When you think about it, the simple action of walking–most of the time anyway–involves balancing on one foot, leaning forward, landing on the opposite foot, then doing it all over again.  Walking seems easy enough.  When I was in the tenth grade of high school I took three 50 mile hikes and never fell once.  But many years later, an engineer who designed robots informed me that one of the hardest things to design is a humanoid robot that can walk without falling over.  Balance is central to walking and running, and it is not an easy thing to accomplish by using electronic circuits. 

Ambulatory movement  is not simple.

Humans learn to walk by repetition.  At some point in their development, babies learn to balance on two feet then start the complex tasks of putting one foot forward, heel first, then pushing back against the floor with the toes, followed by the other foot doing the same thing.   After a short time, a part of their brain called the cerebellum takes over and automates things, so the toddler can choose which direction to go.  Meanwhile, deep inside both inner ears, a set of connected tubes called the semicircular canals sends signals down the spinal column through a set of nerves that help automate things even further, making sure that the toddler can maintain their balance automatically. 

To help things along, tissues all over the body called proprioceptors ensure that we are able to maintain our balance, despite changes in the various surfaces upon which we are walking or running.  Over time, we subconsciously catalog the requirements for walking on these different surfaces in order to be able to navigate them properly at our next encounter with them.  Jumping from rock to rock in a quarry strewn with boulders requires a different set of walking procedures than is found while navigating a city sidewalk.  Not only that, but while navigating an unfamiliar environment, our eyes are tasked with looking for objects in our path that might cause us to fall.  That is, our vigilance is more fully engaged than it is when we are in a familiar environment.

Despite our ability to navigate all kinds of different surfaces, our expectations of what will be required of our internal navigation apparatus–our proprioceptors, the semicircular canals inside our inner ears, and the rest of the vestibular system, including our vigilance system– play a large role in how we navigate our environment. 

If we assume there will be no dips, bumps, holes or protruding objects projecting from an otherwise flat surface, say in a grocery store, we will simply keep our eyes on the produce and dial back our vigilance.  After all, it’s not easy to check the produce price tags when your eyes are aimed at the floor.   This is where automation comes in.

Life being complicated enough, the brain does its best to automate the things that are repetitious.  It is why, for example, once you learn how to direct your car (or bicycle) down the road you can do it later without consciously being concerned about steering properly.  Your brain already knows how to do this and has somewhat automated the process.  This is the result of what clinical psychologist (and chemical engineer) E.R. Hilgard has termed divided consciousness. Your brain’s job of automating the driving process is what allows you to listen to a favorite song on the radio, or to consider what kinds of challenges your job will bring you after you reach your workplace.  Given a repetitive task like driving or walking and the brain acts like any software program designed to automate a task (for example, Excel’s macros.) 

It all works great, most of the time.

As a bonus, unlike some Excel macro, if the brain sees something out of place during the automation process, it will let you know it.  If a deer jumps onto the road directly ahead of you, you will likely step on the brakes immediately. This is because your consciousness isn’t completely divided–some parts are still being vigilant–watching for potentially dangerous stuff up ahead, whether you are driving your car or walking through your neighborhood grocery store.

And yet, falls happen.  Often, it is because something in the environment was different from what your brain expected. 

No one plans to fall, either by a slip, where the foot suddenly slides forward; or a trip, where the foot is stopped from forward motion, causing the rest of the body to pitch forward.

As a safety professional, I occasionally encounter instances where people were injured as a result of a fall of some kind.  In none of my cases did the victim see it coming.  The fall came as a total surprise. 

The statistics show that a lot of people fall.  According to U.S. emergency room statistics in 2023, more than 8.8 million people visited hospital emergency departments for injuries related to falls.  And, according to the National Safety Council, in the same year (2023) 47,026 people died in falls at home and at work.[i] 

Injuries due to falls are extremely common.  Falls can be also be deadly, and they can happen to anyone.

So, if the brain is watching for dangerous things in our path, why do we still drive, run or walk into things that end up damaging our car if we’re driving or lead to a fall if we’re walking?

Much of the reason is because the automatic part of your brain only has the same fund of knowledge about dangerous things that you do.  And most people who go about their lives, don’t know much about such things as the coefficient of friction between the soles of shoes and an asphalt tile surface, shoes and the oil/water interface within plant stems, shoes and wet leaves on a hard surface, or the fall risk from stepping on some random abandoned polyethylene lid resting on tiny pebbles about the size of individual grains of cat litter.  Nor are the brains of average people aware that the leading edge of a well-worn carpet-covered stair tread may LOOK as solid as the rest of the stair tread–and it may not be (years ago, I was fooled by THAT one, and in my own home, to boot.)

In fact, some paint jobs can fool the brain into making all kinds of incorrect assumptions about walking surfaces.  Every now and again you may see a set of outdoor steps where both the tread (the horizontal part) and the riser (the vertical part) are painted the same color–often yellow (for safety?).  As a result, the nosing or edge, where the tread meets the riser may seem to simply vanish, especially for folks with relatively poor eyesight. I once had a case where this paint design was in place on steps leading to the door of a community hospital. 

The brain is doing its level best (no pun intended) to map our path ahead.  Often it goes back to the past encounters with similar objects in the environment.  The last time you walked down a flight of stairs, you probably didn’t consciously notice that each step was exactly the same as the one that came before–and the one that came after.  That is, the width and treads (the horizontal part) were all uniform–similar throughout the flight of stairs.  Similarly, the risers (the vertical parts) were uniformly the same heights.  The International Residential Code (R311.7.5.1) specifies that risers must not be more than 7 and ¾ inches in height (excluding carpets, rugs or runners) and the tread shall not be less than 11 inches (if no nosing is included.)  For industrial sites, the OSHA standard 29 CFR 1910.25 allows a bit more leeway with a chart allowing the rise/run ratio to vary based on the angle of the staircase to the horizontal,  from 6.5 inches/11 inches (at 30 degrees, 35 minutes) to 9.5 inches/8 inches (49 degrees, 54 minutes.)

So, the takeaway is that the stair rise/run ratios will be generally within a certain range around 7 and ¾ inches for the rise and around 10-11 inches for the run.  And uniform.  No rise/run surprises on the way up or down the flight. 

One way to fool the brain is to have a stairway that varies a great deal from the above rise/run values.  These can show up in hard surface paths between differing elevations.  Contractors may throw in a few steps along the way that vary from both the IRC Code and the OSHA standard.   Here, the pedestrians are faced with is essentially a set of non-uniform stairs–long runs of several feet followed by vertical drops of only a few inches.  Something like this can throw people off balance and they won’t know why.  In some instances, they may blame themselves for not “paying attention” when the problem was an irregularity that fooled the brain’s vestibular system.

Coefficient of Friction

For our purposes here, the coefficient of friction is simply a measure of how much force is necessary to move one surface (for example, the hard rubber sole of a shoe) over a second surface (such as a tile floor.)  The more force it takes to move one surface (say a neoprene shoe sole) against the other (usually, a floor) surface, the safer it is to walk on.  Intuitively, the *less* force it takes to move a neoprene sole over a floor surface, the easier it is for one’s shoe to lose traction.  Generally, property owners do not want customers to be able to slide their way across the floors of their various properties.  Entire books have been written about the coefficient of friction, and there are precision devices that can be used to determine the coefficient of friction between two surfaces.  There are also lists available that detail the coefficient of friction between a variety of surfaces, from aluminum and mild steel (0.61) to waxed wood and wet snow–0.14[ii]. You can find a comprehensive list of coefficients of friction for various surfaces at www.engineeringtoolbox.com . 

The  coefficient of friction between two dry surfaces will change considerably if other materials are placed between the surfaces, i.e. in the interface.  The addition of water, oil or even small particles of concrete will usually decrease the overall coefficient of friction.  A neoprene or hard rubber shoe sole on a concrete garage floor covered with tiny bits of gravel such as cat litter might have a high coefficient of friction.  But by placing a random hard plastic lid between these two surfaces, the overall coefficient of friction could drop considerably, resulting in a hazardous walking surface.

Tripping Hazards

Other factors involved in falls are objects and imperfections in the path upon which people will stub the toe of their shoe.  That sudden coming-to-a-stop of the forward foot disrupts the walking rhythm and can lead to a fall.  When it doesn’t lead to a fall, the poor individual is seen galloping several steps ahead at a fast clip in an effort to regain their walking rhythm.  Small imperfections in the walking surface are often a factor in trips, and, depending on the walking pattern of the individual, the imperfection doesn’t have to extend much above the surface before it poses a problem. 

One case where I served as a safety expert involved an electrical conduit that extended a few inches above the floor surface.  Apparently, the employees had all been warned of the hazard and they still tripped over it on a regular basis.  While slipping hazards may be nearly invisible, tripping hazards may be so small that they are hard to see.  And, those who have not been informed of the problem would likely not spend much time scanning the floor looking for such things in their path.  Besides conduits, improperly set brick floors and irregular surfaces can lead to tripping hazards.  

Falls are common in the workplace, and especially in the construction trades, where approximately 20,000 non-fatal fall-related injuries are reported each year.

Falls from Heights

Up to now, the falls that have been discussed are generally those that involved one level, i.e. fall to the floor or ground.  Another category of falls are those that occur from one level to another, i.e. falls from heights.  In the construction trades fatal falls from heights occur at a rate of around 600-700 per year.[iii] [iv] [v] The circumstances are varied, but often involve an absence of fall protection systems.

Children can fall from heights

An 1989 article in Journal of Trauma by JR Hall, H.M. Reyes, M Horvat, JL Meller and R. Stein, “The mortality of childhood falls,” noted that falls represented the seventh leading cause of death in all children 15 years of age or younger, and the third leading cause of death in children 1 to 4 years old.”

“Forty-one percent of the deaths occurred from ‘minor’ falls such as falls from furniture or while playing; 50 percent were falls from a height of one story or greater; the remainder were falls down stairs.”[vi]

Children also can–and do–fall from bleachers.  I cover that kind of fall in the next chapter.

[i] https://injuryfacts.nsc.org/all-injuries/deaths-by-demographics/top-10-preventable-injuries/?

[ii] This sounds about right.  I once went skiing in wet snow at a place called Snoqualmie summit in Washington state.  Thanks to a bad skiing maneuver (and NOT the coefficient of friction) I broke my left tibia in 26 places.  And snapped my fibula as well.

[iii] https://safetyequipment.org/working-at-heights/

[iv] https://blog.dol.gov/2024/05/02/taking-a-stand-to-prevent-falls

[v] https://www.nsc.org/workplace/safety-topics/slips-trips-and-falls/slips-trips-and-falls-home

[vi] Hall, JR et al “The Mortality of Childhood Falls” J. Trauma Sep 29, 1989 pp 1273-5.