by Anne Kessler
According to the fossil record, endurance running capabilities first evolved roughly two million years ago, approximately four million years after the initial evolution of bipedal walking – a divergent moment in evolutionary history leading up to the human species. Fast forward to present time. In America alone, over 500,000 people complete at least one marathon in a given year. Because many people run races of distances that exceed 26.2 miles (ultra-marathon) and many runners never register for official races, the actual number of endurance runners in America likely exceeds one million people. Despite the fact that the majority of human beings find endurance running an abnormal and entirely painful behavior, it has only continued to rise in popularity across people of all ages and abilities since the initial “running boom” of the 1970’s.
So, why do so many people participate in endurance/marathon running? Perhaps one reason is because we are the only species that can.
Most mammalian species can out-sprint humans with ease because of their ability (and our inability due to our bipedalism) to gallop; however, regardless of their size, these mammals exhaust after a very short distance. Even the specialized mammalian quadrupeds (i.e. canines) that can trot for relatively long distances begin to overheat and fatigue once a gallop pace is sustained for a reasonable amount of time, regardless of the temperature and/or environment. According to the work of Lieberman et al., humans are able to successfully engage in endurance running because we are the only living species capable of handling both the biomechanical and physiological demands of a marathon, which include proper energetics, stabilization, and thermoregulation.
Certain derived physical features, altogether unique to humans, play an important role in endurance running. First and foremost, humans have large tendons (i.e. Achilles, IT, etc.) in their legs, which are absent from primates, that store the elastic energy responsible for pushing forward during the second half of a runner’s stance. Similarly, a number of lower body muscles, including the gluteus maximus, are enlarged in humans (sorry to be the bearer of ‘inherently human’ bad news). The g. maximus is one of the strongest muscles in the human body, and although it rarely contracts while walking, it plays an important, powerful role in running. Looking also at upper body anatomy, humans have both a narrow waist (relative to the rest of the body) and a mobile thorax that functions independently of the neck, which not only stabilize the center of mass of the trunk but also allow for full arm and trunk rotation. These features further provide the balance, stabilization, and upper body relaxation required for marathon-distance running.
Although these features are all important for endurance running, the human method of thermoregulation likely plays the most important role in managing the physiologic demands of a marathon-distance run. Over evolutionary time, humans emerged with no fur and a large number of high density endocrine sweat glands that, in combination, allow us to release heat from our bodies via evapotranspiration. These heat dissipation mechanisms are efficient and far superior to those used by most other mammals, which often involve ‘panting’ – a process that is not only less efficient but also greatly interferes with an important component of any type of running: respiration.
Although it is quite intriguing that humans are the only modern species capable of endurance running, the true physical beauty of the sport is that given the appropriate amount of time and proper training, almost any person who is capable of running has the potential to improve their endurance running abilities. And not just a little, but by a lot. Both routine endurance running and marathon training, which involves runs of various times and distances over a three to four month span, result in major physiologic adaptations to the imposed ‘running stress,’ improving and preparing the body for future endurance endeavors.
Kevin and Keith Hanson, founders of the Hansons-Brooks Distance Project, owners of Hansons Running Shops, and creators of the Hansons Marathon Method, refer to this phenomenon as the ‘Overload Principle,’ which is the idea that regular exposure to a specific exercise will enhance certain physiological functions and therefore induce a training response. This means that over time, various changes in tiny, biological functions of the human body result in rather large changes in muscle adaptation and both oxygen and energy utilization – ultimately leading to improved performance.
Routine endurance running stimulates positive adaptations in both the heart and its periphery. Looking at the heart, endurance training yields increased coronary circulation, left ventricle thickening, ventricle chamber growth (endurance runners literally have bigger hearts), and decreased pulse. This means that more blood is able to reach the heart, and more importantly that the heart is able to pump more oxygenated blood to the arteries with not only a much greater force but also a significantly reduced effort. The impact of these cardiac adaptations is further enhanced by changes that occur in the blood itself. Studies have shown that total blood volume increases in endurance runners, typically resulting in a higher number of red blood cells in an increased, less viscous total volume. As a result of these changes and the increased capillary density that occurs in the muscles (in this case, the legs), more oxygenated blood reaches the exercising muscles in an improved, more efficient exchange.
In addition to increased capillary density, many important adaptions happen in the skeletal muscles as a result of endurance training, including mitochondrial growth, increased mitochondrial density, and improved mitochondrial enzyme levels and function. Since mitochondria are the “cell’s powerhouse,” utilizing fatty acids or carbohydrates to produce energy, these improvements give endurance runners ‘more bang for their buck’ – producing more energy at the same rate and maximizing fatty acid utilization all while conserving the body’s precious glycogen stores.
In addition to the aforementioned sub-cellular adaptations, routine endurance running also alters both slow- and fast-twitch muscle fibers. Despite their namesake, slow-twitch fibers play a key role in endurance running due to both their high fuel efficiency (resulting from high capillary density, high mitochondria density, and a high oxidative capacity) and extreme fatigue resistance. A runner’s basal level of slow-twitch fibers is genetically determined; however, endurance training fosters the development of the aforementioned fatigue-resistant slow-twitch fibers and also aids in the development of type I fast-twitch fibers (which play a key role in endurance running once the slow twitch fibers have been exhausted). In comparison to other fast-twitch fibers, type I fibers not only have a higher fatigue resistance, but also have higher mitochondria density, capillary density, and oxidative capacity. These specific muscle fiber adaptations can occur in as little as 10 weeks of routine running, yielding improved endurance/performance and preventing a runner’s legs from hitting the previously unavoidable ‘wall.’
In the field of endurance running, an individual’s anaerobic threshold – the moment when aerobic pathways provide some energy for muscle contraction, but they are no longer fast enough to provide all of the energy required for the exercise – is more often than not the best predictor of their endurance performance. Taken together, all of the aforementioned adaptations improve performance by increasing a runner’s anaerobic threshold. Through a large volume of aerobic training, endurance improves resulting in a reduced reliance on less efficient anaerobic pathways. By increasing the anaerobic threshold, endurance runners are better equipped to utilize fatty acids as energy (an oxygen-dependent process), ultimately preserving carbohydrate stores and preventing the body from ever reaching the point of total exhaustion.
But why did humans start running in the first place?
Lieberman et al. loosely hypothesize that endurance running evolved as a necessary method of hunting, as hominids and early Homo species had to run down and track their ‘moving food’ over long distances, as they lacked the tools (tipped spears, bow and arrow) required to efficiently kill from afar. As their mammalian prey fatigued, our evolutionary ancestors needed to maintain enough energy to approach closer for the kill. If this hypothesis held and ‘endurance hunting’ was truly a means of survival, I wonder how many pre-human Homo species would qualify for Boston?
- Lieberman, DE and Bramble, DM. Sports Medicine 37(4-5), 288-290 (2007).
- Lieberman, DE et al. J of Human Evol 53, 439-442 (2007).
- Humphrey, L. 2012. Hansons Marathon Method: A Renegade Path to Your Fastest Marathon. Boulder, Colorado, USA. Velo Press.
Annie got her start in science at the University of Wisconsin-Madison where she earned a BS in medical microbiology and immunology. Currently, she is a PhD candidate at Albert Einstein College of Medicine, studying pediatric cerebral malaria with a focus on host immunology and outcomes in HIV and malaria co-infection. In collaboration with the Blantyre Malaria Project, Annie travels to Malawi during the ‘rainy season’ to collect and process the patient samples required for her thesis work. Outside of the lab, she enjoys running, searching for NYC’s best cup of coffee, and rooting for the Wisconsin Badgers.