How does the human heart react to the most extreme forms of stress?


Using sports science technology, three very different riders at the International North West 200 motorbike festival, put this question to the test.

With top speeds hitting 208mph, on closed public roads around the coast of Northern Ireland, the North West 200 is one the fastest road races in the world. The motorbikes are often just inches from each other.

Three competitors were fitted with a heart strap and wireless sensor to measure their heart rate over every inch of the course.

The riders were 'the Champion' Alastair Seeley, 33, 'the Novice' Gareth Keys, 22, and 'the Veteran' Jeremy McWilliams, 49.


Heart stress

Dr Sean Roe, from the Centre for Biomedical Science Education at Queen's University Belfast, explains what causes the heart stress and some of the factors that influence the results.

"The heart is controlled largely by two centres in the brain," said Dr Roe.

"The sympathetic 'fight or flight' response or the parasympathetic 'rest and digest' response.

"On occasions that require extra cardiac output the sympathetic nervous system kicks in, which speeds up the heart. Cardiac output, which is the amount of blood put out by the heart every minute, increases allowing the additional demands to be met.

"Alternatively when someone is relaxing, the parasympathetic nervous system is to the fore, reducing cardiac output by reducing heart rate, and increasing blood flow to the digestive system, taking it away from the muscles.

"Anything that increases fight or flight, such as exercise or emotional stress, will increase stress on the heart."



Harder, faster, stronger

Cardiac function is related to fitness and age. The better your heart is at pushing blood around the body, the less relative stress it will be under when your heart rate goes up.

Dr Roe explained how exercise can help the heart cope with stress.

"One of the effects of training is to increase the volume of cardiac chambers. To increase cardiac output a certain amount, a fit heart would need to increase heart rate only a small amount compared to an unfit heart, because the fit heart is pumping more with each beat."

He considers emotional stress to be among the main factors increasing heart rate for competitors in road racing.

"How stressful you interpret the situation to be would contribute to how fast your heart rate goes," he said.



The resultsDeadly risks

No matter how meticulously a rider prepares it is impossible to cover every eventuality. There is a thrilling unpredictability to road racing and this race was no exception.

The Novice was faced with a dog running out in front of him, causing his bpm to peak at 204bpm, while the Veteran had to contend with a fellow rider coming off his bike at high speed just metres in front of him, resulting in a huge spike of 220bpm.

Good core muscular strength is vital in road racing.

"A rider's core muscles have to withstand the forces of acceleration and braking and after a number of laps those muscles get tired. Tired muscles are harder to control and can lead to more mistakes later in the race," said Dr Roe.

Minimising the risk of mistakes is imperative in a road race, where the slightest error can have serious, even fatal, consequences. Dr Roe puts the risk road racers take in stark terms.

"Human reaction time is between 150 and 300 milliseconds. If a rider is travelling at 200mph, he is travelling 90 metres per second. In the one-fifth of a second he takes to react, he will have travelled about 18 metres. This means that if anything unexpected happens within 20 metres of a rider, a high speed collision can be unavoidable."



Preparing for the unexpected

"It is important to train to certain zones to be within 160, 170, 180 beats per minute. Train your heart to the particular zone you're going to be in when you're racing," explained Alastair Seeley, who won the feature Superbike race at the North West 200 in 2012.

Many competitors also ride motocross bikes in deep sand during the winter to prepare themselves for the physical rigours of road racing.

Gio Capello, a strength and conditioning coach who worked with Seeley, explained the benefits of carefully tailored training.

"During races, a rider's heart can get up to 180bpm at certain points so you want to develop more capacity to cope with those rates. We worked with Alastair on improving his aerobic fitness and increasing his VO2 max [the volume of oxygen you can consume while exercising at your maximum capacity]."



Psychological pressure

The Veteran is adamant that stress and pressure are always there in the build-up to a race.

"That's the side of racing that's not pleasant, the nervous energy, not knowing what's around the corner. Once the flag drops it becomes so much easier, all of that is gone."

The Novice prepares for the race by attempting to blank everything out and focus on the task ahead.

"I'll spend about a minute alone with the bike just thinking about who's in front of me and stay as calm as I can."

The Champion explains how there are practical steps a rider can take during the race to keep their heart rate down.

"Round the coast road section of the North West 200, it's quite busy and sometimes you maybe hold your breath for that bit too long, which then upsets the rhythm of the heart. But once I get on the big straights here I consciously take some big gulps of air and calm myself down again."



Life at 200mph

Despite the many factors that influence cardiac performance at high speeds, perhaps the most important thing for any road racer to have in their hearts is a love of the sport. Without this, it's unlikely they would take on such big risks at such high speeds.

Guy Martin, a regular rider at road racing events, said the risk factor goes some way to explaining the sport's attraction: "We all know it's a dangerous sport and we accept that. Probably the danger element is the reason I do it - I like the buzz."

Source - BBC Science - How the heart reacts at 200mph

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