Why Can't You Do Up a Button When Your Hands Are Numb with Cold?
― News from Your Fingers Is Arriving Late
Cold fingers haven't lost their strength. The signals from your fingertips to your brain slow down. On top of that, the sensors in your skin go dull, so you can't tell how hard you are pinching. It is the sensing side, not the moving side, that gives up first.
It is a winter morning on a station platform. You take off your gloves and try to slide one card out of your wallet. Two come out together, and you can't separate them.
You drop a card and try to pick it up, but it lies flat against the floor and you can't pinch it. While you scrape at it with a fingernail, the person behind you picks it up for you.
Your arm is no weaker than usual. Yet your fingertips feel as if they belong to someone else.
There are only two main reasons
How fast a signal travels from fingertip to brain depends on temperature. When your hand gets cold, that speed is said to drop to about half. The information your brain receives is that much out of date.
Your skin contains tiny devices that pick up the smallest slips and vibrations. When it is cold, they go dull, and you can no longer tell how hard you are pinching. So people grip harder than they need to.
Neither reason means your finger muscles have weakened. The information you need to move your fingers has become slower and coarser. Let's take them one at a time.
A nerve is not a wire. It is a relay.
When a signal runs along a nerve, electricity is not flowing through it like current in a copper wire. Tiny doors in the cell membrane open one after another. Each time, the electrical state is rewritten one step, and that change is passed to the next section. Opening and closing these doors is a chemical reaction.
Chemical reactions slow down in the cold. It is the same reason food keeps longer in a fridge. Nerves are no exception. In the nerves of the human hand, the speed of transmission is said to drop by about 2 metres per second for every 1°C fall in temperature.
The skin of your hand is around 35°C in a warm room. If winter air cools it to 20°C, the signal speed falls to less than half. Look at Figure 1. A trip that takes 0.018 seconds in a warm hand takes about 0.040 seconds in a cold one.
You grip too hard because your sense of touch goes dull
When you hold a cup, you don't think about how hard to squeeze. Tiny devices in your skin pick up the faint vibration just before an object starts to slip, and your brain adds force by reflex. This adjustment is normally said to take less than 0.1 seconds.
When your hand is cold, though, these devices respond more sluggishly. Your brain can't tell whether the object is about to slip, so it plays safe and grips hard from the start. The result is the most awkward state of all: plenty of force, but no fine control.
Buttons, zips, coins, fishing line. All of these need you to vary your grip lightly and finely, not to squeeze hard. This is exactly the kind of movement cold fingers lose first.
When your core gets cold, your body narrows the blood vessels in your hands and feet to save heat. Less blood reaches your hands, so they get even colder. If gloves don't warm your fingertips, try zipping up your coat, covering your neck, or drinking something hot. It looks like a detour, but it can work faster.
It isn't only your sense of touch that is dulled. Your ability to sense temperature drops as well. If you press numb hands against a heater or a hand warmer, you may end up touching something hotter than you realise, for longer than you realise. It is safer to use something not too hot, such as lukewarm water or your own armpits.
Summary
The reason numb fingers don't work well lies in the sensing side, before the muscles. Signals arrive late, the sensors go dull, and the brain has to move your fingers using old, coarse information. That is why you can apply force but can't do fine work.
Numb fingers haven't lost their strength.
The news from your fingers is simply arriving late.
How much fine detail your skin picks up is covered in How can you tell "rough" from "smooth" just by touching with your fingers?, and the body's response to cold itself is covered in Why do we get goosebumps when it's cold?. What happens when the blood vessels in cold fingertips tighten and then don't fully recover on warming is explained in "Why do chilblains appear on days with big temperature swings rather than in the depths of winter?".
- Put just one hand in cold tap water for about 2 minutes. If it starts to hurt, stop there. Do not use ice water.
- Dry your hands with a towel. With each hand in turn, pick up ten 1-yen coins one at a time and stack them, and time how long it takes.
- Next, close your eyes and guess whether the coin on your palm is a 1-yen or a 10-yen coin. The cold hand will find it harder to tell.
The cold hand will often take about 1.5 times as long to stack the coins. When you have finished, warm your hands up well.
For those who want to know more ― terms, formulas and links to textbooksFrom junior-high science to university specialist courses, each part is labelled with its level
- Junior highCovered in junior-high school science
- High schoolCovered in high-school "Basic Biology / Basic Chemistry"
- HS+Advanced high-school material, or textbook sidebar content
- UniversityUniversity specialist content (physiology, neuroscience) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught even at university as settled
Junior highTerms: this phenomenon has names
- Nerve conduction velocity: how fast a signal travels along a nerve. In the thick nerves of the human hand, it is said to be around 50 metres per second.
- Mechanoreceptor: a device in the skin that turns changes in force, such as pressing, slipping or vibrating, into signals. They are packed especially densely in the fingertips.
- Temperature coefficient: a number showing how many times faster a reaction becomes when the temperature rises by 10°C. For biological reactions it is said to be about 2 to 3.
Junior highHigh schoolChecking with a formula: how late do cold fingers' signals arrive?
The subject is the time a signal takes to get from fingertip to brain. We find the speed from the temperature, then the time from the speed.
| In symbols | v ＝ v0 － k × ( T0 － T ) |
| In words | Speed when cold = usual speed − drop per 1°C × temperature fall |
| Where it comes from | Chemical reactions of the membrane doors opening and closing drive the signal, and reaction speed falls as temperature falls. Over a narrow temperature range, that fall is treated as a straight line. |
| Formula for time | t ＝ L ÷ v |
| In words | Travel time = distance ÷ speed |
| Usual speed v0 | 55 metres per second (a guide for thick nerves in the hand) |
| Drop per 1°C, k | 2 metres per second (for each 1°C), it is said |
| Skin temperatures T0 and T | 35°C in a warm room, 20°C in winter air |
| Distance L | About 1 metre from fingertip to brain |
| Temperature fall (°C) | 35 － 20 ＝ 15 |
| Speed drop (metres per second) | 2 × 15 ＝ 30 |
| Cold hand's speed (metres per second) | 55 － 30 ＝ 25 |
| Warm hand's time (seconds) | 1 ÷ 55 ≒ 0.018 |
| Cold hand's time (seconds) | 1 ÷ 25 ＝ 0.04 |
| Delay (seconds) | 0.04 － 0.018 ＝ 0.022 |
A delay of about two hundredths of a second never reaches your awareness. Even so, the reflex from sensing a slip to adding force is built on trips back and forth over this distance. The delay builds up with each round trip, so the finer the adjustment, the more easily it falls apart.
High schoolHS+Why does temperature slow the reaction?
High schoolAs you learn in chemistry, reaction speed changes a great deal with temperature. The more energetically molecules move around, the more of them can get over the hill a reaction needs to cross. The opening and closing of the doors in a nerve membrane is also a process of getting over this hill.
HS+This temperature dependence is described by the Arrhenius equation, and physiology uses a simplified temperature coefficient instead. For the opening and closing times of nerve doors, the temperature coefficient is said to be about 2 to 3, so a 10°C fall slows them to one half to one third of their speed. The temperature coefficient of conduction velocity itself is said to be gentler than this.
UniversityThe speed of the membrane doors sets the speed of the signal
A signal running along a nerve is called an action potential, and its shape and speed are described by the Hodgkin–Huxley equations. These equations contain time constants for the opening and closing of the sodium and potassium channels, and a temperature coefficient is applied to those time constants. Lower the temperature and the time constants lengthen, the signal's rise becomes blunted, and it takes longer to excite the next patch of membrane. This shows up as slower conduction velocity. At still lower temperatures, the next signal cannot arrive before the doors have fully closed, so the upper limit on how often a nerve can fire repeatedly also falls.
📖 Derivations and further reading: Action potential (Japanese Wikipedia)
ResearchWhat is still not clear
- Which part matters most. How much of the clumsiness in the cold comes from slower nerves, lower skin sensitivity, stiff joints and tendons, or weaker muscle output is still being teased apart.
- Those who adapt and those who don't. People who work long hours in the cold are reported to show a stronger response in which blood vessels reopen after narrowing when the hand is chilled. Whether that difference is inborn or acquired through habituation is not settled.
- Protecting versus sensing. Gloves keep heat in, but they also dull the sense of touch. How to combine protection and sensation is still a matter of trial and error in the design of work gloves.
In other words, this article too is "an explanation of what is understood so far." The figures vary greatly between individuals, so please read them as rough guides.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science / How animal bodies work (stimulus and response) | The whole idea of signals reaching the brain from the fingertips |
| High school | Basic Biology (nerves) / Basic Chemistry (reaction speed) | The section on reactions slowing as temperature falls |
| HS+ | Chemistry (Arrhenius equation) / Biology (conduction of excitation) | The temperature coefficient |
| University | Physiology and neuroscience (mathematics of membrane excitation) | The Hodgkin–Huxley equations section |
| Research | Environmental physiology and ergonomics | Studies of cold and hand performance |
| ― | Connection to daily life | Fine work in winter, choosing gloves, ways to warm your hands |
- Hongo, T. et al. (eds.), 『標準生理学』 (Standard Physiology), Igaku-Shoin, chapter on nerve excitation and conduction
- Denys, E. H. "The influence of temperature in clinical neurophysiology." Muscle & Nerve, 1991
- Heus, R., Daanen, H. A. M., Havenith, G. "Physiological criteria for functioning of hands in the cold: a review." Applied Ergonomics, 1995
- Johansson, R. S., Westling, G. "Roles of glabrous skin receptors and sensorimotor memory in automatic control of precision grip." Experimental Brain Research, 1984
- Action potential (Japanese Wikipedia)
※This article is a general-audience science explainer. The figures given are rough guides to help you understand how things work, and they vary between individuals. If feeling does not return to your hands or feet after prolonged cold, or you notice changes such as colour changes, follow the advice of a medical professional rather than judging for yourself.