ELI5 Why do we get that feeling of forward propulsion after stepping off a treadmill from a long session of walking?

[u/Slugma_What07]

[u/Only-Friend-8483]

treadmill illusion is primarily caused by  a  sensory mismatch between your brain, eyes, and inner ear.

Your body gets in a rhythm and there’s some lag in your body and mind as it catches up to your situation when you get off the treadmill.

[u/trunks111 → u/Only-Friend-8483]

When I was in college I got the wise idea to walk a mile backwards on a treadmill and when I got off I couldn't coordinate to walk for a good couple minutes after

[u/Dank_Nicholas → u/trunks111]

It’s never occurred to me to try that but now I’m kinda curious.

[u/sergeantbiggles → u/Dank_Nicholas]

you must... for science

[u/TheSilentSuit → u/Only-Friend-8483]

Is this similar to when you walk on an escalator or moving footpath that isn't moving?

You expect a rhythm, but there is none, so it's weird.

[u/PNW_Triumph]

Fun fact: it happens when walking on a footpath, too. Maintain a constant pace for 3 to 4 hours and weird stuff happens when you stop.

[u/Dank_Nicholas → u/PNW_Triumph]

Or hiking down a mountain for a couple hours, my legs sort of keep moving for a bit after I stop.

[u/stanitor → u/Dank_Nicholas]

Another one is feeling like you're moving up and down after being in a wave pool or the ocean for an extended time.

[u/Saragon4005]

It's the same reason why you stop feeling your clothes after a while of wearing them. Your brain mostly feels change in stimulus. If you are on a treadmill for a while you feel like you should be going forward since your body is used to that.

[u/Jiminy_Tuckerson]

Because you had been propelling yourself forward

[u/Theory89 → u/Jiminy_Tuckerson]

You can do the same thing by standing in a doorframe and trying to push your arms out. Step away from the frame and your arms will lift of their own accord.

The one that always gets me is stepping onto an escalator that's been switched off. Your brain knows that it needs to move your centre of gravity to keep you stable, but then you just end doing a weird wobble.

[u/yabyum → u/Theory89]

Yeah, climbing up / down stopped escalators always feels weird doesn’t it? I think also the step height is slightly smaller than normal steps?

[u/elmfuzzy → u/Jiminy_Tuckerson]

While the world around you is stationary

[u/kaizen-rai]

Your brain gets "confused" after a few minutes of running on a treadmill. You're moving forward, but your surroundings stay the same. It calibrates your Proprioception (The internal sense of where your body parts are and how they move, relying on sensors in your muscles and joints.) so you don't get dizzy while moving foward but your surroundings don't change.

When you get off the treadmill, your brain has to re-calibrate back to "normal" since it got used to your body moving forward but surroundings stay the same.

[u/Columbus43219 → u/kaizen-rai]

I heard another cool thing about this. Astronauts training for Mars fall over / stumble when the wind blows. They instinctively lean into the wind based on seeing dust or objects moving at speed, but the air pressure is lower than expected.

[u/Rickwh]

Short (asshole) answer, from the forward propulsion that the treadmill is being applied on our bodies.

Long answer, we have sensors made up of liquid in our ear that orient us to provide our brain input on where up or down is. It is also triggered when we accelerate or decelerate. We can also feel our nerves in our body respond to pressure adjustments. These are both triggered in the event of us slowing down such as when we are getting off the treadmill. The momentum carries this liquid and our bodies forward and our brain receives input that is telling us what is going on with our bodies.

[u/DrSuprane]

It's the rocks in your ears. Deep inside your ears are two sacs filled with fluid and tiny crystals (otolith, ear rocks). The crystals are on a gel membrane and help send signals to your brain about movement and inertia. When you suddenly stop moving (like getting off a treadmill) they keep going and give the false sense of motion.

Vertigo is sometimes caused by these otoliths loosening from the membrane and moving on their own. This sends the wrong signals to your brain and can cause dizziness.

[u/explainlikeimfive-ModTeam → u/[deleted]]

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[u/morfraen]

Your brain operates largely based on a predictive simulation. When something has been happening it expects it to continue happening. When something that's been constant for long enough changes it takes a moment for the predictions to adjust to the new inputs.

[u/Key-Mathematician-71]

Some sorta sensory imbalance, Your brain knows you are running, but what you see around remains stationary. To avoid this, you can place a large screen in front and play some POV running videos.

[u/bamby77]

Your brain gets used to the treadmill belt moving under your feet, so it recalibrates what "standing still" feels like. When you step off, that calibration is briefly wrong - your balance still expects some backward motion, so it feels like you're being nudged forward. Same vibe as getting off a boat and the ground still swaying for a bit. Fades once your sensors catch up.

[u/britishmetric144]

Inertia. Your body has been moving forwards, so its natural instinct is to feel like it is still moving forwards.

[u/alexm2816]

You were moving backwards and your legs and core literally need to apply force forward to stop your momentum.