HEART RATE RESERVE (HRR) – if heart rate measurement is available, e.g. smart watch.

HRR = (HR MAX – HR RESTING)
HR MAX = 220 – Age

Light Intensity: 30-39% HRR
Moderate Intensity: 40-59% HRR
Vigorous Intensity: ≥ 60% HRR

For example:

A 50 years old man with a good health a comes for  an exercise prescription and decided to start a
moderate intensity at 40% of his HRR with his resting HR, 78 bpm

Desired intensity = (HR max – HR resting) x % intensity (in decimal) + HR resting

40% HRR= (170 – 78 bpm) x 0.4 + 78 = 115