How Can the Heart Adapt to the Tour de Ski?

When racers like Therese Johaug and Ivan Babikov reach the final day of the Tour de Ski, their bodies are already exhausted from day after day of racing. Their legs are tired, are they’re not alone: a recent study found that some aspects of heart function can not only decline, but remain suppressed after several days of consecutive races.

But thanks to the human body’s remarkable ability to adapt to repeated stress, Johaug, Babikov, and their competitors can continue pushing to the top of the hill climb, and a tired heart won’t even slow them down.

The paper, published in the European Journal of Applied Physiology, examined how heart function changes in response to multi-day stage racing, and found that while the stress of such events decreases some measures of heart function, the heart can actually compensate in other ways so that efficiency is maintained.

The team of South African researchers led by Dr. Tanya Oosthuyse embarked on moderately new territory with their study; while much research has been conducted on the effects of habitual exercise or single competitive bouts on heart health, very few studies have looked at how repeating that high-intensity activity day after day can change physiology.

Many of the previous studies had found that changes in heart function – for instance, a smaller diastolic dimension (the size of the chamber when it expands to fill with blood), a greater systolic dimension (when the heart contracts to push blood out), and differences in the velocity with which the chamber expelled blood – can last for varying periods of time. Some studies found that the heart returned to normal in less than a day; others found that the heart took several days to recover. The length of time seemed to depend on the specific measure of heart function, as well as the type and duration of intense exercise.

These findings inspired the South African team to ask a single underlying question: what would happen if the competition was repeated day after day? Would the heart recover more or less quickly, and what if it hadn’t returned to normal by the time the next day’s race started?

To answer these questions, the scientists worked  with cyclists. They chose test subjects in their 20’s and 30’s who rode an average of about two hours per day and competed regularly. They then told the men to bring their bicycles into the testing lab and set them up on a trainer with a video feed of the Tour of Flanders race course. Each day for four days, the athletes biked their way through Flanders, attempting to cover as much ground as they could in three hours and watching real-time footage of the course as they went. The testing was performed in pairs to add a sense of competition.

Overall, the researchers reported that diastolic function, or the way that the left ventricle filled with blood, did not return to normal after 24 hours, while systolic function, or how the left ventricle pumped blood out, did.

The athletes tended to perform worst on day three of the challenge, covering less distance and creating less power as they pedaled. They also had lower average and maximum heart rates on the third day. But by the fourth day, their performances once again matched those at the beginning of the trial. While heart rates on the fourth day were not as low as on the third, they still averaged eight beats per minute lower than on the first day.

How could the athletes perform just as well with a heart that was beating more slowly? This finding foreshadowed other changes in how their hearts functioned over the four-day period.

For instance, the amount of blood the heart was pushing out of the left ventricle in each heartbeat was the same before each day of competition. On the first two days, less blood was expelled after the “race”, showing that the heart was fatigued, but on days three and four there was no difference; the heart had adapted to the time trials.

The scientists also reported that after each day’s trial, the heart had reduced the rate at which it passively filled the ventricle. The ventricle draws in blood first by expanding in volume, then by using pressure; it was this first step that became less effective. However, the researchers found that the heart compensated by allowing a longer period of passive filling, and that on the third and fourth days of the trial, the athletes’ hearts showed a more forceful atrial contraction, which would push more blood into the ventricle. These two changes meant made up for the slower passive filling.

A last interesting finding had to do with how blood replenishes itself. After exercise, the volume of blood in the body actually decreases due to a loss of plasma, but the body regenerates plasma quickly, often within an hour or less. When the athletes were recovering on the fifth day of the study, their blood volume actually increased. This might suggest that the body increased its plasma as it had for the first four days, even though there was no actual race on the fifth day – in other words, the body had adapted to the stress of repeated competitions and hadn’t un-adapted.

What does it all mean? The South African team revealed a number of seemingly contradictory findings. First, they showed that in suggests that in multi-stage events like the Tour de France or the Tour de Ski, competitors are starting most of their competitions not only with tired muscles, but with tired hearts. They also showed, however, that the human body is capable of adapting to repeated exertion, and that it can change physiologically to compensate for fatigue.

So by the time Tour de Ski racers make it to the final day of competition, they don’t have to worry about their hearts; those will just keep chugging along. They may beat differently than they had before, but the skiers’ hearts will get them to the top of the hill – as long as their muscles can cooperate.

The Study: T Oosthuyse, I Avidon, I Likuwa, and A Woodiwiss, 2o11. Progression of changes in left ventricular function during four days of simulated multi-stage cycling. European Journal of Applied Physiology 14 October 2011 – published online ahead of print.