How tendons work | Their relationship with muscles and joints and the importance of injury prevention

[Digest video] Basic knowledge of joints and tendons that athletes should know / Learn with illustrations

"Joints" that convert muscle power into movement and "tendons" that transmit power

Do you know how your body moves? I think you can imagine that muscles are involved in our body movements.

However, our bodies cannot move with just muscles. The force generated by the muscles is transmitted to the bones through the "tendons," and that force is converted into movement by the "joints," and only then can we "move" our bodies. Here, we will take a closer look at what "joints" and "tendons" are.

The parts that connect bones to bones are called "joints," and the parts that connect muscles to bones are called "tendons" (Figure 1).

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Joints – the connections between bones

A joint is the connection between bones. Movements such as walking, squatting, and grasping objects occur when bones move around the joints. Therefore, in order to move smoothly, the joints need to move smoothly. For this reason, the ends of bones are covered with cartilage, which acts as a cushion. Cartilage has a mesh-like structure of collagen that is folded over, and by storing hyaluronic acid and water within it, it provides cushioning and a smooth surface. This allows joints to move smoothly.

Tendons – the connections between muscles and bones

Tendons are the connections between muscles and bones (Figure 1). Tendons are made up of bundles of evenly aligned, tough fibrous proteins called collagen, which allow muscles to withstand the force of pulling on bones. A typical example of a tendon is the Achilles tendon in the lower calf, but in fact, most muscles are connected to bones by tendons. For this reason, there are many tendons throughout the body.
Joints and tendons are the key players that transmit muscle power to bones and convert that power into movement, making complex movements possible.

The role of muscles, tendons, and joints

Joints and tendons, which are essential for moving the body, are also extremely important for sports performance.

The importance of joint range of motion and flexibility

It is said that "athletes have flexible and supple bodies." In fact, flexibility is an essential element not only for improving athletic performance, but also for preventing injuries. Flexibility refers to the degree of range of motion of a joint, and high flexibility means that muscles and tendons can stretch easily and that muscles can move joints dynamically. If the range of motion of a joint is sufficient, it becomes possible to reach objects farther away and perform larger movements, which also leads to improved athletic performance (Figure 2). However, loose joints and a joint range of motion that is too wide can lead to injuries, so it is important to maintain an appropriate range of motion.

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Also, moving beyond the range of motion of a joint can lead to injury. However, if the range of motion is wide enough, you can move within the range of motion, which helps prevent injury. Also, as we age, the range of motion of our joints decreases, which can lead to falls and injuries.

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Tendon elasticity and its impact on athletic performance

The function of tendons may not be widely recognized, but they play an important role in athletic performance. Tendons are elastic, meaning that they have the property of stretching and contracting like a spring. By utilizing the force of this spring's expansion and contraction, we can generate force more effectively. For example, let's take the Achilles tendon, a typical tendon. When the Achilles tendon lands, it is stretched and then, like a spring, tries to return to its original shape. By using the properties of the Achilles tendon as a spring, we can run with less force (Figure 4).

Furthermore, it is said that the harder the tendons are (scientifically called stiffness), the higher the jumping ability (Figure 5). In this way, tendons affect athletic performance by transmitting strong forces to bones during exercise and by stretching and contracting. Also, as we age, tendons become harder to stretch, which is related to a decline in balance.¹⁾It is also known to cause tendon fragility.^{2), 3)}.

1) G. Onambele et.al. J Appl Physiol 100: 2048–2056, 2006.
2) S. Arya, et. al. J Appl Physiol 108: 670-675, 2010.
3) Yasuo Kawakami, Clinical Sports Medicine: 37: 590-599, 2020.

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In this way, joints and tendons have essential properties for utilizing the force generated by muscles for athletic performance.

Stress on tendons and joints that occurs during exercise

We have already introduced the important role that joints and tendons play in athletic performance. However, because they are such important parts of the body, they are subjected to a large load during exercise. When that load becomes too great, it can cause discomfort or pain in the joints and tendons. Let's take a closer look.

Stress on joints due to impact from landing

One of the stresses on the body during exercise is the impact when landing on the ground. It is said that when walking slowly, the impact on the legs is slightly heavier than body weight, but when landing while running, the impact on the legs is said to be 1.5 to 5 times body weight, and when landing after jumping, the impact on the legs is about 10 to 12 times body weight.⁴⁾.

Tendon stretching stress

Tendons are also subjected to stress when stretched during exercise. For example, it is said that the force applied to the Achilles tendon during running can reach 1 ton.^{5), 6)}.

Tendons can also become stressed when they rub against surrounding tissue during movements such as throwing, which is seen in baseball and handball, or twisting, which is seen in golf and tennis.

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Chronic problems caused by overuse of tendons and joints

As such, exercise places a great deal of stress on the joints and tendons, and if you do not wait long enough before exercising again, in the worst case scenario, it can lead to overuse, which can lead to chronic problems.⁴⁾.

For example, Achilles tendonitis, plantar fasciitis, and chondromalacia patellar are common joint and tendon conditions, especially among runners, and some studies have shown that as many as 70% of runners suffer from them.⁴⁾.

In this way, the stress on joints and tendons may initially cause discomfort, but eventually it can cause pain and loss of movement, leading to reduced performance and injury. For example, the surface of articular cartilage is normally smooth, allowing the joint to move smoothly and for smooth exercise (Figure 7). However, when stress accumulates on articular cartilage, the surface of the joint becomes rough and smooth movement becomes impossible (Figure 8).

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On the other hand, when tendons are exercised continuously without sufficient intervals, inflammation occurs around the tendon, leading to pain and problems. The Achilles tendon is a very strong tissue, so experiments have shown that it does not get damaged even when subjected to loads that exceed the normal range of exercise.^{6), 7)}However, fatigue and lack of conditioning of the Achilles tendon can lead to injury.^{6), 8)}When an injury occurs, inflammation occurs, which causes the uniformity of the tendon structure to be lost and the tendon's hardness to decrease. In this way, the load of exercise accumulates, leading to a decrease in tendon function.

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Rest allows tendons and joints to heal

In this way, joints and tendons are subjected to stress through exercise, and this stress accumulates with daily training. However, if you leave adequate time between exercises and take rest, this will not lead to pain.⁴⁾However, I think that athletes, aiming to improve their competitive performance, often begin the next training session before fully recovering from the stress of the previous exercise.

Prevention and care of tendons and joints for performance

Continuing to push yourself in this condition will lead to injury. If you injure yourself, not only will you be unable to train while you are undergoing treatment, but even after treatment, you will have to wait a long time before you can resume full-scale training due to rehabilitation. Even after recovery, there is a risk of the injury recurring. For this reason, it is important to take daily care of your joints and tendons even before you become aware of any abnormalities, both for the sake of your performance and to reduce the risk of injury.

Furthermore, as we age, the cartilage in our joints wears down and the properties of our tendons change, increasing the risk of injury.¹⁾As you age, you will need more careful care.

As mentioned above, keeping your joints and tendons in good condition is extremely important when exercising.

4) A. Hreljac et.al. Med. Sci. Sports Exerc., Vol. 36, No. 5, pp. 845–849, 2004.
5) PV Komi et. al. Clin Sports Med 11: 521-531, 1992
6) Yasuo Kawakami Clinical Sports Medicine: 37: 590-599, 2020.
7) TF Huang et. al. Ann Biomed Eng 32: 336-341, 2004
8) H. Langevin et. al.: Imaging: Ultrasound. Fascia: The Tensional Network of the Human Body, Schleip R, et al, eds, Elsevier Health Sciences imprint Churchill Livingstone, London, 483-487, 2012

Key points to protect your joints and tendons and maintain optimal movement

It is important to know the following characteristics of our joints and tendons, which are important for moving our bodies.

• -Joints connect bones to bones, and tendons connect muscles to bones.
• - The force generated by the muscles is transmitted to the bones by the tendons, and the bones rotate around the joints, causing the body to move.
• - Cartilage and tendons in joints are made primarily of a protein called collagen.
• - To achieve optimal athletic performance (ideal movement), it is important to keep your joints and tendons in good condition.
• - Problems can occur in the joints and tendons due to increased stress during exercise, increased frequency of exercise, and lack of conditioning.
• -If joints or tendons are injured, a long period of rest and rehabilitation will be necessary.
• - Daily care is important to prevent damage to your joints and tendons.

Joints and tendons are very important parts of the body for achieving "ideal movement," but they are also subject to stress and stress buildup easily. This is why daily care is so important.

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<Supervisor>Yasuo Kawakami

Professor, Department of Sports Sciences, Graduate School of Sports Sciences, Waseda University.
Bachelor of Arts (1981), Master of Education (1983), Doctor of Education (1995) (University of Tokyo).
After working as an assistant professor at the University of Tokyo and an assistant professor at the School of Sports Sciences at Waseda University, he assumed his current position in 2005.

He is conducting research into the form and function of the human body, focusing on biomeasurements. In particular, he is using techniques such as motion analysis, tissue image analysis, and biosignal analysis to study the behavior of skeletal muscles and tendon tissues during exercise, the effects of training, and changes with growth and aging. He is also researching the determining factors of sports performance and ways to improve it from the perspectives of exercise physiology (physical strength characteristics) and biomechanics (characteristics of techniques and equipment). Together with researchers both inside and outside the university, he presides over the Human Performance Research Institute, a consortium that aims to provide scientific support for improving the competitive performance of athletes and to apply research results to the general public.https://www.waseda.jp/inst/ori/other/2017/05/30/551/