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The Karvonen method sets targets as a percentage of heart rate reserve rather than of maximum, so it accounts for individual resting rate: target = RHR + intensity x (MHR - RHR). For a 35 year old with a 60 bpm resting rate, 60-75% intensity means 134-149 bpm - noticeably higher than taking 60-75% of maximum directly.
Karvonen target
target = RHR + intensity x (MHR - RHR)
Heart rate reserve
HRR = MHR - RHR
This is an estimate from population-average equations, not a medical or diagnostic assessment. Individual results vary with genetics, training history and technique. Seek advice from a qualified coach or clinician before making significant changes to training.
Because percentage of maximum ignores resting rate, which sets the floor of your working range. Karvonen zones align much better with measured lactate thresholds.
Typically 60-70% of heart rate reserve for aerobic base, 70-80% for tempo work and above 85% for intervals.