Why Do Your Legs Go Numb When You Sit on Your Heels?
― The Pins and Needles Are Your Nerves Waking Up
At a memorial service or a tea ceremony, you sit on your heels for a long time, and the feeling in your legs slowly fades. Then, the moment you try to stand, tingling and prickling rush in. These are actually two separate events. The first is "nerves short of oxygen". The second is "nerves that have got their blood back, running on empty".
At a family gathering, you have been sitting on your heels for about 20 minutes. At first, the tops of your feet only hurt a little.
Before long, everything from the ankles down feels heavy and vague, as if it isn't your foot. Even if you pinch it, you can hardly tell.
When you finally stand up, countless needles seem to prick the soles of your feet. You stagger without meaning to.
The reason is two events, one after the other
When you sit on your heels, the weight of your upper body rests on your shins and the tops of your feet. That pressure becomes greater than the pressure inside the thin blood vessels. When the blood stops, the nerves run out of oxygen and cannot send signals.
When you stand and the blood comes back, the nerves begin to recover. On the way, they fire off signal after signal, even though nothing is touching them. Your brain takes this as being "pricked by needles".
So the "loss of feeling" while you sit and the "tingling" after you stand are events that run in opposite directions. Let's take them in order.
While you sit: nerves go quiet without oxygen
When you sit on your heels, your bottom rests on your heels. The weight of your upper body passes to the floor through your shins and the tops of your feet. Right there, just under the skin, run thin blood vessels and nerves.
The blood in the thin vessels (capillaries) does not flow under much pressure. When the force pushing from outside exceeds that pressure, the vessels are flattened and blood can no longer pass. Use the slider in Figure 1 to change the body weight and compare it with the pressure on the floor.
For a typical adult, the pressure on the floor is about twice the pressure in the capillaries. And over bony bumps, the push is even stronger. This is how blood flow almost stops at the tops of the feet and the outer sides of the shins.
To send signals, nerves constantly keep an electrical charge ready. That preparation needs energy made with oxygen. When the blood stops, within a few minutes the preparation can no longer keep up, and the nerve cannot send signals.
At this point, the thick nerves are said to go quiet first. Thick nerves are the ones that report "I'm being touched" and "my ankle is at this angle". So you lose the sense of touch and of foot position first. That is what lies behind the feeling that "it isn't my foot".
After you stand: waking nerves run on empty
When you stand up, the pressure disappears and blood rushes back. Once oxygen arrives, the nerves hurry to restart the electrical preparation that had stopped.
But nerves in mid-recovery are unstable. Though nothing is touching them, they fire signals again and again on their own. Your brain takes this burst of signals as "being pricked with needles". That is the tingling and prickling. In Figure 2, look at the order in time of the blood flow and the nerve signals.
The key point is that the tingling is not a sign that things are getting worse. It is a sign that things are coming back. It usually settles within a few minutes.
The nerves that report the angle of your ankle are still silent. Your brain doesn't know how the soles of your feet are pressing on the floor. If you walk off suddenly in this state, you can easily twist an ankle or fall. It is safer to rotate your ankles before standing, and to put a hand down and rise slowly.
Now and then, swap which big toe is on top, or lift your bottom slightly. The spots under pressure change, and blood gets through. And if the numbness does not fade after tens of minutes, or you cannot lift your ankle, it may be "palsy" from a nerve being pressed for too long. Please see a medical professional early.
Summary
The numbness from sitting on your heels starts when your weight squashes thin blood vessels and nerves without oxygen go quiet. The tingling after you stand is a sign of recovery: nerves that have got their blood back are firing signals on their own.
Lost feeling is nerves short of oxygen.
Tingling is nerves waking up.
Cold fingers that will not do what you want also come down to a change in how nerve signals get through. See also Why can't you do up a single button when your hands are numb with cold? And Why is it so tiring to carry a load away from your body?, which calculates the forces on the body from a heavy load, is linked by the same view of force and area.
- Sit on your heels and lightly stroke the top of your foot with a fingertip every 5 minutes. Note when the sense of touch starts to get vague.
- When feeling dulls, close your eyes and, without moving your ankle, try to guess which way your toes are pointing. You will find the sense of position goes first.
- Put a hand down and stand slowly. Use a clock to time how many seconds pass before the tingling starts, and how long until it fades.
If the pain is strong, stretch your legs out right away. Do not try this if you have a pre-existing knee or ankle condition.
For those who want to know more ― terms, formulas, and links to textbooksIt states clearly what level each part is, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school biology and physics
- HS+Advanced high school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university specialist courses (neurophysiology)
- ResearchNot taught even at university as settled fact; researchers are studying it now
Junior highTerms: this phenomenon has a name
- Pressure: the force pushing on a surface divided by its area. With the same weight, the smaller the contact area, the greater the pressure.
- Capillary: a blood vessel much thinner than a hair, joining arteries and veins. Here oxygen is handed over to the body's cells.
- Paraesthesia (abnormal sensation): feeling prickling or tingling when nothing is touching you. The numbness after sitting on your heels is one kind.
Junior highHigh schoolChecking with a formula: how many times the capillary pressure is the pressure under the shins?
We estimate the pressure on the floor when sitting on your heels with the pressure formula taught in junior high. The share of weight on the legs and the contact area are rough values, chosen to show how the mechanism works.
| In symbols | P ＝ F ÷ S 、 F ＝ m × g × k |
| In words | Pressure ＝ force on the floor ÷ contact area with the floor. Force on the floor ＝ body weight × gravitational acceleration × share on the legs |
| Where it comes from | It is simply the definition of pressure. The force the legs put on the floor balances the weight (gravity) of the body they carry |
| Symbol | Meaning and unit |
| P | Pressure under the shins and tops of the feet (pascals) |
| F | Force the legs put on the floor (newtons) |
| S | Contact area with the floor (square metres) |
| m | Body weight (kilograms) |
| k | Share of body weight on the legs (rough value) |
| Body weight | 60 kg |
| Share on the legs (rough value) | 0.8 |
| Gravitational acceleration | 9.8 m/s² |
| Area where shins and feet touch the floor (typical adult) | 0.06 m² (600 cm²) |
| Pressure inside capillaries (arterial end) | About 4000 Pa (about 30 mmHg), it is said |
| Weight on the legs | 60 × 0.8 ＝ 48 kg |
| Force on the floor | 48 × 9.8 ≒ 470 N |
| Pressure under the shins | 470 ÷ 0.06 ≒ 7833 Pa |
| Compared with capillary pressure | 7833 ÷ 4000 ≒ 2.0 times |
Even as a flattened-out average, it is about twice the capillary pressure. Over bony bumps the area is smaller, so it is greater still. That the vessels are squashed and the blood stops is only natural, as this calculation shows.
| 7833 Pa as a depth of water | About the same as the water pressure at a depth of 80 cm |
| Even a child of 30 kg | 30 × 0.8 ＝ 24 kg is carried, nearly reaching the dotted line |
High schoolHS+How nerves go quiet, and how they wake up
High schoolA nerve cell carries a negative charge inside and a positive charge outside (the resting potential). What keeps up this difference is the "sodium-potassium pump", which pumps sodium out and potassium in. The pump runs on energy (ATP) made from oxygen.
HS+When the blood stops, the pump stops, the electrical difference gradually breaks down, and the nerve can no longer fire signals (action potentials). When the blood returns, the pump suddenly starts working. During that start-up, the state of the cell membrane is thought to be unstable, so action potentials fire repeatedly even without a stimulus. It is also known that thick nerves (myelinated nerves) are more vulnerable to pressure and lack of oxygen.
UniversityWhat is it called technically?
The loss of feeling while you sit is a conduction block caused by nerve ischaemia. The numbness after you stand is called "post-ischaemic paraesthesia", and it has long been studied in experiments in which a cuff on the arm stops the blood and the sensation is examined after it is released. If prolonged pressure damages the nerve itself, it can lead to conditions such as peroneal nerve palsy, in which the ankle cannot be lifted.
📖 Derivation of the formula, and what lies beyond: Paraesthesia – Wikipedia (Japanese) ／ Sodium-potassium pump – Wikipedia (Japanese)
ResearchWhat is still not clear
- Where is the "trigger" for the tingling? Which channels (ion channels) in the nerve cell membrane cause the bursts, and exactly how, is still being studied.
- Individual differences in who goes numb. With the same weight and the same way of sitting, some people go numb easily and others do not. What matters, such as how the vessels run or how thick the fat under the skin is, is not well understood.
- What getting used to it means. Whether people used to sitting on their heels go numb less because they sit better and spread the pressure, or because the body itself changes, has no clear answer.
So the contents of this article too are "an explanation within what is known today". In particular, please read the figures for area and share as rough values for seeing how the mechanism works.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science Year 1 "Force and pressure" / Year 2 "Blood circulation" | The pressure calculation in Figure 1, capillaries |
| High school | Biology "Nerves and neurons" | Resting potential and the pump |
| HS+ | Advanced biology "How action potentials work" | Bursts from recovering nerves |
| University | Neurophysiology, clinical neurology | Post-ischaemic paraesthesia, nerve palsy from compression |
| Research | Ion channel research on peripheral nerves | The trigger of the tingling |
| ― | Links to daily life | Rotate your ankles before standing; stand slowly |
- Merrington, W. R. & Nathan, P. W. (1949) A study of post-ischaemic paraesthesiae. Journal of Neurology, Neurosurgery and Psychiatry 12
- Guyton & Hall, Textbook of Medical Physiology (Japanese edition: ガイトン生理学) (capillary pressure, resting potential of nerves)
- Kandel et al., Principles of Neural Science (Japanese edition: カンデル神経科学) (conduction in peripheral nerves and compression)
- Wikipedia (Japanese), "Paraesthesia (錯感覚)"
※This article is a general-audience science explainer. The figures given are rough estimates meant to help you understand the mechanism. If numbness lasts a long time, keeps coming back, or you cannot put strength into your leg, do not decide for yourself; consult a medical professional.