When I began my research on knee injuries in aikido, while myself recovering from knee surgery, it was clear that despite a lot of anecdotes and a general concern regarding the subject, there was very little data specifically addressing it. The purpose of this article is to take some things that are known about knees, take some things that are known about aikido, and toss them together, with an admittedly large serving of my own beliefs about healthy practices. Two caveats apply:
One, if you are young and healthy and have great knees, you are not living proof that commonly encountered aikido situations are low-risk. You are what we call "lucky."

Two, if you are older with some joint disease, or have had the misfortune to have an acute injury, please don't take anything in this article as the gospel or as a substitute for your own common sense combined with input from your own health care providers.

I. Meet Your Knee, Which Doesn't Exist

It's a source of minor frustration when somebody says, "My knee hurts" and expects her doctor or other provider to know just where and what she means. There is no one thing that is a "knee," just as there is no one component that is a "car." So we start with a very basic overview of the complex system of structures we call the knee. Like all joints, it is a place where bones meet and need to move in relationship to each other without spinning out (kind of like nage and uke). A variety of things help with this:

Cartilage, which covers the ends or facets of many bones, is a connective tissue without its own nerve or blood supply, which limits its ability to heal. It acts as a cushion, like the little round plastic discs you put on the bottom of table legs so they don't scratch your floor. When cartilage is worn away, exposing bone, that's degenerative joint disease or osteoarthritis.

Ligament is a connective tissue that connects bone to bone. It too has limited ability to heal. Its like strapping tape: strong but not a lot of give.

Tendon is a connective tissue that usually connects muscles to bones. Yep, limited ability to heal. But some injuries only create inflammation in the sheath around it, which will heal completely given proper rest and treatment. Look at the tendons in your fingers as you move your hand and its clear that they are like marionette strings, allowing movement in one place (muscle contraction) to cause movement elsewhere (a bone further out from the body).

Bursa is a sac found in or near a joint. It contains a fluid that reduces friction in a joint, kind of like a bit of slightly leaky bubble wrap.

Here's how it fits together to make a knee:

The thigh bone (femur) ends with two large round bumps. The larger of the two lower leg bones (tibia) offers the bumps a plateau: not the makings of a stable relationship. So some specialized structures are added. First to provide cushioning on top of the plateau of the tibia, there are 2 big flat donuts of cartilage, the medial meniscus and the lateral meniscus. They give the bumps something round to sit on. They are prone to wear and tear, especially when the knee joint is compressed, and can be torn outright by sideways or shearing movement.

To help hold the bones together, there are collateral ligaments like strapping tape running vertically outside the bones, both on the inner leg (medial collateral ligament) and the outer leg (lateral collateral ligament). For added stability, another pair deep within the knee cross each other to connect the bottom of the femur to the top of the tibia in an "x" shape. The front one, the anterior cruciate ligament (ACL) is the main stabilizer and keeps the tibia from turning and sliding forward. Behind it, the posterior cruciate ligament (PCL), keeps the tibia from sliding backwards.

Ligaments can be stretched or torn, creating pain, swelling, and the possibility of a very unstable joint that may refuse to hold you up or may function askew so that you develop uneven cartilage wear and tear and eventually osteoarthritis.

The large thigh muscles front (quadriceps) and back (hamstrings) with their associated tendons provide for stability, strength, and movement. They are essentially the first line of defense for the knee, providing the flexibility and stretch that the ligaments cannot. If you absolutely must overstretch something, the muscle should be it, not the tendon and surely not the ligament! In practical usage, the hamstrings seem to be more protective of the knee because tightening the quads puts tension on the ACL, and several studies of college athletes suggest that hamstring strength is often neglected in favor of building up the quads.

Finally, riding in front is the little bit of bone called the patella or kneecap, floating in the tendon of the quadriceps and anchored to the tibia by the ligamentum patellae, a continuation of that tendon. It can be broken or dislocated under stress. Inflammation and swelling in the joint can cause it to float up, creating a lot of pressure and discomfort. Sometimes chronic pain is due to a strength imbalance between the inner and outer quadriceps making the patella deviate to one side (usually weak medial quads with deviation laterally). If so, relaxing the stronger side and focusing strength training on  the weaker side may move it back to a proper, pain-free position.
 

II. Movement In Aikido

Traditional aikido includes a fair amount of static and dynamic kneeling on the mats. Whether sitting or standing, movement from the center and the taking of balance in aikido is characterized by circular movements. Most commonly this involves lateral turning of the body; that is, pivots in combination with steps or slides. When standing, it often blends with vertical and/or spiral turning, via bending of the knees or a complete drop to kneeling. In the course of a one hour class a student might perform technique 80 to 200 times and receive technique (be pinned or thrown) the same number. For the knees, this presents many opportunities for injury, which comprise two main types: pressure on the joint due to hyperflexion/compression, and torquing the joint during pivoting/turning.

In general, hyperflexion and compression of the knee joint can damage the menisci, the patella, and create general wear and tear. Examples of this type of activity in aikido include: kneeling to bow in/out of training, warmups involving kneeling then lying back, and seated technique (suwariwaza and nagewaza for hamni handachi). Many people do these for years without any acute injury being evident and without complaints of pain or impaired function. Every now and then, an unfortunate person will have an acute meniscus tear just from crossing her legs yoga-style or doing a couple of suwariwaza shomenuchi ikkyo. The problem is that we can neither watch the slow accretion of wear and tear in any individual's knees, nor predict who walking into the dojo is at risk for damage before it happens.

Torquing injuries, which can damage tendons, ligaments and the menisci, tend more often to be acute and obvious. They are common in a variety of sports, including skiing, American football, and basketball. Aikido, in the absence of accidents (collisions and falls), should not pose a problem if done correctly. Most people, however, will sometimes use poor body mechanics that result in knee torquing. Beginners are especially prone to letting the knee turn inward as they turn or drop their centers, or letting the upper and lower body move separately when they turn. More advanced students may continue at risk if the poor body mechanics go uncorrected and become set into muscle memory. The frequency of errors hopefully lessen over time, but show up when students ramp up their practice in both speed and intensity, focusing more on "doing technique" than on what is happening to themselves. Seminars, randori, anything out of the unusual that increases a sense of pressure in training have been cited anecdotally to the author by those who have had acute torquing injuries.
 
 

III. Aikido For The Knee-Injured

This section assumes that any acute injury has resolved. The only aikido that is really knee-safe during the weeks it takes for soft tissue injury to heal is "eye waza" in combination with a regimen of RICE and slowly graded return to training.

For all the risks that aikido poses to the knees, most folks do train without incurring serious knee damage. What is notable is that many people, from mudansha such as I through Shihan level instructors, do continue to train with damaged knees. How they do so runs the spectrum form making no accommodation at all for their knees to radically altering their practice. Two examples from my dojo follow:

One of my sempai does not have an intact ACL. There is a standard tests orthopedists use to test the ACL; he knows the test and demonstrates how he fails it quite startlingly. Yet he trains fully and without a brace because functionally his knee is very stable. As is the case with soccer pros who continue their careers with blown ACL, he has worked diligently on strength training so that the muscles completely pick up the slack for the missing ligament.

In addition to strength-training (with a focus on the hamstrings, which seem especially knee-protecting), one can learn plyometric exercises/drills from an athletic trainer to develop proper body mechanics during movement, dynamic balance and coordination (these generally involve hopping, jumping, performing upper body tasks while balancing on moving surfaces, and running figure-eights, backwards, and sideways).

My own situation is different. I had surgery to replace my ACL with a graft, and had part of my medial meniscus removed. Following rehab and slow return to training, there is some laxness in the ACL graft (though its intact) and pain in the medial meniscus area with any prolonged weigh-bearing activity. Strength training helps, but my training now flows from the assumption that I have osteoarthritis/bad medial meniscus.

I wear a rigid hinged brace on the mat. It gives some protection against torquing and hyperextending. It is NOT capable of decreasing internal pressures caused by hyperflex ion (kneeling). I believe that the internal pressure and shearing forces of seated technique put them in "high risk" category for meniscus damage and osteoarthritis, and simply won't do them anymore.

Also off my list are the few techniques that may involve loading of the partner's weight in combination with torquing (koshinage, aikiotoshi). Ukemi variations are chosen for ways to get up without kneeling. For back ukemi, its easy to do either slapping side/backfalls or a rolling form that goes across the shoulders and uses both arms and legs to come up into a stable horse stance. Front rolls easily transition in midroll to "breakfall" style side landings.

As a precaution,  I let my partners know my limits (that it is difficult for me to do rapid, large pivots, and prefer a slower pace on such techniques). When I visit dojo for seminars, I check in with the dojocho and with the seminar instructor regarding my knee. I bow out if my meniscus pain tells me to, and watch the rest of the class.

The question arises of people coming into aikido as beginners who have already damaged their knees elsewhere. I think that it is possible  to do so. But the beginner needs to be fully cognizant of his own limits and of the risks inherent in aikido, because nobody else is going to take that responsibility. One can be very competent instructor of aikido but rely on memorized exercises and historical anecdote in place of a grounding in human anatomy and sports physiology.

The attitude of dojo/instructor is of paramount importance for the student with special needs. If a legitimate need to avoid certain postures and techniques is met with denial, argument, or even rolled eyes, it is not a good sign. If the instructor straight out says not to expect to do everything the others do, and that its ok to sit down or to find a different way, that's a good sign. Observing classes taught by the chief instructor will show if people seem to treat each other with respect and care, and if there are older people on the mat or people with disabilities.

It should be noted that many dojo teach beginners to do front and back rolls from a kneeling position and may not have instructors willing to teach beginners from alternate positions (such as low squats or lunges to start standing forward rolls, or simple side and back falls instead of back rolls). Again, what is important is whether the person instructing considers the an impossible burden or an interesting challenge.

In the final analysis, each of us is responsible for our own training. Having an injury or disability merely underscores this demand to be aware moment-to-moment of one's position, abilities, and limits, and to be implacable in respecting them and expecting others to do the same.