THE MECHANICS OF STANDING
The body stands like an inverted pendulum.
When it looks still, the body is working. The ankle keeps shifting the point of pressure under the feet so the centre of mass stays above the base of support.
for a 0.5° sway to grow to 5°, a tenfold increase in the no-correction model at 1.75 m.
The static figure shows an aligned body. The formula and result remain below without animation.
A fall held in place
Gravity pulls. The ankle answers.
Picture the body as a long ruler pinned at the ankle. The centre of mass sits above the joint. As long as its weight line falls between the feet, the load stays inside the base of support and the body remains upright.
A small sway moves that weight line. The calf muscles change the ankle torque, and the centre of pressure shifts across the foot. That short movement brings the body back towards its chosen position.
The model is used in biomechanics of quiet standing. Winter and colleagues describe the body as an inverted pendulum controlled by muscular stiffness, with the centre of pressure closely linked to the centre of mass.
Change the scale
How much time does your pendulum have?
Height changes the distance between the ankle and the centre of mass. Choose a height and a starting sway, then stop the correction in the simulation. The result is a physics approximation, not a prediction about your body.
For a tenfold increase
0.72 seconds
t = √(h / g) × ln(θfinal / θinitial)
h = 0.96 m, estimated as 0.55 × chosen height. 5.0° / 0.5° = 10.
When the room runs out
The ankle gets the first chance. The hip and the step follow.
In a small sway, the body can stay close to a single segment. As the sway grows, the strategy changes: the hip moves the trunk, the knee and hip share the movement, and a step moves the base of support under the centre of mass.
Balance is therefore not a perfectly vertical line. It is a choice of correction suited to the size and speed of the sway. The nervous system combines vision, vestibular information and proprioception to estimate body direction, as Peterka describes.
An experiment on your own body
How long can you keep one foot in the air?
Stand beside a wall or a stable chair. Lift one foot and start the timer. Stop when you put the foot down or touch the support. Repeat on the other side if you want a second observation.
Safety: clear the area, stay close to support and stop if you feel dizzy. This is a personal reading for this attempt, not a medical assessment.
Instrument target: 20 seconds, a round interval for observation.
Outside the frame
A pendulum explains the idea. The real body adds many layers.
The equation uses one rigid segment, a small angle and an estimated centre-of-mass height. The real body has deformable feet, several joints, delayed muscles, senses that sometimes disagree and a base of support that can move.
The 0.72-second result says how quickly sway grows in a no-correction model at the default height. It does not say when a person would fall or assess fall risk.
Where the model comes from
- Winter, Patla, Prince, Ishac and Gielo-Perczak, 1998. Stiffness control of balance in quiet standing, Journal of Neurophysiology. The inverted-pendulum model, centre of pressure and sagittal-plane control.
- Peterka, 2002. Sensorimotor integration in human postural control, Journal of Neurophysiology. The integration of visual, vestibular and proprioceptive information.
Calculation: for a small sway, the model uses θ¨ = (g / h) × θ, where g = 9.81 m/s² and h = 0.55 × height. The solution grows as et√(g/h). The time for a tenfold increase is √(h / g) × ln(10). Values are rounded to two decimal places.