Running a marathon is physically hard, but not necessarily for the reasons that some of us might expect, according to a new study of the relative physiological impact of completing a 42.195km race versus a 21.098km half-marathon.
The study's findings provide new insights into how runners might best prepare for a marathon. They also underscore possible reasons to consider a half-marathon instead.
Few past studies have examined what makes long-distance running events so demanding, especially for recreational runners.
So for the new study, in the Journal of Sports Medicine and Physical Fitness, researchers in Spain decided to track runners participating in the annual Madrid Marathon and half-marathon, both of which occur at the same time and on the same course for the first 21.098km.
The researchers hoped to assess what happens to recreational runners physically while running those distances and how any strain might affect their performance.
They examined the role that dehydration and muscle soreness might play in runners' finishing times.
The scientists recruited 11 experienced runners preparing for the marathon distance and another 11, similar in age, weight and racing experience, enrolled in the half. All were following training programmes provided by the race organisers, with the marathoners completing more weekly mileage.
The day before the race, the scientists asked the runners to visit their lab, where they drew blood and checked for baseline indications of dehydration and biochemical markers of muscle damage.
Each runner performed several standing vertical leaps. This test indicates how much force the leg muscles can generate. When muscles are tired, people cannot jump as high. The scientists noted the maximum height for each runner.
The next morning, just before the race start, the runners reported to a medical tent where they were weighed and fitted with patches to measure perspiration rates.
Then, they ran. All completed their respective race distances and returned to the tent to be weighed, provide more blood, answer questions about how they felt and repeat the vertical leaps.
Finally, the researchers compared data. The half-marathon runners, not surprisingly, proved to have had an easier time. They had lost body weight from sweating but were not seriously dehydrated. They also had elevated levels of blood markers related to muscle damage, compared to the day before.
But their levels were much lower than those who completed the full marathon. The half-marathon runners were able to jump higher after the race than the marathon racers. Their legs were relatively fresher.
Perhaps most interesting, the half-marathon runners had managed to maintain a steady pace throughout their race. In fact, most had sped up slightly near the end. Their average pace for the final 5km tended to be a few seconds faster than for their opening 5km.
The marathon runners, on the other hand, had slowed. Most had run at a steady pace for the first 21.098km, but then began to brake progressively, so that their final 5km times were much slower than the same distance at the start.
That may seem predictable, but Juan del Corso, a professor of physiology at Camilo Jose Cela University in Madrid, who led the study, said there were surprises in the data. For one thing, none of the volunteers in either race became clinically dehydrated. So dehydration had not made the marathoning difficult.
More fundamentally, the marathon racers, on paper, should have been prepared for the rigours of the distance, he says. They had run more mileage during training than the half-marathoners.
Despite the rigorous runs, however, their legs were not fully prepared for the marathon, he says. Their muscles became slightly damaged and grew sore, particularly in the second half of the race, and their pace slowed.
The implication of these results, he says, is that "just running long distances is not enough to prepare the leg muscles for the great demands of an endurance event like the marathon".
Instead, he says, targeted strength training of the lower body might be needed to inoculate the muscles against the slight tearing and other damage that occurs during the hours of pounding in a marathon.
Of course, this study was small, short-term and examined only a few aspects of distance running. But its underlying message seems applicable to all of us training for a distance race.
"Gym training", including the use of machines and free weights to develop strength and power in the muscles of your legs, "can very much help to prepare muscles for the stress imposed by these long races", del Corso says.