What’s the Deal With the Achilles Tendon Anyway?

Kambiz has already done a good job of dissecting some of the claims made concerning this story about recent research on bipedalism so this post is kind of redundant. Having said that, I have a few things to say on the subject as well. Let’s start at the beginning.

There are a number of muscles in the calf. For the purposes of this post two are important. The first is the gastrocnemius. The gastrocnemius muscle from the lateral and medial femoral condyles. Below is a picture of the femur so you can see where the condyles are located:
In anatomical parlance these (the two origins of the gastrocnemius) are known as heads. In humans and gorillas the medial head is larger and extends further distally (towards the feet). In chimps, the heads are approximately the same size.
The second muscle is the soleus. In humans the soleus originates on the back of the fibula and the soleal line (I couldn’t find a good picture of this). In apes the soleus originates almost exclusively on the fibula, although it may also attach to the tibia. (Note: the gastrocnemius and soleus are also referred to collectively as the triceps surae). These two muscles unite in a single tendon variously called the Achilles tendon or the tendo calcaneus – so called because it inserts on the calcaneal tuberosity of the calcaneus. Here are some pictures of that:
In humans the Achilles tendon is about 65% of the total muscle length (so it is quite long) in apes it is quite a bit shorter. An additional muscle associated with the triceps surae is the plantaris. I bring this muscle up because of the way it is distributed among hominoids. The plantaris originates from the lateral condyle of the femur near the gastrocnemius. It is almost always present in humans, frequently absent in chimpanzees and orangutangs and always absent in gorillas and gibbons. Interesting, no?
At any rate the triceps surae are twice as heavy in humans as in chimp. So, what do these muscles do? For our purposes, the most important fuction is to plantarflex the foot (they have others, the gastrocnemius flexes the knee, for example). What is plantarflexion?
Sit in a chair and extend your leg, your foot should be pointed at the ceiling. To plantarflex your foot keep your leg extended but aim your toes towards the wall instead of the ceiling. If that didn’t help you visualize it here is a picture:
In bipedal locomotion plantarflexion occurs quite frequently. To understand this, stand up and begin to take a step with your left foot. Freeze before it hits the ground. At this point your right foot will be plantarflexed (it will also be supporting and lifting your entire weight – which explains why the calf muscles are so big).
As Kambiz pointed out one of the first people to clue into the importance of the Achilles tendon in bipedal locomotion was Adrienne Zihlman, most notably in the 1979 paper – cowritten with Brunaker – Hominid Bipedalism: Then and Now. Shortly thereafter R. Alexander elaborated on and extended Zihlman’s ideas. He proposed that the Achilles tendon stores energy at one stage in the cycle and releases it in the next (he even wrote a book called Elastic Mechanisms in Animal Movement) so the idea reported on by Science Daily and others that this is somehow a new or novel idea is a little off. To be fair to Bill Sellers, we don’t know the contents of his presentation and hence don’t know if he made proper attribution of these ideas. Especially because several of the articles I have read have portrayed the research as conformation of the Achilles-tendon-as-spring idea rather than as a new theory…


2 Responses

  1. You mentioned that the plantaris is “almost always” present, what are the consequences for people who lack one?

  2. Due to its position, the plantaris has the potential for flexing the knee, plantarflexing the ankle, and inverting the subtalar joint. However, because of its small size, these actions are rather trivial. In fact, orthopaedic surgeons often use the plantaris tendon as a graft for repair of damaged tendons in the hand.

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