Question on doing HIIT, by senior. I'm 60. & I have a total knee replacement & limited to no-impact exercise. My primary cardio workouts are on Arc trainer or elliptical. My max HR is 160, resting HR is 58 & target cardio at 135 PBM (based on normal charts & calcs). My question is how high should I let my heart rate go during a HIIT cycle. As an example I can, raise resistance from 6 (normal) to 10, & accelerate from 6mph to 10 mph for one minute. My pulse rate will go up to 145 to 150 bpm. Typically lower at the beginning & higher end toward the latter part of a half hour work out. Recovery cycle of 2.5- 3 minutes lets my HR return to 130-135 bpm. I don't have any adverse effects, not dizzy feeling faint. & I feel fine next days. So the question is am I going hard enough?