Heat does not add time to your finish the way a long climb does. It destabilizes the systems your body depends on to stay moving, and it does so in ways that build quietly before they become obvious. This is not about training protocols. This is about race day.
performance drops begin
the same pace
in hot editions of WS
for heat stroke
What Heat Does to Your Body
Your body's priority in the heat is keeping core temperature from damaging tissue. To do that, it redirects blood from working muscles toward the skin for cooling. Your heart compensates by beating faster, climbing 5 to 10 bpm at the same pace — what exercise scientists call cardiac drift. If you are running by pace, the cost is invisible until it is not. Research has identified roughly 40°C (104°F) as the threshold where your central nervous system begins suppressing voluntary muscle activation. The slowdown is not weakness. It is a hard biological ceiling.
A 2011 study by Parise and Hoffman on 161km ultramarathons found something counterintuitive: faster runners are more severely impacted by heat than slower ones. Because faster runners complete more of the race during peak afternoon temperatures, they absorb a greater heat load. A 2020 review in Sports Medicine put the performance penalty for hot editions of Western States at approximately 8 percent — roughly an hour on a 13-hour race.
"There were times too, I'm struggling in the heat just like everyone else, around Green Gate, through those five-mile splits on Auburn Lake Trails and Quarry Road's tough times." — Jim Walmsley, 2021 Western States champion interview, iRunFar
Pacing
Plan for 4 to 6 percent slower than your goal pace on a moderate heat day. More on a genuinely hot one. The more reliable frame is effort-based: in the heat, your heart rate is already elevated at the same pace, so chasing a pace target pushes effort higher than intended and accelerates core temperature rise. Run by feel and HR. Accept the slower splits.
The first third is where most races are lost in the heat. The morning start feels cool relative to what is coming. Back off 10 to 15 seconds per mile from what feels easy. The heat debt you avoid early cannot be repaid in the canyon miles. Abby Hall won the 2025 Western States running controlled through Foresthill while others slowed hard. Her crew reported she had spent months training in the heat of the Grand Canyon specifically for this reason — her canyons felt cool because she had already been in worse.
Cooling
The goal is reducing core temperature, not skin temperature. Cold water over your head feels good and has limited physiological effect. The four targets with the highest return are the areas where major blood vessels run close to the surface: neck, armpits, groin, and wrists. Ice or cold water applied here cools blood before it circulates. Ice slurry ingestion — crushed ice in a drink — cools from the inside through the phase change from solid to liquid. Both are more effective than a bottle poured over your head.
- Neck (carotid arteries): Highest impact per second spent.
- Armpits (axillary artery): Best during walking breaks at aid stations.
- Wrists (radial artery): Easy to do while moving through an aid station.
- Ice in the hat or buff: Sustained cooling for the next mile, not a brief spike.
Pre-cooling before the start — 10 to 20 minutes in shade, cold towels on the neck and armpits, cold fluid ingestion — creates additional heat storage capacity. Research on ice vests during warm-up found a 2.6°C average core temperature reduction and performance improvements of 5 percent or more. Do not skip aid station stops to save time. In a hot race, 30 seconds of effective cooling at mile 30 returns more than 30 seconds of running.
Your race vest matters in the heat. Our race vest reviews cover which packs have the capacity for ice at aid stations.
Hydration and Sodium
Drink to thirst, not on a schedule. Drinking plain water at fixed intervals regardless of need dilutes sodium in the blood, causing exercise-associated hyponatremia (EAH). Research in ultramarathons puts EAH incidence at up to 51 percent in some events. Symptoms mimic heat exhaustion. The treatments are opposite. Getting this wrong at the medical tent is dangerous.
Sodium loss in sweat ranges from roughly 500 to 2,000mg per hour. Standard sports drinks do not replace it at the high end of that range. Supplement with salt tabs or electrolyte capsules throughout the race. The practical check is urine color and frequency: very clear and frequent means overhydrating, no urine for several hours means under-hydrating. Our fueling guide covers electrolyte targets by race length.
Recognizing Heat Illness
Know the difference before you toe the line. One is manageable at an aid station. The other is a medical emergency.
Heat exhaustion: Heavy sweating, rapid but weak pulse, pale or cold skin, nausea, dizziness, cramps. The athlete is still responsive and sweating. Move to shade, apply ice to the neck and armpits, give fluids if the athlete can swallow.
Heat stroke: Core temperature above 104°F, hot and red skin, little or no sweating, confusion, possible loss of consciousness. This is a medical emergency. Call for help immediately. Begin aggressive cooling while waiting. Do not give fluids to someone who is not fully conscious.
The distinction is sweating. Heat exhaustion: still sweating. Heat stroke: stopped. If you see someone confused and dry-skinned in the heat, do not walk past.
In a hot year at Western States, the winning time can be an hour slower than a cool year. Give yourself permission to run the race in front of you, not the one you trained for.