1. Why does bumping your shin hurt so much?
The reason for the pain is explained on a general information page: the thin ridge on the front of the bone runs just under the skin, so there is almost nothing to cushion the impact. According to this explanation, the shock is said to be transmitted to the periosteum, the membrane covering the bone, causing strong pain [2]. This is an explanation from a general audience page.
This spot is also called the "Benkei’s crying spot" in Japan. As the name suggests, it is a place where even strong people might cry out in pain [1]. But is this the only bone below the knee? No, there are actually two.
2. The tibia and fibula: the two shin bones that form the ankle joint with the talus
The shin consists of two bones: the thick, long tibia and the thin fibula next to it. Together, they form the lower leg from the knee to the ankle [1]. Try finding the bumps on both sides of your ankle with your fingers. The inner bump is the end of the tibia, called the medial malleolus. The outer bump is the end of the fibula, called the lateral malleolus.
The ankle joint is formed by these two bones and a third bone called the talus. Several ligaments connect them in a ring shape to keep the ankle stable [3]. Imagine the ends of the tibia and fibula holding the talus between them. The thick tibia mainly bears the body’s weight. So, what is the thin fibula for?
3. How the thin fibula shares weight depending on ankle position
The thin fibula does not bear zero weight. A paper by researchers at Kobe University states that when the ankle is in a neutral position, the fibula bears 6.4% of the load. This percentage increases when the ankle is bent upward (dorsiflexion) and decreases when it is pointed downward (plantarflexion). The share changes depending on conditions like ankle orientation [4]. In the neutral position, it is about 6 out of 100, meaning the tibia bears most of the rest.
Even if the number is small, the role is not light. In an experiment using leg bones from chimpanzees, baboons, and humans, researchers cut parts connected to the fibula and applied compressive force. As they cut more, the strain on the tibia increased step by step [5]. This result shows the fibula shares some of the load, but it is a summary of an experiment on primates, not a conclusion only for humans.
4. How running puts about 9 times body weight on the lower tibia
A study investigating compressive forces on the lower end of the tibia during running reported an average peak of about 9 times body weight in the middle of the time the foot is on the ground [6]. For a person weighing 40 kg, a simple multiplication shows this is about 360 times the weight of a 40 kg person. That much force was concentrated at the bottom end of the thick tibia.
In this study, the main source of the force compressing the bone was said to be muscle contraction, not the ground force [6]. This is the view that muscles contracting push on the bone. However, this 9 times figure is an average peak during running, not walking. Muscles do not just push bones; they also pull them. What happens where they pull?
5. How the bump below the knee receives the pulling force of muscle
Touch the area just below your kneecap with your finger. You will feel a small bony bump. This is the tibial tuberosity, where the patellar ligament from the kneecap attaches. The tibia also serves as an attachment point for muscles [1].
During the growth period of ages 10 to 15, pain can occur here when the tendon from the kneecap pulls repeatedly and strongly. This is called Osgood-Schlatter disease, and it causes pain in the front of the knee, just below the kneecap [7]. The MSD Manual Home Edition explains that symptoms usually disappear in weeks to months, and the condition heals naturally when growth ends. However, if pain continues, it is recommended not to judge by yourself but to tell an adult and consult a medical institution.
6. How to find these bones and bumps on your own shin
You can check the places mentioned in this article on your own leg. Do not press hard; touch gently. Trace the front of your shin from the ankle toward the knee. The hard, thin ridge is the front edge of the tibia. Find the bumps on the inner and outer sides of your ankle. The inner one is the medial malleolus of the tibia, and the outer one is the lateral malleolus of the fibula. Touch just below the kneecap to find the small bump (tibial tuberosity).
Connecting these three spots helps you feel where the shin bones run. You cannot measure the forces inside the bone, but knowing the locations might change how you see the number "9 times body weight." Papers on bone loading and MSD Manual explanations can be read in the source list.