Hot Weather Running: Heat Acclimation, HR Drift, and Pace Adjustments
Summer turns “easy pace” into a heart-rate argument. The conflict is performance ego vs thermoregulation: holding winter splits in a heat wave isn’t toughness, it’s a different workout. Slow down, respect drift, and acclimate. Cap easy days with the Zone 2 Calculator, translate effort with the Pace Calculator, and follow fluid strategy in the hydration guide.
Heat Changes the Cost of Pace
Higher skin blood flow and sweat raise cardiac load at the same speed. Expect HR to climb across a steady run (cardiac drift) even when pace is honest. Humidity and direct sun make it worse than the thermometer alone suggests.
Numeric scenario: Cool-weather easy pace 6:00/km at 140 bpm. At 30°C / high humidity, the same 6:00/km hits 155 bpm by minute 25. Shift the goal: hold 140–145 bpm and accept ~6:20–6:35/km (example — yours will differ). Over 10–14 days of regular heat exposure, the same HR often buys back some pace as you acclimate — but race-pace targets still need heat-adjusted expectations.
Acclimation is gradual with repeated heat sessions; it isn’t a single sweaty long run. Early-morning routes, shade, and lighter clothing help. Dizziness, chills, or stopping sweat are stop-and-cool signals — not “push through” moments.
Decision rule: if HR is >10–15 bpm above your usual easy ceiling at a pace that feels only mildly warm, slow or walk until HR returns — don’t defend the pace split. Cancel intensity if you can’t finish a short warm-up without undue HR spike and poor feel.
Workout Adjustments
- Replace pace targets with HR/RPE on easy and long runs
- Shorten intervals or add recovery in heavy heat
- Move key sessions to cooler hours when possible
- Pre-cool with shade/fluids; never start a long run already dehydrated
Watch “training effect” badges often look huge in heat because HR is elevated — that does not mean you earned a breakthrough VO2 day.
Carry fluid on longer efforts, and treat salt stains plus lingering fatigue as feedback for the next day’s plan. Quality sessions in extreme heat can be shortened by a rep or two and still count — unfinished heroic sets in 35°C heat rarely make you fitter.
Practical Takeaway
In heat, effort is the prescription. Preserve the training stimulus by slowing down before you cook the session.
FAQ
Should I race all-out in a heat wave?
Adjust goals. Many runners add minutes honestly rather than forcing a cool-weather PR plan.