Paris-Roubaix performance: What do cobbles do to the human body?

The brutal pavé of Paris-Roubaix is witness to staggering power outputs and shuddering vibrations. We examine the physical demands of conquering the cobbles

Mat Hayman’s 2016 Paris-Roubaix victory was one for the ages, coming off the back of an injury-tainted build-up that meant hour upon hour on Zwift. Clearly it worked, Hayman combing tactical nous with impressive and calculated surges in power output to win The Hell of the North at the 15th time of asking.

Hayman’s stats were recorded and analysed by TrainingPeaks (albeit the speed sensor stopped functioning at 177km, likely due to the cobbles) with some startling top-line figures, Ahead of Sunday’s race, it’s relevant to unpick what it takes to win arguably the hardest one-dayer in men’s cycling:

  • Over six hours, Hayman averaged 313 watts or 3.82w/kg. For TrainingPeaks users, his normalised power came in at 351w
  • In the final kilometres, several attacks played out including two within two minutes. The first saw Hayman generate 1,198w, the second 1,294w, or 15.5w/kg, which was his peak wattage of the day.
  • When Hayman attacked over the top of Tom Boonen in the last 2km, he maxed out at 1,145w and average 540w for 30 seconds. His cadence averaged a rather rapid 100rpm.
  • In the velodrome, his victorious sprint required one last huge effort, averaging 884w over 20 seconds, or 10.8w/kg, and peaking at 1,234w.

Strava reveals all 

Hayman is not the only rider whose data has been pored over, of course. Strava is a fertile ground for acting velo voyeur over the professionals’ wattage and km/hr. Take 2021’s surprise package, Florian Vermeersch of Lotto-Soudal, who finished runner-up behind Sonny Colbrelli. Vermeersch’s moving time nestled at 6:15:45 over 263.65km, consuming an elevation of 690m, at a startling average power of 343w including a rather ridiculous peak of 1,476w.

Vermeersch (c) at 2021 Paris-Roubaix (Credit: Getty Images)

‘Highlights’ include holding 628w for over a minute coming out of the first cobbled section at Troisvilles; churning out 492w for more than three-and-a-half minutes when the break was forming; and unleashing 993w for 18 seconds during the near-successful final half lap of the velodrome.

It’s clearly impressive stuff but it only tells half the story. Despite the omnipresence of power meters, rise of blood-glucose-measuring devices (particularly Supersapiens) and even tyres that can change pressure through a race – which will be used by Team DSM at Paris-Roubaix this weekend - we’re not explicitly told how debilitating the cobbles are on both the muscular system and the metabolic system. The nearest we get is arguably total work done, which for Oliver Naesen at the 2019 edition of Roubaix was 5,438kj. Compare that with a similar duration ride sans cobbles and you can begin to speculate on the cobbles’ impact.

Impact of the cobbles

Speculation, of course, isn’t in the sports scientists’ vernacular, which is why amateur rider and scientist Sébastien Duc of Reims University signed up to the 2015 Paris-Roubaix Challenge and acted as a vibration guinea pig; in other words, lead researcher Duc and his team loaded his Specialized Roubaix with two tri-axial accelerometers – one on the stem and one on the seatpost. These small devices measure the vibrations shuddering through the carbon Roubaix and Duc himself. They’re extra weight and drag, which is why professional riders are reluctant to sign up to these sorts of experiments, and they’re not perfect. “In all honesty, it would have been better to measure the vibrations at the interface between tyre and road but, not surprisingly, that wasn’t possible,” Duc tells us.

The sportive measured 139km and included 15 of that’s year’s 27 stretches of cobbles, ranging from ‘easy’ (two stars) to ‘very hard’ (five stars). Duc weighed just 68kg and was 1.80m tall. He set tyre pressure to 73psi, plus measured power output and heart rate.

And after a long day in being thrown about by 28.1km of cobbles, what did Duc and his accelerometers discover? Firstly, that he’s not a bad recreational cyclist, covering those cobbles at an average speed of 28.1km/hr, his speed fluctuating between 19.1 and 27.1km/hr, just like his beating heart, which varied from 122 to 155bpm. Cadence came in at a pretty metronomic 79 to 87rpm while wattage ranged from 167 to 235 watts.

As for the vibrations, “Not surprisingly, the ‘vibration dose value’ measured at the stem (hand) and seatpost (whole body) increases with the cobbles’ difficulty,” he says. “More specifically, over the most taxing cobbles, the vibrational value reached 35m/s2 at the stem and 28m/s2 at the seatpost.” As comparison, the UK government-run Health and Safety Executive regulates that, “Exposure limits of 5m/s2 should not be exceeded in the workplace, like working with drilling machinery.”

Can we please see the risk assessment? (Credit: Offside/L'Equipe)

In short, Paris-Roubaix should be cancelled henceforth along with the cobbled season as a whole. Alternatively, says Duc, the secret to beating the cobbles is to become a stronger, faster cyclist. Simple. “To reduce the deleterious effects of vibrations and make it more bearable for the cyclist, you have to ride it as fast as you can and with an adequate gear ratio (not too small, not too big) and at a cadence of around 80-85rpm,” Duc advises. “If the cyclist moves too slowly, they’ll feel a greater impact of the cobbles.”

According to Duc, experiencing perineum that pleaded for pity and forearms that still seek an apology days later is common, even with today’s dampening technologies, wide tyres, and the old trick of multiple layers of bar tape.

“Vibrations can induce many symptoms and injuries in cyclists, including numbness in the fingers (due to ulnar nerve compression), loss of gripping force, muscle aches and pains in the arms, shoulders and back,” says Duc. “It’s also possible to develop sagging of the skin on the hands and buttocks due to overpressure and repeated shocks.” Nice.

Bad vibrations

Bent Ronnestad, professor in exercise physiology at Norway University of Applied Sciences, is another academic who investigated the impact of cobbles on the cyclist – more precisely, vibrations. Ronnestad realised that unless you’re based near stretches of deep cobbles or could afford the time and money for frequent recons, it was a difficult task to recreate the feeling of riding roughshod over rickety terrain from the comfort of tarmac. But not impossible.

“We wanted to help professional riders prepare for the cobblestone races, so we created artificial vibrations,” Ronnestad explains. “We mounted a bike on a trainer that sat on a vibration plate, and then we tried different vibration amplitudes and frequencies to create a combination that was most similar to that felt when the pros rode over the cobbles.”

It’s why 10 elite-level riders, averaging a VO2max of 78.6mL/min/kg, were put through six intervals of five minutes at over 90% of VO2 max and over 90% of peak heart rate. They followed this high-intensity template with and without the vibration plate.

“Although not exact, we tried to imitate the last stretches of Roubaix, timing the duration similar to that seen in the race between sections of cobbles,” Ronnestad continues. “As for power outputs, that varied in both the test and the race according to the section, of course, and weight of rider. But there’s always a higher power output before reaching the cobbles because of the importance of being in a good position. The power output usually ranges from 350 to 650w with spikes even higher.”

Arguably, that’s to be expected – positioning requires power in every race, whatever the surface – but what was more unexpected was the fitness-boosting benefits of the cobbles themselves. “We observed increased EMG activity with the vibrations,” says Ronnestad. “This can be interpreted as a higher muscle activation and then we can likely assume that this is at least partly due to increased fast-twitch muscle activity.

“We also saw increased oxygen consumption, which could be related to fibre activation. Based on this acute increase, one could suggest that the vibrations give an extra training stimulus, leading to an improved training adaptation when repeating such sessions over a prolonged period.”

Just think about all those gains (Credit: Peter Stuart) 

In essence, much like Colombians like Egan Bernal and Nairo Quintana have benefitted from being born and raised at altitude whether racing ‘high’ or ‘low’, those raised on the cobbles enjoys benefits that transcend any surface. Maybe it’s how four-time Roubaix winner Tom Boonen could win the sprints classification at the Tour de France in 2007 upon smooth tarmacked roads. That’s reductionist, of course, but it does highlight that if you can avoid saggy hands and buttocks from overdosing on cobbles, ultimately you’ll become a stronger cyclist.

Why does all this matter? Because rolling out meticulous training plans that lead to peak performance and choosing gear that’s optimum over those deep recesses requires reverse engineering. You must identify good behaviour, model it and then improve upon it.

Alternatively, you can simply race old school, displaying the grit of Roubaix’s very first winner all the way back in 1896. German Josef Fischer was nearly kicked off his bike by a horse and held up by cows but maintained his composure, and a very fetching coiffured moustache, to win by 25 minutes. All in all, science or no science, one thing’s clear: Paris-Roubaix is pain on a saddle. Good luck to the men and women this weekend.

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