prosthetic foot (SACH and energy-storing)
/ SACH = 'sak' /
A real foot does an astonishing amount in each step: it lands, rolls, adapts to the ground, and then springs you forward off the toes. A prosthetic foot is the component at the bottom of an artificial leg that tries to reproduce as much of this as possible without any muscles or nerves. How well it does so shapes how smooth, tiring, and capable the person's walking will be.
The classic, simplest design is the SACH foot — Solid Ankle Cushion Heel. It has no moving ankle joint; instead a soft, compressible wedge of rubber in the heel cushions the impact when the heel lands, and a stiffer foam body lets the person roll forward over it. It is cheap, durable, light, and reliable, and for a slow, careful walker it works perfectly well. What it cannot do is give anything back: the energy that goes into compressing it is simply absorbed, not returned. The more advanced answer is the energy-storing foot, usually a curved carbon-fibre blade that flexes and loads up like a diving board during the step, then springs back at toe-off to give a gentle push forward, returning some of the energy and easing the work of walking.
Matching the foot to the person is the art here. An energy-storing carbon foot helps an active, faster walker or a runner, returning spring and reducing effort; for a frail, slow, household-distance walker it offers little and adds cost. Some feet add a movable ankle or hydraulics that adapt to slopes and stairs. Crucially, a prosthetic foot has no sensation and cannot truly feel the ground or balance itself, so the person still relies on vision, hips, and the sound leg — the foot springs, but it does not sense.
A young, active below-knee amputee is fitted with a carbon-fibre energy-storing foot: as she pushes off, the blade that bent under her weight springs back and nudges her forward, so a brisk walk feels noticeably less tiring than it did on her first, simple SACH foot.
An energy-storing foot returns some spring at push-off; a SACH foot only cushions and absorbs.
Fancier is not always better: an expensive energy-storing foot offers little to a slow, household-distance walker, while a simple SACH foot serves that person reliably — the foot should match the walker, not the catalogue.