Both will be in their THR zone because the heart rates will be 153 when decreased by 20 bpm. When comparing this to the THR zones, that falls within both of their zones.
The right answer is both of them since 153bpm (173 - 20bpm) is included in both intervals (140-177bpm and 148-181bpm)
In general, your maximum heart rate is about 220 minus your age. The maximum heart rate is the upper limit your cardiovascular system can cope with during a physical activity.
If the exercise is vigorous, target a heart rate of 70 to 85 percent of the maximum heart rate.
These figures are only estimates intended to give an average value. Many people have a higher or lower heart rate, sometimes 15 to 20 beats per minute. It is not easy to estimate accurately the maximum heart rate and the average value that should be targeted.
