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RUNNING ECONOMY

How to make headway on improving running efficiency: can freeloading African women provide some important clues?

When Kikuyu men from Kenya compete in international running competitions, they often achieve superlative performances. When Kikuyu women carry heavy weights on their heads, they defy the laws of exercise physiology.

You might well expect us to focus exclusively on the first phenomenon and treat the second as a fascinating curiosity. Instead, we’ll explore the physiology of walking Kikuyu women in detail and then utilise our explorations to understand a fundamental aspect of running economy. We’ll then give you tips on how to walk like a Kikuyu woman and – more importantly if you use running in your training – how to improve running economy. Finally, we’ll even offer a theory as to why Kikuyu runners are so good.

Our story begins in north-central Kenya, where Kikuyu women literally carry society’s load. The burden might be wood, corn kernels, corn meal, vegetables, live animals, water, or cooking oil, but it is bound to be heavy, and it is usually suspended behind their backs, supported by a strap which runs across their foreheads The Kikuyu females are known to carry up to 70% of body mass with their head bindings: if this does not impress you, feel free to place a barbell or large stone equal to 70% of your weight on top of your head – and then attempt to walk in a relaxed and carefree manner for 10k or so, as the Kikuyu women often do! Those Kikuyu ladies are strong! However, their strength is only half the story; the other half involves a rather amazing physiological display of efficiency during movement. Several years ago Geoffrey Maloiy and his colleagues from the Department of Animal Physiology at the University of Nairobi in Kenya, Harvard University in the United States, and the University of Milan in Italy measured the oxygen consumption rates of Kikuyu and Luo women (the Luo tribe hails from the western edge of Kenya, along Lake Victoria) as they walked along with and without loads on their heads. Maloiy and his co-workers had the women walk with loads equivalent to 20% and then 70% of body mass attached to them(1).

How Kikuyu women differ from western army recruits

Bear in mind that when tough young army recruits are asked to carry extra weight around in their backpacks, their rate of oxygen consumption increases by about 13% with the 20% loads, and 70% objects raise their oxygen consumption rates by nearly 100%. As a student of exercise physiology, this should make sense to you: carrying around extra weight while maintaining the same rate of movement forces the body’s muscles to work harder, and thus energy and oxygen are consumed at higher rates. Enter the Kikuyu. When Kikuyu women load up with an extra 2% of body weight, oxygen consumption rate increases barely at all and energy cost also fails to increase. Give them 70% of body weight on their heads, and oxygen consumption increases by only 50% (so they’re not perfect after all!), compared with the 100% spike seen in the gritty recruits with their backpacks. As noted physiologist R McNeill Alexander points out, we should not conclude from this that the most efficient way to carry something heavy is to place it squarely on top of the pate(2). In short, don’t try to move through the airport or station with your suitcase on your head, especially during these troubled times, because experiments show that the cost of carrying a load is the same whether it is supported by the head or back. For most of us, carrying weights with our heads wouldn’t be practical anyway; our neck muscles are too weak and uncoordinated. Experiments in which bicycle helmets are filled with small lead weights and then placed on the head reveal that most Europeans and Americans can support no more than about 15% of body weight on their heads without experiencing real trouble; as the weight increases above 15%, most individuals begin to stagger and feel as though their heads are going to be ripped off

None the less, the key to Kikuyu economy is not the use of the head; it is something the load bearing Kikuyu women do mechanically as they move along. These mechanical adjustments permit Kikuyus to move more efficiently than the average person – and to carry extra weight with little or no increase in energy cost. If we can discover what it is that makes the Kikuyu so economical, we might be able to use the finding to improve our efficiency when we walk and run. To understand what the Kikuyu women are actually doing, let’s think about why moving along the ground burns up energy in the first place; we’ll focus on walking first and then turn our attention to running. As we walk along, we move like a pendulum. If this seems a curious analogy, bear in mind that a pendulum is simply a device which transforms the kinetic energy of motion (the swing of the pendulum) into gravitational potential energy (present at the end of the pendulum’s arc, when the pendulum stops for a millisecond and then starts its downward fall and progression towards the other end of the arc). Of course, the gravitational potential energy present at the end of the arc is transformed into kinetic energy, which goes back to potential energy at the other end of the arc, and so on

The pendulum approach to walking efficiency

Note that when a pendulum moves through the bottom of its arc, the pendulum’s velocity and thus kinetic energy reach their maxima. Kinetic energy is expressed by the mathematical formula:

KE = mv2/2 (mass of the object times velocity squared, divided by two). At the top of its arc, the pendulum slows to a temporary stop, at which point all kinetic energy is lost. However, at that very point potential energy (mass x gravitational force x height) is at its apex. As the pendulum falls, potential energy flows back into kinetic energy, and if the pendulum is a ‘good one’, the transfer of energy should be almost 100%, with just a bit of energy lost due to air friction and friction in the bearing from which the pendulum is hung. The pendulum’s great ability to swing back and forth between potential and kinetic energy without losing much energy explains why a hard push on a pendulum will keep it moving for a long time. What does all this have to do with walking? With each step that you take during the act of walking, you actually become a pendulum, albeit an inverted one. As you walk, you pivot on the foot which is on the ground ahead of you as if you were using the leg attached to that foot to swing your body through a forward arc. In effect, your foot is the bearing, your body is the arm of the pendulum, and your head is the pendulum’s body. Of course, as you plant a foot on the ground ahead of you, the ground exerts a force on your leg which slows you down, and you continue decelerating as you rise up on that foot to the top of your arc, the point at which your centre of mass is directly over the foot. At that point, your kinetic energy is at a minimum, since you have decelerated to the greatest possible extent, but your potential energy is now at its maximum, since your centre of mass is ready to fall forward as you take the next step. As you step (fall) forward, the potential energy is converted to kinetic energy, just as with a traditional pendulum, and you accelerate in a forward direction. If the body were a perfect pendulum, it could convert kinetic and potential energy back and forth without losing energy, and walking would require no energy expenditure at all. Naturally, it would take you a certain amount of energy to reach a certain velocity, but once that speed was attained you could simply keep falling forward, converting your potential energy to kinetic energy and back again, without slowing down. Unfortunately, the body is far from perfect. In fact, noted physiologist Norman Heglund of the University of Louvain in Belgium estimates that about 35% of the energy required to create movement is lost with each forward swing of your body through its pendulous arc (ie with each step). Thus, 35% of the energy needed for each step must be supplied by the carbohydrate, fat, and protein you eat. When such energetics are compared with those of other animals, humans look like energy wastrels. As they fly, birds burn less energy than humans do per unit distance, even though they can’t take advantage of the energy savings associated with pendular movement. Fish must struggle through dense liquids to move, but they are also more efficient than humans. So why is it that humans spend more energy moving than other animals, and why is it that we can not function as decent pendulums, even though we routinely move in pendulous fashion? For one thing, there are some key differences between our pendulations and those of an

inanimate pendulum. Remember that a regular pendulum reaches maximum kinetic energy at the centre of its arc and maximum potential energy at each end of the arc; humans reach maximum kinetic energy at the ends and greatest potential energy in the middle. Furthermore, a normal pendulum is suspended in air, and its bearing (the point of attachment of the pendulum to some stationary object) does not have the responsibility of holding up weight. By contrast, a human-pendulum’s bearing (ie the foot) and its attached ‘arm’ (ie the leg) must expend some energy to support the body above and keep it from collapsing in a heap.

2001 ISSUE 156

Kikuyu women are at the more perfect end of the spectrum

None the less, some humans are more like a perfect pendulum than others, and the Kikuyu women are certainly at the more perfect end of the spectrum. (Those who have watched Katherine Ndereba run will have no problem with this assertion.) What makes the Kikuyu women better able to inter convert kinetic and potential energy – and use less fat and carbohydrate energy to move along? Why doesn’t it cost them very much to carry heavy weights? Maloiy and his colleagues suggested that the answer to the latter question might be that the Kikuyu women do not allow the loads on their heads to rise and fall very much. At first glance, this is an attractive hypothesis: a significant part of the energy which muscles consume during walking is ‘burned’ in order to lift the body up to the top of its arc as it pendulates above the foot; if the load on the head ‘tracks’ in a straight-ahead horizontal line, with little vertical oscillation, then the energy expenditure associated with carrying the load will be minimised. In theory, if the load moves steadily along, as if on wheels, it could be carried without energy cost. One thinks here of the great Ethiopian marathoner Abebe Bikila, who stated often that one of the keys to his success was learning to balance his head on top of his body, so that he could run as economically as possible. Did he mean that he wanted to minimise vertical movements of his head, or was he simply talking about controlling side-to-side and front-to-back motions so as not to force his neck muscles to spend energy re-positioning his crown (or both)? Maloiy’s idea is attractive, but – at least in the case of heavy loads – it is probably wrong. As Alexander points out, walkers usually take about two steps per second. Thus, the back of a Kikuyu woman would have to be compliant enough to bring the natural frequency of the load below around 1 Hertz to reduce vertical movements significantly. In that case, the back would have to be springy enough to be compressed by 0.25m (almost 10in) by the load! Such compressions would no doubt buckle the Kikuyus’ backs. So we are left with a baffler: the energy cost of walking and running is associated with the generation of muscular force. As a result, an animal which carries a load of x% of body mass must increase its energy consumption rate by roughly x%, as classic research at Harvard University once demonstrated(4). Rats, dogs, horses, and most humans follow this pattern, but not Kikuyus: you will recall that they can carry 20% more and increase energy breakdown by 0% and lug 70% more with an increased cost of just 50%! Forget about giving these women heavy hands or weighted vests to help them burn more calories – they won’t work! A 120-pound Kikuyu woman can carry at 24-pound weight for free (ie with absolutely no increase in caloric expenditure)!

Footstrike, rather than genetics, proves to be the answer

If you suggest a unique genetic configuration is behind the Kikuyu advantage, you are well off the mark: Maloiy also measured the energy expenditure of Luo women walking with weights, and they exhibited similar parsimony. There are probably over six million Kikuyus within Kenya and almost four million Luo, but Luo and Kikuyu rarely intermarry, and the two groups have dissimilar ancestral bases, so it would be difficult to imagine that both tribes have captured an identical super gene, or even some set of super genes.

Fortunately, Heglund has a different explanation. In research which followed the initial explorations of Kikuyu economy, Heglund and his friend Giovanni Cavagna of the University of Milan asked two women to carry loads Kikuyu style (with straps on their heads), two to carry loads Luo style (with weights directly on their head), and one to use both styles of support. For comparison, six male and six female Europeans carried similar loads in backpacks. All individuals walked across a force platform mounted at ground level in the middle of a walkway with and without the loads, and gravitational potential energy and kinetic energy were measured from the measured forces (5). Heglund and colleagues focused closely on the conversion of kinetic energy to potential energy and back again during the various walks. Interestingly, the ability to convert energy back and forth without losing it increased with increasing load in the African women, explaining why they could carry extra weight without increasing their basic energy expenditure. Under normal circumstances, walking individuals can preserve about 65% of their total energy of movement per step, but the African women could hold onto over 80% of their movement energy as their loads increased to 20% of body weight. How did the women manage to be so efficient? According to the researchers, the answer can be found in what happens during foot strike. As individuals move through the ‘top’ of one stride (remember the pendulum analogy; the top of the walking stride is when the centre of mass reaches its greatest height and potential energy attains its maximum) and start to fall into the next step, most pause for a few milliseconds before beginning to fall. Muscles in the legs are contracting and fighting the fall, trying to preserve balance. This decelerates the body (causing a loss of energy during the potential-kinetic transfer) and, of course, increases energy expenditure. By contrast, the Kenyan women shorten or eliminate the pause at the top of the stride. Thus, there is no added deceleration, no loss of potential energy, and no unnecessary increase in energy expenditure associated with muscular contractions (since the muscles aren’t trying to exert a braking effect at the stride’s top). The Kenyan women are, in effect, more perfect pendulums. In effect, the Kenyan women eliminate braking actions during the foot strike portion of gait and thus improve their efficiency. As runners, we can do the same thing. If we take off the brakes and eliminate the pause during foot strike, we’ll be better able to preserve potential and kinetic energy, and our energy cost – expressed as either calories or millilitres of oxygen per minute – will be lower. Since we’ll be operating at a lower percentage of VO2max, our efforts will feel easier, and so we will be able to step up our training and race paces. But how do we actually take the brakes off ? Two factors must be at work: first, our nervous systems must be highly reactive, so that muscular actions which inhibit forward propulsion can be tightly controlled from the moment of impact, and muscular actions which boost propulsion can be instigated without hesitation. Second, our movements must be well coordinated, so that there is no need to spend extra time (and energy) restoring the body’s equilibrium position. Foot strike must be an explosive time, not a period in which weakly controlled joint movements must be corrected prior to toe-off, or in which the leg muscles ‘throw on the brakes’.

What we want to achieve is speed and control.

Exercises that will improve that all-important footstrike

To dramatically enhance speed, abbreviate the duration of foot strike, and decrease energy wastage during the foot strike portion of walking and running, carry out the following routine several times a week.

1. Springy jogging: jog along with very springy, short steps, landing on the mid-foot area with each contact and springing upwards after impact. As you move along, your ankles should act like coiled springs, compressing slightly with each mid-foot landing and then recoiling quickly – causing you to bound upwards and forwards. Move along for one minute with quick little spring-like strides, alternating right and left feet as you would during regular running. After this minute is completed, jog in your regular manner for about 10secs, and then ‘spring-jog’ for about 20m, alternating three consecutive spring-like contacts with your right foot with three contacts with the left (eg three hops on your right foot, three hops on your left, three more on your right, etc, until you have travelled about 20m). Jog in your usual manner for 10secs again, and then spring-hop along for 20m on your right foot only, before shifting over to 20m on the left foot alone (making certain that you land in the mid-foot area with each ground contact).

2. One-leg hops on the spot: two sets of 40secs on each leg. Stand in a relaxed position, with your full body weight supported on your left foot only. Lift your left heel slightly, so that the force of body weight is passing through the ball of the left foot (your right knee is flexed so that your right knee is off the ground). Then, hop rapidly on your left foot at a cadence of 2.5 to 3 hops per second (25 to 30 foot contacts per 10secs) for the prescribed time period, while maintaining relaxed, upright posture. Your left foot should strike the ground in the area of the mid-foot and spring upwards rapidly, as though it were contacting a very hot burner on a stove. Your hips should remain fairly level as you do this; try to minimise vertical displacement of the upper body.

3. ‘Box-hop’ with ‘sticks’: for 60secs on your right foot, rest for a few seconds, and then shift over to 60secs of box hopping on your left foot. After resting for a moment, repeat with each foot. The box utilised for this exercise should be sturdy and about six inches in height. To perform the exercise, stand about two metres away from the box, and then hop forward quickly towards the box on one foot only. As you near the box, hop up onto the box surface (continuing to hop on only the chosen foot), and then hop quickly off the ‘far’ side of the box. When you land on the other side, hop forward explosively, ie with as little ground-contact time as possible. In this explosive hop, try to avoid significant vertical oscillation of your centre of mass; you are trying for length, not height. When you land from this explosive hop, continue hopping on the same foot for four more hops, and when your foot touches down after the fourth hop, ‘stick’ your position, ie stop movement completely while remaining relaxed and nicely balanced on your single foot. Jog back to the starting point on both feet, and then continue the exercise on the chosen foot until the time limit is up. Following a short rest, do the hopping routine on the other foot.

4. High-knee explosions: complete 10 with your right leg, rest for a few seconds, and repeat with your left leg. To carry these out, stand with erect but relaxed posture with your full body weight supported on your right foot. Begin by jumping very lightly on the spot on your right foot only, but then suddenly – while maintaining fairly erect posture – jump vertically while swinging your right knee up towards your chest (your left arm should swing forward as your right knee comes up). Land back on your right foot in a relaxed and resilient manner, jump lightly for a few moments, and repeat nine more times, before resting briefly and continuing the pattern on your left leg.

 

5. Shane’s in-place accelerations: to carry these out, stand with erect but relaxed posture with

your feet directly below your shoulders. Begin by simply jogging on the spot, but then – when you feel ready – begin to dramatically increase your in-place ‘stride rate’, building up fairly quickly to as rapid a rate of striding as you can sustain (remember that you are not moving forward to any significant degree). Keep your feet close to the ground as you do this; you’re not shooting for high knee lift but rather for dramatically minimised foot-contact times. Maintain erect but relaxed posture. As you accelerate up to ‘top speed’, it sometimes helps to turn your legs slightly outwards at the hips. Perform 3 x 20secs. To minimise the risk of injury, at least at first, please make sure that all of these activities are completed on a ‘forgiving’ surface (soft dirt, grass, cushioned artificial turf, or wooden gym floor). NOVEMBER 2001 ISSUE 156

This group of quicksilver exercises will, of course, enhance the reactivity of your nervous system and thus help to minimise foot strike time. Naturally, strength and coordination of the weight-bearing leg are also needed to ensure that energy will not be wasted correcting non-optimal leg and body movements associated with foot strike. As mentioned, the overall idea is to create quick-to-act legs which channel all available energy towards forward propulsion, without the need to correct anti-propulsive movements. Towards that end, the following exercise strengthens the legs tremendously and improves balance and coordination to a close-to maximal extent.

Partial Squats: one set per leg. Stand with your left foot directly under your left shoulder, keeping your left knee just slightly flexed and maintaining relaxed, fairly erect posture. Hold the barbell (initially with no weights attached) so that it rests on the top-back of your shoulders just behind your neck; you may incline your upper body just slightly forward for balance. Most of your body weight should be directed through the heel to mid-portion of your left foot. Your right leg should be flexed at the knee so that the foot is not touching the ground at all – your right foot is literally suspended in air (although, you may occasionally need to ‘spot-touch’ the floor for balance with your trailing leg). From this position, if you were carrying out a traditional one-leg squat you would ordinarily bend your left leg at the knee and lower your body until your left knee reached an angle of about 90 degrees between the backs of your thigh and lower leg (usually at this point your thigh would be almost parallel with the ground). However, for the partial squat you should just go down about half-way – so that the angle between the back of your thigh and lower leg is just 135 degrees or so. Then return to the starting position, maintaining upright posture with your trunk. That’s one rep! So far so good – but you have lots more work to do! Continue in the manner described above until you have completed 10 reps (10 partial squats). Then – without resting – descend into the 11th partial squat, but instead of rising back up hold the partial-squat position (the 135-degree position) for 10 full seconds. We’ll call your body alignment during this 10-second period the ‘static-hold’ position. After completing 10secs in the static-hold position, immediately – without resting – rattle off 10 more reps, maintain the static hold on the 11th rep for 10secs again, hit 10 more reps, and then hold statically for 10 more seconds on the 11th rep. That’s one set!

To summarise, a set proceeds as follows (with no recovery at all within the set):

(A) 10 partial squats

(B) 10 seconds of holding your leg and body in

the down position

(C) 10 partial squats

(D) 10 seconds of holding

(E) 10 partial squats

(F) 10 seconds of holding

If you complete these six exercises two to three times a week and emphasise intensity of effort in your regular walking or running training, you will soon develop springy, explosive legs which refuse to dawdle during foot strike and put all available energy into moving you forward. Soon you’ll be moving more efficiently, like a Kikuyu woman, even if you are a white male. The good news is that such efficient movements will make you a truly faster runner, whether you are a soccer player, rugby enthusiast, basketball player, cricketer, or marathon runner.

Owen Anderson

 

Thursday July 17, 2003 06:43 PM -0400