Field notes on things that run themselves
It Is Never Quite Stiff Enough
Stand up, put your feet together, close your eyes. You have not locked into position. You are swaying — small, irregular, continuous — and you will go on swaying for as long as you stand there. Stillness is not a state the body reaches. It is a loop the body runs.
The trouble is the shape. Most of a person’s mass sits high, over a base no longer front to back than a foot. Hinged at the ankles, a standing body behaves like an inverted pendulum: something heavy pivoting near the ground with its weight above the pivot. Lean a degree and gravity’s torque tips you further; lean two and it is larger still. Every departure from upright feeds itself.
The obvious fix is stiffness. Calf muscle, Achilles tendon and connective tissue span the ankle, resisting rotation the way a spring does. Stiff enough to out-torque gravity degree for degree, and standing would need no attention at all. In 2002 Ian Loram and Martin Lakie measured the ankle’s intrinsic stiffness during quiet standing, using perturbations brief enough to exclude any neural response. It came to about 91 percent of what gravity demands. Later work puts it near 94, across studies ranging from a third of the requirement to a little over all of it.
Ninety-one percent is a telling number. This is not a body that failed to solve the problem; it is one that gets within a few percent and stops. Below 100 the arithmetic is unforgiving — however close, the lean still grows. Those last few percent cannot come from the tissue. Something has to supply them, continuously.
This is contested ground. An influential model from David Winter and colleagues held that ankle stiffness alone could do the job, letting muscle mechanics answer a disturbance instantly without the nervous system; Pietro Morasso and Marco Schieppati argued it was nowhere near sufficient. Direct measurement has since favored the second camp, but the argument runs on, not least over how much the size of the test perturbation changes the answer. That the stiffness falls short is not disputed; how far short, and what fills the gap, still is.
What fills it is stranger than a spring. Loram, Constantinos Maganaris and Lakie put ultrasound on the calf during quiet standing, resolving length changes down to ten micrometres, and caught the fibres doing the reverse of what a spring does. As the body sways forward and the whole calf muscle-tendon unit lengthens, the soleus and gastrocnemius fibres shorten. The Achilles tendon is the reason: compliant rather than rigid, it stretches while the muscle pulling on it contracts. Muscle and body move in opposite directions.
Nor does it happen smoothly. Instead of one continuous correction the calf delivers small, discrete, ballistic adjustments — roughly 2.6 every second, about 2.8 for each one-way sway of the body’s center of mass. Loram and Lakie describe this as a series of small throws and catches: you do not hold the load up, you toss it and intercept it.
The whole business is smaller than it sounds. The median ankle rotation in undisturbed standing is about an eighth of a degree. Sway figures need care, though, because two quantities get quoted interchangeably: the center of mass, which is where the body is, and the center of pressure, which is where the ground pushes back. The second travels further and faster, precisely because it is what chases the first.
Four streams feed the loop — vestibular organs, vision, proprioception from muscle and joint, and pressure under the soles. Close your eyes and sway roughly doubles, reliably enough to have been a bedside neurological test for two centuries. But the tempting conclusion, that vision runs the show, does not survive the data: the effect depends on what you were looking at, and against a distant enough target it nearly vanishes. Across most conditions proprioception better predicts how much a person sways.
It is natural to read sway as error — leftover wobble from a loop that cannot quite converge. That may be wrong. A serious body of work treats it as partly exploratory: movement is what makes the senses informative, and a perfectly motionless body would be one that had stopped asking where it was. The question is open. What is clear is that the sway does not stop when the disturbances do. Take away the wind, the noise, the moving scenery, and it continues.
Damp sand holds its shape without storing anything, but there the sea does the work from outside. Standing inverts that. The correction is internal, against a load that never lets up, on a sub-second clock, in a body never told it has succeeded. Upright is not a position you settle into but a place you keep passing through, in both directions. Stand still for a minute and nothing about you is at rest. You are running, the whole time, a loop whose only visible output is the appearance of having stopped.
One loop I’m watching
Next: a blast furnace. Some run for a decade without stopping, and cannot simply be paused — let the burden stop descending and the gas stop rising and the column inside sets. Ore and coke go down, hot gas comes up, and between them a stack of chemical zones holds fixed positions in space while every atom composing them is on its way through.
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