Treadmill hypothetical

(no airplanes here)

Say you have a treadmill that is level to the ground with no railings around it. Centrally located in an open room with nothing around it.
A runner is going at a fast steady pace. Not sprinting but a high maintainable speed.
If the treadmill were to suddenly stop, not slow down, but actually come to an abrupt stop, what happens to the runner?
Would they suffer immediate damage to their legs or knees? Would they fall in any particular direction? Immediately collapse? Be left standing up?

If there is no slipping, and the floor is similar to the treadmill in terms of surface, I would expect the runner to continue running on with no adverse effect.

His legs keep moving forward, but there’s no inertia helping his upper body to keep up. I say he falls on his rear.

Probably reasonable guess. When the first foot falls after the sudden stop of the treadmill, it will attain traction on the immobile surface and try to accelerate his body without the benefit of the body’s normal acceleration stride: the initial lean forward, the relatively shorter and less powerfull first push-off step, the reflexive balance and kinesthetic adjustment.

The other possibility is that the runner doesn’t fall down, but strains something due to trying to keep balance or absorb the acceleration shock of the footfall gaining traction on the now-immobile surface.

Huh…I am not a physics guy, but I’d expect the runner to surge forward and likely jam his ribcage into the handlebar/control panel section of the treadmill…if there are no handlebars/control panel (like with the treadmills in a gym), then I’d expect him to surge forward and possibly fall forward.

you would run right off the stopped treadmill.

That’s exactly what would happen…in a perfect world. In our world, the runner would go flying forward and either end up just a bit ‘shaken up’ or with some bruises or strains from trying to stop before he hits a wall or from falling. If he wasn’t expecting it, I’d be surprised if he didn’t fall on his hands and knees.

It would be normally for him to try to stop when he ‘hits the ground’, but with all that momentum still pushing him, he’ll probably fall over.

If he was expecting it and had plenty of space in front of him, he’d just run right off the treadmill and slow down at his own speed.

If he expected it, it would be equivalent to entering one of those moving walkways they have at airports. If he wasn’t expecting it, it would still be like that, for someone who wasn’t expecting it. Surely there are YouTube videos? My Google-Fu is weak…

He would start flying…

The runner has no forward momentum with respect to the ground; if the treadmill stops abruptly, it effectively becomes the same as the ground - he’s not going to zoom off like a clockwork mouse - there’s no reason for that to happen.

I think Keeve got it right - his legs run out from under him and he probably falls backwards - he’d be trying to do the human equivalent of a wheelspin start.

What Mangetout said. It is the reverse of someone jumping off the back of a slow moving truck. If you stand at the back and just step off, your body will keep moving even though your feet stop, and you will fall on your back. If you run off the back at the right speed, you could land stationary on the ground, but you would have to stop running. If you kept going, you would, again, fall backwards.

I don’t think this is true. When you jump off a truck, your body has momentum while the ground does not. When you’re running on a treadmill, this is not true. No part of your body really has any momentum that has to be compensated for except the foot that is in contact with the treadmill, but that doesn’t matter because your other foot is already in the air and you can proceed to continue running as though you had been on stationary ground throughout. I think it would make no/minimal difference.

It’s not equivalent to walking onto a moving sidewalk because you definitely have horizontal inertia when you step onto a moving walkway.

It’s not equivalent to stepping off the back of the truck.

I have had this situation happen to me almost exactly as described on a treadmill, and I can tell you what happens.

Once, while running on a treadmill, the belt had a small tear in it, and what happened was it caused the belt to fold in on itself and jam, bringing it to a stop very quickly and jamming it entirely.

What happened to me is that I lost my balance and almost fell over on my next step or two down, and had to grab onto the handlebars. It was very scary. I was running at 7mph I believe. Definitely no slower than 6mph. I heard the loud noise of grinding to a halt, lost my balance and freaked out. I would have fallen on my ass or face if I didn’t have something to grab onto.

Unfortunately my memory is not good enough to state for certain whether I would have fallen backward or forward, but I seem to recall falling forward, as if the rug had been pulled out from under me, leading me to feel like I was falling forward. But it happened in an instant and I caught myself, so I can’t be sure.

My WAG is that you’d fall flat on your face.

Running (and walking) is sometimes compared to falling forward and catching yourself each time because you know how to predict the fall: controlled falling. On the treadmill you are not doing that controlled falling forward; you are staying above the same point. Suddenly hit a stationary point and you drive yourself into the forward fall but are totally unprepared to catch yourself. Kersplat.

You do, but so does the moving walkway. If you’re moving at the same speed as the walkway, when you first step on it, you’ll be stationary WRT to it, even though you’re moving relative to the ground.

I was thinking there must be surveillance videos of someone distracted happening to walk onto one of them.

But you’re not totally unprepared. Your other leg is moving into position to come up in front of you.

But that’s not the same thing either. In the treadmill system, the only things that have any momentum are your legs and the conveyor belt. If the treadmill switches off and comes to an abrupt halt, your legs should be able to switch from belt moving backwards to upper body moving forwards without any difference. In fact, I don’t think anything changes.
As a thought experiment, consider the kind of treadmills that don’t have a motor. If you were running on that, with nothing in front of you except ground at the same level, and someone suddenly jammed a brake on the treadmill, you would just continue running on the ground instead. Pretty much the only difference with motorised treadmills is that there’s a motor that was pushing the belt back, and since that’s stopped, you’ll have to put in that much extra energy to maintain the same speed while running. I doubt that the difference would be so large as to cause a fall.

drewtwo99 has it close; what would happen is the person would lose their balance on the first step onto the now stationary surface, quickly try to re-establish it, and stop trying to run by the next step; perhaps falling perhaps not.

The manner in which they’d lose their balance would be for their upper body to fall back (actually straight down) while their hips would thrust forward during the first running step; depending on how hard their were running they may or may not actually fall before regaining their balance.

You could approximate this by standing in place on one foot in a running stance as though you were about to take the next stride (not in a starting position, but as though you were already in motion and standing pretty well straight up). When you then pull that supporting foot back keeping the foot in place your hips will come forward, your leg will bend and collapse, and your upper body will fall back as a result.

However, unless your were in a full sprint and really forcefully using your leg, I doubt there’d be enough power in the movement to actually pull your hips clean out from under you in one stride; your brain would catch on that something was wrong and cut the running motion/shift weight around within a fraction of a second.

Leg moving forward is not the same as being prepared to catch yourself falling forward.

On the treadmill I am prepared to stay in the same place balancing my center of gravity vertical as my leading foot is pulled backwards and I move the trailing foot into a forward position at which time I basically hop and then balance vertically on the other foot. Repeat.

Running I am using my leading foot and previous inertia to drive my center of gravity forward, ahead of my vertical plane, push forward with it as it becomes the rear foot to provide more forward inertia, and then catch myself with the other foot that I have driven forward.

They are very different. Transitioning from one to the other instantaneously? From expecting my foot to be dragged backwards as I maintain my balance over it, to trying to throw my center of gravity forward over the foot? Equals falling forward.

A moderately close personal experience: my old treadmill belt began to wear out and stuck, briefly stopping while running (the tension had gotten loose so it hitting the right spot caused the friction between the belt and the platform to be greater than the friction between the rollers and the belt, so it stopped suddenly, albeit briefly). I fell forwards towards the control panel more or less collapsing onto my leading leg as I caught myself.

I have a somewhat related question, about the mechanics of “climbing” a StairMaster.

I put a new thread here about it.