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Feature - Achilles tendon a blessing, not a curse

Feature - Achilles tendon a blessing, not a curse

Sprinters lining up for the start of the Women’s 100-meter at the Beijing Olympics. Image courtesy LIM CK, under Creative Commons license.

Compared with other runners on this planet, humans are feeble.

If Olympic sprinters competed against mammals of comparable size, they would never even qualify for the finals. The top speed for an in-shape male human is normally between 15 and 18 miles per hour (24 to 29 kilometers per hour). The world record is 27 mph (43kmh), and that was sustainable for only a few seconds.

Meanwhile, horses have been clocked at about 48 miles per hour, wolves about 42, and the speed champion — the cheetah — at 70 miles per hour. (That’s about 77 kmh, 68 kmh, and 113 kmh, respectively.)

Even warthogs are faster than us.

But in the field of endurance racing however, we leave everyone else in the dust. Over long distances, a well-trained human can outrun a horse.

What is the key to this gift? Placement of our muscles? Proportion of our bones? Flexibility of our tendons? New evidence from the Animal Simulation Laboratory (ASL) out of the University of Manchester, UK, suggest that our Achilles tendon is the key feature that allows us to be efficient running machines.

Diagram showing the individual muscles used in the reconstruction from an anterior and lateral view. Full path definitions for these muscles are in the human-readable XML file available at the Animal Simulation Laboratory. Image courtesy ASL

Key to human endurance

Many questions on the mechanics of human running are unanswered. The Achilles tendon has long been believed to be a key structure in allowing us to run efficiently, acting like a spring to store and release energy. Other primates, such as the great African apes, are poor runners and are essentially missing this structure.

In a paper published this spring in the International Journal of Primatology, Bill Sellers and his colleagues report on a virtual experiment which tested the importance of the Achilles by analysing computer models. The models, powered by NW-GRID, created frames with the most important muscles, tendons and bones. By altering the elasticity in the tendons of their models, the experimenters were able to see that a tight Achilles would make running more exhausting and necessarily slower.

The team had successfully used this “direct modelling approach to bipedal motion” before, in studies of things such  as dinosaur locomotion (see 27 January 2010 iSGTW ).

By making many repeated computer runs, they found that they could determine parameters such as optimal muscle use, a computationally intensive process. Even relatively simple models can have as many 61 dimensions in its search space, and the more realistic the model, the greater the computational requirements. (For the dinosaur locomotion studies, about 170,000 core hours of computation were performed in less than half a year, using NW-GRID Beowulf clusters supplied to the consortium by Sun Microsystems and Streamline-Computing.)

Performance came from AMD Opteron Model 275 processors, each of which has two processing cores. Typically 40 or 80 cores would be used in a modelling run, although up to 300 cores could be used at once were that many available.

The modelling approach has been exploited in several collaborative ventures, such as working with Robin Crompton of the University of Liverpool’s department of human anatomy to find the most efficient — and thus the most likely — form of bipedal motion for Australopithecus afarensis, our ancient ancestor. (The fossil known as Lucy is one of the best-known examples of this early species of humans.)

Using this robotics model, the team found that elastic storage is required for efficient, high-performance running. Elasticity allows both energy recovery to minimize total energy cost, and also power amplification to allow high performance. The most important elastic energy store on the human hind limb is the Achilles tendon: a feature that is at best weakly expressed among the African great apes.

Sellers wrote: “By running simulations both with and without this structure we can demonstrate its importance, and we suggest that identification of the presence or otherwise of this tendon — perhaps by calcaneal morphology or Sharpey’s fibers — is essential for identifying when and where in the fossil record human style running originated.”

—Danielle Venton, for iSGTW

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