Your Heart Rate Monitor is LYING to You (Pt III)
In the last two training logs (here and here), we analyzed the impact of heat on heart rate during intervals and between warmups and cooldowns. Today we will demonstrate this same phenomenon in a longer easy run.
All of my easy runs are performed on the same treadmill in the same cool apartment gym, which usually sits at a crisp 69 degrees Fahrenheit. However, when entering the gym this morning around 4:30 am, it felt like a sauna with the temperature sitting at a toasty 86 degrees, the result of a broken A/C. Nice.
Not surprisingly, this had a significant impact on my heart rate during the run. Here you can visually see my heart rate decouple from ventilation. Although ventilation remains essentially flat, heart rate continues to progressively increase.
- Date
- 9/10/26
- Duration
- 50 min
- Pace
- 10:55/mi
- Avg VE
- 85.4 L/min
- Avg HR
- 120 bpm
Compare this to a nearly identical run I completed back on March 15 on the exact same treadmill but with the air conditioner working. In this case, although average ventilation was identical at 85.4 L/min, average heart rate was 3 bpm lower overall.
- Date
- 3/15/26
- Duration
- 60 min
- Pace
- 10:55/mi
- Avg VE
- 85.4 L/min
- Avg HR
- 117 bpm
The table below shows the progression of average HR and VE for both runs by 10 minute blocks. As you can see, average HR for today's run accelerated more rapidly than the March 15th run, eventually reaching 130 bpm at the end of the run.
| Run time | March 15 HR bpm | Sept. 10 HR bpm | March 15 VE L/min | Sept. 10 VE L/min |
|---|---|---|---|---|
| 0–10 min | 110.1 (+0.0) | 107.6 (+0.0) | 79.8 | 82.0 |
| 10–20 min | 116.5 (+6.4) | 115.1 (+7.5) | 90.6 | 84.8 |
| 20–30 min | 116.8 (+6.7) | 119.9 (+12.2) | 89.0 | 86.2 |
| 30–40 min | 117.3 (+7.2) | 123.2 (+15.6) | 91.1 | 88.2 |
| 40–50 min | 118.0 (+7.9) | 126.6 (+19.0) | 85.9 | 86.8 |
| 50–60 min | 120.6 (+10.5) | — | 85.2 | — |
One takeaway for heart-rate-only users is that your VT1/VT2 ‘Expected Drift’ settings in the sidebar may need to allow more leeway for heat-related drift compared to ventilation.
