHRR').innerText = hrr;
document.getElementById('resTHR').innerText = thr + " BPM";
var zoneMsg = "";
if (intensity < 0.6) zoneMsg = "This is a Light Intensity zone (Recovery/Warm-up). Great for basic health and metabolism.";
else if (intensity < 0.7) zoneMsg = "This is the Weight Management zone (Fat Burning). Ideal for building aerobic endurance.";
else if (intensity < 0.8) zoneMsg = "This is the Aerobic Zone (Cardio). Improves cardiovascular fitness and lung capacity.";
else if (intensity < 0.9) zoneMsg = "This is the Anaerobic Zone (Hard). Increases high-intensity performance and speed.";
else zoneMsg = "This is the Red Line Zone (Maximum). For short bursts of effort during interval training.";
document.getElementById('zoneInfo').innerHTML = zoneMsg;
document.getElementById('hrr-results').style.display = 'block';
}
What is Heart Rate Reserve (HRR)?
Heart Rate Reserve (HRR) is the difference between your measured maximum heart rate and your resting heart rate. In exercise physiology, heart rate reserve is used to calculate the Karvonen Formula, which provides a more personalized target heart rate range for aerobic training than the simple "220 minus age" method alone.
Suppose you are 40 years old with a resting heart rate of 60 BPM and you want to exercise at 70% intensity:
MHR: 220 – 40 = 180 BPM
HRR: 180 – 60 = 120 BPM
Target HR: (120 × 0.70) + 60 = 144 BPM
Why Use HRR Instead of Just MHR?
The primary advantage of using heart rate reserve is that it accounts for an individual's actual fitness level by incorporating the resting heart rate. A person with a lower resting heart rate (often a sign of higher cardiovascular fitness) will have a larger reserve, allowing for more precise exercise intensity adjustments compared to methods that only consider age.