The scientists also asked each runner to perform 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 each runner could achieve.

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 finished their race, whether it was the half or full marathon. Afterward, they returned to the tent to be reweighed, provide more blood, answer a few 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 in the blood of the racers completing the full 26.2 miles. The half-marathon runners also could jump higher after the race than the marathon racers. Their legs, by all indications, were still relatively fresh.

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 five kilometers tended to be a few seconds faster than for their opening five kilometers.

The marathon runners, on the other hand, had slowed. Most had run at a steady pace for the first 13.1 miles, but then began to brake progressively, so that their final five kilometers were significantly slower than the same distance at the start. Those runners with the highest subsequent blood markers of muscle damage had the greatest decline in pace. They also reported much more soreness in their legs after the race than the half-marathon runners.

These findings, on their face, may seem predictable. Most of us might expect that running a full marathon should be more tiring than striding half as far, and that the more your muscles hurt, the slower you will be.