1. The main parts of a prosthetic limb: socket, suspension, joints, and foot
Artificial limbs, called prosthetics, are built by combining several components. A medical manual lists the main parts: the socket that fits the body, a suspension system to hold it in place, control systems, joints, and the foot or hand at the end [2]. This article focuses on three key parts of a leg prosthetic: the socket, the knee joint, and the foot. They work in order from the body outward: the socket supports, the knee handles bending, and the foot meets the ground.
2. The socket: why it must fit a body shape that changes during the day
The socket covers the remaining part of the leg, called the residual limb. It supports the body's weight and passes movement forces to the limb. Medical guides call this the most important component [1]. The difficulty is that the size of the residual limb changes throughout the day. This is a normal process affected by activity levels and weather [1]. It is similar to how feet can swell in the evening, making shoes feel tight. Because the socket carries the body's weight, a poor fit causes pain and makes movement hard. Therefore, sockets are usually made to fit each person's body and are adjusted later.
3. The artificial knee: how it keeps the leg from bending too much
People who have lost the leg from the thigh down, including the knee, need an artificial knee for their prosthetic. The challenge is not just bending, but preventing it from bending too much. Older prosthetic knees could buckle under weight, causing the knee to collapse. This made it impossible to go up or down stairs by alternating legs. Reports from the Ministry of Health, Labour and Welfare note that falls and injuries were common when going down slopes because of this buckling [3]. One solution is the NAL-Knee(S). It uses no power or electronic control, only a hydraulic cylinder. It is said to work for slow walking and running [4]. Features like buckling prevention, slow-bending control, and a locking mechanism allow users to climb stairs by alternating legs [4]. Computer-controlled knees and ankles are thought to improve safety and stability, while reducing energy use and strain on nearby joints and the spine [1].
Note: Computer-controlled knees and ankles can improve safety and stability, lower energy use, and reduce strain on other joints and the spine [1].
4. The prosthetic foot: how the carbon fiber blade appeared in the 1980s
A major change for prosthetic feet happened in the 1980s. Engineers developed an "energy-storing foot" made mainly of carbon fiber. This is a flat spring. When you step down, it bends and stores energy. When you lift up, it releases that energy. Today, these feet are designed for a wide range of people, from those who are less active to high-level Paralympic athletes [5]. For running, carbon fiber blades were adopted in 1984. Records improved significantly when they were used seriously in competitions in the 1990s [6]. The core idea is that the stiffness and shape of the blade are chosen based on the user's activity level.
5. Running blades: why they have no heel and how their length and stiffness are chosen
Running prosthetics differ from everyday ones because they have no heel. They run by bouncing on the toes. International rules state that the length is based on a formula, no power source is allowed, and the device must be a commercial product [6] [6]. A new blade design by Mizuno and Imasen Gijutsu Kenkyusho features a wavy curve from the toe to the arch. It is designed to keep the contact area constant, with the aim of stabilizing the push from the ground [7]. Blades are chosen based on body weight. For example, the entry-level KATANAα has sizes for weight groups. The L1 size is for users up to 50 kg (about 110 lbs) and weighs about 285 g (10 oz). The L2 size is for users up to 65 kg (about 143 lbs) and weighs about 310 g (11 oz). [8] The material is carbon fiber reinforced plastic [8]. Aligning the blade angle and position to the user's weight and sport is a difficult task called "alignment adjustment." This product includes a system to match the selection and assembly information [8].
6. Does a stronger spring always mean a faster runner?
When running, human legs act like springs, bouncing with each step. Research cited in papers shows that prosthetic blades can return over 90% of elastic energy [10] [10]. However, this number comes from previous studies. A doctoral thesis from Bournemouth University reported that the energy return efficiency of existing Flex Run feet exceeded 99%. The same thesis showed that values from earlier studies varied widely, from 63% to 100%, depending on how they were measured [9]. Studies on actual running show more complexity. In elite sprinters with a prosthetic on one lower leg, the average vertical force during ground contact on the prosthetic side was about 9% smaller than on the natural leg. The time to swing the leg back was not significantly different between the two sides [11] [11]. It seems that high energy return does not simply mean faster speed. Choosing a spring that fits the body and adjusting it to the running style is essential.
7. A simple experiment: feeling how your ankles work like springs
Today you can try this: sit on a chair, place your feet on the floor, and slowly lift and lower your heels. You can feel your ankles stretching and working like a spring. Running prosthetics are designed to let the blade handle part of this movement. Stop immediately if you feel pain. You can also visit a local library to find books about prosthetics and Paralympic sports. You might see the shape of running prosthetics at local Paralympic events. Official information is available from the MSD Manual Home Edition page on prosthetics and reports from the Ministry of Health, Labour and Welfare on assistive technology.