Why are only hands and feet “soaked” after a bath? These folds can reflect your health

Introduction: Have you noticed that after taking a bath, there are many folds on the skin surface of our hands and feet, but other parts of the body such as forearms, The skin on the torso, legs, and face did not. For decades, scientists have been searching for the root cause of this water-induced skin fold on the fingertips and toes. Scientists recently discovered that this fingerfold fold reflects our health.

Out of curiosity, scientists explored external triggers for wrinkling of hands and feet, and finally found that: under the optimal water temperature of 40°C, the fingers were soaked in water for 3.5 minutes Wrinkles can appear; under the condition of 20 ℃ water temperature, it may take 10 minutes; after soaking for about 30 minutes, the wrinkle degree of the skin will reach its peak[1]. But why does this happen? Why does it only appear on the skin of hands and feet? What health problems can these folds reflect?

Small folds, big question marks

What’s the reason?

The reasons for the appearance of wrinkles on hands and feet in water have been studied for many years. At present, there are three main explanations.


Wrinkles originate from skin absorption: osmosis

A common explanation for the appearance of wrinkles on hands and feet is as follows: When the skin is in contact with water, the concentration of cell fluid in the skin is higher than that of water, and it has semi-permeable membrane properties. In order to balance the concentration difference between the two sides, the cells of the epidermis allow water to enter the cells of the surface layer of the skin through osmosis, and the stratum corneum of the epidermis absorbs water and swells. Due to the large number of stratum corneum layers on the hands and feet, water can only penetrate layer by layer, resulting in the cells not swelling outward as a whole, but the outer layer swells more and the inner layer swells less [2]. The combination causes the skin to fold in the water-soaked part.


Wrinkle formation originates from vasoconstriction: neural regulation

As early as 1935, scientists suspected that the wrinkling process of hands and feet in water was more than osmotic and might be related to nerves. As one of the main nerves running from the arm to the hand, the median nerve has many functions, one of which is to control sympathetic nerve activity, such as regulating sweating and vasoconstriction. Scientists studying patients with ruptured or damaged median nerves have found that their fingers do not wrinkle when soaked in water. From this, it is concluded that the wrinkling of the fingertip skin after immersion is controlled by the nervous system[3].

Figure 1 Schematic diagram of the median nerve (Source: Three rivers orthopedic associates)

In 2003, NUS neurologists Einar Wilder-Smith and Adeline Chow measured blood circulation in the hands of volunteers whose hands were immersed in water. They found that blood flow to the fingers dropped significantly as the skin on the volunteers’ fingertips began to wrinkle. When the blood vessels in the fingers of healthy volunteers were temporarily constricted by the application of a topical anesthetic cream, the degree of finger wrinkling was significantly different from that of soaking water.to a similar extent.

In response to the test results, Wilder-Smith said: “When a hand is immersed in water, the sweat ducts in the fingers open up to let in water, which can lead to an imbalance of salt in our skin. This change in salt balance triggers the firing of nerve fibers in the finger, causing the blood vessels around the sweat ducts to constrict. This in turn causes the fleshy area of ​​the fingertip to lose volume, which pulls the overlying skin down and deforms it into folds. “


The formation of folds arises from evolutionary necessity: a theory of evolution

Nick Davis, a neuroscientist and psychologist at Manchester Metropolitan University who studies fingertip folds, said: “Fingertip folds happen for a reason. Through penetration alone, our skin It takes 20% expansion to achieve visible finger folds. But If the wrinkles are controlled by our nerves, it means our body is actively responding to the water environment, which is good for our survival brings advantages.

In 2020, Davis measured how hard it would take 500 volunteers to grab a plastic object. It was found that without creases, those with dry hands were able to grasp objects with less force than those with wet hands; while volunteers dipped their hands in water for a few minutes and formed fingertip folds, grasping The same item requires less grip than a person whose hands are wet but not wrinkled [4]. To that end, Davis said: “Wrinkles increase the friction between the finger and the object. Our fingers are sensitive to changes in surface friction and use this information to apply less force to grip objects firmly.

Figure 2 Walking in wet areas, the folds of the fingertips help us grasp better (Credit: Alamy)

Davis’ discovery has been recognized by evolutionists. They argue that folds are an evolutionary expression, and that folds in human hands and feet are like grooves in a tire or the sole of a shoe. With the “gully” between these folds, the water between the point of contact between the finger and the object is better able to drain. Humans may have evolved finger-toe fold mechanisms at some point in the past to increase surface friction and help humans hold slippery objects.

Tom Smulders, an evolutionary neuroscientist at Newcastle University, said: “Other primates may also operate in wet environments, but only humans can catch underwater Grips objects. Folds increase underwater grip, a phenomenon associated with two human activities: moving in very wet environments and transferring objects underwater. For example, when The wrinkling mechanism provided a stable balance advantage when our ancestors were navigating slippery rocks or grasping branches. Furthermore, it made it easier for our ancestors to find aquatic food such as shellfish.”

Figure 3 Japanese macaques bathing in hot springs (Source: Pablo Daniel Fernandez)

Why only the hands and feet are wrinkled

No other parts of the body?

Wrinkling occurs on other parts of the body after prolonged soaking, but it’s just not noticeable. The subcutaneous fat of the skin of the human body is different. The skin other than the hands and feet, such as the legs, arms and upper body, is thicker and has relatively strong tension, which can support the skin well, so it will not appear wrinkled; The subcutaneous fat on the hands and feet is relatively thin. After soaking in water, the subcutaneous fat has poor tension and cannot support the skin surface well, so there are raised and sunken folds; in addition, relative to the hands and feet (especially fingers and toes) , and the stratum corneum is thinner in other parts, so folds do not occur [5].

Picture 4 When taking a bath, only the skin on the hands and feet is wrinkled, and other parts of the body are not wrinkled (Source: Alamy)

Fingertip folds

What health problem?

Study finds Fingertip folds reveal health information. People with skin conditions such as psoriasis and vitiligo take longer to develop wrinkles; people with cystic fibrosis have excessive wrinkling of the palms and fingers, which is evident even in genetic carriers of the disease; Patients with type 2 diabetes sometimes show significantly reduced levels of fingertip folds in water; reduced fingertip folds have also been found in people with heart failure, possibly due to some disruption in their cardiovascular system control Asymmetric folds of fingers with the same soaking time but less folds in one hand than the other are considered an early sign of Parkinson’s disease because asymmetrical folds indicate dysfunction of the sympathetic nervous system on one side of the body [6] .

Regarding the crease between the fingers, Why do women take longer to soak in water to wrinkle than men? Why does the skin return to normal within 10 to 20 minutes after wrinkling? Can fingertip folds significantly affect our grip on dry objects? Wrinkles on the fingers can increase grip when hands are wet, but do folds affect grip when hands are dry? Why aren’t the fingers folded all the time? These unsolved mysteries are still waiting to be answered by researchers.

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[2]Sáez P, Zöllner AM. Mechanics Reveals the Biological Trigger in Wrinkly Fingers. Ann Biomed Eng. 2017 Apr;45(4):1039-1047. doi: 10.1007/s10439- 016-1764-6. Epub 2016 Dec 2. PMID: 27913950.

[3]Wilder-Smith EP, Chow A. Water-immersion wrinkling is due to vasoconstriction. Muscle Nerve. 2003 Mar;27(3):307-11. doi: 10.1002/mus .10323. PMID: 12635117.

[4]Water-immersion finger-wrinkling improves grip efficiency in handling wet objects. PLoS ONE 16(7): e0253185. https:https://doi.org/10.1371/journal. pone.0253185

[5]Afsaneh Khetrapal. Why Does Skin Go Wrinkly in Water? June 9, 2016. https:https://www.newsmedical.net/amp/health/Why-Does-Skin -Go-Wrinkly-in-Water.aspx

[6]Djaldetti R, Melamed E, Gadoth N. Abnormal skin wrinkling in the less affected side in hemiparkinsonism-a possible test for sympathetic dysfunction in Parkinson’s disease. Biomed Pharmacother. 2001 Oct;55 (8):475-8. doi: 10.1016/s0753-3322(01)00088-9. PMID: 11686582.