What Happens to the Human Body When Cut by a Japanese Sword? The Legend of Its Sharpness and the Limits of Its Material

When people hear "Japanese sword," most probably picture a curved blade such as an uchigatana or a tachi. According to the video, however, there are many types depending on shape, size and the era in which they were made, and the wakizashi, the ken and the naginata are also counted as Japanese swords. The video defines the Japanese sword as "a blade made using a forging method that has existed in Japan since ancient times," and states that the essence of the Japanese sword lies in that forging method.

Iron in nature is bonded with oxygen and other elements, and in that state it does not exhibit the properties of a metal. Ironmaking is the technique of extracting metallic iron, and in Japan the best-known method is tatara smelting. Many Japanese swords are made from tamahagane, a steel obtained through tatara smelting. Steel is the general term for alloys of iron and carbon. The swordsmith distinguishes subtle differences in the carbon content of the tamahagane by feel and arranges the pieces so that they form the shape of a sword. From there, the sword goes through numerous steps, including folding, before the Japanese sword is complete.

Since the purpose of a sword is to cut people and things, the edge that makes contact must be hard. In addition to adjusting the carbon content and removing impurities, the swordsmith uses quenching—rapid cooling from a high temperature—to create an especially hard steel structure on the surface.

The video explains it like this. When the heated blade is plunged into water, the surface cools so rapidly that the carbon in the steel has no time to gather. The carbon left trapped in the iron structure acts like a wedge, preventing the structures from sliding past one another, which produces an extremely hard structure. This is martensite, which the video introduces as the hardest steel microstructure still used in steel today. In an era with no means of examining the microscopic structure of steel, Japanese swordsmiths arrived at this answer through experience and practice—that is the video's assessment.

However, a blade that is merely hard is brittle and breaks easily. The interior of a Japanese sword cools more slowly than the exterior, so the carbon has time to escape from the iron structure and gather. The result is pearlite, a structure in which iron-rich and carbon-rich layers alternate. Pearlite is softer than martensite, but it is supple and tough. Because the core is made of this structure, Japanese swords are relatively resistant to breaking.

The video notes that this idea itself is not unique to Japan, and that swordsmiths in other countries have also made similar blades. What distinguishes the Japanese sword, it says, is that it was produced on such a large scale. Furthermore, at the boundary between the hard martensite and the soft pearlite appears the beautiful hamon, a hallmark of the Japanese sword. The video states that centuries of ingenuity by swordsmiths enhanced both the practicality and the artistry of the sword.

Masamune and Muramasa are swordsmiths said to have left behind many famous blades. According to the video, no surviving sword can be definitively attributed to Masamune himself. On the other hand, a number of Muramasa's swords survive, and some of them have had their blades analyzed by modern science. The results of this analysis of a Muramasa are used in the examination later in the video.

When the body takes a sword strike unprotected, the skin is virtually its only defense. According to the video, skin is the most resistant to mechanical force of all the soft tissues in the human body. Although it varies by location, skin is in a state as if it were being stretched in a particular direction. It is crisscrossed by countless Langer's lines, which indicate the direction of tension; cutting along them is easy, but tearing across them becomes much more difficult.

According to research cited in the video, penetrating the skin of the chest requires a force of up to 261 N. That is equivalent to pressing a weight of 27 kg onto a single point. The video considers that a Japanese sword would easily break through this.

Most Japanese swords have a curved blade rather than a straight one. The video explains that this is because adding a pushing or pulling motion makes it possible to cut with less force than simply pressing the blade against something. With a curved blade, starting from the point where it contacts the skin, a push-pull force is naturally applied in addition to the pressing force. The video states that skin alone would probably not be able to stop this.

Once past the skin, the internal tissue is cut with almost no resistance, according to the video. Subcutaneous fat offers practically no resistance, and even muscle is easily severed by a sharp blade. It is at this stage that life becomes endangered. In some places in the human body, important blood vessels run just beneath the skin. Examples include the common carotid artery in the neck, which supplies blood to the brain, and the femoral artery, which runs beneath the muscles of the inner thigh. The video explains that if these are cut, the person would go into hemorrhagic shock and probably die without being able to do anything. In other words, if it strikes the wrong spot, a blade that has pierced the skin can easily take a life.

So is the scene so common in fiction—"cutting through bone with a blade"—actually possible? The video's assessment is that even a legendary Japanese sword would have difficulty severing bone.

One basis for this is the analysis of the Muramasa mentioned earlier. It has been confirmed that the outer part of the blade is martensite—that is, the hardest structure in steel. But martensite is brittle. According to the video, the force needed to break bone is at least 900–1,200 N, equivalent to applying a weight of 92–122 kg to a single point, or roughly concentrating the body weight of an adult onto one point. The video acknowledges that it would not be surprising for the energy of a downward-swung Japanese sword to reach this level.

The problem lies with the blade. The video points out that the shock of suddenly striking hard bone could chip the edge, and in the worst case could even crack the blade. Even legendary swords have the same martensite structure as other Japanese swords, and the video's view is that however keen the edge, the constraints of the material's properties would probably come into play first.

The video also introduces an actual case report. It describes an incident in which a man was attacked with a Japanese sword and killed following a drunken fight. There were at least 23 wounds to his head and neck, and part of his skull was missing. His condition was so gruesome that many muscles, including the trapezius, the splenius capitis and the semispinalis capitis, were completely severed. Even so, it is reported that his cervical vertebrae remained intact despite repeated blows from the sword. The video treats this as evidence that cutting all the way through bone with a Japanese sword is extremely difficult.

From all this, the video concludes that the Japanese sword is a dangerous weapon that can easily take human life, but that it cannot exceed the limits of physics. As long as the blade is made of brittle martensite, it cannot cut down one person after another as in fiction. The video further states that even on the battlefields of the past, the Japanese sword was never the main weapon; it was a means of attack when close to the enemy and was not intended for cutting down many people.

Then is the Japanese sword merely an antique around which exaggerated legends have taken on a life of their own? The video's answer is "no." Without modern facilities or analytical instruments, experience and wisdom accumulated over hundreds of years arrived at the combination of martensite and pearlite. The video regards this as remarkable.

The video acknowledges that it would not be surprising if modern swords made with modern technology surpassed the Japanese swords of the past in every respect, including the use of microstructure, thorough removal of impurities, and the timing of quenching and cooling. But that is the result of marshaling all of modern materials engineering knowledge. What is more astonishing, the video states, is that within the range of what human hands could accomplish, there is not much difference from modern industrial products.

To the question of how the Japanese sword differs from other blades, the video answers: "Not performance itself, but performance under constraints." In an era with no analytical instruments and no knowledge of materials engineering, people reached the same optimal solution as modern science through human senses alone. The video closes by asking whether it is precisely this fact that elevates the Japanese sword from a mere weapon to a crystallization of human wisdom—and, by extension, to a work of art.