Standing Still

reflex delay push back (× gravity) resist speed body noise

the body (lean drawn 6× larger than life)

which settings keep you up at this delay (tap to choose)

Centre-of-mass sway, last 10 seconds, in millimetres. A quiet stance sways a few millimetres. The body: 70 kg, centre of mass 0.9 m above the ankles; left alone, a lean doubles every 0.22 s.

The model: second-order Langevin dynamics, upside down and late

	angle	How far the body leans at the ankles. The lean of the centre of mass is h·θ (the trace, in mm).
	9.51 s⁻²	Gravity's pull towards falling: m = 70 kg, h = 0.9 m, I = 65 kg m² about the ankle. Left alone, a lean doubles every ln 2 / √a = 0.22 s. Fixed.
	reflex delay	How late the brain sees the lean: the time from a sensor to a muscle. Human loops run at roughly 0.1–0.2 s.
	push back	How hard the muscles push against the lean they see, per unit of lean. It must beat gravity (P > a); the slider shows P / a.
	resist speed	How hard they push against the speed of the lean: the friction of the Langevin equation, but delayed like everything the brain does. The slider shows D / √a.
	body noise	Random kicks: breathing, heartbeat, muscle tremor. ξ(t) is white noise; σ is set so the default stance sways about 4 mm, as a real one does. The slider scales σ.

In the textbook Langevin equation a particle feels friction, a force from a potential and random kicks. Here the potential is upside down (standing is the top of a hill, not the bottom of a valley), and the restoring force and the friction arrive τ seconds late. No choice of P and D can hold the body once τ ≥ √(2/a) = 0.46 s (Stépán): that is why the map empties as the delay grows.