I think I am missing something when it comes to how this works for dissipating energy.
If you fall completely vertically onto level ground, you still have to stop the entire vertical component of your speed. I see how keeping your body straight in the air and curling up once you hit the ground can help, but how does adding the horisontal movement help?
One thing that comes to mind is it allows different body parts to absorb the impact, which could bring any one impact below a critical level where something breaks. The roll would be needed to engage these other parts. It also allows the core to transfer right to the ground, so that does not have to transfer through the legs.
You redirect what would otherwise be completely vertical deceleration into the horizontal, thereby giving you an additional distance to decelerate. You can also dissipate additional energy through other body parts, reducing the impact on the feet and legs.
It’s the redirection I don’t completely understand. If you subtract the initial vertical velocity vector from the horizontal velocity after you start rolling, you get a longer vector than if you had just stopped.
Coming in from and angle would be different though.
I don’t think the roll is used when dropping vertically. Parkour folks seem to use the roll when they have horizontal velocity. The roll lets them continue horizontally, while absorbing the shock of the vertical component. They don’t have to stop both movements at once.
Without the roll, they either have to come to a standstill, or be instantly running after the land, which won’t be possible from the crouch they need to be in when absorbing the vertical drop. From an energetic roll, it’s easy to get right back up and moving.
Isn’t that the advantage? In practicality it means literally more area and time to spread the force over.
When landing from a parachute jump, you are initially taught to fall sideways with your knees bent and ankles loose. This has two benefits: one is that it helps prevent getting tangled in the rigging if you haven’t completely killed forward speed (beginning parachutists often land too soon into the flare instead of playing it out), and second it prevents hyperextension injuries to knees and ankles or breaking the tibia by keeping the legs loose. The energy and momentum are ultimately the same but get distributed throughout the fall instead of concentrated in a sharp impulse. (More experienced parachutists develop the skill to control their landing such that they walk right out of it but I’m not that good, nor do I care to be.)
For parkour/freerunning athletes, rolling out of a fall is as much about maintaining and using forward momentum as it is cushioning the blow. By going into a directed roll they can pop up with continuous movement or transition into a vault. It’s as much about looking pretty as being functional because static “superhero landings” often end with residual hopping for balance, something even experienced gymnists struggle with.
Stranger
I am sharing this video (not entirely SFW due to language, so be warned), from one of my favorite youtube series THE SCIENCE! which breaks down the actual science of a wide variety of video game elements. It’s not always as accurate as a professional scientist would like, but it is very entertaining. This episode deals with parkour and has a pretty good explanation.
It discusses wall runs, but starts to focus on falls quite soon, falling roll discussion starts at 4:20ish. The discussion is impact vs impulse. The short version, is if you increase the time spent slowing, you reduce the impact and spread it over a wider portion of your body.
Most of the episode is about energy needs to accomplish the activities of the game though, which is also fun, but not about the subject of the thread.
I’m trying to figure a situation in which a falling body has ONLY vertical direction, with no horizontal. I mean - maybe if you are falling off a building. But in such situations, I doubt many folk are able to roll at the moment of impact. Parachutists generally are moving in some horizontal direction, no?
We used to practice falling A LOT in martial arts. Of course, all of our applications would have involved considerable horizontal motion. The main concerns were to protect your head, and to achieve progressive contact with as much of your body as possible. For example, better if you can land on your leg, then trunk, then slap the ground with your arm - as opposed to bouncing down on your hip. I remember one teacher saying, “You may never get in a fight, but at some point you ARE going to fall.” When you don’t panic, and your main thought is to protect your head and NOT try to brace your fall with outstretched arms, you realize how much time you actually have when you find yourself in the act of falling.
I fall more often than average, to the point my gf makes fun of me, and yet I haven’t gotten as injured as I’d expect to get. I do roll and I do think about the fall. It’s almost as if I’m falling in slow motion. My last “bad fall” I had my phone in my left hand and stumbled on the last two steps going to the basement. When I got up I was fine, my phone was fine, and I was able to play back the fall in my head.
You can’t roll out of a vertical drop from great height. At lower heights, say from the top of a step ladder, you have the movement of your body to redirect your motion towards horizontal. You also don’t want to land flat on your back if you want to roll, you need to hit the ground rolling, your body already curved and turning into the roll. Just like your martial arts training (I took judo, we did nothing but duckwalk and fall), you want to use your arms and legs to slap the ground to dissipate some of the force, but also to bounce and roll off them to redirect it. Also, slapping the ground makes it sound like a much bigger impact than it is.
Professional wrestlers have a lot of expertise at this. They will sometimes hit the mat hard for effect, and it hurts. But they are also adept at rolling out of drops from between 5 and 8 feet off the ground. After practicing it a 1000 times and doing in matches another 1000 times their calloused up bodies don’t notice the impact at all and they roll through the falls without thinking about it. How high a fall could someone survive like that? In real life it gets pretty dangerous around 10 feet even when you land on something less than solid.
My all time worst “fall” wasn’t even much of a fall. I was walking down some tiled steps in my home. I was wearing shoes w/ somewhat slippery soles that were wet from having stepped outside, and had both hands full of books I was going to take back to the library.
My left foot slipped off the front edge of a step. The toe struck the next step down, and I lost my balance, causing me to essentially just slowly sit down on the stairs. But as I did so, my left ankle just bent further and further backwards. A couple of plates and 13 screws later… 
You should see me now - I grab for EVERY bannister! 
A friend of mine who was in the military was taught that there should be three main points of contact: feet, hip, shoulder, but all his jumps ended up being feet, ass, back of the head. He didn’t think it was funny when I pointed out that that’s still three main points of contact.
He also has wrecked knees.
Jump out of enough airplanes and you’ll wreck pretty much everything. After one particularly spine-swashing landing, I decided that while the freefall part was kind of fun, the landings were hard and the trip up (in a rickety Twin Otter that always seemed to be straining to take off, and jammed asshole to elbow with pot-addled ‘recreational’ jumpers) was harrowing to my delicate sensibilities. I much prefer swimming with sharks, running Class V rapids, and even glacier travese to parachuting.
Stranger
I’ve got no experience parachuting, but pretty much the most important consideration is to tuck your head - whether falling forward or back. Taken a slight step further, you roll over one shoulder or the other, not right over your head.
There were nights that we would do NOTHING but a hundred or more forward or backward rolls for a significant portion of that night’s training. It just becomes an instinct, rather than something you think about doing.
But yeah - the most common military injury I see medical records of is knee/back injuries from parachuting.
On the first day of judo they teach you to slap the mat. As for mae ukemi, there are techniques with or without rolling but you are supposed to rotate your head and/or direct the roll down the length of the arm to cushion the blow. Caveat: something like that, in any case you do not go straight over forward and strain your neck.
One way to intuit this is to hold a banana by the stalk and drop it from a short distance onto a table - the tip of the banana hits first, but because it’s curved, the banana rolls along its convex edge, coming to a stop in a longer time than if you just drop it from the same height so it lands flat, where it stops all at once.
It wouldn’t be possible to do this if bananas or humans were point masses or rigid straight rods, but we are neither.
This may tie in to the Presidential fitness thread, but when I was in primary school, in gym they had training where everybody had to climb on top of these towers (just a few feet, I suppose) and jump off to practice landings and rolls. Do they still make kids do that today?
The banana example is interesting, but I think it is useful only because it is a rigid body, and it is the only way to distribute the impact over a wider area.
For humans there is another option, that is to start by being perfectly straight and then sort of collapse on ourselves like a spring being compressed. Maybe it will involve a little bit or rolling, but am I correct the resulting horizontal movement isn’t useful in itself when dropping vertically?
You have to absorb all the energy with your legs that way. Just falling over or rolling in any direction will redirect some of that impact into your body and arms. It’s the basic parachute landing described above. Certainly good if your practiced at it. If not, well my old judo instructor would tell you (after his rant about why judo was the best martial art superior to all the others) that people landing on their feet will tend to break foot and ankle bones, snap their shin bones in half, destroy their knees and their hip sockets, and then fall forward or backward with their hands out to break their fall but actually breaking their hands, wrists, elbows and shoulders. Then your head hits the ground causing brain damage. The guy was demented, seriously.