The Journal of Experimental Biology has published an interesting paper about some unique features in sprinters: longer toes and shorter ankle joints. The only one flaw is that their sample size is limited, they only compared 12 collegiate sprinters with 12 non-athletes of the same height. Regardless, from a physical anthropological point of view, this comparative & biophysical analysis informs us what traits help humans sprint faster.
The significance of long toes and short ankle joints can be explained from a purely physics perspective. From the start of a sprint, the only way a human can accelerate is through the transfer of energy from the force of the leg muscle to pushing on the ground. The advantage of longer toes provide maximum contact with the ground just a little bit longer than shorter toes.
The ankle joint is shorter because there is an inverse relationship between tension force and distance — think torque and angular momentum. Sprinters have a 25% shorter distance between the Achilles tendon and center of rotation of the ankle. The Achilles tendon is the common attachment of the gastrocnemius and soleus muscles into the calcaneus. When contracted, these two muscles flex the knee and plantar flex the foot. With a shorter ankle joint, these muscles shorten less for the same joint rotation. If muscles shorten less, then they shorten more slowly. This facilitates them to produce greater force that more than compensates for the reduced leverage.
When these two adaptations are combined, the authors figured that the greatest acceleration is achieved when the Achilles tendon lever arm is the shortest and the toes are longest. Comparing these anatomical features to other sprinting animals, like ostriches, greyhounds and cheetahs, they authors observed that they also have feet built for sprinting with similar features.
The authors, who are not physical anthropologists, state in press releases that they think these adaptations could have had some evolutionary backing. They raised the tired hypothetical scenario where early human ancestors, now those with longer toes and shorter ankle joints, were better able to run away from the saber tooth tiger or marauding tribe and reproduce that trait. But I disagree, while there certainly is an inherited component to the size and shape of our bones, muscles, and joints, our bodies are malleable and depending on training, our bones and muscles can change!
Furthermore, the majority of humans are not sprinters, as I understand it. In fact, most of us are good at long distance motility. Our bodies are extremely inefficient at sprinting but we’re really good at staying the course! Most of us have lots of Type I muscle fibers, slow but fatigue resistant fibers. Anyways, I don’t mean to rag them on this concept, as I mentioned they aren’t physical anthropologists and they seem to only be speculating on this last point. Either way, I believe the observation they made is interesting!
- Knight, K. (2009). SHORT HEELS GIVE ELITE SPRINTERS THE EDGE Journal of Experimental Biology, 212 (22) DOI: 10.1242/jeb.039735
- Lee, S., & Piazza, S. (2009). Built for speed: musculoskeletal structure and sprinting ability Journal of Experimental Biology, 212 (22), 3700-3707 DOI: 10.1242/jeb.031096