Treadmill vs outdoor for sub-threshold sessions in hot/humid conditions
Hi everyone, I am looking for advice on choosing between two training methods for my sub-threshold sessions: (1) treadmill vs (2) outdoor.
My main concern is that I live somewhere that's typically around 30°C / 70% humidity year round, and I'm finding a pretty significant difference between treadmill and outdoor sessions.
For example, here is a comparison of 2 sub-T sessions that I did this week just 2 days apart:
Treadmill: Air-conditioned gym (~20°C, 0% incline):
- 12 min reps @ 4:15/km: HR roughly 165 → 175
- RPE feels comfortably sub-threshold
- I doubt the speed calibration is significantly different from what is shown as cadence and stride length feels just about right compared to outdoors. The main difference is the cardiac drift is SO much lower and stable
- However, it does not replicate the same stimulus as running outdoors esp when racing
Outdoor: ~30°C/70% humidity:
- 5×6 min @ 4:16/km: HR roughly 170 → 185
- Same pace feels significantly harder, not to mention the reps are shorter
- Even slowing down / using 1% treadmill incline adjustments, the outdoor equivalent tends to push HR into the high 170s/low 180s
For context, my LTHR is ~184, max HR 200.
My question is would you recommend doing the majority of my sub-T sessions on the treadmill under controlled conditions, or should I run outdoors and simply slow the pace down until HR/RPE is equivalent?
Thank you.