A study by Mooses et al., (2015) found that the reason Kenyan runners are so good is due to thinner lower limbs, smaller body mass concentrated on distal segments of the leg, and shorter Achilles tendon arm moment.
In regards to body mass, previous studies found that aerobic cost increased by 1% for every additional kilogram carried on the trunk. Makes sense.
Why Kenyan Runners are so Good
American distance runners have greater muscle mass than most Kenyans, obviously due to genetics, but also because of their training program which includes intense strength training (i.e weight lifting) and is based on the premise that strong muscles are less prone to injury.
However, injury rates are higher in American runners than in Kenyan runners, therefore certain modes of strength training appear to have little effect on injury prevention.
The use of intense strength training for injury prevention is under-utilized in Kenyan runners, meaning they spend more time running and less time lifting weights, mainly because they incur fewer injuries.
How Come Kenyan Runners Injure Less?
The answer becomes evident in their running technique, which is a forefoot running style learned early in life, under barefoot conditions. Most Kenyan runners learned to run forefooted the correct way, without shod-interference and it was barefoot running that was the basis for a shorter Achilles tendon moment arm.
- A 2009 study by Lee and Piazza found that sprinters had an Achilles tendon moment arm that was 25% smaller compared to non-sprinters. And like most Kenyan distance runners, sprinters are forefoot strikers.
- The current study found that the Achilles tendon moment arm in the Kenyan runners was strongly related to running economy and accounted for 28% of the variance in running economy.
- Other work found it to be 58-64% of the variance in running economy. Therefore, a shorter Achilles tendon moment arm is imperative to running economy, but what is it?
The amount, high or low, of the Achilles tendon moment arm represents the orientation of the foot-ankle complex relative to the ground throughout the gait cycle of running.
Kenyan runners have a shorter Achilles tendon moment arm upon and at touchdown because the ankle is slightly plantarflexed, or in a neutral position per se. This also means that Kenyan runners have a flatter foot placement at foot strike, shown below.
A flatter foot placement at touchdown means the ankle is plantarflexed where at touchdown, a mechanical advantage of the plantarflexors it to increase joint angular velocity which increases muscle shortening speed, decreases muscle force generation and allows the Achilles tendon to recoil.
In contrast, a higher Achilles tendon moment arm means that initial strike position occurs high up on the toes, or squarely on the heel, causing initial strike position of the foot to be less flat, shown below.
The researchers suggested that a shorter Achilles tendon moment arm is advantageous in running because it allows for efficient storage and release of elastic strain energy, and therefore allows for better running economy.
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Cardinale, M., Newton, R and Nosaka, K. Strength and Conditioning: biological principles and practical applications. Wiley-Blackwell. 2011.
Mooses et al. Dissociation between running economy and running performance in elite Kenyan distance runners. J Sports Sci, 2015; 33(2):136-44.
Myers, M. J., & Steudel, K. (1985). Effect of limb mass and its distribution on the energetic cost of running. J Exp Bio, 116, 363 – 373.
BSc Neurobiology; MSc Biomechanics candidate, ultra minimalist runner & founder of RunForefoot. I was a heel striker, always injured. I was inspired by the great Tirunesh Dibaba to try forefoot running. Now, I'm injury free. This is why I launched Run Forefoot, to advocate the health & performance benefits of forefoot running and to raise awareness on the dangers of heel striking, because the world needs to know.
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