Magic in Motion

Why Moving People Don't Fall Over

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Humans are bipeds. One of the unique features of our species setting us apart from virtually all other sentient organisms. Like other mammals, we have four limbs but only two are dedicated to getting us from here to there and back again. Some other creatures – witness the kangaroo or the ostrich – also walk on two limbs exclusively, but in very different manners. A number of animals, including many primates among them our closest relatives the chimpanzee, can walk on their hind limbs from time to time should they chose to do so, but lack one important characteristic – grace.

Bipedalism is thought to be the defining feature of human evolution, with suggestive evidence in our African birthplace as far back as 6 million years or so, appearing first as a facultative condition, meaning our distant ancestors could walk on two legs, but kept four-limbed locomotion – including climbing ability – a viable option for several million years. At some point however, members of our lineage chose two-legged movement as the best – and only – way to get around. We became obligate bipeds. At one and the same time we both freed our hands for many other purposes and enslaved our feet for but one.

Scholars have put forward several hypothetical models to explain this very profound transition – hands not committed to movement could carry items, fashion and use tools; being upright enables better surveillance over open ground, and minimizes heat load in a tropical environment. It is also said to be more metabolically efficient – bipedalism costs less energy whether walking or running. None of these models are mutually exclusive, and any or all of them could have factored into this dramatic evolutionary shift over time. But here is the thing – humans are truly bipedal only when we are standing still!

In walking, our gait consists of two primary phases: (1) the stance phase in which one leg has contact with the ground through a sequence of events transferring body weight in motion, and (2) the swing phase in which the opposite leg is moving forward through the air. The stance phase is initiated when the swing leg meets the ground (called heel strike) and our forward motion transfers our entire body weight to that supporting foot. As that foot absorbs the full weight of the body (at mid-stance, and the primary arches of the foot take up that force) we are effectively unipeds! The stance phase ends with heel lift and toe-off for the supporting foot, and the (once) swing leg makes heel strike with the ground. That moment – toe-off of the stance leg and heel strike of the swing leg, is the one and only moment in walking when we are truly bipedal. The duration of this bipedal moment is determined by how fast we are walking – it lasts longer if we move slower. And that moment never occurs when we are running!

All of this begs the question: if we are effectively unipeds, and more so when moving quickly, why are we not falling over? Our body center of gravity runs right down the midline thus, when we support our weight on one leg the natural tendency will be for that line to shift toward the unsupported side and all else being equal, we should fall over in that direction.

But we don’t – why? We owe this little mechanical miracle to two things developed over many millennia of evolution in the transition from four-leg walking to two-leg walking: a re-design of our pelvic skeleton, and a re-purposing of the action of two of three magical butt muscles, Gluteus minimus and Gluteus medius. (the third gluteal muscle, Gluteus maximus, as the name suggests, is that large fleshy muscle you spend a chunk of your day sitting on and fills out the seat of your pants!).

Chris Stringer, Peter Adams, Public domain, via Wikimedia Commons

So what happened? Let’s begin with some basic anatomy. The ape pelvis (ours, chimpanzees, gorillas etc.) consists of paired bones formally called the os coxae (aka the hip bone or the innominate) which meet at the front of the body as the pubis and are joined at the rear by the sacrum. The hip bone itself is formed from the fusion of three separate bones during growth: the ilium, ischium and pubic bones. Our concern here is the ilium, the largest of the three which provides a considerable flat surface area for many muscles to attach around the hip and lower back. In non-human apes such as the chimpanzee and gorilla, the ilium is very elongated, flat and facing rearward (posterior). In ourselves, and our bipedal ancestors, the pelvis has been reconfigured: the ilium is shorter and curved such that much of its outer surface area now facing toward the side of the body (lateral). Metaphorically, we contrast a pelvis with two large, long rearward-facing plates (chimp) with a compact bowl having a laterally facing expanse (humans) to which the Gluteal muscles attach.

One effect of this re-design is its impact on the action of G. medius and G. minimus. Action refers to what muscles do when they contract. In animals such as chimpanzees these muscles, attaching to the back of the ilium and top of the thigh bone (femur) but oriented to the posterior of the animal act as extensors – they help to lift (extend) the leg backwards during locomotion. In humans, modifying the shape of the pelvis changed what these muscles do - they now act on the side of our hip as abductors. Abductors? Literally think abduction, as in ‘taking away’. Hip abductors take the hip away from the midline of the body (that is, the line defining the center of gravity). In a standing posture these muscles contribute to lifting your leg to the side, away from the midline (rather than backwards as in the chimp).

But here is the crux: in movement, these abductor muscles contract on the stance side of the body, resisting the tendency for your torso to fall toward the non-supporting swing leg. In doing so they re-position the body toward the midline and center of gravity. And there you have it.

With every step you take these muscles work their little magic. Left, right, left, right, left…. and you don't fall down! Thankfully, this is something you don't have to think about it.