On Monday, April 17th, 27,221 runners filtered through Hopkinton, MA to run the 121st Boston Marathon. Many of these runners were anxious about the forecasted temperature with estimates climbing higher and higher as race day approached. After a long winter of freezing temps and a cold start to spring, it looked like Marathon Monday was going to be a pocket of warm weather.

While the heat wasn’t as bad as the 87° day back in 2012 (when nearly 5,000 runners chose to defer), the temperatures this year still hovered in the 70s with a glaring sun on a minimally shaded course. At the end of the race, local news station WCVB reported that 2,597 runners sought medical treatment during or after the race, with at least 52 people taken to hospitals by ambulance. Reports also mention some runners had fevers up to a dangerous 108° when they crossed the finish line.

To the marathon spectators, 70-degree weather is just another nice day. But for marathoners heavily exerting themselves for hours at a time, it’s incredibly important to pay attention to the body while properly hydrating and fueling throughout the event, especially when race conditions are different than the conditions you trained through.

For runner and WHOOP Product Associate Emma Van Emmerik, the heat certainly took a toll on her performance. While her time was still impressive, the race itself was a mental and physical battle to combat cramps, dehydration, and self-doubt. Emma’s WHOOP performance data over 26.2 miles completes the story by quantifying her experience.

Emma Van Emmerik

Age: 24
Marathons run: 4
Personal Best: 3:12
Boston 2017 Finishing Time: 3:37

The morning of the Boston Marathon, Emma was prepared to take on the conditions. She had been taking a lot of salt and electrolyte supplements, and her physical fitness was peaking. However, due to the heat, it was clear that she wouldn’t be hitting a personal best.

“I had so many important people out there along those 26.2 miles that I did not want to miss seeing. When I heard that it was going to be hot, I decided to focus on enjoying the experience, rather than aiming for a time,” said Emma.

Before we review Emma’s Boston Marathon data, here’s a look at one of her typical long runs:

This WHOOP activity chart highlights a 17-mile run averaging a 7:30/mi pace completed on Saturday, March 4th when it was 21° in Boston. Emma’s average heart rate is 156 throughout the duration of the run, which is a moderate aerobic effort–not recovery pace, but not marathon race pace either. She slows down around about 3/4 of the way in and pauses to stretch, and her heart rate drops about 30-40 beats. Then she picks it back up to finish the run.

For marathoners, a typical long run is somewhere between 15 and 20 miles depending on how training is going. Although long runs should not simulate racing a marathon, they are still good indicators of fitness and one’s ability to sustain 26.2 miles, both physically and mentally. Emma said most of her long runs were between 7:20 and 7:30 mile pace, and she generally felt comfortable and strong. This would translate to somewhere between a 3:10 and 3:20 finishing time for marathon day.

However, Emma’s data the day of the Boston Marathon tells a different story.

Over 26.2 miles, Emma averaged a heart rate of 178 beats per minute, reaching up to 201 at certain points of the race. For more than 3 hours, Emma was essentially operating at a near maximum aerobic capacity, but she was averaging 8:16 per mile. Based on her long runs, 8-minute pace should have been a breeze, and her heart rate should have been 130-140 bpm.

“I could feel myself getting woozy, and my legs were tight from dehydration. The medical staff had me sit down, take my hat off, and drink broth,” said Emma. “I felt like I was not sweating enough–I was so so thirsty and all I wanted was something cool and salty.” You can see where Emma took a break about halfway through the race when her heart rate dips down as she rests.

Despite slowing her pace down, her body was still losing electrolytes quickly under the sun. When your body sweats, it must increase blood flow to the skin’s surface to release heat. As a result, less blood is available to your muscles and heart, which is why you have to work harder when you exercise. When you aren’t sweating enough to compensate for your effort due to dehydration, that’s when exercise can become dangerous as your body overheats.

For an experienced marathoner like Emma, she knew Boston 2017 wasn’t going to be a fast race, and she knew she’d have to dial it back. Her focus was simply getting to the finish line.

“Throughout the course I stopped at several medical tents. I didn’t want to pass out or get cramps and not be able to finish. I would essentially pop my head in the med tent doors, ask if they had any broth, and quickly drink some before moving on,” she says. “I would feel my legs loosen up and my spirit rejuvenated after a combination of broth, GU packets, and Gatorade.”

After 3 hours and 37 minutes, Emma crossed the iconic finish line on Boylston Street. Thanks to prioritizing hydration and staying mindful of her body, she was able to complete the race on her feet with a smile on her face. Best of all? She had the energy to go out later and dance with her friends to celebrate!