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Function of the Achilles Heel Tendon

The function of the Achilles heel tendon is to transmit forces between the calf muscles and the foot and vice versa. These forces can apparently be up to 12.5 times bodyweight which explains why the tendon is the largest in the body – see Anatomy of the Achilles Tendon post.

Something I learned only a few years ago came as a rather surprise – namely that the calf muscles work hardest as your foot hits the ground and not as you push off!!

Another thing that I did not fully appreciate was the role of the Achilles heel tendon in controlling movements at the subtalar joint.

To explain this requires going into some detail about what happens during the running gait cycle and also a quick resumé of the subtalar joint.

The Running Cycle

The running cycle consists of 3 phases

  1. stance
  2. float
  3. swing

Look at the 3 phases for the RIGHT leg below:

Stance PhaseFloat PhaseSwing Phase

(Hover over the picture with your mouse for the picture title)


I am only interested in the stance phase for now. This phase has 3 markers

  1. initial contact – when the foot makes first contact with the ground
  2. mid stance – when the knee and ankle are at their maximum flexed angles
  3. toe off – when the foot loses contact with the ground

Initial ContactMid StanceToe Off

These markers give us two sub-phases of the stance phase

  1. ABSORPTION – storing the landing forces in the tendon and muscle
  2. PROPULSION – using the stored landing energy to propel the body forwards

Subtalar Joint

subtalar jnt


The subtalar joint is the lower portion of your ankle area. Your ankle (talo-crural) joint allows dorsiflexion and plantarflexion movements. The subtalar joint allows inversion and eversion of the Calcaneum on the Talus. When the joint between the Talus and the Navicular bones (Talo-Navicular joint) is included then you get Supination and Pronation of the hind -foot at the Talo-Calcaneo-Navicular joint.
If any of these terms are confusing see Foot Fundamentals and Ankle Joint – a balanced view.


The subtalar joint is a very complex joint. If you look at the picture on the left this is the foot with the leg and the Talus removed. You are looking down on the Calcaneum and forward to the Navicular bone. The blue areas are the joint surfaces where the Talus joins with the Calcaneum and the orange where it joins with the Navicular bone – you can see the complicated nature of the joint.

Look at the video below and check the complicated movement at the subtalar joint.

Get the Flash Player to see this content.


Right STJ AxisTA Supination

If you draw a line through the axis of rotation of the Talo-Calcaneo-Navicular joint (the red line in the diagram of a right foot) then the Achilles tendon attaches medial (inside) to this axis (green area in the diagram).
The importance of this is that it helps to produce supination at the subtalar joint – you can see this in the diagram.
Imagine if you were to pull on the green area – the Calcaneum will tilt inwards and upwards (supinate the hind-foot).
If you now imagine landing on the outside of your heel then you can see how your Achilles heel tendon can help to control the rate at which your hind-foot pronates (slows the Calcaneum tilting outwards and downwards).








Achilles and Running Gait

Initial Contact -

Achilles heel tendon

The hind-foot is inverted by about 10 degrees.
The ankle is in 10 degrees of dorsiflexion.


As described above the position of the attachment of the Achilles heel tendon to the Calcaneum means that the Soleus muscle stabilises the lateral foot onto the ground as it lands in a supinated position.


In other words it stops the heel and foot from collapsing down into pronation.
It doesn’t matter whether you land on your heel or nearer the front of your foot the action of the Achilles is the same.




Absorption Phase -

TA Slows Tibia

During the absorption phase the foot pronates and becomes flexible.

To achieve this the hind-foot everts through to 10 degrees of eversion with the Achilles controlling this movement of pronation at the Subtalar joint by continuing to act as it does above.

The ankle dorsiflexes further to 20 degrees of dorsiflexion – the Soleus muscle decelerates this forward movement of the leg on the foot.
To understand this think of it this way :-

  • if the muscle contracts with your foot off the floor it points your foot down
  • if your foot is fixed on the floor then the same contraction now will take your leg backwards on the fixed foot OR in this case slow it going forwards…

Does that make sense?

Propulsion Phase -

TA Recoil

During the propulsion phase the role of the Gastrocnemius and Soleus muscles is to control the joint positions.
Creating stiffness here allows the tendons to recoil and release their stored energy.
The Subtalar joint supinates to provide a rigid propelling lever through the action of the Achilles heel tendon.
At propulsion the Soleus slows tibial momentum forward (this would increase the ankle dorsiflexion).


The Gastrocnemius helps produce knee flexion which creates forward movement.
Gastrocnemius simultaneously plantarflexes the ankle, flexes the knee and supinates the Subtalar joint.


At toe off the ankle is plantarflexed to 25 degrees.


Summary

You should now a good understanding of

  • the 3 phases of the running gait cycle
  • what the Stance Phase is
  • structure of the Subtalar joint
  • what pronation and supination of the hind-foot are
  • what the Absorption Phase does
  • what the Propulsion Phase is
  • action of the Achilles heel tendon during each phase
  • how elastic recoil of the Achilles heel tendon provides energy to walking and running

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