PHYSICS BEHIND PARKOUR

By Jerry Li Grade 11

Have you ever seen parkour in an ac3on film, where the tracers are sprin3ng on top of buildings and take massive drops without injuring themselves? One of my favorite things to do in the city is parkour, and I enjoy physics the most at school. I’m here to explain parkour in physics terms with the best of my knowledge. Let’s jump right in!

WHAT PEOPLE THINK ABOUT PARKOUR?

Allow me to clear something important first, in order to get you the right mindset to understand this content. Here’s a logical fallacy that people tend to easily fall into: the phrase “common sense” was defined in the English language to describe something that everyone is agreed with, but there’s really no existence of absolute common sense in this universe as it’s always depending on the unique personal backgrounds and experiences different people possess. Since parkour is not “common” for most of the popula3on in our daily lives, people who don’t know what parkour is tend to get it wrong. It’s neither an “insane dangerous sport” nor an “internet deep fake” as some might call it. Given an analogy: if a person doesn’t know how to walk, but tries to walk from standing posi3on without any knowledge and protec3on, that person is going to bang his head straight into the ground; It’s the same thing as someone who doesn’t know how to do parkour trying to leap a rooXop gap and end up falling off the building. I’m going to explain parkour in more detail using physics instead of giving the conven3onal “convincing” examples of people who already succeeded in parkour to “debate” about parkour in a subjec3ve way.

What The Physics Is Parkour

According to Wikipedia, Parkour (French: [paʁkuʁ]) is an athle3c training discipline or sport in which prac33oners (called traceurs) aaempt to get from point A to point B in the fastest and most efficient way possible, without assis3ng equipment and oXen while performing ar3s3cgymnas3c maneuvers. Couple of word equa3ons in my own understanding of the two main types of parkour:

2. Freerunner = Crea3ve Body Movements. Tracers purely train for gegng from point A to point B as efficiently fast as possible, while freerunners train for as many crea3ve movements as possible from anywhere possible. But that was the old

1. Tracer = Escape Mar3al Arts; days when the parkour communi3es were very isolated. In the 21st century that we live in, the parkour community has grown larger around the world, thanks to the internet, the film industry, social media… Tracers and freerunners started to train together, become friends, and learn from each other, combining all possible skills into what is known as parkour today.

Movements In Parkour

In the two main types of parkour, tracing and freerunning, all of them can be useful when implemented correctly. I know this because I do it myself, that the more movements you master, and the more you think about how you can implement these movements, the beaer you’ll be. For instance, a flip is oXen recognized as a freerunner’s maneuver, but in a real 3me escape, doing a flip is way beaer than doing a vault (a tracing skill) over an obstacle.

First of all, with good leg strength, it’s completely possible to just jump over an obstacle below chest height, but why sacrifice a lot of kine3c energies to gain those unnecessary gravita3onal poten3al energies instead keeping the center of mass low enough to go way faster?

How can physics be exploit so that a flip be more efficient than a vault?? Well, if you can keep running on a surface, then it must have fric3on force to support wherever you’re going: in this case, if you know how to do a flip and land a flip, in theory, your success rate over the obstacle is 100% 3mes the accidental rate –– the probability that you can make a mistake in a move; while your success rate in vaul3ng over the obstacle is (100% - x) 3mes your accidental rate, as “x” represents the probability of failing on the obstacle surface. Therefore, the accidental rate cancels out on both sides of the inequality, leaving 100% > (100% - x), a higher chance of success for doing a flip. The reason behind that is, you know the surface that you’re running on beaer than the surface that you’re about to vault over: what if the obstacle is not strong enough for your body weight; what if there isn’t much fric3on on the surface of the obstacle that you just can’t tell that you’re going to slip and fall…

Learning how to flip and improve flexibility are actually very helpful, as they get you a beaer understanding of the feel of the movements you can do with your body, while dealing with gravity on earth. For example, if you slip somewhere, or clip your foot on a curb between a huge 2 stories drop, knowing how to flip and being flexible can save your life! Think about it, if you’re not so flexible, you’d most likely get your tendons stretched before you could do anything else to control your muscles; and if you don’t know how to flip, you’d land a 2 stories drop on your head or back, which would not be ideal for your skeleton.

Speaking about landing, I would like to introduce to you… The great, fabulous, and pres3gious PARKOUR ROLL, being one of the most important moves if not THE most important move in parkour. The parkour roll is similar to a side roll in mar3al arts, but it’s a liale bit different, they have similar purposes, but different usage in terms of the needs for certain movements.

To do the parkour roll, place your hands on the opposite side of the shoulder that you’re going to roll over; keep your chin to your chest (looking to the side where you’re placing your hands would help), as you want to roll over from one side of the shoulder down across to the other side just before rolling on your hip bones. Since I’m right handed, in this case, I’m rolling over on my right shoulder. To do that, I have to place my hands to the mid-leX, look down to my chest and to the leX as I roll over. This way, I’m rolling over the soX parts of my body supported by adequate muscle groups. I also won’t hit my head because I’m 3l3ng it to the leX. (As shown in the images.)

WHY ROLL?

The purpose of Parkour rolls and mar3al arts rolls are both to reduce the impact for landings. Parkour rolls are generally done faster horizontally than mar3al arts rolls, since it’s usually used to land a speedy drop (over a gap or to the ground ) so that the pressures on the knees can be properly reduced; while mar3al arts rolls are designed for connec3ng fight moves; it is also commonly used to dodge an aaack in mar3al arts.

How exactly does the roll reduce impact? One way to think about it in physics terms is the study of mo3ons: rolling is the process of transi3oning impact from linear momentum into angular momentum. Bear with me. This is simply momentum, here’s a formula to set everything up: p

=

mΔv

(momentum = mass · the change in velocity)

The momentum formula studies how much mass of an object is in how much mo3on. Momentum is a vector, because velocity is a vector. If a scalar (mass) mul3plied by a vector quan3ty (velocity), it gives out a vector quan3ty. It’s important to understand the rela3onships these variables in physics even if the equa3on doesn’t make sense to you just now, which is completely normal. But what’s really intui3ve is that, once the rela3onships of these variables are crystal clear in mind, we can then apply them in the real world. In this case, it’s very easy to see that if either mass or velocity increases on the right side of the equa3on, with the premise that the rest of the variables stay the same (control variable), then the momentum increases. Let’s think about the change in momentum derived from the ini3al momentum equa3on:

p = mΔv –> Δp = |mvf - mvi |

Since we’re dealing with the mass of a human body, it will remain constant; Velocity however, changes, as shown in the formula. There’s an ini3al velocity (represented by the leaer “i” next to “v”), and a final velocity (represented by the leaer “f” next to “v”). All I did was opened up the Δv from the momentum formula to show that the change in velocity is the absolute value of the difference between the final velocity and the ini3al velocity. Now let’s move on to another formula that is defined to represent the rela3onship between the effect of the force ac3ng on an object over 3me and the momentum of that object––scien3fically known as impulse. As being said about the change in momentum, impulse is simply equivalent to the change in momentum, basically another angle to look at the same problem:

Δp

= |mvf - mvi | –> Δp = Jnet = Fnet Δt

Impulse, represented by the leaer J, is equal to the net force (the resultant of all forces ac3ng on the object) 3mes the change in Gme (from start of the landing to finish). We know that the smaller the force is, the beaer it is to our landing. Although, technically knee strength can be prac3ced properly to absorb quite large impacts, that is irrelevant for our mo3on analysis right now. This is the only rela3onship we need here: the greater the impulse, the harder the landing, the more force involved. What can we do to change this, in order to reduce the impact? Again, but last 3me, I promise. Let’s rearrange the equa3on and see what can we find in terms of the rela3onships of these variables from all the formulas we derived and went through. If you’ve no3ced, the only equa3on that contains force (F) is the impulse formula, so let’s use that:

Jnet = Δp = Fnet Δt –> Fnet = Jnet /Δt = Δp/Δt = |mvf - mvi | /Δt

By dividing on all sides of the equa3ons by “Δt”, we can see that, the greater “Δt” is, the smaller the force is, rela3vely in the equa3on. Note, that the net force is the average force during the landing, not the gravitaGonal field strength (the accelera3on of “gravity”: always about 9.81m/ s^2 on planet Earth) which is just a constant for accelera3on. Since a drop into a roll is a transi3on from linear momentum into angular momentum: it would result in a stretch in the 3me axis, which shrinks the peak force. Peak force reduced. Problem solved!

That is literally the whole purpose of rolling: crea3ng more 3me on the landing, and use that 3me to equally spread out the forces to different parts of the body to reduce impact.

Pressure On The Landing

Another important reason why a parkour roll can reduce so much impact and injuries is that, there’s way less pressure on your landing. Let me explain this:

P = F/A (Pressure = Force / the surface area that the force is applied)

This is a formula for pressure. The rela3onship here can be represented such that, the larger the surface area that the force is applied on, the smaller the pressure - meaning less impact. Vise versa, the larger the force applied, the greater the pressure - meaning more impact, which is not what we want in this situa3on. From the impulse formula, we could already minimize the net force by increasing the 3me variable. Now, we can plug that minimized force into this equa3on corresponding to pressure. The only thing that we can really mess around with in this equa3on is the surface area. Since the surface area of human’s back is larger than the surface area of the shoes under the 3p of the toes, rolling on soX parts of the back can avoid all the bones that are connected to the skeleton. This can mange to take down quite a bit of impact.

The rela3onship between A (the surface area) and P (pressure), is inversely propor3onal ––meaning the greater A is, the smaller P is. And we’ve minimized our impact. Done! I know, it’s just that simple. AXer learning the parkour roll, you’ll be so much less painful in a situa3on where you have to jump off from a good amount of height in order to escape for your life.

DID YOU KNOW?

The famous Newton’s (II) Law “F = ma” (force = mass · accelera3on) actually came from the intui3ve momentum and impulse formulas:

Fnet = Jnet /Δt = Δp/Δt = |mvf - mvi | /Δt = m · Δv / Δt = m · a

If we can spot that the change in velocity over the change in Gme is actually in the formula, as it just might not be so obvious while the change in velocity is already jammed in the momentum part (mΔv), we can actually discover that it is just mass 3mes acceleraGon. Isn’t that FUN.

Parkour Parkour

Physics does the work to assure safety concerns in parkour; It’s also fun to calculate something and implement it in reality. Knowing how to do something and understanding the purpose of doing something are two very different things. Some3mes, there are movements that I think are just too easy for me, but when I actually try to do them, it’s really not that simple. Hopefully you’ve got a beaer understanding of what parkour is aXer reading this ar3cle.

References, further reading, and links

Universal Parkour Defini3ons: haps://en.wikipedia.org/wiki/Parkour

Defini3on of Mar3al Arts Rolls: haps://www.akban.org/wiki/Roll

Parkour Roll Tutorial: www.youtube.com. (n.d.). Ryan Doyle - Parkour Roll Tutorial. [online]

Available at: haps://www.youtube.com/watch?v=RgPd29MlHpg [Accessed 13 Feb. 2023].

Momentum & Impulse Defini3ons: haps://byjus.com/jee/impulse/