Lee Saxby's Wisdom of Coaching Blog

The skill of creating pendulums and springs

The beginning of wisdom is the definition of terms’


Locomotion can be defined as a biomechanical behaviour that results in the displacement of an organisms centre of mass from one point of reference to another. This biomechanical behavior can be further defined in terms of kinetics (forces producing the movement) and kinematics (description of the movement in relation to space and time). Therefore, a logical definition of locomotive skill would be the ability of an organism to select an appropriate kinetic and kinematic strategy to get from point A to point B within a desired time frame with minimum metabolic cost and minimum risk of injury.

There are two, fundamentally different locomotive strategies available to humans which are based on the mechanical and energetic properties of ‘pendulums’ and ‘springs’ and human locomotive skill can be defined as the ability to select a suitable pendulum or a spring strategy based on the speed of locomotion and the substrate under-foot.

An inability to select a suitable pendulum or spring strategy due to a compromised sensorimotor system (as a consequence of reduced sensory input because of shoes or reduced motor output due to lack of strength) inevitably leads to inefficiency and injury.


When ‘GOOD’ science is ‘BAD’ coaching

“the progress of science is not based on description of phenomena but by uncovering the laws governing them”
Ivan Pavlov

The ability to differentiate between ’cause’ and ‘effect’ is probably more important for coaches (and their clients!) than ‘scientists’.



[1] Kugler. F,Janshen.L. Body position determines propulsive forces in accelerated running. JBiomech (2010)

[2] Weyand et al. The biological limits to running speed are imposed from the ground up. J Appl Physiol (2010)

[3] Hayes P, Caplan N. Foot strike patterns and ground contact times durig high-calibre middle-distance races. Journal of Sport Sciences (2012)

[4] Kong P.W, de Heer. H. Anthropometric, gait and strength characteristics of Kenyan distance runners. JSSM (2008)

[5] Robbins S.E, Hanna A.M. Running-related injury prevention through barefoot adaptations. Med. Sci. Sports Exerc (1987)

[6] Lieberman DE. What can we learn about running from barefoot running: an evolutionary medical perspective. Exerc Sport Sci Rev (2012)

The Extensor Paradox – but not really Part 3 of 3

Motion is created by the destruction of balance, that is, of equality of weight, for nothing can move by itself which does not leave its state of balance, and that thing moves most rapidly which is furthest from its balance” (Leonardo Da Vinci, 1452).

Part 2 of this series closed with a statement that gravity is the motive force in terrestrial locomotion. Parts 1 and 2 provided evidence disproving the existence of an active push in accelerating the center of mass forwards. In this final part of the series, gravity, or more specifically, torque created by gravity, will be shown to provide forward acceleration of the center of mass according to the undisputed-fundamental laws of physics.

Destruction of balance creates acceleration by gravitational torque.

btr-drestuction-of-balanceThe object in A is in balance / stationary, with its center of mass (the cross) vertically aligned over its stationary point of support. In B, the center of mass of the object is out of alignment with the stationary point of support. Gravity acts on the center of mass, creating a turning force (torque) about the point of support as an axis. Left alone, the center of mass will continue to accelerate at 9.81 m/s every second (i.e. constantly increasing speed), until the object hits the ground. The further the center of mass of the object from its point of support, the greater the gravitational torque and the faster the center of mass will fall. This is GRAVITATIONAL TORQUE.

Picturing gravitational torque in locomotion.


The figure left is adapted from the work of Morton (1935). He labeled the tilted (away from the gravitational vector) line drawn from the point of support through the center of mass the ‘angle of instability’. From left to right, the figure shows a state of balance, moderate instability in walking and maximum instability in running. The forward acceleration in walking and running is provided by gravitational torque, acting on the center of mass around the stationary-supporting foot.

While the figure is useful to conceptualize how forward movement can result purely from gravity, without an active push, it is a gross oversimplification and has some critical problems that do not agree with observation. Nevertheless, this conception of how gravity can be a motive force has been used as the cornerstone of some schools of running, in particular, Pose technique (Romanov and Fletcher, 2007). While the general idea of gravitational torque as the motive force is correct, the mechanisms by which it acts have been incorrectly described.

The problem of constant acceleration.

As stated earlier, gravity accelerates mass continuously, so the idea of holding a fixed angle of lean at a constant velocity and simply increasing or decreasing the angle to accelerate or decelerate is flawed. Suppose a runner adopts an angle of instability of 15 degrees. As soon as this happens, his center of mass is accelerating under gravitational torque at an ever-increasing rate. Unless he is able to accelerate the recovery of his feet to match the rate at which his center of mass is falling away from his feet, the angle of instability will constantly increase and an imminent face plant is inevitable!

So near, yet so far, but the truth is out there.

The simple model of gravitational torque outlined above cannot account for constant-velocity running. By now, it should be clear that a theory at odds with evidence is, by definition, wrong. The ‘concept’ of gravitational torque must be true based on fundamental laws of physics. It is simply the case that the mechanism by which it operates in locomotion has been oversimplified. The true mechanism by which gravity acts as the motive force in locomotion is well accepted, but apparently not well known. It is the ‘Virtual-Pivot-Point model’. It explains all observations, has been used by engineers to create human-like running robots, and also explains why inherently-unstable bipeds are not as unstable as predicted.

For an explanation of the TRUE application of gravity as the motive force in locomotion, watch out for the ‘Science of Falling’ post coming along soon.


Maus, H.M., Lipfert, S.W., Gross, M., Rimmel, J and Seyfarth, A. (2010). Upright human gait did not provide a major mechanical challenge for our ancestors. Nature Communications, 1(70), 1-6.

Morton, D.J. (1935). The Human Foot: its evolution, physiology and functional disorders. New York: Columbia University Press.

Romanov, N and Fletcher, G. (2007). Runners do not push off the ground but fall forwards via a gravitational torque. Sports Biomechanics, 6(3), 434-452.

Development of running skill

wickstrom-1Fundamental Motor Patterns by Ralph L. Wickstrom (1983) has been a major source of wisdom for the BTR coaching system. The summary of the developmental changes in running patterns and performance is pure coaching gold:

1. An increase in the length of the running stride

2. A decrease in the relative amount of vertical movement in each stride

3. An increase in hip, knee and ankle extension at takeoff

4. An increase in the proportion of time in the nonsupport phase of the stride

5. An increase in the closeness of the heel to the buttock on the forward swing.

6. An increase in the height of the forward knee at takeoff

7. A decrease in the relative distance that the support foot is ahead of the center of gravity of the body at contact




The Extensor Paradox – but not really Part 2 of 3

“If there is an exception to a rule, and if it can be proved by observation, that rule is wrong” (Richard Feynman, 1963) 

The ‘rule’ addressed in this series of posts is that ‘runners push’ to propel themselves forwards. In part 1, experimental data showing the leg extensor muscles to be silent / switched off at the point of the stride cycle where they ought to be providing the ‘push’ was discussed. These data are the ‘exception’ to the pushing ‘rule’ and have been consistently repeated. So the rule is WRONG, there is no paradox. To quote from one of the key studies, “These experiments have, however, shown that the notion of an extensor thrust – with plantar flexors, knee extensors, and hip extensors all being active in late support to generate forward and upward thrust – is in need of modification” (McClay et al., 1990). And yet, some 25 years later, it is still the common belief that running involves actively pushing into the ground!

In addition to the absence of extensor-muscle activity some 30ms after ground contact, measurement of ground-reaction force data provides a further nail in the coffin of the ‘push’ theory. Newton’s 3rd law states that each action has an equal and opposite reaction. Thus, there is a force of equal magnitude but opposite direction produced by the ground in reaction to the runner’s impact with it. The size and direction of the ground reaction force can be measured and visualized using a vectogram. To understand a vectogram, it must be understood that a force has both magnitude (size) and direction (forwards and backwards, vertical, and lateral). The latter is generally ignored in analysis of human movement, as the largest forces occur in the vertical and horizontal planes. A vector of vertical and horizontal forces can be calculated and plotted. A vectogram of a single ground contact in a skilled runner is shown below along with the phases of the stride cycle. The magnitude of force is expressed in multiples of bodyweight with the vertical axis representing one bodyweight.


The figure shows many notable features:

  1. On impact, the ground reaction force is angled back at the runner and produces a braking effect that must be controlled using eccentric activity of the leg muscles, which also stores elastic energy in tendons.
  2. The magnitude of force rises as the runner’s centre of mass falls over the supporting foot, reaching a peak when the two are vertically aligned, the centre of mass is at its lowest point, and the runner is squashed between gravity and the ground-reaction force. This is also the point of peak muscle activity as the compression is resisted.
  3. After midstance, the extensors switch off, and force (from passive elastic recoil) rapidly falls
  4. When the direction of the ground reaction force is forwards, its magnitude is only a fraction of bodyweight and therefore insufficient to ‘propel’ the body weight. Leg muscle activity at this point is to recover the trail leg, to catch the runner on the next stride.

There is no push.

The ‘push’ paradigm states that runners actively create a downwards and backwards force with the leg extensors to drive them forwards and up into the next stride. For this ‘rule’ to be true, we should observe highly active extensors after midstance, and a ground-reaction force vector in excess of bodyweight when angled in the direction of travel. Instead, experimental data show the extensors to be silent and forces too small to ‘propel’ the runner in the direction of travel. There can be only one conclusion from these observations, THERE IS NO PUSH, just as there is no extensor paradox!

So what causes acceleration in running?

As unstable-terrestrial bipeds, bound by the biological imperative, it makes sense that we use the strongest force on the planet to cut the energetic cost of locomotion. In running as in walking, GRAVITY provides the motive force for movement. The final post in this series will explain how skilled runners use gravity to move forwards at great speed, and thus will shed light on how runners should condition themselves to run faster instead of wasting time on triple-extension drills.

This way to Part 3


Feynman, R. P. (1963). The Meaning of it All. Strand, London: Penguin Books.

Mann, R.A., Moran, G.T. and Dougherty, S.E. (1986). Comparative electromyography of the lower extremity in jogging, running and sprinting. The American Journal of Sports Medicine. 14(6), 501-510.

McClay, I.S., Lake, M.J. and Cavanagh, P.R. (1990). Muscle activity in running. In P.R. Cavanagh (Ed.), Biomechanics of Distance Running (pp 165-186). Champaign, Illinois: Human Kinetics.

The Extensor Paradox – but not really Part 1 of 3

“How wonderful that we have met this paradox. Now we have some hope of making progress” (Niels Bohr, 1957)

A fundamental and ubiquitous belief among runners, biomechanists, strength and conditioning experts and running coaches is that a runner must push into the ground to propel themselves forwards. The faster a runner wishes to go, the more force they must push with. This belief is FALSE. It originates from:

  1. A failure to view observations in the context of evolutionary biology
  2. A failure to adopt a telelogical view of the function of muscle in locomotion
  3. Confusing cause and effect

This ‘push’ mindset is the basis of the obsession with so-called ‘triple-extension drills’ and coaching cues of driving the rear leg and slamming the foot into the ground.

This series of three posts will explain how the ‘paradox’ came to be and how, by viewing the existing evidence through appropriate filters, the ‘paradox’ ceases to be a paradox. A paradox in science really means that the current theory disagrees with experiments, which really means the theory is FALSE. If an alternative theory agrees with the same experiments, it is the one that should be believed while the old one has been falsified and should be binned (see earlier confirmation bias post). Claiming a ‘paradox’ is simply a reluctance to accept that the current theory has been falsified and to think about / consider alternative theories. As Niels Bohr said, a paradox is an opportunity to progress understanding. Sadly, as Thomas Kuhn recognized, scientists are human and are reluctant to give up their pet theory.

So what is the extensor paradox?

Studies recording electrical activity of muscles during running have consistently shown the extensor muscles of the stance leg (glute, quadriceps and calf) to be silent / switched off immediately after mid-stance. BUT, this is the very time when they ought to be highly active and forcefully extending the leg to push the runner forwards and up into the next flight phase, IF the theory that runners push back and down to go forwards and up is correct.


The diagram illustrates activity in the leg muscles with the intensity of colour reflecting the extent of activation of the muscles. The third image from the left is mid-stance. Image four, when the runner should be ‘pushing’ clearly shows the quads and glutes to be completely inactive while calf activity is also decreasing.

At first glance, and with a ‘runners-push’ mindset, this appears to be impossible. Where is the propulsion coming from? After all, Newton’s third law states that for every action there is an equal and opposite reaction, so to go forward and upwards, there has to be push backwards and downwards, right? And it is the job of muscles to create the push, right? WRONG!

The next post on this topic will show why this belief is wrong and will use evidence that is cited to support the ‘runners-push’ mindset to show how this simply cannot be the case.

This way to Part 2


Kuhn, T.S. (1962). The Structure of Scientific Revolutions. Chicago: Chicago University Press.

Mann, R.A., Moran, G.T. and Dougherty, S.E. (1986). Comparative electromyography of the lower extremity in jogging, running and sprinting. The American Journal of Sports Medicine. 14(6), 501-510.

McClay, I.S., Lake, M.J. and Cavanagh, P.R. (1990). Muscle activity in running. In P.R. Cavanagh (Ed.), Biomechanics of Distance Running (pp 165-186). Champaign, Illinois: Human Kinetics.

Weyand, P.G., Sternlight, D,B., Bellizzi, M.J. and Wright, S. (2000). Faster top running speeds are achieved with greater ground forces not more rapid leg movements. Journal of Applied Physiology, 89, 1991-1999.

BTR Training Zones

Bulletproof running entails good technique and training at the right intensity. The BTR method emphasises training at opposite ends of the intensity spectrum. Specifically, this means very short duration high-intensity exercise emphasising running-specific reactive ability, and long-duration low-intensity work to encourage mitochondrial-metabolic conditioning. Most runners experience the symptoms of ‘overtraining’ from prolonged activity in the metabolic-destruction zone. Exercise in this zone rapidly depletes energy stores, exhausts adaptive mechanisms and creates a systemic inflammatory response. Excursions into this metabolic minefield should be minimised and reserved only for competition and the final preparatory phase before the competitive season begins.

If you want to know more about the science and art of training and how to become a bulletproof runner, book your place HERE:


The BTR Running Performance Matrix


The foundation of running performance and injury prevention is running technique. Once running technique is mastered it is simply a matter of strengthening this foundation to withstand the increased biomechanical and physiological loads of running faster and/or longer. The BTR method achieves this by combining running-specific strengthening exercises and metabolic conditioning with an individual-specific training and recuperation strategy.

BTR workshops have been designed as a practical, applied coaching experience for runners interested in improving their running technique and learning the necessary skills and conditioning required to acquire and maintain this technique.

Click HERE to find a BTR workshop near you

Additional locations will be available in 2016.

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