The marathon world record-holder is always mentioned in my introductory lecture in exercise physiology as an example of the astonishing things that humans can do physically. While few students can run a mile in less than five minutes, elite marathoners are doing that for 26 miles.
However, with the appropriate training and diligence most healthy people can complete a marathon. It’s not surprising when you consider for millennia our feet were our primary mode of transportation. Our physiology is geared to respond elegantly to the physical stress of running.
When the gun goes off, a runner ramps up energy systems, especially the aerobic energy system. The rate and size of breaths is increased to bring in more air and oxygen to support the aerobic system. The cardiovascular system increases blood flow through the heart, beating faster and with greater force. Blood flow is specifically increased to the lungs and to the exercising muscles to deliver oxygen.
The marathoner’s primary fuel source comes from carbohydrates stored in the muscle in the form of glycogen and from the liver breaking down glycogen to keep up blood glucose. Stored fats are also mobilized for fuel. Proteins can be broken down into amino acids for fuel, but the marathoner’s body prefers carbohydrates and fats. Over 26 miles, runners often will consume gel packets and sports drinks to supplement the carbs on board.
One byproduct of expending all this energy is heat, making body temperature go up. Sweating and blood flow to the skin helps get rid of heat. Warm and humid conditions make the body heat up even more, and if the pace stays too high the runner may overheat, as the cardiovascular system may not be able to meet the demands of both exercise and thermoregulation.
The sheer distance of a marathon taxes the capacity of these systems. When a runner hits “the wall,” it could be from depletion of stored carbohydrates or not enough taken in along the way. Fat stores can’t be metabolized quickly, so the pace must slow down. Runners fail to finish the marathon for many reasons, including cramping, the physiology of which is poorly understood.
The stress of running 26 miles causes microscopic injury to muscle fibers as well. An inflammatory response occurs that results in the muscle soreness and stiffness felt in the days after the race. Given adequate time, the marathoner’s body will recover, repair and adapt.
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