The MAX heart rate formula used is the following:
MAX HR = 210 - (0.5 * age)
It does sometimes yield an aggressive (HIGH) number but MotiFIT uses it because the heart rate halo closes at this number. If you exceed this number you have no more visual feedback.
This value can be edited.
Here is an alternate formula that can be used for a less aggressive (LOWER) number.
MAX HR = 220 - age