Smart runners are always trying to improve their 'running economy' (the rate at which oxygen is used to run at a particular pace), because better running economy (lower oxygen consumption) almost always translates into better race times and lower perceptions of effort during quality running.
For years, exercise physiologists have believed that the key ways to upgrade economy involve (1) doing intervals on the track at faster-than- race speeds, and (2) carrying out lots of hill training, which includes running up hills at about race pace with regular running form and also 'bounding' up steep inclines with exaggerated knee lifts. The theory is that the fast intervals and hill running boost leg muscle power. Improved power then makes it possible to recruit fewer muscle fibres while running at quality speeds. The lower recruitment diminishes oxygen consumption (heightens running economy). In addition, many physiologists speculate that the vigorous intervals and hill training also improve neuromuscular coordination during running, leading to fewer wasteful body movements and lower oxygen usage.
Runners who do a lot of work on hills and carry out frequent race-speed intervals do have better economy than runners who train differently, but researchers at the Laboratory for Functional Anatomy and Biomechanics at the University of Copenhagen in Denmark have recently identified two key ANATOMICAL factors which can account for differences in economy between runners, too. These two factors are Achilles tendon length and Achilles tendon area. Basically, the longer and slimmer the Achilles tendon, the more economical is the runner, say the Danish researchers.
The Danes had previously noticed that running economy can be nearly doubled in humans just by improving the elastic energy return' of the tendons in the legs. To understand elastic energy return, think about what happens to the Achilles tendons during running. As one of your feet swings forward prior to footstrike, the Achilles tendon attached to that foot is stretched out greatly. As your foot hits the ground and you roll forward over the ball of your foot just prior to toe-off, your stretched Achilles tendon 'recoils' (shortens), just as an extended rubber band snaps back into a shorter configuration when it is released.
This recoil of your Achilles tendon is good, because it pulls up on your heel and helps you pitch forward onto your toe for toe-off. Best of all, it doesn't cost you anything - in terms of energy or oxygen. The energy comes from 'stored energy', put into the Achilles tendon when it was stretched prior to footstrike. In other words, your poor abused calf muscles don't have to use up glycogen or gobble up oxygen in order to get you into a powerful toe-off position. That makes you more economical! Because your Achilles tendons are good at 'returning' energy, you're a better, more efficient runner.
It follows that if your Achilles tendons get even better at returning energy, your economy should improve and your race times should slim down, too. But what separates great Achilles tendons from shabby ones? To find out, the Danes studied a group of young athletes and found that if an athlete has Achilles tendons which are about 10-per cent shorter than average, mechanical efficiency (economy) is hurt by about 6 per cent. On the other hand, if the Achilles tendons are 1 per cent longer than average, economy is enhanced by 8 per cent!
Why would this be true? Longer Achilles tendons can be stretched to a greater extent than short tendons. The greater the length of a stretched Achilles tendon, the greater the force it can apply to the foot when it 'snaps back'. The greater the force, the lower the reliance on glycogen and oxygen and the better the economy.
The Copenhagen researchers also noted that thick Achilles tendons were bad for economy, although the effects weren't quite as pronounced: thickening the Achilles tendons by 10 per cent harmed economy by about 4 per cent, for example, while thinning the strange things assisted economy by 3 per cent.
Is Achilles tendon length set at birth? If so, running economy would at least be partially genetically determined. Observations of elite Kenyan distance runners - currently the best in the world - reveal that they tend to have very long, very slim Achilles tendons, while runners from other parts of the world are more likely to come from the 'short and broad' Achilles tendon factory. The Kenyans are also extremely economical. In fact, they often possess a rare mix of physiological attributes - great economy AND superb aerobic capacity - which is rarely seen in endurance athletes (usually, an individual with superior aerobic capacity will be mediocre in economy, and vice-versa).
However, that doesn't tell us a thing about genetics, because the Kenyans spend a lifetime running up and down hills and gambolling along on the soft, red-dirt trails of Kenya. Running uphill puts tremendous stretch and pressure on the Achilles tendons, because each footfall allows the heel to descend to a level lower than the rest of the foot, elongating the Achilles tendon dramatically. Likewise, running on soft dirt permits the heels to gouge out little foxholes on each footstrike. This foot-in-foxhole phenomenon also elongates the Achilles tendons mercilessly. It could be that the constant hill and dirt-trail efforts - not the genetic endowment from mum and dad - lead to the greater length and stretchiness of the Kenyans' Achilles tendons, and thus their incomparable economy.
Whatever the reason for the Kenyans' efficiencies, the lesson for runners is clear: stretch out those Achilles tendons! The more elastic they become, the more powerfully you will run. And the way to extend them is to run on hills, train on dirt trails, stride on soft ground in bare feet, and stretch them carefully but thoroughly AFTER your workouts are over, when your Achilles tendons are warm and distensible.
('Influence of Achilles Tendon Variables on Mechanical Efficiency, ' Paper Presented at the Second Symposium of the International Society of Biomechanics Working Group on Functional Footwear, June 230,1995, Cologne, Germany)