The Karvonen Method
The standard method of calculating target heart rates (just taking a percentage of your Max HR) has a major flaw: it completely ignores your current fitness level.
The Karvonen Method solves this by introducing your Resting Heart Rate (RHR) into the equation. Your resting heart rate is a massive indicator of cardiovascular fitness; elite endurance athletes often have resting heart rates in the 40s, while untrained individuals sit in the 70s or 80s.
Calculating Heart Rate Reserve
The Karvonen formula calculates your Heart Rate Reserve (HRR), which is the exact difference between your absolute maximum heart rate and your resting heart rate. This represents the actual "working range" of your heart.
The Formula
Target HR = [(Max HR - Resting HR) * Intensity%] + Resting HR
Why It's Superior
If two 30-year-olds want to train at 70% intensity, the standard formula gives them the exact same target (133 bpm). But if one is an elite marathoner with a resting HR of 45, and the other is untrained with a resting HR of 80, 133 bpm feels completely different to each of them.
The Karvonen method personalizes the target. The marathoner will get a target of 146 bpm, forcing them to work harder to achieve true 70% intensity, while the untrained individual will get a target of 143 bpm, matching their physiological reality.