How your muscles and joints in your legs interact and function is very important in staying pain free. If there is an imbalance or inefficiency in your lower limb biomechanics it can cause pain in soft tissues such as muscles, ligaments or tendons or cause joint pain through excessive loading and cause accelerated wear and tear (osteoarthritis).
First of all we should take a step back and start with a definition of biomechanics. Biomechanics is defined as a “motion study” of the “mechanical forces acting on a body”. So in the case of lower limb biomechanics we are looking at how the muscles, soft tissues and joints function in the lower limb.
To fully understand why some people have ideal biomechanics and others have poor lower limb biomechanics we need to have a basic look at what biomechanics for the feet and lower limb are, and how our lower limbs work.
We will start at the foot – as this is the first point of contact with the ground.
Basic Biomechanics of the foot
The foot is a very mobile structure and has movement occurring through several different joints and planes (or directions). The complexities are beyond the scope of this report but I will aim to give you a good picture of what happens.
In simple terms the foot has two primary roles:
- To absorb shock during weight bearing activities (standing, walking and running)
- To provide a rigid lever to propel the body during walking and running
In order to do these functions the foot is a very unique structure that allows the joints in it to behave in two ways:
- To absorb shock the foot allows mobility and movement through its key joints as a way of dissipating or absorbing load and impact associated with weight bearing. This movement is called “pronation”.
- To provide a rigid lever the same joints are able to assume a position that effectively “locks” the joints of the hind foot into a firm or rigid position and allows propulsion. In other words a stiff lever to push off with. This movement is known as “supination”.
The bones of the foot form an arch when viewed from the side. To help support the bones in this position the body relies on soft tissue support from ligaments, fascia (fibrous tissue) and also the support of muscles and tendons.
A simple analogy is to consider that the joints and bones of the foot act a bit like the bow (of a bow and arrow), and the soft tissue in the arch of the foot (the plantar fascia and ligaments and supporting muscles and tendons) act like the string of the bow – they provide the tension needed to hold the bones in the arched position.
What happens at the foot when we weight bear (stand, walk, run)?
During shock absorption (weight bearing) the bones of the foot start to unlock and allow the arch of the foot to flatten out. This flattening of the arch is called “pronation”. It is important to note that it is completely normal for a foot to undergo a degree of pronation. As stated before this is how the foot absorbs ground impact and shock forces from walking and running
The amount of pronation (or the extent that the foot flattens out, and the time that it stays pronated during weight bearing) is a more important factor. Pronation places increased tension through the soft tissues in the arch of the foot. It can also increase load in muscles further up the leg and place other joints higher up the leg in less than optimal alignment.
In a “normal” foot type the degree of pronation results in an optimal loading of the supporting tissues in the arch of the foot, as well as optimal loading of the muscles in the leg and optimal alignment of the joints in the leg.
In a foot that flattens out too much (or over pronates) the increased tension and load placed through the soft tissue support structures in the foot and leg and may overload these tissues. This can result in the development of painful symptoms developing and dysfunction. The pain or dysfunction can happen in the foot or further up the leg.
A foot that has an excessively high arch (called pes cavus) can also cause problems. A foot with a high arch has reduced ability to flatten out (pronate). It appropriately has the opposite problem. It does not allow for shock absorption during weight bearing activities. This can cause problems in the foot, or the shock transmission further up the leg and can overload soft tissues and joints further up the chain.
How much force goes through our feet?
Walking alone puts up to 1.5 times one’s body weight on the foot. Running has been seen to put 3 times your body weight through every inch of your foot when you make contact with the ground. The foot is a very complex structure which when functioning optimally supports and balances the weight of the entire body.
Since the forces going through are feet are quite astonishing – especially when you consider the forces that will go through your feet and legs over their lifetime – even more so if you are a very active person. It is vital that you give your body the best chance of minimising these forces.
Our highly trained team at Blackwood Physiotherapy – Sports and Spinal can give you the best advice and recommendations on how to optimise your lower limb biomechanics.
Basic biomechanics at the knee
We have looked at what the foot does during walking and weight bearing in terms of pronation and supination.
With each of these movements at the foot there is movement that occurs further up the leg (the kinetic chain).
What happens during pronation?
During pronation of the foot where the foot flattens out, there is a rotation movement that occurs at the knee. The knee joint internally rotates. You can test this movement yourself when sitting in a chair. If you place your feet shoulder width apart on the floor, deliberately make your foot flatten out and then make an arch with your foot. You should be able to see your shin rotating or twisting. This resultant twist is what causes rotation through the knee joint.
A certain amount of rotatory movement is normal, but what happens when the movement is excessive?
When the rotatory movement is excessive there is excessive loading of the knee cartilages and meniscus in the knee. On top of this, the optimal alignment of the bones in the knee is compromised and this can cause accelerated wear and tear of the cartilage and meniscus (which leads to accelerated or early Osteoarthrits), and places increased load on the muscles and other soft tissues around then knee which can lead to common problems such as:
- Patello-femoral joint pain (pain under the knee cap)
- ITB friction syndrome
- Pes anserius bursitis
- Patellar tendinopathy
- Medial ligament pain
The following picture shows the effect of excessive pronation and the subsequent internal rotation of the lower leg and rotation and the knee.
What happens with a high arched or supinated foot?
With a foot that does not pronate enough (i.e. has a high arch that does not flatten out sufficiently) there is a reduced capacity in the foot for shock absorption. Weight bearing tends to focus on the outside border of the foot and the reverse kind of rotation occurs in the lower leg and knee. With a high arched foot there is external rotation of the lower limb and also external rotation of the knee.
The following picture shows the different rotatory effects of pronation and supination of the feet.
What are the possible problems associated with a high arched or supinated foot type?
Those with a supinated foot type are more likely to experience an ankle sprain on the outside part of the ankle. This is because of the foot positioning during weight bearing tending to be more on the lateral or outside border of the foot. An ankle sprain occurs when the ankle is forced out of its normal position due to instability while walking, tripping or falling. The ligaments, which connect bones together, on the outside of the ankle joint are injured due to over-stretching or tearing.
Excess supination of the foot can lead to injuries that result from reduced shock absorption. This can lead to over-use injuries, such as peroneal tendinpoathy. The peroneal tendon sits behind the outside bone of the ankle joint. Its job is to turn the foot and ankle in an outwards direction. A supinated foot tends to angle in the opposite direction thereby placing a tension/load on these structures. This increased load can cause problems to develop.
Those with a supinated foot type can also suffer from recurrent stress fractures due to the reduced amount of shock absorption. A stress fracture can occur when the muscles are over-used and in the case of the legs when the feet do not absorb shock properly, transferring the forces to the bone further up the chain. The repetitive stress can cause a small crack or fracture in bones of the foot or ankle. The pain of a stress fracture appears suddenly and goes away with rest. Over time, the pain can become debilitating, requiring complete rest of the affected leg or foot including the use of crutches in order to get around.
Basic biomechanics at the hip
We have seen how the movement of the foot into pronation and supination affect the knee joint. But what happens even further up the chain at the hip joint?
With pronation of the foot, there is internal rotation (twisting of the knee) and this results in the knee moving inwards. This movement in turn causes the hip to internally rotate and tilts the pelvis forward.
The following diagram demonstrates the flow on effect of a pronating foot further up the kinetic chain.
What effect does this have on the hip and other structures?
The excessive internal rotation of the hip can affect optimal loading of the hip cartilage during weight bearing. With increase loading of the cartilage over a prolonged period of time there can be accelerated wear and tear of the cartilage – thereby causing premature osteoarthritis of this joint – resulting in pain and stiffness.
The internal rotation of the hip also places increased load through the soft tissues around the hip (muscles, tendons and fascia). Increased load can produce problems in these structures.
The most common conditions that can result from this are:
- Gluteus medius/minimus tendinopathy (lateral hip pain)
- Troachanteric bursitis (pain on the side of the hip/thigh)
- Anterior impingement of the hip (pain in the groin or front of the hip)
- Iliotibial band pain (ITB pain) – pain on the side of the thigh anywhere from the top of the thigh to the side of the knee