Built for speed

7 May 2013 by Malcolm Campbell, posted in Biology

An ordinary cheetah can go over 60 miles an hour. A human can hardly do a quarter of that. What a joke! Hobbes in Calvin & Hobbes, by Bill Watterson

In the fable of the tortoise and the hare, a great many of us relate to the tortoise. Slow and steady wins the race. This said, there’s likely a strong contingent who relate to the hare. Eager to get a move on, cover ground, get to where they’re going fast. These are folks who are generally built for speed, both physically and mentally.  They are quick of mind and body, agile, and fleet of foot. They share with the hare an innate ability to make haste.

Innate ability to run fast abounds amongst non-human animals. Running fast is a clearly adaptive trait. A quick sprint can get you dinner, or prevent you from being on someone else’s menu. No small surprise that evolution has honed some particularly fine examples of fast fauna.

Cheetahs are perhaps the most frequently touted examples of fast-footed animals. It is easy to understand why. With their ability to reach speeds in excess of 105 km/h, cheetahs are certainly the fastest felines, and, over short distances, leave other large land animals in the dust.

Like other fleet quadrupeds, cheetahs have a special version of the gallop that enables them move so quickly. Most people are familiar with what is known as a “transverse gallop”. In the transverse gallop, the hind limbs and fore limbs exhibit a similar pattern in the order that they strike the ground:  first the right hind, followed by the left hind, followed by the right fore, then finally the left fore. The transverse gallop is a single-suspension gait – all four legs are only off the ground once in each footfall cycle. The transverse gallop is familiar to many people, not the least of which because it’s how thoroughbred racehorses run.

Cheetahs are able to kick the gallop into a higher gear. In addition to the transverse gallop, Cheetahs have a “rotary gallop” in their running repertoire. Unlike the transverse gallop, the rotary gallop is a double-suspension gate  - all four legs are off the ground twice in each footfall cycle. Here, the footfall cycle occurs in the order: right hind, left hind, then a stretched suspension, followed by left fore, right fore, and finally a compact suspension. The compact suspension provides additional propulsion during the stride, while the extended suspension enables a very long stride. In fact, in comparison to body size, the cheetah’s stride is twice that of a horse’s stride.  This enables cheetahs to cover much greater ground per stride.

What is truly amazing about cheetahs is that they achieve faster speeds using the rotary more effectively than comparably-sized rotary gallopers.  For example, greyhounds, which have roughly the same size and physique as cheetahs, are not able to reach cheetah speeds.

Greyhounds only clock in at about 60-70 km/h. Why can’t greyhounds run as fast as cheetahs? It turns out that cheetahs have an additional trick up their sleeve – or at the end of their sleeve as the case may be.

Slow motion video analysis of the greyhound versus the cheetah gallop reveals that cheetahs are well grounded.  That is, cheetahs hit the ground for longer, and with greater frequency, than greyhounds. In the world of fast runners, greyhounds are, comparably speaking, the tortoises – slower and steadier. The greyhound rotary gallop stays at a relatively constant cycle rate. By contrast, cheetahs are able to increase the rate of the cycles of their rotary gallop – increasing the tempo with which they hit the ground. Cheetahs also leave their feet on ground for a fraction of a second longer than greyhounds, using this extra time to propel themselves forward that much better.

Cheetahs are able to accomplish this feat by powering their runs with a phenomenal engine. The cheetah has three main components to its running engine – legs, back and abdomen.

The cheetah’s legs are astonishing engines in and of themselves. The hindquarters in particular have powerful thighs that propel each stride. The cheetah’s back functions as extension of these hind legs – bending forward during each stride so that the cheetah’s rear paws reach the ground almost beyond its ears.  The strong abdominal muscles then kick in to push the rear and fore quarters in opposite directions leading to the fully extended stride.

We know that muscle mass is extremely important to power such an engine on the basis of another rotary galloper: the whippet. The whippet is a medium-sized sight hound breed, closely related to the greyhound. Like cheetahs and greyhounds, the whippet is also is able to attain very fast speeds using a rotary gallop, reaching over 55 km/h over short distances.

A number of years ago, people found that some whippets are decidedly faster than others. These whippets are characterised by having high muscle mass.

People who desired fast whippets found that the high muscle mass trait was inherited, but in an interesting manner. When a pair of these high muscle mass, fast whippets were bred together, half of their offspring were like their parents, but one quarter of the pups were like normal whippets, while the final quarter was like no normal whippet. This final quarter of the puppies looked like body builders.

The latter group of whippets are known as “bully whippets” due to their resemblance to bull terriers. Bully whippets were stocky, incredibly muscular dogs. Bully whippets have a trait called “muscle doubling”. They literally have double the muscle tissue found in normal whippets. Some meat-producing cattle breeds, and the occasional human, have the same trait.

Muscle doubling in whippets, cattle and humans arises due to mutations in a gene that is involved in controlling muscle development. The gene encodes the growth factor known as mysostatin. Myostatin restricts muscle development to ensure that the right amount of muscle is made. Bully whippets carry two copies of a dysfunctional myostatin gene. Consequently, muscle formation is not kept in check by myostatin. This poses no advantage to bully whippets in terms of speed. In fact, bully whippets are hindered by their extra muscle, and are not as fast as even normal whippets.

By contrast, fast whippets carry only one mutated version of this gene. Consequently, they produce more muscle – not so much as to be a hindrance, but, instead, an amount that is an advantage for running fast. Regular whippets have two normal versions of the same gene – so they can run fast, but not as fast as the mutants with one copy of the mutated gene.  Recent evidence indicates that some racehorses may be advantaged in a similar way with mutant versions of their myostatin gene. This is particularly true for sprinting horse breeds, like the Quarter Horse. The Quarter Horse acquired its name as this breed was bred to “sprint” a quarter of a mile.

Like speedy whippets, cheetahs have muscle distribution in all the right places. Their upper limbs, particularly their hindquarters, are equipped with muscle that enables them to make rapid, harder foot strikes as they run. Cheetahs have a high propensity of fast-twitch muscle fibres in these muscles. Fast-twitch muscle fibres, in contrast to slow-twitch muscle fibres, are the cells responsible for the cheetah’s ability to move its muscles quickly. Between 60-85% of cheetah limb muscle fibres are fast twitch. Contrast this to the average human, who has approximately an equal ratio of fast-twitch to slow-twitch fibres.

Is there anything that we humans can learn from cheetahs so as to hasten our own running pace? Humans are unique amongst animals in that we only really have two natural gaits – walk and run. We don’t have anything equivalent to a gallop, let alone a rotary gallop. This said, some of the biophysics that makes for a fast cheetah, also makes for a fast human.

As is the case with cheetahs, increasing the rate of foot strikes, as well as the power at which the foot hits the ground, can increase human running speed. Muscle mass is important to achieve this, especially the proportion of fast-twitch muscle fibres. No small surprise that the best human sprinters are highly muscled. Analysis of their leg muscles shows that they can be up to 80% fast-twitch muscles. Contrast this with marathon runners who may only have 20% fast-twitch to their 80% slow-twitch muscle fibres. Like cheetahs, being able to pound the leg mass into the ground to increase the force of the footfall is crucial for a sprinter’s speed.

Cheetahs are clearly built for speed. Some humans appear to be built for speed also. Where our species differ is that cheetahs live out their lives dependent on this speed to survive. And this is a survival that has become increasingly precarious.

Generally speaking, humans do not need to sprint to survive. For humans, sprinting has become recreational – a pastime, a sport. Those of us who do sprint do so because we can’t imagine getting from point A to point B any other way, or to placate a competitive urge. As we make our way about in haste, it might be worthwhile to pause for thought - nature has afforded us the opportunity to sprint as a luxury, for other creatures on this planet, it is still a matter of life or death.

References:

Binns MM et al. (2010) Identification of the myostatin locus (MSTN) as having a major effect on optimum racing distance in the Thoroughbred horse in the USA. Animal Genetics 41: 154-158

Bower MA et al. (2012) The genetic origin and history of speed in the Thoroughbred racehorse. Nature Communications 3: 643

Hudson PE et al. (2012) High speed galloping in the cheetah (Acinonyx jubatus) and the racing greyhound (Canis familiaris): spatio-temporal and kinetic characteristics. The Journal of Experimental Biology 215: 2425-2434

Mosher DS et al. (2007) A mutation in the myostatin gene increases muscle mass and enhances racing performance in heterozygote dogs. PLoS Genetics 3: e79

Petersen JL et al. (2013) Genome-wide analysis reveals selection for important traits in domestic horse breeds. PLoS Genetics 9: e1003211

Williams TM et al. (1997) Skeletal muscle histology and biochemistry of an elite sprinter, the African cheetah. Journal of Comparative Physiology B 167: 527-535

Images: All photographs by Malcolm M. Campbell.

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