Biomechanics of the Knee and Common Knee Injuries

A common place to have problems in the body, ranging from aches and pains to more serious injuries is the knee. These injuries can be sudden and seemingly come from nowhere like an ACL tear, or they can be wear and tear problems such as the wearing down of the cartilage inside the joint. But why do these problems occur and how can we help prevent them, or recover from them once they happen?

Knee Anatomy

The picture above shows the structure of the knee. If you’re mechanically minded, you will see it’s primarily a hinge joint, with potential for a bit of rotation, and some clever person has designed it with a nice pair strong strap down each side to keep it stable whilst allowing free movement. So what causes problems with it?

Knees. Vulnerable, especially to arrows…

The knee is effectively a passenger – no matter what it wants to do, it’s stuck in the middle of the leg between the hip and the foot and it pretty much has to go where they tell it to. This is fine if everything is working right, but what about it either the foot or the hip is in the wrong place, or even worse if both the hip and foot are trying to get the knee to do opposite things?Contrary to what you may think, knee pain is often not an issue of a problem with the knee joint. Long term injuries such as wearing away of cartilage are of course problems located within the knee, but even these are often a symptom of bad alignment of the knee for years. Most of the muscles connecting near the knee are concerned with flexing and extending, but when it comes to rotation and lateral movement the knee is often at the whim of the hip and the foot.

“Stuck between a hip, and a foot place”
(the other option for this picture was some kind of ‘stuck in the middle with shoe’ gag, so think yourself lucky)

What Can Effect The Knee Joint

So now we’re seeing that the knee is often a downtrodden victim, bowing to the whims of the foot and hip, let’s see what commonly goes wrong with those areas. Here’s a quick list:

• Incorrect hip function
• Incorrect foot function
• Weak muscles in the and hip
• Imbalance between muscles of the leg

Incorrect Hip Function

The hip can often cause problems with the knee due to incorrect function causing the knee to be put in the wrong position, twisting and torquing which can cause excessive wear and tear or in extreme cases sudden snapping of the ligaments holding the knee stable.

The Piriformis muscle

The muscle to look at first for hip function is the piriformis. This little blighter sits right in the centre of the body, and can often protest about it’s tough working conditions by going into spasm. This can cause all sorts of problems within the body, but at the knee specifically it can cause problems by transmitting stress that should be taken up in the bigger, stronger hip joint down into the knee. This type of problem will be all the more obvious if you perform repetitive impactful exercise such as running. To reduce spasm in the piriformis anti-spasm exercises, massage and stretching can be beneficial (usually in that order in terms of effectiveness)

Incorrect Foot Function

Another major contributor to knee pain is if the foot is not working properly. The knee is delicately balanced on top of a stack of bones consisting of the shin (tibia/fibula), then the talus and calcaneus (ankle and heel), with a whole host of other bones in the foot held together by tendons, ligaments and muscle. The way it should work is that the weight is mostly supported on the heel when standing, and the toes are like controls, steering which way the whole unit further up. If there is a problem with the foot (collapsed arch etc.) the issue doesn’t just affect the foot but is transferred up the body. In this way a foot problem can be known to cause a neck problem in extreme cases! In some cases the foot may need support from orthotics or the ankle may need to be mobilised, but sometimes the foot function is affected by the strain of carrying too much weight. This is usually an issue of a hip weakness.

Weak Muscles In The Hip

As suggested in the previous paragraph, weakness in the hip can cause problems further down. The hip has some of the biggest and strongest muscles in the body, and has a large part in the side-to-side and rotational movement of the knee. These movements in turn can put more or less stress on the knee and foot.

Gluteus Medius

The most commonly weak muscle in the hip is the Gluteus Medius. This muscle runs down the outside of the hip and when contracted in a side-lying position lifts the leg out sideways from the body. However this is not the major function in normal life. When walking, running or jumping, every time the foot hits the ground the Gluteus Medius (amongst other muscles) must work to stabilise the knee and prevent it from collapsing inwards. This is a very common problem for women especially, due to their wider pelvis causing a mechanical disadvantage for the gluteus medius. This ‘falling in’ of the knee puts a pressure on the surfaces of the bone that rub over each other, and can wear away the cartilage and cause arthritis in the long term.

Knee falling in due to weak glutes

The best ways to strengthen the gluteus medius are generally standing exercises such as squatting with a band around the knees providing something to push against, standing abduction of the leg (i.e. pushing the leg out to the side) against cable or band resistance, and various single leg stability exercises such as squats, reaches, deadlifts etc. I also like using more simple exercises such as clamshells (lying on your side and pushing your knees apart with band resistance) and glute bridges with a band (lying on back and pushing the hips in the air) in the warm-up in order to activate the glutes and get the mind-muscle link working. A stability band is ideal for this type of work.

Imbalances Between The Muscles Of The Leg

The last common problem I will mention is an imbalance between the muscles of the leg. This is less common that some of the others but can certainly cause or contribute to knee pain. The basic story is this – the muscles in the front of the leg attach to the front of the shin, the muscles in the back of the leg attach to the back of the shin. Therefore, is the muscles on one side are stronger or tighter than the muscles on the other side, they are able to create a pulling force on the knee which can move it out of optimal position. This means, similarly to the last point, that there is more stress on certain areas of the knee that are designed to rub together.

Great image showing how the muscles of the leg affect the knee

The solution to this is to strengthen the muscles that are weak and/or stretch the muscles that are tight. Again, I prefer to use as ‘functional’ a movement as possible for this so would prefer a front squat or a lunge to strengthen the quads as opposed to a leg extension, but some people could benefit from isolation exercises at certain times.

In Closing…

I hope this article has given you some ideas as to the wide range of factors that can affect the knee and cause pain and injuries. As you can see it is a complex subject and this has only really scratched the surface, but the main take-home point is that it is often not just a problem at the knee. Often I see people running or playing sports wearing knee supports whilst still having incorrect biomechanics and movement patterns – this will not solve the problem long term and could even make things worse by making you reliant on outside aids to perform without pain. Please comment or ask questions below!