Human foot
A perfect suspension
A car crashing a wall is a great analogy for demonstrating how fore-foot striking is superior to heel striking in running. Let me talk you through it…
Human foot evolved to be an integral and complete part of a natural suspension system for his heavy body. It also evolved in a way that promotes a fore-foot strike running style. Despite those natural predispositions most runners today still heel strike and wear heavily wedged shoes, as if feet weren’t made for running and needed aid. This fore-foot strike vs heel strike debate has been going on for decades now and I felt I needed to chip in.
Disclaimer
Perhaps I should also preface the upcoming discussion with a disclaimer that I’m not a physician nor an anthropologist nor any kind of shoe or human body specialist. I am however a mechanical engineer and suspension systems are indeed a part of what I specialise in. This I hope should give my opinion a unique perspective. That of a physicist watching a mechanical structure and considering the dynamic forces acting upon it. Besides, I’m a passionate ultra-runner. The thousands of foot-miles I’ve covered so far plus the tens of pairs of running shoes I used in the process should also count for something. But leaning on authority isn’t a great practice in any kind of discussion so let’s see if we can come up with some agreeable line of argumentation here, shall we?
Ahh one more thing. Fore-foot striking, and heel striking are going to be talked about a lot in this post. The story will read lighter if we have them two running styles represented by two runners instead. We’ll call them Voytek — fore-foot striker and Joe — heel striker. With this out of the way let’s start with a fundamental question. Why does a runner need shoes?
Two reasons
There are two main reasons. Protection — all runners (both Joe and Voytek included) wear shoes to protect the foot both from a mechanical injury like a sharp stone or a thorn or from the elements like heat or cold. Aid — some runners, like Joe, wear shoes to aid their running style (cushioning) or correct their posture (pronation) or orthopaedic defect (arch support). That’s it. Protection and aid. Simple. True, there are a host of other factors that come into play when it comes to choosing a running shoe, like comfort, durability or weight of the shoe to name a few, but they are all secondary in the sense that they are a factor in deciding which shoe to buy but not a reason to wear shoes in the first place. So let us skip over that rabbit hole for it’s simply too deep for this article and go back to the two fundamental functions footwear can serve. The protection function remains mostly uncontested among the contemporary running community, except for an obscure group of barefoot runners (remember barefoot Ted from Born to Run?), who spend years growing enough callus and fat pad under their feet to be able to run 100 miles in rough terrain with no shoes at all. For the rest of us mortals though, it’s the shoe that provides the necessary protection, period. The aid function of a shoe, on the other hand, is much more controversial and that will be the narrow focus of the discussion that follows.
Humans are lazy
Let me base my first argument on Christopher McDougal’s popular theory that humans are born to run barefoot (I really recommend reading his book Born to Run for a more complete picture). In short summary, anthropologists generally agree that we had a hunter-gatherer past. And in that past we chased down our food for survival. Running was a life-sustaining skill then, similar to how we have to make money to sustain ourselves today. This was long before humans were intelligent enough to invent fabric or cover their foot for aid in running or even for protection. So, for generations our feet evolved to run barefoot, and we became good at it. In fact, we could consistently outperform our four-legged food and stay fed. It’s not that we were faster than our food, we were not. We were running upright which meant we could breathe out-of-sync with our pace. We could also sweat which greatly improved our heat management, especially in motion. Those and other differences gave us the edge that meant we could last long enough to hunt down the animal and kill it when it eventually collapsed out of exhaustion. That’s it for the book summary. Oh wait, did I mention Barefoot Ted yet? Anyhow, back to the core subject. Unfortunately for us, the contemporary folk, because we evolved into an upright creature the anatomy of our foot had changed to allow it. Our heel grew further back and lower to eventually touch the ground behind our toes (compare that to say a wolf or a cat whose “heel” is always high above ground even when standing still). We evolved to walk by way of rolling our foot from heel to toe which is natural and good because it allows our calf muscles to rest. Walking is a low impact activity, so this heel strike doesn’t cause any extra strain to our joints. It was also natural for the primitive man to lift that heel when he wanted to run because anything else was just painful and uncomfortable. If you doubt that try taking your trainers off and go for a run on a firm ground like track or meadow. You’ll notice that the faster you go the more uncomfortable it is to heel strike and you should naturally transition to a fore-foot strike. It won’t be your second nature yet, because of years of unnatural and aided running style, but it should still feel more comfortable than heel striking. So, it’s the shoes that make heel striking possible. The other aspect is that we’re lazy and we naturally lean towards whatever is easier. Keeping that heel up during running requires more effort so whenever possible we’ll naturally heel strike. When barefoot heel striking is uncomfortable, so we naturally strike fore-foot. Put the cushy shoes on however and all of a sudden, it’s possible to be lazy, i.e. keep the heel down and still run. The problem is that no amount of cushioning can damp enough of that heel impact, and we’ll talk about why that is in a second, but first let us consider the biomechanics of running. Not that I’m trained in this discipline specifically, but it really just is structural mechanics wrapped up in organic material — how much more complicated can that be?
Biomechanics
Running is a high impact exercise. The impact happens every time a runner strikes the ground with his foot. Its amount depends on a few factors, most importantly our weight and the vertical amplitude of our stride. While we can work on improving our vertical amplitude somewhat, there certainly isn’t much we can do about our weight once it’s optimal. The point is there’re limited ways we can reduce the total amount of impact our body takes during running. The good news is, it’s not the total amount of impact that matters for the wellbeing of our joints, rather how this impact is distributed over our suspension chain. The suspension chain consists of suspension links and each one of those links dissipates its part of the total impact. We can distinguish at least five links in the human suspension chain: ball of the foot, ankle, knee, hip and spine. Each one of those links (or an array of links in case of the spine) is a mostly frictionless mechanical joint wrapped up in soft tissue, i.e. muscles and ligaments. It’s up to the soft tissue to spring and damp the foot impact while running, for instance calf muscle provides the springiness and damping to ankle joint. An engineering analogy that helps me think about it is a car suspension (consisting of a spring and damper per wheel) that isolates its occupants from the road vibration in a similar way to how the soft tissue is critical in isolating upper parts of a runner’s body from the running-induced impact. Now let us look more closely how Voytek’s and Joe’s running styles compare in this context. The ball of Voytek’s foot takes the most beating as it’s the ground zero where the impact takes place. The impact induces a shockwave that travels through Voytek’s body. The foot is designed to take this impact raw (unsprang), and it has the fat pad underneath to take the edge off of it. The shock is partially dissipated through the complex structures of the foot to then arrive, already reduced, at his ankle. The ankle dissipates it further with the help of lower limb soft tissue (muscle and ligaments, mainly calf) passing the remainder to the knee and further up the suspension chain. As the shockwave travels up Voytek’s body its energy is gradually dispersed. Eventually it reaches his head at which point it is merely a residue of the original impact energy incapable of disturbing his focused eyesight. Now, on to Joe and his running style. For Joe ground zero for the impact is his heel. The cushioning in his shoe takes the edge off of the impact but most of it finds its way straight through his ankle and into his knee. There’s no mechanism between the heel and the knee that could effectively disperse it. Joe’s knees evolved to take a heavily reduced impact energy, but his running style means his knee has to cope with almost raw impact energy — the amount of energy that in Voytek’s case was last seen somewhere between the ball of his foot and his ankle. This near-raw impact is more than evolution had prepared the knee for. Put it simply Voytek is using his natural ability to damp each stride while Joe attempts to achieve the same using external aid, i.e. cushioned shoes. Human foot is a spring and damper system with most of its spring and damping in the foot and ankle. Joe’s bypassing of his natural ability to absorb the impact is similar to a mountain biker going full blast downhill with his bike’s suspension in “locked” position. The inflated tyres relative to the full suspension provide a similar amount of damping to Joe’s cushioned shoes relative to Voytek’s organic full suspension system. So, what is it about the shoe cushioning that isn’t as good as our natural ability to cushion the impact? Let me introduce you to passive safety engineering and the concept of “ride-down”.
Ride-down
A discipline that specialises in keeping the human occupant safe in a car crash event is called passive safety. A foot striking the ground, in a sense is similar to a car crashing a wall in that they both dissipate kinetic energy and the role of them both is to decelerate the passenger to a stop safely during impact. There are big differences of course. Car structure dissipates the impact energy through plastic deformation (i.e. deforming or breaking components) while foot dissipates it through damping. The foot does it in a cyclical way while the car is designed to do it only once. The scales of weight and velocity and therefore the energy involved are hugely different too, but those differences don’t matter for the point I’m trying to make. A well-known truth within this very narrow community of engineers is that
speed doesn’t kill, it’s the sudden loss of it.
In other words, high speed isn’t dangerous on its own because, as per the Newton’s first law there’s no inertial forces when speed is constant. It’s the rate at which it changes (deceleration or acceleration) that can be dangerous for a human body because of the inertial forces it introduces. If we decelerate slowly, no matter the initial speed, we’ll be fine. But if we decelerate suddenly, like in a car crash, the inertial forces acting upon us are so great they can break or tear a weak link, no matter the initial speed. So, a way to save life in a car crash is to make sure the deceleration peaks are reduced, and the passenger is brought to a stop as smoothly as possible using all the available ride-down distance of the crumple zone (most of what’s under the car’s bonnet).
Ride-down, simply put, is the distance (or time) from the moment a car hits an obstacle to the moment it rebounds.
The importance of it in passive safety is that this is the distance over which the human occupant should be brought to stop in as smooth a way as possible in order to reduce the risk of injury. The longer the ride-down the easier the job for the engineer. This is why a Smart isn’t as safe in crash as say VW Golf or a VW Golf isn’t as safe as say E-Class. Back to running. Let us consider the ride-down of Voytek’s foot, i.e. the distance between the lifted heel and the ground when the ball of his foot first hits the ground. Depending on how tired he is (and perhaps a few other factors) it could be 2 inches or more. The most padded shoes I’ve seen have an inch of cushioning which equates to an inch of ride-down, but most heel strikers run in shoes with much less cushioning than that. That’s a half or less of what evolution gave us in terms of ride-down. The passive safety guys would love to be able to just make the bonnet longer in their quest of making a vehicle safer as this is the easiest and the surest way of reducing the risk of occupant injury in a full-frontal crash event. The takeaway here is that the more distance we allow for our body deceleration the lower the impact energy that reaches our knee and the rest of our suspension chain is and hence the lower the overall risk of injury. Don’t be fooled by the marketing bs behind cushioned shoes. No matter how advanced the material or technology used in a cushioned shoe it can never be as effective as lifting the heel and giving your foot enough room to cushion the body. That is first principles physics. I’ve used car crash analogy because the consequences of a poorly decelerated occupant are far more gravely and thought provoking than a poorly decelerated runner. In running the damage is more subtle and it deteriorates our tissue more gradually, but over time fatigue will accumulate and eventually the weakest link will give.
Closing thoughts
I have three loose thoughts to close this off.
From my own observations and common-sense reasoning, I suspect both Joe and Voytek will likely get injured as they ramp up the intensity and volume of their running alas slightly different types of injuries are probable for them. Over time however it is likely that Joe’s frequency of injuries will increase while that of Voytek’s will decrease. This is because Voytek’s running style is in line with his nature and training strengthens his suspension system. Given time both hard and soft tissue will adapt to the amount and intensity of running Voytek does and eventually he should cease to have any injuries at all. On the other hand, the unnaturally high impact caused by Joe’s running style is abusive to his tissue (mainly cartilage and ligaments) causing damage at a rate potentially beyond the healing capability of his body. I’d be keen to see a study that tests this theory — let me know in the comments if you know one.
I used to heel strike myself as a matter of course and it wasn’t until a friend of mine pointed out the obvious to me that I started my transition to fore-foot strike. Since then, I’ve been paying attention to the running styles of my fellow runners more closely. I have met many runners who have transitioned from heel strike to fore-foot strike, some of whom I even coached myself through the process. On the other hand, I’ve yet to meet a runner making the transition in the opposite direction. No runner I know has ever come to a conclusion “naah, this ain’t working for me, I’d better go back to my heel striking style” or even more strangely “I’ve been a fore-foot striker my whole life and it came to me naturally, but now I’ve tried heel striking I feel like it suits me more”. True, the few tens of runners I have met personally aren’t a decent enough statistical sample, however I think the observation is indicative of a trend. I suggest you look around amongst your own crowd to see if you can observe a similar trend.
It’s not all negative about Joe and his running style. Heel strike has its merit too and can and should be a part of a competent runner’s acumen. It engages different groups of muscles to fore-foot strike and so in a long-distance run it can provide a great relief to his tired calves. It makes sense to switch to a heel strike especially during those slower uphill sections when there’s not much of a stride impact anyways and so there’s no significant downside to switching. Also heel strike can provide a better traction. The shoe-ground contact surface is larger and so more studs are engaged. In a low-traction conditions like snow, mud or sand it makes sense to alternate between running styles. Such bulky surface types are soft and they themselves provide enough cushioning to significantly reduce any impact shock before it even enters the runner’s body. Going back to automotive and crash perhaps you noticed those barrels with sand (or sometimes water) around the motorway exits. Their purpose is to disperse the kinetic energy of an impacting vehicle that hasn’t quite made the exit and bring the vehicle to a safe stop. Again, different scale but the same first principles physics.
