Did you know that a single strand of hair can be as strong as steel?

Like steel, though, it can break under pressure. However, unlike most steel, hair faces a constant barrage of different stresses–like friction and pulling from brushing, combing, and tossing and turning at night, to the tension of tight hairstyles, like braids.

Every time hair moves or is manipulated—whether pushed or twisted—is a potential moment of damage. A quick brush through wet hair. That slept-in ponytail. Even the rubbing of strands against a cotton pillowcase. All of these actions can create mechanical stress that weakens hair over time.

Today, we'll explore the science behind hair strength and mechanical damage. You'll learn about the effects of brushing, what happens when you pull your hair into a tight hairstyle, and even why certain sleeping positions might cause damage. Plus, we’ll share how to keep hair strong, healthy, and beautiful–no matter what the type or style. 

Hair architecture: why strands are strong + delicate

Hair is a masterpiece of biological engineering. Each strand is built like a sophisticated rope that is resilient to pulling and twisting and can stretch and bounce back to its initial position. These properties are made possible by the remarkable materials hair is made of and a complex network of chemical bonds. 

Hair fibers are made mostly of keratin–the same tough proteins found in many natural materials including fingernails and animal horns. The tough outer layer– the cuticle–acts like protective armor. Beneath it lies the cortex, where thousands of strong, flexible keratin polypeptide chains form an intricate structure resembling tightly coiled springs. The cortex gives hair strength, as well as the remarkable ability to stretch and return to its original shape.

According to scientists, a single strand weighing between 0.005 and 0.010 grams can hold about 80 grams before breaking. A dry strand of hair can stretch up to 1.5 times its original length. Wet hair can stretch up to 2.5 times its length, but there's a catch: Wet hair loses about 30% of its strength, making it more vulnerable to breakage. This is why even a quick brush through damp hair can create unseen damage.

Forces at work: how everyday actions break down hair structure

Every time hair is brushed or styled, mechanical stresses are applied that can weaken its structure. These forces–tension (pulling), compression (pushing), and friction (rubbing)–create damage over time. 

Let's break down how these different forces affect the hair:

Tensile stress
This happens when hair is pulled in opposite directions. For example, when you brush or comb, especially if you hit a knot, hair stretches between the brush and scalp. If too much force is applied, it can break.

Traction + tension
These are forces that pull on the hair’s roots. Tight styles like sleek ponytails, buns, and braids fall in this category–constantly tugging at the scalp and applying mechanical stress. 

Friction
Aka silent damage. When hair strands rub against each other, pillowcases, or a rough comb or brush, tiny cracks can form in the protective cuticle layer. Over time, these cracks grow and break down the protective cuticle, and expose the cortex–weakening hair and making it more prone to further damage and split ends.

Water
When water is involved, the situation becomes more complex. While washing hair, water penetrates its cuticle and enters the cortex, disrupting the complex network of bonds that make it strong (including the hydrogen bonds in keratin proteins, which make each strand more susceptible to stretching and deformation). The disruption to the bonds also causes the hair shaft to swell, which in turn lifts the cuticle–like shingles lifting on a roof. 

This can make the hair feel rougher and cause an increase in friction forces, also contributing to mechanical damage. This is why wet styling can be extra damaging, with that quick brush through damp hair further adding stress to the already weakened strands.

Chemical services
Hair treated with chemical services–like perming, straightening or dying–has been weakened by the chemicals involved, making it even more vulnerable to mechanical stress.

The breaking point: daily stresses cause continual damage

While a single high-stress event, like tugging on a knot, can overstretch hair and damage the protective cuticle layer and even the cortex below, greater damage comes from the cumulative effect of daily stresses. Regular actions like tight hairstyles, brushing, heat styling, and friction add up over time, gradually weakening your hair's structure. 

When combined with chemical treatments (like coloring, perming, or straightening), which already weaken hair, the damage becomes even more severe.