I had a long layoff from consistent running (over four years) and have recently put together 6 months of consistent training. Just three weeks ago I started adding a weekly tempo run.
In college my threshold HR seemed to be around 174-176. This was at least the repeated HR for hard, sustained running.
On my first tempo, my HR was at 165 at the end of the 15:00 tempo. I assumed that given my age (now 27), it was reasonable to assume that max HR had decreased and thus the threshold BPM would decrease too.
On the second (a week later), it was at 169 at the end of the 15:00. Today, it was 172 at the end of the 18:00 run. All three efforts felt very similar in intensity.
As I'm gaining fitness, does the threshold BPM increase? Should I expect to be still around 175 for optimal threshold training, or should I assume this has declined given the long layoff and being 27 instead of 22?