Karvonen et al. 1957, Heart-rate reserve method

The original work behind the Karvonen method, which prescribes intensity as a percentage of heart-rate reserve (the span between resting and maximal heart rate) added to resting heart rate. Percentage of reserve tracks percentage of oxygen-uptake reserve more closely than percentage of maximal heart rate does, and yields a higher absolute target heart rate than the same percentage of maximum. Moderate (primary source paywalled; method well established).