Final answer:
The onset of sweating is not a useful method for prescribing exercise intensity, unlike %HRR, %VO2R, and RPE which are standardized methods allowing for individualized exercise intensity prescriptions.
Step-by-step explanation:
The method that is not useful for prescribing the recommended intensity of exercise training is the onset of sweating. This is because the onset of sweating is not a standardized method and can vary greatly from person to person based on individual factors such as environmental conditions, personal fitness level, and hydration status.
On the other hand, %HRR (Percentage of Heart Rate Reserve), %VO2R (Percentage of VO2 Reserve), and RPE (Rating of Perceived Exertion) are recognized methods for determining exercise intensity. RPE, in particular, allows individuals to assess their effort based on how they feel, which can be more accurate than heart rate measurements that can be influenced by external factors.