Elastic tendons let ostriches run faster using less calories
October 29, 2010
Runners are about to suffer from a new condition. Ostrich Envy is likely to spread now a study just published has found why ostriches burn less calories than humans while running. Assistant Professor Jonas Rubenson of the Biomechanics Group in UWA’s School of Sport Science, Exercise and Health used reflective markers on limbs to capture movement. Subjects consisted of five human males and five tame ostriches although only two of them “were amenable to the procedures required for full three-dimensional gait analysis.” The study doesn’t mention how researchers discovered which were amenable but perhaps some poor undergrad volunteered to stick reflective markers on three intractable ostriches. The human subjects ran at a comfortable speed they chose themselves and trials where ostrich and human speeds matched were analyzed.
Although humans and ostriches use similar amounts of energy when walking, one aim of the research was to find why humans use 50 percent more calories than ostriches when running. Rubenson was surprised to find both groups used comparable amounts of mechanical power to swing the limbs at the same sort of speeds. Although we share similar limb mass, ostriches have a lower limb moment of inertia and the research cited recent studies showing approximately 20-30 percent of the metabolic cost of running is used for limb swing.
It seems the secret to ostriches efficient running style is their tendons which store and release much more elastic energy than our own. This occurs during the stance phase, the point when one foot is in contact with the ground while the other is swinging. During this time tendons act like springs, storing energy from landing then returning it for propulsion. They found ostriches can generate an amazing 120 percent more stance-phase mechanical joint power via release of elastic energy compared with humans.
The researchers discovered most of the elastic energy in the ostrich occurs the tarsometatarso-phalangeal (TMP) joint. This joint elevates the toes creating the tip-toe running and walking stance of the ostrich, whereas in humans the metatarsal bones are kept in contact with the ground. They also found the store of elastic energy allows ostriches to use 35 percent less muscular power than humans. We also have an elastic contribution to running although this occurs in the Achilles tendon and arch of the foot. The possibility of more economical force production in ostriches was suggested to explain a small part of the metabolic cost difference between species. Rubsenson said “animals such as the ostrich are specialized to run both fast and remarkably economically” using tendons “to store and return twice as much elastic energy per step than us, reducing the work required by their muscles.”
One group of humans that may be close to ostriches are barefoot runners. Although humans and ostriches appear to have similar ground reaction forces when running, the vertical component of the ground reaction force didn’t exhibit a transient at foot contact in ostriches. This has also been observed in barefoot runners and runners with mid and fore foot strike. The researchers suggest the lack of a vertical ground force transient could be because ostriches don’t heel strike. Ostrich feet also look more scary than VFF’s, but this probably has no effect on their running efficiency.
If you still haven’t got ostrich envy then consider this. The research was performed with subjects running at around 7 mph since at this speed ostrich and human gait is similar. But ostriches can run at speeds of 37mph compared to our sedentary 18mph. Even Usain Bolt couldn’t keep up, having hit just under 28mph over 20 meters of the 100 meter sprint. It’s possible that at faster speeds ostriches are even more efficient since other muscles may add to the elastic energy!
Rubenson hopes to use knowledge gained in the UWA lab for improving human health and performance and sees this study as having a role in development of better prosthetics and bioinspired robotics. He says his future work aims to apply information from the Lab to “technologies for enhancing human gait, such as improving locomotor economy and reducing muscle injuries”. Ultrarunners will be paying close attention as they’re already at the cutting edge when it comes to bioengineering. With ostrich feet for inspiration maybe toenail removal is just the beginning. As Rubenson says “It’s all in the spring of their step!”