Are women the stronger sex?

On average, men have bigger hearts, lungs and muscles, so how are women seemingly closing the endurance gender gap? 220 investigates…


The sores on my neck and body were excruciating. I’ve never known water like it. It’s something I carried with me for three weeks. Why did I do it? I love a challenge.” The words of ultra-endurance triathlete Claire Smith when reflecting on becoming the first British athlete – male or female – to complete the continuous Double Deca in Mexico last November.


Brit Dave Clamp completed it in the old format of one iron per day multiplied by 20, while Smith ‘tamed’ the 48-mile swim, 2,240-mile bike and 524-mile run in 660hrs, 28mins and 58secs. So long is this ridiculous event that Smith started on 4 October.

Along the way, she experienced the most toxic water she’s swum in, saddle sores and an ankle injury. “But no hallucinations,” she laughs. “Not this time. I used to, though, terribly before realising it was linked to dehydration. I remember the Double Enduroman up the road in the New Forest. I stamped my foot in a puddle but there were no ripples. My son, Jake, was with me. He said there was no puddle. But it was so vivid. I saw soldiers in the bushes as well.”

Soldiers or not, Smith’s consistently trampled all over male competitors. And she’s not the only one…


The past few years have seen women outperform men at the extremes of endurance. In December 2018, American Camille Herron claimed the world record among men and women for the fastest 24hr race, covering 162.9 miles. Then there’s the remarkable exploits of Brit Jasmin Paris, who thrust herself into the media spotlight when beating everyone at the 2019 Montane Spine race – a 268-mile winter assault on the Pennine Way. During pitstops, she breastfed her daughter Rowan.

Anomalies? Maybe not. There’s mounting evidence that the tougher and longer the event, the better women do. Most recently, a study by and the International Association of Ultrarunners concluded that female ultra runners are faster than their male counterparts over 195 miles beyond. “The average pace was 17:19min/mile,” lead author Paul Ronto tells us. “That was 0.6% faster than the men’s 17:25min/mile.” That compares to 5km where men are 17% quicker. The study was comprehensive, Ronto exploring ultra trends over 23 years that involved analysing over five-million results from 15,451 ultra events.

It was also run-specific, but this closing of the gender gap as distance grows is reflected in triathlon, too. Professor Romauld Lepers of Burgundy University in France revealed that between the mid-1990s and 2012, Ironman World Champ elite females closed in on the men from 15% back to 11%. Fast-forward to 2018. Daniela Ryf wins the Ironman World Champs for the fourth year in a row, finishing 25th overall in a time of 8:26hrs and thus finishing just 7.1% behind men’s winner Patrick Lange (7:52hrs). Brit Lucy Charles-Barclay regularly hits top-10 exiting the swim, including second overall at Challenge Roth 2019.


So what’s going on? We crossed the channel to find out. Guillaume Millet is professor of exercise physiology at Saint Etienne University. He’s an expert on gender-specific physiology and suggests there are two reasons behind the closing of the gap: muscle fatigue and fat-burning capacity. “Before I elaborate, I must stress that there’s no definitive answer and to what degree the gap is closing, if at all,” Millet warns. “But it’s a question that deserves to be asked because women can win scratch races.

“Anyway, we undertook a study in 2012 to understand the area more,” Millet continues. “We tested a number of male
and female athletes at the 2012 UTMB event in Chamonix [legendary 170km run in the Alps]. It was reduced to 110km that year because of adverse weather but it was still tough.
We electrically stimulated their quadriceps before and after the race and discovered that the female runners’ neuromuscular system was fatigued to a lesser extent than the men. Their muscles were seemingly more fatigue resistant.”

Millet concedes he’s not 100% sure why but suggests it could be down to women possessing a higher percentage of type I slow-twitch fibres than men. Men have a higher percentage of type II fast-twitch fibres. Type I are better for endurance; type II are more suited for speed and strength.

Then there’s the notion that women excel at ultra triathlons and other events due to being more efficient at burning fat for fuel. Studies show that their FatMax, the exercise intensity at which the highest rate of fat oxidation is observed, is higher in women than men. The resulting conclusion is that females can not only spare more precious glycogen stores for intense parts of the race, they can race fast and still predominantly tap into a near exhaustible supply of
fat-burning energy, too.

“That’s the theory but I speculate that there’s another reason,” says Millet. “If you can tap into more fat, you’ll preserve amino-acid status as you won’t be drawing on protein for fuel. This prevents muscle breakdown and strengthens the muscle-fatigue resistance argument.” To that end, Millet’s currently analysing data from another study of his at the 2019 UTMB event to either prove or disprove his theory. He hopes to draw a conclusion soon. But, anecdotally, this propensity to burn fat and block fatigue’s echoed by Smith.

“The longer I go, the stronger I feel,” says Smith. “Take the Double Deca. I had a nasty injury halfway through the run. It broke my heart as I thought I’d have to stop. It worsened for a few days and reached heavy limping stage. But I flipped my mindset and thought, ‘I’m here long enough, this could heal’. And it did. As an event grows, I strengthen mentally and physically.”


Ahh, the mental. There are many reasons mooted why women might be stronger than men, including the idea that childbirth bulletproofs a female’s mind to future painful exploits. Millet disputes this, citing no empirical evidence. As did Jasmin Paris in 2019 when asked about childbirth boosting her pain threshold:
“My daughter was born backwards but I don’t think one experience trains you for other unpleasant experiences.”

Sports psychologist Dr Carla Meijin is currently studying pain experiences of ultra female athletes. But the results aren’t yet in. They’re also not comparative with men, which reflects the sports psychological field as a whole. “There exists very little research that’s focused on differences between endurance male and female athletes from a psychological perspective,” says Meijin. “Much of the media coverage centres on the anecdotal.”

To that end, Smith concludes that she has a high pain threshold and quite an extreme personality. “I’m all or nothing. If I aim for something, I won’t give up.” That’s arguably symptomatic of many ultra athletes, not just women. You can also speculate that men’s egos have a more detrimental impact the longer an event, especially when it comes to pacing. Take an American study of 92,000 marathon results at 14 races that showed while both sexes slowed during the second half, men slowed more. Their grand egos sent them shooting from the start line before falling backwards.


It’s similarly unclear when you deconstruct each discipline. During the period 1983-2012, Lepers showed that the sex differences in elite Ironman Hawaii performance remained relatively stable at around 12.5% for swimming and cycling, while running dropped from 13.5% to 8%. Why remains unclear although a more refined pacing strategy’s been mooted. You’d also think women’s naturally higher fat percentages would provide greater buoyancy and more speed in the swim. But reverting back to the 2018 race, Ryf came in 13.5% slower than Lange and only 3.9%
back on the bike.

“And that clashes with the protein fat-burning theory,” says Millet. “Cycling doesn’t really damage the muscle so clearly muscle-resistance to damage isn’t the only reason. It’s a complex subject!” Other theories of gender closing include lighter female triathletes being naturally more economical on the run. The female’s natural lightness also pays off when dissipating heat, says Lepers.

Ultimately, it seems that females excel at the true extreme fringes of endurance events. Why could be down to physiological and psychological advantages when racing for days on end. The evidence is inconclusive. Or it could simply be a numbers game and outliers. The Spine race that Paris won counted just 11 women among its 126 starters. The next woman came in two days behind Paris. “Small sample sizes are a problem in making any conclusions,” says Millet. “All I know for certain is that whatever the gender of anyone who attempts such an undertaking, these extreme athletes deserve a medal. They’re inspirational.”

Four female athletes who’ve rewritten the endurance record books…

In 2009, the Brit claimed her third straight Kona title in a course record 8:54:02. In the process, she finished just 33:41mins behind men’s winner, Craig Alexander – the shortest difference between the two genders in the history of the race. She also beat all but one man at the Alpe D’Huez Tri in 2008.

“She’s the current female record holder for the Arch to Arc [87-mile run, 21-mile channel swim, 180-mile bike ride] in 66:56hrs,” says Claire Smith. “I’m attempting it this year. She’s an amazing athlete and so down to earth.”


In 2018, 37-year-old Sarah Thomas was treated for breast cancer. Twelve months later, the American become the
first person to swim across the English Channel four times non-stop.

“CCC is a 100km event held during UTMB that features 6,000m of climbing,” says Guillaume Millet. “Corinne finished first overall at the first one in 2006, despite there being more than 1,000 entrants!”

Last August, the 24-year-old German won the 4,000km Transcontinental bike race from Bulgaria to France in 10 days, 2hrs and 48mins. She finished over 6hrs ahead of the nearest man.

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