The Mechanics Of Plantar Fasciitis
Why is plantar fasciitis such a common cause of heel pain?
An easy way to understand why plantar fasciitis can develop is to consider the mechanics of the anatomy (biomechanics) of the foot and ankle.
Consider a wheelbarrow working as a lever system that you might have learnt at school.
A wheelbarrow utilises a type 2 lever system. The position of the handles behind the barrow means that the effort used to lift any weight in the wheelbarrow has a proportionally greater mechanical advantage on rotating around the axis of the wheel than the weight within the barrow does.
This is because rotational forces depend on both the size of the force applied and the distance the force is from the point of rotation.
Thus, force times the distance gives the rotational force.
When forces (or weights) are equal, the force applied further from the axis creates the most rotational force. This makes a wheelbarrow a very mechanically efficient simple machine because the handles are further away than any load in the wheelbarrow.
When you lift the contents of a wheelbarrow you apply lifting force (effort) at the handles and rotation then occurs at the wheel (the fulcrum or pivot). The load (or resistance) is held within the body of the wheelbarrow, which with the handles, provides the lever. This makes lifting the load in the barrow relatively easy when pulling up the handles. With more handle-lifting effort, we can tip the contents out of the wheelbarrow and onto the ground.
This is very similar to how humans lift the heel at the end of a step and shift bodyweight towards the next footstep. Despite the mechanical advantage offered, this is a rare way to move a joint. The reason for this is that it places a lot of force across the anatomy to achieve only a small motion. However, that is perfect for motions like lifting the heel but it is not ideal for moving the leg at the thigh or the knee.
To lift the heel off the ground, we apply effort force at the back of the heel bone by using the calf muscle power via the Achilles. Our toe joints across the forefoot (metatarsophalangeal joints) become our wheel of rotation (fulcrum) allowing foot rotation across the toe joints. The load (resistance) is our body weight moving onto the next footstep.
Effective use of the plantar fascia
The plantar fascia and the muscles under the foot tether the foot together firmly so it can become the body of the wheelbarrow (lever). The plantar fascia tensions while lengthening before and during heel lift. After heel lift, it tightens and shortens due to the toes bending upwards, raising the arch and providing some stiffness as the muscles relax. This allows us to tip our body weight onto the next foot in a stable, powerful manner.
The foot never becomes completely stiff. However, it becomes stiffer and more elastic-like just before and as the heel lifts than at any other time during a step. By remaining semi-stiffened, the foot aids the acceleration power from the calf muscles moving the body weight onto the next step. However, the flexibility that remains, provides some important shock-absorbing capacity to avoid injury by keeping a little ‘give’ in the system.
Once weight moves off the foot, the elasticity developed across the foot is released. With tightening and shortening of the plantar fascia, these actions help to raise the foot back into its off-loaded shape.
Issues with the plantar fascia
Problems can occur if the calf muscles are tight and the heel is forced to lift too early before the opposite heel is ready to contact he ground. This risks putting the plantar fascia at greater strain.
At this time, the foot may still be too flexible to easily act as a stable lever. In this situation it is difficult to safely move body weight onto the next foot, especially as the other foot isn’t yet ready or positioned correctly to receive it, either.
An early heel lift can cause more strain on the plantar fascia because the muscles that help stabilise the foot and protect the plantar fascia, will not be holding the foot together strongly enough.
Equally problematic are weak foot muscles that cannot assist the action of stiffening the foot to protect the plantar fascia.
Weak foot and calf muscles may delay heel lift causing the body weight to travel too far forward before the foot stiffens. A flexible foot in the later stages of a step will cause it to flatten excessively causing increased lengthening strain in the plantar fascia. When the heel finally lifts, body weight drives down on a more mobile forefoot, which squashes outwards and flattens excessively. This again increases the strain on the plantar fascia, mostly towards its attachment to the heel bone.
Finally, if the plantar fascia is too stiff, such as is often found in high-arched feet, there will be no elastic ‘give’ in the system. Elasticity under the foot allows it to flatten a little and then spring back acting like a shock-absorbing spring.
A high-arched or pes cavus foot also makes it harder to move body weight towards the front of the foot, by restricting rotations at the ankle. If the plantar fascia cannot easily stretch out, it will pull harder on its attachments to the heel bone.
The attachment to the heel bone is a mechanically weak point in all these situations. Injuries tend to focus here and this area can gradually become over-strained and degenerate, damaging both the heel bone and fascia and thus causing plantar fasciitis.
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People also report pain relief with the Foot Pump compression socks
Andrew Horwood M.Pod.A, D.Pod.M;
Andy is a clinical textbook author, retired musculoskeletal podiatrist and previously visiting fellow and lecturer at Staffordshire University. He has treated thousands of cases of plantar fasciitis and prescribed, fitted, made, and designed foot orthoses since 1988.