The athleticism of horses is undeniable, but even the most ardent of fans will acknowledge that they are beaten for pace by cheetahs.
What makes this big cat the fastest land mammal? Why aren’t other animals, such as horses, as fast? While scientists have yet to determine all the reasons, they have some idea about how cheetahs make use of a “galloping” gait at their fastest speeds, involving two different types of “flight”.
The first is with the forelimbs and hind limbs beneath their body following a forelimb liftoff. This is referred to as “gathered flight.” The other involves the forelimbs and hind limbs stretched out after a hind-limb liftoff, called “extended flight” (see Figure 1).
Of these, it is the extended flight that enables cheetahs to accelerate to high speeds, although it very much depends on ground reaction forces satisfying specific conditions.
In the case of horses, this extended flight is absent, according to researchers.
Cheetahs show appreciable spine movement during flight, alternating between flexing in the gathered mode and stretching in the extended mode, which contributes to its high-speed locomotion. However, little is understood about the dynamics governing these abilities.
“All animal running constitutes a flight phase and a stance phase, with different dynamics governing each phase,” explains Dr Tomoya Kamimura, an assistant professor at the Department of Electrical and Mechanical Engineering with the Nagoya Institute of Technology in Japan, who specializes in intelligent mechanics and locomotion.
During the flight phase, all feet are in the air and the center of mass (COM) of the whole body exhibits ballistic motion. Conversely, during the stance phase, the body receives ground reaction forces through the feet.
“Due to such complex and hybrid dynamics, observations can only get us so far in unraveling the mechanisms underlying the running dynamics of animals,” Kamimura says.
Consequently, Kamimura and his fellow researchers have turned to computer modeling to gain a better dynamic perspective of the animal gait and spine movement during running and have had remarkable success using fairly simple models. However, few studies so far have explored the types of flight and spine motion during galloping (as seen in a cheetah).
Against this backdrop, the study team has now addressed this issue in a recent study published in Scientific Reports, using a simple model emulating vertical and spine movement.
The team employed a two-dimensional model comprising two rigid bodies and two massless bars (representing the cheetah’s legs), with the bodies connected by a joint to replicate the bending motion of the spine and a torsional spring. Additionally, they assumed an anterior-posterior symmetry, assigning identical dynamic roles to the fore and hind legs.
By solving the simplified equations of motion governing this model, the team obtained six possible periodic solutions (repetitive actions), with two of them resembling two different flight types (like a cheetah galloping) and four resembling only one flight type (unlike a cheetah galloping).
Researchers then verified these criteria with measured cheetah data, revealing that cheetah galloping in the real world indeed satisfied the criterion for two flight types through spine bending (see Figure 2).
Additionally, the work also revealed that horse galloping involved only gathered flight due to restricted spine motion, suggesting that the additional extended flight in cheetahs combined with spine bending allows them to achieve such great speeds.
Indeed, many animals, including horses, exhibit galloping with just one type of flight, they said. Previous research indicates horse galloping involves only gathered flight and rarely or never involves extended flight, with observational studies conducted by others suggesting that horses do not exhibit substantial bending of the spine.
“While the mechanism underlying this difference in flight types between animal species still remains unclear, our findings extend the understanding of the dynamic mechanisms underlying high-speed locomotion in cheetahs,” Kamimura says.
“Furthermore, they can be applied to the mechanical and control design of legged robots in the future.”
Kamimura, T., Aoi, S., Higurashi, Y. et al. Dynamical determinants enabling two different types of flight in cheetah gallop to enhance speed through spine movement. Sci Rep 11, 9631 (2021). https://doi.org/10.1038/s41598-021-88879-0