Are Marathon Runners Really Endurance Animals?
by Peter Weyand
Covering the 26.2 mile distance from Hopkinton to Boston on foot is an intimidating prospect for most. Yet, this coming Patriots Day thousands of runners will toe the starting line in Hopkinton at 12 noon and arrive at the finish line in downtown Boston before the dinner hour. The top runners will make it to the Boylston Street finish line between 2:00 and 2:30 pm, middle of the pack runners will arrive sometime between 3:00 and 4:30 pm, and the remainder will straggle in sometime after 4:30 pm. Top runners, and other athletes as well, are often described as "animals" when their athletic feats approach or exceed the perceived limits of human performance: there is sound basis for this vernacular. As impressive as the marathon times that today's top runners manage are, they are unremarkable when compared to the performances of animal runners. An average dog toeing the starting line in Hopkinton at noon could arrive at the downtown finish line by 1:30 pm. A trained sled dog would arrive before 1:10 pm. Even a less capable animal runner, the miniature pig, would arrive downtown before 3:00 pm, and nature's swiftest endurance runner, the pronghorn antelope, could arrive comfortably before 12:50 pm and return to Hopkinton to pick up personal belongings before many of the top human runners reach the 20-mile mark.
While those who know me may think I made these comparisons for the comedic value that the image of short sows galloping by human runners on Boylston Street provides, this is only partially true. The larger question is whether comparisons of human and animal runners are any more informative than comparing apples and oranges. Indeed, these comparisons would have limited value if differences in endurance performance resulted from differences in the design and function of the equipment humans and animals use to run. However, this is not the case. Evolution has endowed all running species with the same figurative engines, transmissions, and wheels. The respiratory, cardiovascular systems that provide the engine power and the musculoskeletal systems that produce the running movements are made up of the same structural components, and these components work in the same way. The virtually identical nature of the machinery involved begs a simple question: what confers the greater speeds sustained by the top endurance runners?
The answer is equally simple precisely because the equipment involved is so similar. The differences in the sustainable speeds of different runners are determined predominantly by the size of their engines. We can describe these engine sizes accurately and intuitively in terms of the "metabolic horsepower" they provide. In the scientific literature, and sometimes in the popular literature as well, this horsepower is referred to as the aerobic power or VO2max of a runner. Aerobic power is a measure of how quickly oxygen can be extracted from the air and transported to the working muscles via the lungs, heart, and blood vessels to provide the energy for running. Because each liter of oxygen provides the same amount of energy in both human and animal runners (5 calories of energy per liter of oxygen*), simply measuring how much oxygen a runner takes in when running a high speeds provides the maximum number of calories a runner can burn in a given period of time. These burn rates, in turn, determine the speeds a runner can sustain, because the energy the muscles consume is directly proportional to the speed provided. World class human runners typically burn about 25 calories per minute at the limit of their horsepower; a burn rate that allows them to maintain a speed of 12 miles per hour for the marathon distance. However, pronghorn antelope can crank up to a metabolic power output of nearly 50 calories per minute, and therefore run twice as fast.
What then is the basis of these horsepower differences? In this specific comparison, and in general, metabolic horsepower is determined by the size of the engine and components. For example, the heart of the antelope is twice as big, and pumps twice as much blood per heartbeat, as that of the competitive
marathoner. Similarly, the antelope also has twice the number of blood vessels to deliver the greater flow of blood and oxygen provided by the heart to the working muscles. And, at the ultimate site of action, the working muscles, the antelope has twice as many of the structures that convert the oxygen delivered into the energy needed for running. Each of these structures, the mitochondria, converts oxygen into usable energy at the same rate, whether located in the muscles of a running antelope, human, or other species. Thus, the antelope's running muscles must have twice as many mitochondria to keep pace with the greater flow of blood and oxygen delivered by its high-powered cardiovascular system. Without these additional mitochondria, the antelope could not burn all the oxygen delivered, and its horsepower would not be so high. Conceptually, this story is remarkably simple. Because the amount of function provided by the engine components is the same in different runners, greater function is achieved simply by building larger components and assembling a bigger engine. This is true when comparing fast and slow endurance runners, whether dog vs. goat, horse vs. cow, or faster vs. slower humans. This concept not only provides insight into issues of animal design and function, but has considerable applied value, because the increases in function that are brought about through endurance training are the same as those responsible for making faster animals faster. Thus, athletes, coaches, therapists and clinicians also benefit from recognizing that the functional increases in endurance induced by training occur via increased heart size and increased number of both blood vessels and muscle mitochondria.
Perhaps the next time you witness the passing of thousands on Marathon Monday, you will appreciate that all the runners before you are indeed endurance animals, and that even the fastest humans, by virtue of their modest engine size, are stuck squarely in the middle of nature's pack.
*in scientific terms one liter of oxygen provides five thousand calories
or five kilocalories. However, in lay usage situations such as the labels
on food items, the calories provided are actually kilocalories.
Note: Here, I have focused on the primary factor responsible for the
differences in endurance running capabilities of different animal and
human runners. In largest measure these speeds are conferred by engine size, and engines with differing horsepower are constructed with virtually identical components. In comparing humans and antelope, I have chosen similarly-sized animals to simplify the comparison. The large differences in body size in nature have important consequences for running performance that I have not addressed here. These include the energy cost of running, and the portion of the maximum horsepower that runners can utilize for different lengths of time. I will save these topics for later discussion.