How the Nervous System Helps Control Balance

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Have you ever stepped on ice, a wet surface, or simply off an uneven curb and felt the sheer terror of losing your balance? I have. In fact, not so long ago I was innocently frolicking around in some water and noticed a fallen log. Obviously, I needed to conquer said log the only way I knew how—by climbing it, of course. In hindsight, this wasn’t the best idea, as both the log and I were wet. Sadly, my girl repeatedly telling me to get down was not sufficient forewarning for what proceeded to happen.

If you haven’t already guessed, I slipped. Yep, one misstep and my wet paws slipped right out from under me. I tried to recover—I really did. And when that didn’t work, I made a last-ditch effort to leap to safety. Alas, my efforts were futile, and I ended up stuck in the mud. Where did I go wrong?

Humans and other animals are able to effectively balance their body while moving, and we do so with what seems like very little effort. Provided the nervous system is functioning as it should, we are able to effortlessly maintain an upright position while we negotiate our world.

What exactly is meant by “balance”? 

Balance, or postural stability as it is sometimes referred to, is defined as the ability to control the body’s center of mass in relation to the base of support, or the area of the body that is in contact with the support surface (usually your feet). Basically, this means keeping the majority of your body’s mass within the boundaries of your base of support. This sounds simple; however, when we walk, our center of mass and the base of support are constantly changing.

How does the nervous system maintain balance?

As we move, the body has a sort-of intuitive way of correcting for postural disturbances. The nervous system is continuously making small postural adjustments to ensure our center of mass stays within our base of support. These include aligning the body to minimize the effect of gravity and maintaining good muscle tone to prevent the body from collapsing. Even when we are standing still, our muscles are constantly making small adjustments to maintain an upright position. This becomes more difficult as we move, especially when we manipulate objects or encounter obstacles.

When we are faced with more challenging tasks, the nervous system needs to use different strategies to ensure we do not lose balance and fall over. Depending on the situation, there are a number of ways the nervous system accomplishes this.

Strategies for maintaining balance

One of the strategies the nervous uses is a pre-emptive postural adjustment to minimize the risk of falling. These are known as anticipatory postural adjustments (APAs).  For example, if you’ve ever experienced walking on icy snow, it is probably safe to assume that you know it’s slippery—something I learned quickly after moving to Canada. To ensure that you’re not falling all over the place every time winter comes around, your nervous system adapts how you walk in the snow versus how you would walk on less treacherous surfaces. For instance, when walking on slippery surfaces, humans typically walk slower and more flat-footed. This reduces the angle and speed at which your foot contacts the surface, essentially reducing the risk of falling. Thus, knowledge of and experience with an upcoming slippery surface also leads to changes in how you walk.

APA’s don’t just adjust posture for different walking surfaces, but also for different tasks. For instance, after my less-than graceful plunge into the mud, I needed a bit of assistance getting out. When my girl came to help me out, she had to adjust her posture to ensure we didn’t both end up stuck in the mud. Research has shown that when we anticipate a perturbation, there is increased activity in the muscles that will help maintain balance. For example, pregnant women seem to intuitively walk while leaning backward. This is to offset the additional weight of their precious cargo and keep their center of mass within their base of support.

What happens when we don’t anticipate the perturbation?

When you encounter an unexpected disturbance, or when the nervous system is simply trying to control for natural postural sway, there are two movement strategies that the nervous system typically employs to prevent falling. These include fixed-support strategies (e.g., when your feet are fixed to the floor) and change-in-support strategies (e.g., when you can move your feet). The fixed-support strategies are known as the ankle or hip strategies and entail activating the muscles surrounding the respective joints to oppose a perturbation.

People typically make use of smaller ankle strategies first and if these do not suffice, then the larger hip strategies are usually engaged. If the hip strategy is still not sufficient to oppose the perturbation, then people will usually try increasing the size of their base of support to compensate for the movement of their center of mass. For instance, people may employ the step strategy. This is exactly what it sounds like: people take a step to increase their base of support and ensure their center of mass does not move outside of it.

For instance, say you’re standing on a bus on your typical commute to work, when the bus comes to an unexpected stop. As the bus slows, the body continues moving forward due to inertia, causing your legs to rotate over your ankle. The nervous system is able to sense that the bus is slowing down using specialised sensors (e.g., muscle spindles) in the muscles surrounding the ankle joint that detect changes in muscle length. This signal is relayed to the spinal cord (and eventually to the brain), and the body responds by contracting those muscles to prevent further stretching.

If this strategy fails, the nervous system activates muscles around the hip joint, causing the person to bend forward at the hip joint and push their buttocks backward. This shifts their center of mass to keep it over the base of support. If this strategy fails, the person can take a step forward, increasing their base of support and preventing them from falling. An additional response may also be to reach out and grab something to help you stabilize your body. Sadly, this one is limited to animals with opposable thumbs and therefore didn’t offer much help in my recent tumble.

What are the factors that affect balance?

As you’ve probably guessed, there are numerous things that can affect our ability to maintain balance. As our base of support gets smaller, for instance, if you stand with your feet together, it’s more difficult to keep the body’s center of mass within the base of support under an external force (like someone bumping into you).  When your base of support is larger, however, it’s easier to maintain balance as you have more room to play with. For instance, when you widen your stance on a train, you are more resistant to the forces produced by the train when it speeds up or slows down to a stop (assuming that you’re facing the windows).

The position of our center of mass also plays a big role in balance. For instance, when we lower our center of mass, we become more stable. Picture a double-decker bus and a sports car. The sports car has a lower center of mass and thus can turn corners at higher speeds without rolling, in comparison to the bus which is better off taking those corners with a bit more finesse.

Other factors affecting balance include the characteristics of the support surface you’re standing or traveling on.  In my case, the log was wet, which didn’t exactly offer much grip for my large masculine paws. Additionally, the compliance of the surface can play a role in how easy it is to maneuverer on. For instance, standing on foam or a trampoline is more difficult than standing on a firmer surface.

Last, but not least, is the capacity of the nervous system to detect these changes in posture and correct for them. The nervous system relies on numerous (sensory) receptors to send it information about the position and motion of the body. For instance, if the muscles spindles in the lower limbs are not functioning properly, the nervous system may not detect changes is muscle length fast enough to correct for a postural disturbance. Other import sensory information necessary for maintaining balance comes from the vestibular system, which the brain uses to determine the position and movement of our head. If this apparatus is not functioning correctly, it makes it difficult to maintain an upright posture and move effectively in our environment.

As you can see, the nervous system has quite a job on its hands to keep us upright while we explore our world. For the most part, us pups have it pretty good given our low center of mass and all four legs to work with. Sadly, that wasn’t enough to help keep me out of the mud.

 

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