Running in the Heat: Why You Slow Down and How to Adapt

Every summer, runners face the same problem: the pace that felt easy in spring suddenly becomes a grind. In Japan, temperatures soar from mid-June onward, making even a casual jog feel brutal. If heat has been making your runs harder lately, you’re far from alone.

As a runner myself, summer training is something I genuinely struggle with. The heat and humidity drive up perceived effort significantly, making it nearly impossible to complete high-intensity sessions the way you would in cooler months.

That said, learning to use the heat to your advantage can set you up for a breakthrough once fall arrives.

In this article, I’ll explain the science behind why heat hurts your running performance — then walk you through the best strategies and workouts for the summer months.

By the end, you’ll know exactly what to train and how to train it when temperatures climb.

Why Heat Hurts Your Running Performance

When temperatures rise, several physiological mechanisms combine to reduce your running performance.

Increased Muscle Fatigue

Research shows that exercising in hot and humid conditions leads to higher blood lactate levels compared to cooler environments, even at the same pace※1. Your muscles accumulate more lactic acid while running at the same effort.

Heat also increases the production of free radicals during muscle contraction. These free radicals damage the proteins responsible for muscle contraction, leaving the muscles in a more vulnerable state.

Together, these effects accelerate muscle fatigue and reduce muscle force output, directly lowering your running performance.

Reduced Blood Flow to Muscles and Decreased Plasma Volume

When you run, only about 30% of the energy your body produces goes toward actual movement. The remaining 70% is released as heat, causing your core temperature to rise.

In response, your body sweats and redirects blood to the skin’s surface to cool it down — effectively using blood as a cooling system.

As more blood is diverted to the skin, blood flow to your working muscles drops. At the same time, sweating reduces plasma volume, decreasing cardiac output and the blood’s oxygen-carrying capacity.

Even at the same heart rate, less oxygen-rich blood reaches your muscles. Aerobic metabolism is impaired, and maintaining pace becomes harder.

At the same heart rate, less oxygen reaches the muscles — so the aerobic energy system is compromised even when you feel like you’re working just as hard.

Central Nervous System Suppression

As core temperature rises and dehydration sets in, the central nervous system (CNS) sends signals to reduce motor unit recruitment※5. Motor unit recruitment refers to the proportion of muscle fibers the nervous system activates during exercise.

When motor unit recruitment drops, running performance declines directly — you try to push as hard as usual but simply can’t generate the same force.

Furthermore, in hot environments, muscle fatigue accumulates faster and feeds back to the CNS, triggering an additional reduction in motor unit recruitment. It’s a compounding effect that makes heat far more punishing than cool conditions.

What Happens to Your VO2 When You Run in the Heat?

Running in the heat raises both your perceived effort and heart rate. The elevated heart rate occurs partly because more blood is being redirected to the skin to regulate core temperature, leaving less available for the muscles.

As a result, even when you feel like you’re working hard, your actual whole-body oxygen uptake tends to be lower than in cool conditions. The increased heart rate doesn’t translate to higher oxygen delivery to the muscles.

Since oxygen uptake is proportional to running speed, a drop in pace means a drop in VO2. This is precisely why high-intensity efforts like VO2 max interval training are not well-suited to summer running.

Pros and Cons of Training in Hot Weather

Training consistency matters regardless of the season. Here’s a practical look at what you gain — and what you give up — when you train in the heat. These are qualitative trade-offs rather than absolute physiological effects.

Benefits of Running in the Heat

When it’s hot, blood is diverted away from your working muscles, which means even a slower pace can push you to lactate threshold (LT) intensity.

Improving LT requires increasing mitochondrial content and function in your muscle fibers. When blood flow to muscles is reduced, oxygen delivery to mitochondria also drops, creating a state of relative oxygen deficit.

This oxygen deficit triggers mitochondrial biogenesis — the creation of new mitochondria and improvements in their function. The result is a genuine LT adaptation, even at paces that would feel easy in cool conditions.

Heat also acts as a natural governor: cardiovascular strain forces you to slow down before your muscles accumulate enough damage to cause injury. In that sense, summer can actually be a lower-injury-risk period than you’d expect.

Sprint workouts are also well-suited to hot weather. Long rest intervals allow core temperature to partially recover, so the heat has less impact on the quality of each effort. Warmer muscles also have greater range of motion, which may further reduce injury risk during explosive work.

Drawbacks of Running in the Heat

The increased fatigue that comes with heat means overall training volume typically drops. VO2 max interval training — sustained efforts at or near maximal oxygen uptake — becomes nearly impossible in summer. Even though perceived effort is high, actual oxygen consumption remains limited, making it impossible to train at the intensities needed for VO2 max adaptations.

The psychological toll of heat training — the dread of heading out into the humidity — is its own obstacle and shouldn’t be underestimated.

Heat Strategies for Summer Runners

Here are the key strategies to keep your training on track through the summer heat.

Heat Acclimation

When you train regularly in a hot environment, your body undergoes physiological adaptations in response to heat stress. This process is called heat acclimation.

The primary outcomes of heat acclimation are a lower heart rate during sub-maximal exercise and a lower core temperature. Sub-maximal exercise refers to any effort below your VO2 max ceiling — which covers the vast majority of training runs.

To drive heat acclimation, your body needs to experience elevated core temperature. This requires exercising at ≥50% VO2 max in a hot environment for sustained periods.

The physiological adaptations triggered by heat acclimation include: increased plasma volume, earlier onset of sweating, higher sweat rate, reduced salt loss in sweat, lower skin blood flow, and increased synthesis of heat shock proteins (HSPs).

Heat acclimation also carries a surprising benefit in cool conditions. One study found that after 10 days of heat acclimation, VO2 max in a cool environment (55°F / 13°C) improved by 5%※2. The expanded plasma volume allows the heart to pump more blood per beat, boosting oxygen delivery even when it’s cool. This is one key reason summer heat training can directly improve your fall race performance.

Heat acclimation is largely complete within 7–14 days of heat exposure. The chart below shows how quickly each adaptation develops.

However, heat acclimation adaptations don’t last indefinitely. Once heat exposure stops, heart rate and core temperature adaptations are lost at roughly 2.5% per day, disappearing entirely within 3–5 weeks※3.

A separate study confirmed that increases in blood hemoglobin and sweat adaptations gained from 5 weeks of heat acclimation returned to baseline within 2 weeks of stopping※4.

If you’re targeting a fall race, the takeaway is clear: keep training in hot conditions as close to race day as possible to preserve these adaptations.

Run Early Morning or at Night

Even in summer, temperatures are significantly lower in the early morning and at night.

Early morning in particular — before the sun is fully up — often offers the most comfortable summer running conditions, since temperatures have had all night to drop.

If midday and evening runs feel dangerously hot, consider shifting your schedule to early morning or night running during the summer months.

Speed Training

Sustained efforts become very difficult in the heat. Speed sessions with long rest intervals, however, are much less affected by high temperatures and can be completed with good quality.

Fall, winter, and spring are packed with races — especially for marathon runners, leaving little room for speed development. Summer is often the one window of the year where you can dedicate focus to building raw speed.

The best speed options for summer are hill sprints and rep workouts.

Use a Treadmill

Switching to indoor running is another effective option for beating the heat.

If you’ve never trained on a treadmill before, you might wonder whether it actually works. The short answer is yes — it’s effective.

Elite runners around the world regularly incorporate treadmill training, and research confirms it produces similar adaptations to outdoor running.

Because there’s no air resistance indoors, adding a slight incline (around 1%) is often recommended to better match outdoor effort. But if you’re not chasing a specific pace, it’s perfectly fine to skip the incline.

Sakimoto of Aminosaurs, who broke 2:30 in the marathon training primarily on treadmills, is proof that treadmill-based training can produce elite-level results when used properly.

For an in-depth guide on how to use a treadmill effectively, check out the article below.

How to Adjust Your Training Pace for Temperature

When it’s hot, you need to run slower than you would in cool conditions. Fortunately, you can calculate roughly how much to adjust your paces.

The tool to use is the VDOT Running Calculator. Below I’ll explain how to use its temperature correction feature. For a full guide to using the VDOT calculator, see the article below.

Step-by-Step: Temperature-Adjusted Pacing with the VDOT Calculator

Click “Advanced Features” below the Calculate button, then follow the steps below.

After entering the temperature and clicking Calculate, the tool displays a predicted finish time adjusted for that condition. Don’t forget to click “REVERSE” after clicking Calculate.

Here’s an example: if you ran a 1:14:40 half marathon in 86°F (30°C) conditions, what would that be equivalent to in cool weather (below 59°F / 15°C)?

If you ran 1:14:40 when it was 86°F (30°C), the calculator estimates that in cool conditions your VDOT would be 66.4 — equivalent to a 1:11:33 half marathon.

Summer heat makes it hard to hit fast times. But by using this tool, you can convert a summer result into its cool-weather equivalent — so you know exactly where your fitness stands, regardless of the conditions.

If you skip the REVERSE step, the calculation runs in the other direction: “If I ran 1:14:40 in cool conditions, what would that equate to at 86°F (30°C)?”

The result shows that a 1:14:40 in cool conditions would be expected to slow to 1:17:47 at 86°F (30°C). Assuming 86°F conditions, your adjusted training paces would look like this:

At threshold pace, that works out to 5:52/mile (3:39/km). In cool conditions below 59°F (15°C), threshold pace would be around 5:40/mile (3:31/km) — roughly 8 seconds per kilometer slower in the heat.

Best Workouts for Summer Running

Here are the training sessions best suited to hot-weather running.

LT Intervals

LT intervals are interval workouts performed at or slightly below lactate threshold intensity — roughly between half marathon and marathon race pace.

Continuous tempo runs become hard to sustain in the heat. Breaking the effort into intervals with short jog recoveries makes LT-zone work manageable even in summer conditions.

Sweet spot training (SST) is one popular form of LT interval work. For a detailed guide on how to run SST sessions, see the article below.

Rep Workouts and Hill Sprints

As mentioned above, rep workouts and hill sprints are the workouts to prioritize during the summer months.

Once the marathon season begins, there’s rarely time for this kind of pure speed work.

Use summer to build your raw speed foundation. Strong top-end speed developed over the summer will position you to reach a new performance level in fall and winter races.

For specific protocols on rep workouts and hill sprints, refer to the articles below.

Summary

References

※1 Claremont AD, Nagle F, Reddan WD, Brooks GA (1975) “Comparison of metabolic, temperature, heart rate and ventilatory responses to exercise at extreme ambient temperatures (0C and 35C)” Medicine and Science in Sports

※2 Lorenzo S, Halliwill JR, Sawka MN, Minson CT (2010) “Heat acclimation improves exercise performance” Journal of Applied Physiology

※3 Daanen HAM, Racinais S, Périard JD (2018) “Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis” Sports Medicine

※4 Cubel C, Fischer M, Stampe D, et al. (2024) “Time-course for onset and decay of physiological adaptations in endurance trained athletes undertaking prolonged heat acclimation training” Temperature (Austin)

※5 Nybo L, Nielsen B (2001) “Hyperthermia and central fatigue during prolonged exercise in humans” Journal of Applied Physiology 91(3):1055-1060