Old bones can't talk, but if they could, we'd know far more about the ways our earliest ancestors walked. However, we still have many close relatives that can tell us a thing or two about the evolution of walking. Curious to know how our economical gaits came about, Evie Vereecke, Kristiaan D'Août and Peter Aerts decided to investigate how modern apes walk. Choosing to work with gibbons at their local zoo, the team put the animals through their paces to find out how efficiently the apes walk and to see if they change gait as they shift up through the gears(p. 2829).
Fortunately, the lively animals were very active, although more interested in performing acrobatics than walking across the instrumented walkway. However, once the force plate and four camera video system was installed in the apes' enclosure, it was simply a matter of waiting for the five animals to saunter cross the force plate, at speeds ranging from a stroll to a sprint, to find out whether their movements were more like a walk or run.
After monitoring the animals' antics for two months, Vereecke was ready to calculate each stride's energetics. She explains that most walkers conserve energy by walking with `inverted pendulum' kinetics, where the kinetic energy from the previous step is converted in potential energy as the walker's body moves over their supporting foot, ready to be converted back into kinetic energy as the walker falls into the next step. This efficiently recycles the energy between each step. Runners, on the other hand, move with `spring-mass'kinematics, where the energy from each bound is stored as elastic energy in leg muscles and tendons, before it is reconverted into kinetic energy in the next step. So what was happening to the gibbons?
By plotting the apes' reconstructed kinetic and potential energy functions,calculated from 43 intact footfall force patterns, Vereecke could see that the animals always seemed to move using spring-mass energetics; they were running,even though their footfall patterns were more like a walk. And she never found a clear gait shift as the animals' sped up, they seemed to keep bouncing, no matter how fast they went.
So which structures in the gibbons' legs could possibly store all this elastic energy? Vereecke explains that the team weren't allowed to interact directly with gibbons in the zoo, but sadly, some gibbons died, so the Head of the Zoo gave the team permission to carry out autopsies on the apes. Looking at the Achilles tendon, which powers human running, Vereecke could see that the gibbon's tendon was well developed, but she realised that the animal's movements didn't use the tendon in the same way as a running human. Further investigation of the leg showed that the quadriceps is the most likely structure to store elastic energy in the knee joint to power the ape's bouncing gait.