# Treadmill hypothetical **URL:** \ **Category:** Factual Questions\ **Created:** [March 11, 2014, 8:31pm UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471 "2014-03-11T20:31:38Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 12, 2014, 8:21am UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/21 "2014-03-12T08:21:28Z")
The only confounding issue here is that human legs are a complex system for moving something forward. In terms of momentum: When you’re running on a treadmill, you’re stationary with respect to the ground - if the treadmill suddenly vanishes, you don’t shoot forward. Your legs start to try to move you forward, but in practice, you probably stumble, maybe injure your ankles and knees, maybe fall on your ass - at best you end up standing still wondering what the hell happened. Simplify the system to a motorbike on a treadmill (wheels are simpler than legs) - if the treadmill stops or vanishes, the wheels spin on the stationary surface and the motorbike _ **begins** _ to accelerate forward with respect to the ground. It doesn’t instantaneously start moving forward at the speed it was running on the treadmill. The rider of the motorbike, in this scenario, feels the tug of acceleration just like he would with a standing start. If the engine is powerful and the wheels really grippy, the motorbike runs away from underneath the rider and he falls on his ass. ---
**Author:** ![Pixel\_Dent](https://avatars.discourse-cdn.com/v4/letter/p/bc8723/32.png) [@Pixel\_Dent](https://boards.straightdope.com/u/Pixel_Dent)\ **Post date:** [March 12, 2014, 11:34am UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/22 "2014-03-12T11:34:13Z")
This seems trivial to actually test and gather empirical data. Being a red blooded american my treadmill is, of course, covered in laundry so I’m unable to test it myself. Surely someone has a functional treadmill and can test it? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [March 12, 2014, 12:13pm UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/23 "2014-03-12T12:13:15Z")
> [@bldysabba](#): > > 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. For the treadmill, when someone jams it to make it stop, your body is stationary with respect to it, even though your legs are moving. You’d have to accelerate to continue running on the ground instead. When you first step onto the moving walkway, your body is stationary with respect to it, even though your legs are moving. You’d have to accelerate to continue running on the walkway instead. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 12, 2014, 1:16pm UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/24 "2014-03-12T13:16:59Z")
> [@Pixel\_Dent](#): > > This seems trivial to actually test and gather empirical data. Being a red blooded american my treadmill is, of course, covered in laundry so I’m unable to test it myself. Surely someone has a functional treadmill and can test it? It can be approximated by just jumping off the side whilst running at full speed, but the physics (apart from the mechanics of the legs) is really very straightforward. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [March 12, 2014, 2:06pm UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/25 "2014-03-12T14:06:27Z")
> [@drewtwo99](#): > > 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. I can second this. I had a treadmill stop; in my case the belt just stuck for a fraction of a second, then continued. I too fell forwards. My legs went out from under me and the only thing that stopped me falling flat on my face was that I grabbed the bar. For some reason I didn’t think to hit the stop button but tried to just roll off the machine…got myself some surprisingly nasty cuts for my trouble. But, as to why I fell…a lot of it was the disorientation, and not knowing where to put my next stride. Perhaps with practice you could continue running smoothly through such an event? ---
**Author:** ![TroutMan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troutman/32/6721_2.png) [@TroutMan](https://boards.straightdope.com/u/TroutMan)\ **Post date:** [March 12, 2014, 3:35pm UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/26 "2014-03-12T15:35:30Z")
To continue running smoothly would require your body to instantly accelerate to the speed at which the treadmill was formerly moving, which isn’t possible. With practice you could probably learn to quickly adjust and avoid a tumble, but it wouldn’t be a smooth unbroken stride. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 12, 2014, 4:19pm UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/27 "2014-03-12T16:19:27Z")
If your reflexes are good enough, you’ll break your stride and end up standing in place. If your reflexes aren’t good enough, then you’ll fall onto your back. ---
**Author:** ![Hampshire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hampshire/32/4887_2.png) [@Hampshire](https://boards.straightdope.com/u/Hampshire)\ **Post date:** [March 12, 2014, 6:50pm UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/28 "2014-03-12T18:50:46Z")
Thinking about it more I think it would depend on where you were in your stride. As you run your body remains stationary as each foot takes turns planting out front of your center of gravity, traveling backwardward underneath your center of gravity, and then finally lifted off the ground behind your center of gravity. If the treadmill came to an abrupt stop when your foot is planted ahead of you I’d expect you to fall backward. If it stopped when your foot was planted behind you I’d expect you to fall forward. Somewhere underneath you you may have a good chance of catching yourself standing. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [March 13, 2014, 1:12am UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/29 "2014-03-13T01:12:29Z")
In my treadmill stop it stopped as my forward foot planted and I fell forward onto my leading leg. Again, I think it about what pattern we are expecting to activate and how we respond when that circumstance does not occur. I was, subconciously, expecting that foot to be dragged backwards and planning to keep my center of gravity leaned just enough to stay over it as it did so. But not enough to drive forward over that leg of my own force and to have my center of gravity far enough forward to catch myself on my other leg. When it did not happen I think I tried to compensate and lean forward - hence collapsing onto my leading leg. Without that attempt to compensate I think the physics would have had me just fall straight down as my center of gravity was set back to where I was expecting my leg to be that microsecond, not over where my foot had stopped, so fall down (not back). Definitely did not fall backward. Definitely could not have kept running forward. ---
**Author:** ![Reyemile](https://avatars.discourse-cdn.com/v4/letter/r/46a35a/32.png) [@Reyemile](https://boards.straightdope.com/u/Reyemile)\ **Post date:** [March 15, 2014, 5:10pm UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/30 "2014-03-15T17:10:42Z")
When I accidentally hit the emergency stop button on the treadmill, I end up moving forward and slamming into the bar. However, 1) I’m seldom running full sprinting speed, and 2) the bard is less than two feet from my chest. I suspect that if I were going faster and didn’t have something to slam into/grab onto, I’d end up on my back. ---
**Author:** ![Athabasca](https://avatars.discourse-cdn.com/v4/letter/a/b38774/32.png) [@Athabasca](https://boards.straightdope.com/u/Athabasca)\ **Post date:** [March 16, 2014, 12:34am UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/31 "2014-03-16T00:34:50Z")
> [@Hampshire](#): > > (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? That happened to me IRL. Yes really. While I appreciate the theories above, no one realizes that humans actually have good reflexes and can stop, suddenly. Which is what I did. I did have a bit of a run on, but it wasn’t much. My brain engaged with, ‘hey stop running, the treadmill quit’. Now, full disclosure-- my treadmill had a belt problem which I didn’t get around to fixing for a while so after the first time this happened, I kinda started to expect it, but the first time, I remember it clearly. I gave a slight lurch and two steps forward, and grabbed the front console to steady myself. Human reaction time really is quite a lot better than most folks realize. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [March 16, 2014, 2:13am UTC](https://boards.straightdope.com/t/treadmill-hypothetical/683471/32 "2014-03-16T02:13:46Z")
> [@Athabasca](#): > > That happened to me IRL. Yes really. While I appreciate the theories above, no one realizes that humans actually have good reflexes and can stop, suddenly. Which is what I did. I did have a bit of a run on, but it wasn’t much. My brain engaged with, ‘hey stop running, the treadmill quit’. Now, full disclosure-- my treadmill had a belt problem which I didn’t get around to fixing for a while so after the first time this happened, I kinda started to expect it, but the first time, I remember it clearly. I gave a slight lurch and two steps forward, and grabbed the front console to steady myself. > > Human reaction time really is quite a lot better than most folks realize. My experience was not a theory; I really did fall over. Also **drewtwo99**. So maybe we can conclude that human reactions are really quite poor in some cases. However, I only do fast speeds on treadmills (I just use them for warming up prior to working out). When it happened I probably had it set to about 12.5 km/h (=7.7 mph) [Previous page](https://boards.straightdope.com/t/treadmill-hypothetical/683471.md?page=1)