“The secret of my success over the 400 meters is that I run the first 200 meters as hard as I can. Then, for the second 200 meters, with God’s help, I run harder.” —Eric Liddell
When a sprinter’s legs ease into the starting blocks before a race, they know a few things. Pressure-sensing nerves in the skin of the feet know exactly how dense the ground is beneath their cleats. Stretch receptors in the tendons know just how much tension pulls taught muscles and the bones they lace together. Position-sensing nerves called proprioceptors know where each joint lies and how much further they can bend when the body above them leans over the line. As the runner drops into position, tenses just slightly, and waits for the pop of a starter pistol, her legs are brimming with information. While they don’t know what that first step will look like or how far off the finish line lies, what they—and everyone who is watching them—will be sure of with that explosive first leap forward is that human legs were made to run.
The mantra of experts around the world is that running is nothing more than a controlled fall. But as the breathtaking precision of an athlete’s stride can attest, it is a fall our bodies learn to take faithfully. With each step, the legs seamlessly progress through four phases as a foot plants firmly onto the ground (Strike!) rocks forward to springload the joints (Stand!), thrusts the runner into the air (Drive!), and swings forward to landing position again (Leap!).
But while the propulsion of a stride is powered by the thrust between foot and track, only a small proportion of the time—as little as 30 percent in the best runners—is spent in contact with the ground. The vast majority of a runner’s race is spent in the air in a moment known as the double float—when the hind foot has launched the athlete forward into a parabolic flight and they are connected to nothing. Because it is most commonly followed by another successful footstrike, this phase looks graceful, and has been the inspiration for everything from the winged sandals of Hermes to Superman’s powers. But though this controlled fall looks to many like soaring, physiologists, athletes, and the parents of toddlers alike know it for what it really is: The double float is a crash waiting to happen.
In that instant before the foot finds its way back to earth (or doesn’t, in the case of the crash), a complex net of neurons connecting the toes to the brain to everything in between is processing information, making predictions, and deciding on adjustments in order to ensure that each stride is not the last. The stride of an accomplished runner may seem to be as natural and repetitive as breathing, but the act of running is a complicated blending of both willful and automatic nervous input, with each step being completely unique.
The neural web woven throughout the entire body pulls streams of information from every type of sensory nerve; this not only allows the runner to react to the environment around her but allows minute and precise adjustments in gait and posture in just milliseconds. Touch and pressure receptors in the skin, bone, and joints report on the rigidity and uniformity of the running surface. Those messages incite the legs to react with more toe flexion as they slog through muddy grass or rotate the foot to match uneven pavement. Position sensors throughout the hip, knee, ankle, and toes provide rapid and precise information on the location, rotation, and velocity of each swinging joint, allowing the brain to make predictions on the location of likely footfall and which abdominal and pelvic muscles to activate in order to stabilize the stride. Specialized muscle fibers and tendinous structures called golgi bodies sense the length and rate of contraction of each muscle and the tension being applied upon the bone. At the same time, the eyes are scanning for obstacles and changes in topography while the vestibular system of the inner ear reports on the position of the body in order to maintain balance.
With each step, this data is merged with memories of previous steps, innate reflexes, volitional action, and inborn neural circuits. The result is translated into rapid adjustments in the tone and contraction of more than 200 muscles. But speed of this magnitude is not accomplished by the brain alone; just relaying that information to and from the higher motor cortex of the brain would decrease an athlete’s reaction time. Instead, the body relies on feedback loops throughout the spinal cord, midbrain, and brainstem, the more primal areas of the nervous system, which activate responses that the runner doesn’t even think about. For example, a stretch of the Achilles tendon in the calf activates the golgi bodies within it and shoots a signal to a second neuron in the nearby lumbar spine. This connection then inhibits the movement of the opposing muscles—the plantarflexors—ensuring that it the leg is not working against itself. At the same time, clusters of neurons in the spinal cord and midbrain, chiseled by years of training, are remembering the circular pattern of the sprinter’s stride (Strike! Stand! Drive! Leap!) and are partially taking over the repetitive action, helping to create the most efficient step possible. The result is the churning perfection of the 100-meter dash and the magnificent resolve of the marathon.
These reactions, though, are hard won. While these loops are present in all humans, we still have to learn them. A toddler will fall 17 times an hour on average, but each fall is a lesson, and the very fluid connections of her nervous system are gradually educated into circuits that know a thing or two about where to put a foot. The upper and lower neurons learn where the leg is in space, how it feels to balance atop it, and at just what angles it will support her body. A toddler’s step is perfected not through instruction or observation, but through practice. Each stretch of the leg into space is one of faith, a controlled fall that is often more fall than control. As an athlete trains, these processes of correction, adjustment, and learning are the same, but become faster and more efficient, to the point that they are so finely tuned as to be imperceptible. The biggest difference between the drunken meandering of a toddler and the machine-like perfection of an Olympic athlete is only the number of times that step of faith has been taken.
Lindsay Stokes is an emergency physician living in Albany, New York. She earlier wrote on heart rest (diastole) and automaticity for The Behemoth.
