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July 16, 2026

Reading Between the Lines on HR Data, Biomechanics, RPE + Heat Index

Written by Coach Tom McGlynn

I’ve coached many runners and want to key on one individual who has agreed to share his training data.  I’ve actually coached this runner personally for the last 20 years and he had some pretty terrific coaches previously with some strong results.

The athlete has logged 20,000+ miles and approaches his 54th year on this great earth.  Injuries have plagued him for the last decade, but he continues to gain fitness quickly (when not injured which is infrequent).  Similar to Coach Cally’s post What’s Slowing You Down, this athlete always struggles with peer comparison and past performance as unrealistic measuring sticks.

That’s the background and here are my observations about the athlete who is dealing with a glute injury that shortens his stride.

The runner intends to keep volume relatively high (~35 mpw), while he flosses, rolls and stretches his glutes.  On a Wednesday session, I suggested cutting the 9M @ 7:19/M pace to ~ 6M at 7:30/M pace and running progressively to RPE (rate of perceived effort/exertion) = 5-6.

Here’s a graph of how that run transpired:
Graph_1

Notice the athlete did have a nice progressive run shaping up until 5M.

Now let me overlay HR against pace so I can show you where the problem arose:
Graph_22

Notice the gentle cardiac drift initially, which is exactly what we want.  Then around 2.5M there’s a steeper inflection with a final mile that looks like a cool down (it was not).  Instead the runner over heated and drove RPE + HR too high.  In reality it was a hot day in the SF Bay Area.  When I say “hot day in the SF Bay Area”, we are talking relative to the typical area conditions.  For those of you in the midwest, southeast and Atlantic coast, you are dealing with much worse conditions.

Here’s the synopsis given by the athlete post-run:  After 5M I felt hot, heavy and my glutes were tight.  I had intended to run a gradual progression, but got ahead of myself in M3 and was tired by the slow last mile.

It easy to see this from the splits:
Graph_3

If we look deeper we see a decline in cadence and a subsequent increase in vertical ratio for M6 (both clear signs of fatigue):

Graph_4
Lastly, we see the athlete spent way too much time in Zone 4, for what was supposed to be an easy run effort.

Graph_5

Ok I’m being pretty critical of this athlete so full disclosure, the athlete referenced = ME.

After my failed attempt at <5:00/M on July 2, I recommitted to making my goal.

The problem is that I’m partially injured and instead of listening to the Angel (coach in me) and slowing down with emphasis on rehab, I let the Ego (athlete in me) take control.

The point is that after coaching thousands of runners and spending myriad time on a track, I still fall victim to the same self-sabotage thoughts that derail us. I ignore the weather and my aches & pains.  However, I continue to strive to be better. 

So now I need to go into a Do as I say, Not as I do recommendations list:-)

  1. We want a gradual cardiac drift → that’s a good thing and for the most part it should crescendo to the last steps run or final interval (disregard the cool down after workouts).

  2. We want easy runs to start in Zone 2 and gradually move up through Zone 3.  Ideally we don’t hit Zone 4 on these efforts.

  3. We should see key metrics like cadence (increasing) and vertical ratio (decreasing) on our easy runs.

  4. We need to consider the heat index and adjust accordingly.

The summer heat has been a challenge for most of the world (Europe included), so let’s consider heat index:

  1. It is calculated in the shade; most estimates increase direct sunshine at +15%

  2. Anything over 80 = Caution →  in my opinion this is where performance drops especially if you’re borderline hyperthermic like me

  3. The Rule of 150 =  Air Temp (℉) + Relative humidity (%);  anything over this undoubtedly affects performance and >180 is dangerous

As I (the athlete) reflected on that Wednesday run I realized the Heat Index was ~ 90.  That’s high for the Bay Area and definitely affected my performance. I could’ve stayed at ~ 8:00/M pace and kept HR comfortably in Zone 3, but gee that Ego is powerful and I wanted to run the assigned pace. 

So let’s keep easy = easy, adjust to the elements, pay attention to cardiac drift, vertical ratio and cadence (late in our runs), listen to our bodies when we have challenges, and keep that stress/recovery ratio where it should be.

More in this category: « The Run You Keep Avoiding
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