I don’t recall the circumstances of this question. It probably came up because I’ve been doing a lot of walking recently, in an effort to get myself healthy. But my son asked it. “How do we walk?”
That’s… that’s a really good question. A question that I don’t know I’ve ever contemplated before. I take walking for granted – I can walk, I watched my son learn to walk, it’s just sort of something we do. But, he’s right. How do we walk? I know that it has something do do with having bones (so our legs don’t collapse into puddles of meat when we move), and muscles (to move the legs), and balance (to keep us from tipping over). But… how does it all work?
Unsurprisingly, everything starts in the brain. The whole brain, in a complicated series of feedback loops. Much of what follows is a “chicken and egg” scenario, but you can loosely think of the process as working like this:
- The decision to walk is made in the forward portion of the frontal cortex, which is also sometimes known as the “frontal lobes”.
- The frontal cortex and the sensory cortex (along with other portions of the brain) send impulses to the basal ganglia.
- The basal ganglia activates the motor cortex.
- The motor cortex signals the cerebellum about the intended movement.
- The cerebellum takes information from the motor and sensory cortexes, and then provides the motor cortex with the direction, force, and duration of movement.
- The motor cortex sends commands to the muscles through the spinal cord.
- The muscles contract.
Simple, right? Well, more like simplified. A more detailed look at the process can be found on Brain Connection in The Anatomy of Movement. Suffice it to say that there is a lot going on in your brain when you move, and I’ve barely scratched the surface of it here.
It’s not all just the brain, though. If it was we’d have grey and white matter instead of muscle and bone. And it turns out that there have been a number of studies about human walking, in order to be able to build robots that walk like humans and in order to develop better prosthetics. I’ll be drawing from three articles from the Journal of Experimental Biology in discussing the mechanics of walking:
- Insights into the evolution of human bipedalism from experimental studies of humans and other primates
- Impulsive ankle push-off powers leg swing in human walking
- Angular momentum in human walking
Human walking involves a relatively stiff-legged gait, comparable to pole-vaulting. Our center of mass rises as we push off to take a step and lowers when our heel hits the ground. Although we usually think of our muscles as doing the work of walking, by contracting when the leg is in the air and then extending when the foot hits the ground, our tendons are actively involved as well. They help store energy in a similar fashion to springs, by stretching as the leg bears the weight of supporting the body and then recoiling when the foot pushes off, amplifying the efforts of the muscles as they contract. This boost from the stored energy in the tendon actually gives our legs more power than they could generate with muscle alone.
A forward shift in body weight helps with walking as well. You lean forward slightly as you walk, meaning your weight slightly pulls you in the direction you want to travel. The faster you move, the more you lean forward – meaning your legs have less mass to work against. Then, as you slow, your body returns to a more upright stance.
What, then, keeps us from falling over? Our brains, of course, drawing on signals from our ears.
The National Institutes of Health’s page on Balance Disorders discusses this in some detail. We have something called the vestibular system, which is primarily a structure in the inner ears called the labyrinth. The labyrinth contains the semicircular canals, the utricle, and the saccule. These organs combine to tell your brain which way your head has moved, the position of your head with respect to gravity, and whether your head is moving. This information feeds into your sensory cortex, and gets incorporated into the feedback loop described above.
So, how do we walk? Through an extremely complicated process of neural and muscular activity. It’s fascinating stuff, and probably something you shouldn’t think too hard about while walking.