A zero gravity treadmill is a specialized treadmill that uses air pressure to lift your body weight while you walk or run, reducing the impact on your legs by up to 80%. Originally developed from NASA research designed to help astronauts exercise in space, the technology has become a staple in physical therapy clinics and sports medicine facilities. The most well-known brand is the AlterG, though the general concept is often called an “anti-gravity” or “body weight support” treadmill.
How the Air Pressure System Works
The treadmill looks like a standard running deck enclosed inside a large, waist-high inflatable chamber. To use it, you step into a pair of specially designed shorts that zip into the opening of the chamber, creating an airtight seal around your lower body. Once sealed in, the system runs a quick calibration to register your size and weight.
From there, the machine pumps air into the enclosed chamber. That pressurized air pushes upward against your body, effectively lifting you and reducing how much weight your legs have to support. You control the level through a touchscreen: pressing a button increases the air pressure, which lifts more of your weight and further reduces strain on your joints. The system can adjust in 1% increments, meaning a 160-pound person could dial their effective weight down to as little as 32 pounds. That precision matters in rehab settings, where a therapist might increase loading by just a pound or two each week as a bone or tendon heals.
From NASA Lab to Rehab Clinic
The technology traces back to a NASA scientist named Robert Whalen, who worked at the Ames Research Center studying how to prevent bone loss and muscle deterioration in astronauts. His original idea was the reverse of what the treadmill does on Earth: he wanted to use air pressure to add weight to astronauts exercising in low gravity, simulating the natural load their bodies would experience on the ground. Whalen patented the differential air pressure concept in 1992.
In 2005, he licensed the patent to a private company in Menlo Park, California, which adapted the technology for the opposite purpose: reducing weight for patients who needed to relearn how to stand, walk, and run. The resulting device, called the G-Trainer, received FDA clearance for medical use in January 2008. Since then, hospitals and universities have studied its effectiveness for conditions ranging from lower-limb arthritis to Parkinson’s disease.
Orthopedic and Post-Surgical Recovery
The most common use for these treadmills is helping people recover from injuries or surgeries involving the legs, ankles, hips, or knees. After an ankle fracture or tibial plateau fracture, for example, standard recovery often requires weeks of partial or non-weight-bearing activity to protect healing bone, surgical hardware, and reconstructed soft tissue. That usually means crutches and very limited movement.
A zero gravity treadmill changes that equation. In one clinical trial, patients recovering from ankle or tibial fractures were randomized to either standard physiotherapy with crutches or anti-gravity treadmill sessions loaded at roughly 44 pounds, two to three times per week for six weeks. The treadmill group could walk with a natural gait pattern instead of hobbling on crutches, which helps maintain muscle strength and joint mobility during the healing window.
Beyond fractures, the treadmill is widely used for ACL tears, Achilles tendon injuries, microfractures, stress reactions in the shin or pelvis, hip and knee replacements, and even lumbar disc herniations. For runners specifically, it allows continued aerobic training during recovery from overuse injuries, since you can maintain your running form and cardiovascular fitness at a fraction of your normal body weight.
Neurological Rehabilitation
The benefits extend well beyond orthopedic injuries. For people recovering from a stroke, loss of balance and asymmetric weight distribution often lead to difficulty walking and a high risk of falls. A study examining anti-gravity treadmill gait training in stroke patients found that 20-minute sessions, five times per week for four weeks, produced significantly greater improvements in dynamic balance, gait speed, and fall risk compared to conventional gait therapy alone. Gait scores and timed walking tests improved in the treadmill group but not in the control group receiving standard treatment.
The technology has also shown promise for Parkinson’s disease, where it can improve gait freezing (those moments when your feet feel glued to the floor) and overall mobility. Patients with cerebral palsy and other neurological conditions that affect walking have used it as well. The key advantage is that the air pressure provides enough support for someone with significant weakness or balance problems to practice walking safely, building the neural pathways and muscle memory needed for independent movement.
How Athletes Use It
Professional and recreational athletes use zero gravity treadmills for two main purposes. The first is maintaining fitness during injury recovery. A runner with a stress fracture, for instance, can continue training at 50% or 60% of body weight, keeping their cardiovascular system and leg muscles conditioned without loading the injured bone. This can shorten the overall return-to-sport timeline considerably.
The second use is overspeed training. By reducing body weight, the treadmill lets runners practice faster leg turnover at speeds they couldn’t sustain at full weight. This trains neuromuscular coordination at higher speeds with less injury risk. Some athletes also use it for high-volume training weeks, substituting one or two runs on the anti-gravity treadmill to reduce cumulative impact stress on joints while still logging miles.
How It Compares to Pool Therapy
Water-based treadmill therapy (hydrotherapy) also reduces body weight through buoyancy, but the two approaches differ in important ways. On an anti-gravity treadmill, you walk or run on a solid surface with normal ground reaction forces, just at a lower body weight. Your gait mechanics stay close to what they’d be on a regular treadmill or sidewalk. In a pool, water resistance changes how your legs move, and the surface underfoot feels different. The anti-gravity treadmill also allows precise, repeatable weight adjustments in 1% increments, which is harder to control in water where depth and movement speed both affect buoyancy. For patients with open wounds, surgical incisions, or conditions where water exposure is a concern, the air-based system avoids those issues entirely.
Where to Find One and What It Costs
Zero gravity treadmills are primarily found in physical therapy clinics, sports medicine centers, and university athletic departments. A clinical-grade AlterG unit costs roughly $35,000 to $75,000 depending on the model, which is why home ownership is rare. Most people access the technology through a physical therapy prescription, where sessions are billed as part of rehabilitation. Some facilities also offer pay-per-session access for athletes, typically ranging from $15 to $50 per session.
There are some safety considerations worth knowing. People with cardiovascular disease, respiratory conditions that limit exercise tolerance, unstable fractures, or blood pressure issues related to cardiovascular hypotension should get medical clearance before using one. The minimum age for use is generally 13 years old.