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 no longer tell how hard you are pinching. It is the sensing side, not the moving side, that gives up first.
It's 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. Now you can't pinch it, because it lies flat against the ground. You scratch at it with a nail until the person behind you picks it up for you.
The strength in your arm should be the same as usual. Yet your fingertips feel as if they belong to someone else.
There are just two main reasons
How fast signals travel from your fingertips to your brain depends on temperature. When your hands get cold, that speed is said to fall to about half. The information your brain receives becomes that much staler.
Your skin holds tiny devices that pick up very small slips and vibrations. In the cold they go dull, and you can't tell how hard you are pinching. So people grip harder than they need to.
Neither is a case of "the finger muscles have weakened." The information needed to move your fingers is arriving late and in coarser form. Let's take each in turn.
Nerves are not wires. They are a relay
When a signal runs along a nerve, electricity is not flowing through a copper wire. Tiny gates 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 gates is a chemical reaction.
Chemical reactions slow down when it gets cold. It is the same reason food keeps longer in a fridge. Nerves are no exception. In the nerves of the human hand, the signal is said to slow by about 2 metres per second for every 1℃ drop in temperature.
In a warm room, the skin of your hand is about 35℃. When winter air cools it to 20℃, the speed of the signal falls to less than half. Look at Figure 1. The travel time of 0.018 seconds in a warm hand becomes about 0.040 seconds in a cold one.
You grip too hard because your sense of touch goes dull
When you pick up a cup, you don't think about how hard to hold it. Tiny devices in your skin pick up the faintest vibration just before an object starts to slip, and your brain adds force by reflex. This adjustment is normally said to happen in under 0.1 seconds.
In the cold, however, these devices respond more weakly. 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 worst combination: plenty of force, but no fine control.
Buttons, zips, coins, fishing line. All of these tasks call for a light, finely changing touch, not a hard grip. 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 your fingertips stay cold even in gloves, zip up your jacket, cover your neck, or drink something warm. It looks like a detour, but it can work faster.
It isn't only your sense of touch that dulls. Your ability to feel temperature drops too. If you press numb hands against a stove 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 armpit.
Summary
The reason numb fingers don't work well lies on the sensing side before it lies in the muscles. Signals arrive late, the sensors go dull, and your brain has to move your fingers using old, coarse information. That is why you can grip hard but can't do fine work.
Numb fingers haven't lost their strength.
The news from your fingers is arriving late.
For how finely your skin senses detail, see How can your fingers tell "rough" from "smooth"?. For the body's reaction to cold itself, see Why do we get goosebumps when it's cold?.
- Hold just one hand in cold tap water for about 2 minutes. If you feel pain, stop right there. Do not use ice water.
- Dry your hands with a towel. With each hand in turn, pick up 10 one-yen coins one at a time and stack them, and time how long it takes.
- Next, close your eyes and guess whether a coin in your palm is a 1-yen or a 10-yen coin. The cold hand will find it clearly harder.
Stacking the coins will often take the cold hand about 1.5 times as long. When you finish, warm your hands up well.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled by level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Basic Biology / Basic Chemistry"
- High school+Advanced high school material, or textbook sidebar content
- UniversityNot taught in high school; university specialist courses (physiology, neuroscience)
- ResearchNot yet taught as settled fact even at university; questions researchers are studying right now
Junior highTerms: this phenomenon has a name
- 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 and vibrating, into signals. They are packed especially densely in the fingertips.
- Temperature coefficient: a number showing how many times faster or slower a reaction becomes when the temperature changes by 10℃. For reactions in living things, it is said to be about 2 to 3.
Junior highHigh schoolCheck it with a formula: how late do the signals from cold fingers arrive?
The subject is "the time for a signal to travel from fingertip to brain." We get 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℃ × temperature drop |
| Where it comes from | The signal is driven by the chemical reactions of the membrane gates opening and closing, and the speed of those reactions falls as temperature falls. The formula treats that drop as a straight line over a narrow temperature range. |
| Formula for time | t ＝ L ÷ v |
| In words | Travel time ＝ distance ÷ speed |
| Usual speed v0 | 55 metres per second (a guide for the thick nerves of the hand) |
| Drop per 1℃, k | 2 metres per second (per 1℃), it is said |
| Skin temperatures T0 and T | 35℃ in a warm room, 20℃ in winter air |
| Distance L | About 1 metre from fingertip to brain |
| Temperature drop (℃) | 35 － 20 ＝ 15 |
| Speed drop (metres per second) | 2 × 15 ＝ 30 |
| Speed of the cold hand (metres per second) | 55 － 30 ＝ 25 |
| Time for the warm hand (seconds) | 1 ÷ 55 ≒ 0.018 |
| Time for the cold hand (seconds) | 1 ÷ 25 ＝ 0.04 |
| Delay (seconds) | 0.04 － 0.018 ＝ 0.022 |
The delay is about two hundredths of a second, too short to notice. Even so, the reflex between feeling a slip and adding force depends on signals making this trip many times over. The delay builds up with each round trip, so the finer the adjustment, the more easily it breaks down.
High schoolHigh school+Why does temperature slow reactions?
High schoolAs you learn in chemistry, the speed of a reaction changes a great deal with temperature. The more vigorously molecules move about, the more of them can get over the "hill" a reaction needs to cross. The opening and closing of the gates in a nerve membrane is also a process of crossing such a hill.
High school+The Arrhenius equation describes this temperature dependence, and physiology uses a simplified temperature coefficient. For the opening and closing times of nerve gates, the temperature coefficient is said to be about 2 to 3, so a 10℃ drop cuts the speed to one half to one third. The temperature coefficient of conduction velocity itself is said to be gentler than this.
UniversityThe speed of the membrane gates 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 contain time constants for the opening and closing of the sodium and potassium channels, and a temperature coefficient acts on those time constants. Lowering the temperature lengthens the time constants, blurs the rise of the signal, and lengthens the time needed to excite the neighbouring patch of membrane. This shows up as a drop in conduction velocity. At still lower temperatures, the next signal cannot arrive before the gates have fully closed, so the maximum frequency at which a nerve can fire repeatedly also falls.
📖 Derivation of the equations, and further reading: Action potential (Japanese Wikipedia)
ResearchWhat is still not clear
- Which factor matters most. How much of the clumsiness in the cold comes from slower nerves, lower skin sensitivity, stiff joints and tendons, or weaker output from the muscles themselves is still being sorted out.
- Those who adapt and those who don't. People who work in cold places for long periods are reported to have a stronger response in which the blood vessels open up again after a chill. Whether that difference is inborn or acquired through adaptation is not settled.
- Protection versus feeling. Gloves keep heat in, but they also dull the sense of touch. How to combine protection with feeling is a problem that designers of work gloves are still working through by trial and error.
In other words, this article too is "an explanation of what is understood so far." All the figures vary greatly from person to person, 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 story of signals reaching the brain from the fingertips |
| High school | Basic Biology (nerves) / Basic Chemistry (reaction rates) | The section on reactions slowing as temperature falls |
| High school+ | Chemistry (Arrhenius equation) / Biology (conduction of excitation) | The temperature coefficient |
| University | Physiology and neuroscience (mathematics of membrane excitation) | The Hodgkin–Huxley section |
| Research | Environmental physiology and ergonomics | Research on cold and hand performance |
| ― | Connections to daily life | Fine tasks in winter, choosing gloves, warming your hands |
- Hongo Toshinori 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 popular-science explanation for general readers. The figures given are rough guides for understanding how things work, and vary from person to person. If feeling does not return to your hands or feet after a long time in the cold, or if you notice changes such as a change in colour, do not rely on your own judgement; follow the advice of a medical professional.