Anti-Gravity Treadmill

Overview

An anti-gravity treadmill unloads a walker or runner by air pressure rather than harnesses. The user wears neoprene Pressure Shorts, steps onto a treadmill enclosed by a flexible Pressure Bag, and zips themselves into a waist-height Cockpit Ring. A blower then raises the chamber a few kilopascals above ambient. Because pressure acts on the cross-sectional area of the body at the waist seal, the net upward force is simply pressure × area, about 1 kPa over a typical 0.05 m² waist section yields ~50 N of lift, and the control system trims pressure until the user feels anywhere from 100% down to 20% of body weight, adjustable in 1% steps.

The principle, differential air pressure (DAP) unweighting, came out of NASA research, originally inverted, using negative pressure to add load for astronauts, and was commercialized for rehabilitation by AlterG in 2005. Unlike harness systems, DAP lifts through the whole lower body uniformly, so gait kinematics stay close to normal even at deep unweighting, which is why the machines are standard in sports-medicine clinics for post-surgical running progressions and stress-fracture returns.

Pressure chamber

The chamber is the urethane-coated Pressure Bag clamped to the treadmill perimeter below and the rigid cockpit ring above. Clear Viewing Window panels let a clinician watch the legs; gait observation is half the clinical value. The user interface to the chamber is the Shorts Seal Interface: the shorts carry a fabric Sealing Skirt whose Airtight Zipper half mates with the ring, while a compliant Waist Gasket limits leakage at the torso. The seal does not need to be hermetic, the blower runs continuously and the system tolerates steady leakage, it only needs leakage to be stable enough for the pressure loop to hold setpoint. A mechanical Pressure Relief Valve caps chamber pressure independently of software.

Air supply and control loop

A centrifugal Blower Motor in the Blower Housing feeds the chamber through a Supply Duct, drawing through an Intake Air Filter and Intake Muffler. Coarse pressure comes from blower speed; fast trim comes from the servo Pressure Control Valve bleeding excess air, which responds in tens of milliseconds as the runner's bounce changes chamber volume. Feedback comes from Pressure Sensor taps compensated by the Chamber Temperature Sensor.

Calibration makes the percentage display honest. At session start the user stands still while Deck Load Cell gauges under the deck weigh them at ambient pressure; the Sensor Interface Board then maps chamber pressure to measured deck force across a brief pressure sweep. From then on, commanding "60% body weight" means the controller holds the pressure that the calibration showed produces 60% of the measured weight on the deck.

Treadmill base and frame

Inside the bag is a largely conventional deck: the Treadmill Drive Motor turns the crowned Front Drive Roller through a Drive Belt, dragging the Running Belt over a waxed phenolic Deck Board. Speeds reach 20 km/h with incline to 15%, because unweighted athletes run fast. The Support Frame carries the cockpit on telescoping Cockpit Lift Column posts, the ring must sit at the iliac crest, so column height is set per user before zipping in, with Handrail grips for entry and balance.

Console and safety

The operator sets body-weight percentage, speed, and incline on a touchscreen (LCD Panel plus Touch Digitizer driven by a Compute SoC Module); the Console Control Board sequences blower, valve, and belt drive and enforces interlocks, the belt will not start until the chamber is sealed and calibrated. The Emergency Stop stops the belt and dumps pressure in one action. Because the user is zipped to the machine, fall consequences differ from a normal treadmill: the inflated chamber itself catches a stumbling user, which is part of why DAP treadmills are cleared for very early post-operative gait work.

Service notes

The consumable list is short: shorts and zippers (the highest-wear items, replaced like gloves), bag inspection for pinholes, intake filters, belt wax, and annual load-cell recalibration. The blower and treadmill motor are standard rotating-machinery service. A dedicated 220–240 V circuit is required; clinics typically budget the machine at 160–280 kg installed over a 2 × 0.9 m footprint.