Running Coordination and the Goldilocks effect: A Short Story
A short story about running, complexity, and the accidental genius of evolution......
The difference between Complicated and Complex:
A machine may be an intricately engineered marvel of human ingenuity; may be fabricated from the most resilient materials; assembled from precision engineered inter-locking parts, each methodically fulfilling its tightly specified role.
But, machines rely on finely manufactured components. Performance depends on exact tolerances-of-fit and timing precision. If moving parts are imperfectly manufactured —if they do not fit exactly— then the most minor engineering error, leads to friction; which leads to heat accumulation; which exacerbates wear and tear; leading to further friction, quickly spiralling into catastrophic failure. And working life is dramatically foreshortened.
A clock stops because two gear teeth do not mesh perfectly; a space shuttle crashes due to an ill-fitting washer.
Ultimately machine-like functioning is intolerant of error. And, from an evolutionary perspective, machine-like functioning is fragile —excessively exposed to miniscule variability in the ‘manufacture’ and fit of component parts.
'...He's a complicated Man, and nobody understands him...' No, actually, he is COMPLEX
We, on the other hand, come in all shapes and sizes; even changing shape as life progresses. Yet, even though Nature has no mechanism for precisely replicating component parts; despite ill-fitting moving parts; we seem to fit together ok. We can survive the loss of component parts; we can continue our lives even after areas of the brain have been damaged or removed. And although we regularly injure various tissues, we adapt and, in time, recover.
In fact, no dimension of our biology resembles machine-like behaviour. Unlike machines we constantly modify aspects of function —dispersing stress, sharing workloads— in response to changing life demands.
So there is a distinction to be drawn between ‘complicated’ and ‘complex’ systems.
Complicated systems rely on individual components to execute a highly specific task; to fulfil a tightly designated role.
In complex systems, however, components work together in networks to achieve multiple goals, playing differing roles in differing contexts; sharing information through feedback and feedforward linkages; modulating behaviour in response to changing circumstances.
Complicated machine-like systems typically follow one path to achieve a specific end, and as such are highly predictable; but also highly vulnerable. Complex systems achieve their objectives through a process of exploration and on-going adaptation; negotiating obstacles, solving problems through trial- and-error, and flexibly adapting to changing circumstances.
Machines meet force with force and thus are inflexible; not truly robust.
Alternatively a complex system has the in-built capacity to adapt, to be pliable, changing behaviour to circumvent sticking-points and distributing stress throughout networks of dynamically collaborating parts. Consequently, in complex biological systems, when moving components begin to grate, begin to transmit pain or fatigue signals, then multiple levels of compensatory adaptations are initiated to offset impending ‘damage’.
Hence, although our constituent parts are neither as hard, nor as durable, as stainless steel, our advantage is that we can modify function to distribute stress throughout our biological networks. We can modify function so that damaged, or fatiguing tissues, have their workloads temporarily reduced; whilst other tissues shoulder the work burden.
Which brings us to...
Movement variability in healthy human running
The growth in technology over the past two decades has enabled researchers to examine running patterns at a previously unavailable micro-level of detail. Interestingly, when examining the stride characteristics of healthy runners it was observed that each stride, at a steady pace, was always executed in a subtly distinct fashion. In essence, a running pattern that looks —at the superficial eyeball level— like a repeated cycle of replicated units, is actually a sequence of unique movements.
Now, if the runner is travelling at a constant pace, in an un-fatigued state, the differences are subtle. But they are numerous and pervasive. Multiple dimensions of movement —the contributions of various muscles, the populations of fibres recruited within the muscles, the timings of muscular activations, the orientations of the bones and joints, the tensioning of tendons— all consistently, albeit invisibly, vary.
To use a well-worn analogy; in a healthy runner, each stride is like a finger-print. From a distance each looks identical. Under magnification numerous distinctions emerge; making each particular arrangement of variables inevitably unique. Superficially, observable flow appears consistent, whilst beneath the surface underlying currents are ever-varying. Thus, paradoxically, the illusion of technical constancy is underpinned by continuous change.
The variability advantage
The ability to accomplish any physical action flexibly —in a wide variety of ways— is essential for safe and efficient movement. When we run we estimate the ‘requirements’ of the next stride —forces to be produced; impacts to be managed; stabilisation, balance and compensatory postural adjustments to be made, and so on. Although the brain is capable of making fantastically finessed estimations, even the brain cannot accurately predict parameters perfectly matching the dynamically emerging demands of running. And so when we activate muscles we continuously make small errors. We overshoot or undershoot force production; a sudden change in torque at the hip or knee or ankle demands that the other joints must instantly compensate by modulating their behaviour; we slightly over-stride in one step and must accommodate on the next....
... Multiple dimensions of the running action flexibly, and instantaneously, adapt to constantly updated streams of information being emitted, and received, by sensory organs located throughout the nervous system and bodily tissues.
So if you consider the point of initial ground contact, although the foot may impact with a similar force, and at a similar velocity, as the previous footstrike on that side; this similarity is superficial. At the micro-level of observation this similarity has been achieved through a different arrangement of resources —slight differences in the relative positions of knee and ankle, in foot orientation, inclination of the shin; all interacting to ensure that the forces imposed by ground contact are differently dispersed throughout the network of load-bearing biological components. Previously damaged —but still vulnerable— fibres may be de-selected from the workforce through a slight alteration in joint orientation; by a modulation in activation of a co-contracting muscle partner; by the subtlest change in tension resulting from a postural modification; by an unnoticed change in hip rotation.
And so, as much as is possible, within the limits set by movement demands, mechanical stress is dispersed and dissipated throughout the network of biological tissues.
Variability bandwidth and the Goldilocks effect
Accordingly the advantages of movement variability seem clear. In fact, repeatedly executing a movement in an overly stereotypical manner is being increasingly recognised as an indicator of fatigue, damage and/or dysfunction. However, the relationship between movement variability and optimised performance is not that clear-cut.
Consider: experiments using high-speed video technology have revealed that reduced variability typically precedes run-related injury. Furthermore it appears that, in the injured leg, this reduction in movement variability persists beyond the timescales normally associated with standard rehabilitation programmes. And so it seems that reduced variability precedes, and proceeds, injury.
But the story does not end there.
Further analysis has revealed that when movement variability is reduced in one leg, the other leg compensates for this reduction by expanding movement variability on the non-injured side. (Possibly providing a mechanism explaining the commonly observed phenomenon whereby an injury to one leg is frequently followed, once the athlete is back running, by an injury to the previously healthy leg.)
In essence; reduction in variability in the injured leg necessitates an unhabituated expansion of variability in the healthy leg which in turn imposes loading stressors to which the structures in that leg are unaccustomed.
What does seem apparent is that there is an optimal bandwidth of variability that underpins healthy energy-efficient running. This healthy bandwidth of variability facilitates constant modulation of biological function to stem emerging micro-sensations of discomfort.
Somewhere between the boundaries of the excess rigidity of overly-stereotyped movement, and the erratic excesses of uncontrolled variability, there lies a Sweet Spot; an optimal range.
Just like Baby-bears porridge there is a balance to be negotiated; not too much, and not too little.