Well functioning knees are crucial to virtually every martial art. Acute or chronic injuries can seriously interfere with optimum performance. For martial artists working in security, the military and law enforcement, such injuries may even be career threatening. The wise warrior, therefore, would do well to learn how the knee works and how to take care of it.
The knee is the largest joint in the body and its health is crucial to maintaining the full range of mobility and activity that the human body is capable of. A joint can be defined as an articulation between two bones. The knee is a synovial joint. Along with most of the other joints in the human body, the space between the bones which form the knee is lined with a thin, fluid secreting membrane called synovium, hence the name synovial. The joint space is a closed one, and it is filled with a fluid that acts as a shock absorber, a lubricant and a source of nutrition for the cartilage of the joint.
A matrix consisting of collagen and other substances called proteoglycans coats the surfaces of the ends of the bones that make the joint. This is cartilage. Underneath this is the dense mineral substance we call bone. Cartilage is softer and has a small degree of elasticity, which helps to reduce the effects of shock in the joint. The high degree of stiffness of bone is also a contributor to shock absorption.
Wrapping around the ends of the bones and joint space is a fibrous substance enclosing the entire joint in a capsule. What holds the joint together are the ligaments, muscles and tendons. The capsule, ligaments and tendons, like cartilage, are also made of collagen and proteoglycans. Ligaments and tendons are attached directly to bone on the molecular level. Similarly, tendons and muscle are intertwined on the molecular level. These attachments are therefore very strong ones.
Ligaments, tendons and muscle are what give joints their stability during rest and movement. Because the knee bears such large loads, its muscles, ligaments and tendons must be strong as well. It is important to understand that the ligaments of the knee have little elasticity. This is because of the large stresses placed upon them. If they were too elastic it would cause the knee to become unstable. Therefore any effort to stretch knee ligaments is dangerous and should be avoided at all costs. There will be more on this later.
Another component of most joints including the knee is a structure called a bursa. A bursa is a closed fluid filled sac lined with cells similar to those making up synovium. A bursa is usually found close to the joint and underneath a tendon, allowing the tendon to slide smoothly during movement when muscles contract.
Blood vessels penetrate most components of a joint. An exception to this is cartilage, which means that damaged cartilage will not heal. The nerve supply of joints comes from branches of the same nerves that supply their associated muscles as well as from nearby peripheral nerves. This dual nerve supply appears to allow for preservation of joint position sensation, which can be altered in disease or injury.
The anatomy and mechanics of the knee are complex. The knee can actually be thought of as two joints, consisting of the articulation of the femur (thigh bone) and tibia (shinbone), and the articulation between the femur and the patella (kneecap). The fibula is the thin bone lateral to the tibia and is not considered part of the knee. Figure 1 shows the bones of the knee joint and their relationships with each other. Figure 2 shows the ligaments of the knee from a different view.
An important distinguishing feature of the knee is the presence of two menisci within the joint. These are two half moon shaped wedges of fibrous and cartilaginous material, which sit in the slightly hollowed surfaces of the tibia. They are at their thinnest in the middle and are fused to the joint capsule around their edges. The surfaces of these menisci are in direct contact with the cartilage of the joint. In fact, most people mistakenly call the menisci cartilage and in truth there are similarities in their respective structures. The menisci are not completely fixed inside the joint but their movement is limited. The menisci are supplied with nerves but not blood vessels. When a meniscus is torn there will be pain but no bleeding and more importantly, no healing. There may be some regeneration of damaged meniscus tissue at the site of the attachment to the joint capsule (where there are blood vessels) but otherwise a torn meniscus will not repair itself.
Movement of the knee joint is not as simple as it looks. The large muscles of the front of the thigh, the quadriceps, are responsible for extension (straightening) of the joint. The muscles of the back of the thigh (the hamstrings) are responsible for knee flexion (bending) and rotation. Note that muscles contract and relax along a straight line. The manner in which a joint moves therefore depends partly on how a muscle is anchored across a joint. When the knee is completely flexed the joint surfaces are in the least amount of contact, allowing the greatest amount of freedom of movement between the femur and the tibia. As the knee, extends the joint surfaces come into closer contact. As can be seen from figure 1, the surfaces of these bones have definite shapes that direct the sliding, rolling movement of the tibia in relation to the femur. In order to line up the joint surfaces so that they are as close as possible, rotation between the femur and tibia must take place. This rotation is determined not by specific muscles but by the shape of the surfaces of the joint.
In order to fully lock the knee while standing, the femur will spin and slide inwards toward the center of the body in relation to the tibia. The amount of rotation is less than 10° and is passive, only occurring while the knee is being extended. Unlocking the knee requires rotation in the opposite direction. This rotation is provided by the popliteus muscle running behind the knee between the top of the tibia and the bottom of the femur on its outside aspect. Once the knee is unlocked the muscles of the posterior thigh contract to continue knee flexion. All in all, knee movement can be thought of as a hinge with a slight twist needed to open it fully.
While standing with the knees fully extended (locked), gravity alone is needed to maintain this position. The thigh muscles are relaxed. In this position the knee depends entirely upon the strength of its ligaments for stability. In the locked position the tension in all knee ligaments is increased. The anterior cruciate ligament and the posterior capsule are responsible for preventing knee hyperextension.
As one begins to crouch, the quadriceps muscles begin to contract and will provide stability to the joint (preventing rotation and allowing smooth travel). The shape of the femur and the attachment of the quadriceps muscle prevent sideways movement of the patella. The popliteus muscle and the anterior cruciate ligament prevent the femur from slipping forward in relation to the tibia. The iliotibial tract is a long band of tendon like tissue running from the pelvis along the lateral thigh and attaching to the top of the lateral tibia. It contributes greatly to maintaining knee stability during flexion.
By the late 1980s it was established that martial arts practice including tournament fighting, resulted in significantly lower rates of injury in comparison to many other sports including football and basketball. In the medical literature, specific information about martial arts knee injuries is usually taken from tournament statistics and shows similar injury types in comparison to other sports. In activities which require ballistic movements, rapid changes in direction through pivoting, or sudden weight changes, injuries usually resulted from knee hyperextension, rotation and flexion causing damage to the menisci, ligaments and the tendon which attaches the quadriceps muscle to the front of the tibia (patellar tendon). Fractures involving the knee are rare and complete dislocations even more so. Knee dislocation is a true medical emergency because of the potential for damaging major arteries and nerves running down the posterior aspect of the joint (reference six).
The structure of the knee and the nature of its movement will determine the kinds of injuries it is most vulnerable to. Forces applied to the knee from the outside or generated within it can lead to injury. In the martial arts, as with sports and dance, the most frequently damaged structures are the ligaments and menisci. When the knee is locked and weight bearing, it is at its most vulnerable to an outside force such as a kick because its stability depends entirely upon gravity and the strength of the ligaments. If the knee is flexed then the strength of the quadriceps muscle adds to the ability of the joint to resist force. Repetitive stress to the ligaments can result in decreased strength and too much elasticity. Inappropriate attempts to stretch the knee and incorrect stances or stepping practices that place repeated stresses to the ligaments, may also result in chronic injury. This would of course increase the risk of acute injury.
Incorrect stretching, stances and stepping practice occur when there is rotational force applied to the knee. This often happens when the femur is rotated inwardly or outwardly over the tibia causing the knee to appear as if it is pointing one way and the foot another. Stretching or stepping in any way that increases this rotational force can place too much stress on the collateral and cruciate ligaments. For example more harm than good is caused when the feet are planted parallel, the hands placed on the kneecaps and the knees are pushed around in a circular motion.
Although the menisci can be thought of as shock absorbers, their capacity is limited. Being overweight or engaging in any activity that repeatedly places large loads on the joint surfaces (for example jumping) leads to wear of the menisci and eventually the joint cartilage. In turn, this can contribute to the development of osteoarthritis: it has been shown that sports such as basketball, which emphasize jumping also predispose to the development of the knee osteoarthritis.
The medial meniscus (the one on the inside) is the one most commonly affected by acute trauma. Its movement is not well controlled and in the final phase of knee extension as the femur rotates inwardly, the medial meniscus may be caught between the edges of the two bones. Thus, if you are standing with most of your weight on a bent knee and your lower leg is rotated to the outside in relation to your thigh (turning the foot out does this), suddenly straightening your knee could conceivably lead to a torn medial meniscus. In fact this is one of the most common ways in which the medial meniscus is damaged. If the knee is consistently subjected to this kind of rotational force during stepping or while maintaining a stance, there will be too much wear and tear on the medial meniscus.
Regardless of system or style, any form of training that places rotational forces across the knee, particularly during rapid movement, increases the risk of acute or chronic injury to the menisci and knee ligaments. It should be clear from the preceding paragraph that training methods such as repeated, forceful kicks into the air should likely be minimized and correct technique emphasized. As a kick snaps out, the snap is actually the rapid rotation of the tibia and femur locking together forcefully into the position in which the joint surfaces are in closest contact. In order to avoid excessive wear and tear on the joint, avoid extending the knee fully during practice kicks.
Keeping the knee properly aligned is a key factor in safe training. Proper alignment occurs when there are no rotational forces across the knee. In general, this means that during standing and stepping the knee and the foot are pointing in the same direction. While stepping the feet should be parallel, ideally. While stretching, this alignment should also be maintained at all times throughout the entire range of motion of the knee. Remember that tendons and ligaments have limited elasticity. It is therefore possible to over stretch them to the point they will become lax. In fact, stretching knee ligaments will not help performance in any way. The opposite is true.
Strength training of the muscles of the thigh is an excellent way to prevent injury and rehabilitate an injured knee. This has been shown clearly in medical studies of knee injuries in athletes. Strengthening of the thigh muscles will help increase the stability of the knee joint and can even compensate for lax ligaments caused by acute or chronic trauma. Not only muscles respond to strength training. It has been well demonstrated that tendons respond to strength training by thickening and stiffening. This too helps keep the knee stable.
Our current understanding of knee structure and function leads to several training suggestions for the martial artist. Novices in any system would be wise to concentrate on good technique rather than speed and power at first. Unfit individuals who take up martial arts in order to become fit should seriously consider supplementary strength training in order to help avoid injuries caused by poor technique. Indeed, all levels of skill would benefit from strengthening of the thigh muscles. Untrained females are particularly vulnerable to ligamentous injuries of the knee with injury rates up to six times higher than in males in some sports.
A detailed discussion of strength training techniques for martial arts is beyond the scope of this article. However, several points should be made. The use of strength training techniques to improve knee function has become a professional discipline and the subject of rigorous scientific investigation. Different types of strength training will have different consequences with respect to knee function. For example, there is evidence to suggest that squats may be more effective in muscle development than leg presses but squats also stress the posterior cruciate ligament and patellar cartilage significantly more. Weight training in and of itself can be associated with knee injuries and therefore, as with most training activities, good technique is absolutely essential to injury avoidance. Sound advice based on a thorough understanding of knee function and how the knee responds to stress can be found in medical texts, journals or from trained professionals in the fields of rehabilitation and fitness.
Martial arts teachers who wish to ensure safe training methods for their students would do well to study the structure and function of the knee. Understanding how one’s particular style uses the knee can contribute to reducing injuries and also suggest ways to improve training.
Dr. John Painter, the well-known Jiulong Baguazhang teacher and head of the American Rangers Law Enforcement Trainers Association enjoys telling stories of a family of Chinese baobiao (bodyguards) who earned their living guarding merchants against bandits in a time and place with little social security and very limited medical technology. In all the systems they studied including Baguazhang, Taijiquan, Xingyiquan, ground fighting, and weapons, compulsive attention was always paid to keeping the knee appropriately aligned with the foot. This alignment was such that it minimized any stress to the ligaments and joint surfaces of the knee. Strength training methods emphasized the development of strong quadriceps muscles. It seems that the family understood quite clearly how the knee worked and how to avoid injury. What might be considered a minor injury today, easily remediable through a simple operation, could terminate a bodyguard’s career. In those days, an unemployed bodyguard did not eat and therefore the health of his knees was vital for his and his family’s survival. There is a lesson here for all of us.
The knee is a complex and vitally important weight-bearing joint. Virtually every system of martial arts relies to some extent on healthy, strong knees. The anatomy of the knee and the nature of its movement make it susceptible to injuries caused by rotational forces across, and compressive forces on, the joint surfaces. Martial arts training should emphasize keeping knee movements within their normal physiological range as described in this article. When standing and walking the knee and foot must stay appropriately aligned. Specifically training the muscles that move the knee greatly enhances knee function. Applying sound principles of musculoskeletal anatomy and physiology makes for sound practice and a wise warrior.