For me, at least.
So I’ve been capping my training run intensity at about 75% of max HR for the past several months. Pace for these runs is currently 9:00 min/mile at around 135 HR. While that is a modest pace for most, I’m happy with that given my sparse running history and being a heavier runner at 195 lbs. I’m also coming back from a year off from triathlon/running. I’m up to about 25-30 miles/wk running. My plan is to emphasize low HR running until pace starts to plateau (ala. maffetone/hadd/allen/etc).
The intent was to reduce stress on my body and to also see my aerobic pace increase over time. I’m seeing both happen. I had to take last year off from running (used to run at 145-155 HR for “easy runs”) due to various stress-related injuries. None so far this year, knock on wood. My pace is improving nicely, starting from 10+ min/mile at the exact same intensity. Recent improvement might be partly due to the addition of a weekly brick workout at the track with my tri club. There, I’ve been doing anaerobic 1 mile repeats (6:20-6:40 min/mile ~165 HR).
I’ve also applied the low HR approach to cycling, albeit with slightly more intensity at about 78% max HR since cycling is my background. I’m seeing my sustained power steadily improving on my favorite local hillclimb. Being a long time cyclist, I wish I’d had the patience to apply this approach long ago. “Pushing up” threshold from below is much easier than “pulling up” from above, mentally. The plan is to keep adding more volume at this intensity until the power stops improving. Then on to more typical intervals.
I’m not saying that low HR training is the best approach for everyone, especially the already very fit or severely time constrained.
But for those in the process of ramping up, or those who are injury prone, it is a good option IMHO.