Fascia is connective tissue that surrounds various structures within the body. Like tendons and ligaments fascia is made of collagen fibres that run parallel to the direction of pull. Fascia is considered to be a passive structure that can transmit mechanical stress throughout the body. One of the most commonly talked about fasciae is the plantar fascia.
The plantar fascia starts at its insertion in the calcaneus (heel bone) and runs the entire length of the foot. It runs under all 5 metatarsal heads and inserts into the phalangies (toe bones). The thickest part of the plantar fascia inserts into the distal phalanx of the 1st toe (see fig 1 below).
Fig: 1 Medial Side View of Plantar Fascia Anatomy
To feel the plantar fascia in your own foot, hold on to your toes and pull them back. You should be able to feel something like rope in the middle of your foot, this is the plantar fascia.
What Does The Plantar Fascia Do?
Whilst standing the foot acts as a truss to support body weight (fig. 2). Body weight acts on the truss by pressing down on the talus, the bone at the apex of the truss. This causes a ground reaction force, which comes up through the metatarsals at the front of the foot and the calcaneus at the rear of the foot.
The bones in the foot are compressed between body weight from above and the ground reaction force from below. The plantar fascia acts as a tie cord between the forefoot and the calcaneus thus preventing the arch from collapsing.
Fig: 2 Diagram of the Foot as a Truss
What Does The Plantar Fascia Do During Gait?
During gait the plantar fascia becomes part of a windlass mechanism within the foot.
Windlass are a type of winch for moving heavy objects, consisting of a horizontal axle that a chain or rope is wound around. It is the type of mechanism used on boats to raise and lower anchors and in wells to raise and lower the bucket.
In the foot there is a windlass mechanism that pulls the two ends of the foot together. Helping to compress the bones, raise the height of the arch and stabilise the foot.
The windlass mechanism in the foot consists of:
- Plantar fascia – the chain or rope
- The big toe – the handle
- Metatarsal heads – the axle.
What Does The Windlass Mechanism Do?
The windlass mechanism helps to stabilise the foot, making it rigid to improve its ability to act as a lever during gait. During the 3rd rocker the toes dorsiflex which wraps the plantar fascia around the metatarsal heads. As a result the plantar fascia is pulled tight causing the bones within the foot to compress and supinate. The arch rises, the foot shortens and becomes rigid (see fig 3 below).
Fig: 3. A. The foot in a neutral position with a normal arch height.
B. Toes dorsiflexed, arch raised and foot supinated.
C. Diagram of the windlass mechanism in action.
The plantar fascia is a structure similar to tendon and ligament which transmits mechanical stress through the foot. Whilst standing still the plantar fascia enables the foot to act as truss and support the weight of the body. During walking it is a component of a windlass mechanism which helps to stabilise the foot and improve its function as lever.