Anti-Gravity Therapy: How Unweighted Running Works

Anti-gravity therapy is a plain-English name for running or walking with part of your body weight lifted off you. In practice it means an AlterG anti-gravity treadmill: a normal treadmill belt sitting inside a sealed air chamber that closes around your hips. Air pressure inside the chamber takes a set share of your weight, so your legs move through a full running pattern while the forces going through bone, tendon and joint are smaller than they would be outdoors. The technology came out of NASA research into differential air pressure.

The mechanics of it

You put on a pair of neoprene shorts with a zip skirt. Standing on the belt, the shorts zip into the chamber at hip height and the machine calibrates to your standing weight. From there the operator selects a percentage of body weight, and the chamber pressurises until the scale reads that number.

The AlterG unweights to 20% of body weight in 1% increments. For an 80 kg runner, that is a working range from about 16 kg of load up to the full 80 kg. Speed and incline work like a standard treadmill, so intervals, hill work and steady running are all possible once the tissue tolerates them.

What drops is the peak force each footstrike sends up the chain: through the calcaneus, the metatarsals, the Achilles, the tibia, the knee and the hip. What stays is the movement itself. You are still coordinating a running stride, still loading tendons cyclically, still driving your heart rate up. That gap between reduced impact and retained movement is the whole point.

What changes as the percentage drops

At 85 to 90% body weight, most runners barely notice the difference beyond a slight lightness. Around 70 to 75%, stride tends to lengthen and cadence often drifts down, which is worth watching on the gait screen because the aim is usually to keep your normal cadence. Below about 50%, running mechanics start to look noticeably different from road running, and the session is doing more for circulation, confidence and range than for running-specific loading.

The metabolic cost falls too. Running at 70% body weight feels easier for a given speed, so heart rate, breathing and perceived effort become the better guides to session intensity rather than pace alone.

Who it tends to suit

Anti-gravity therapy is used in rehabilitation where the tissue can handle some load but not all of it. Common situations in our Inner West clinic:

• Return to running after ACL reconstruction or other knee surgery, once the surgeon has cleared impact
• Graded return after a diagnosed metatarsal or tibial bone stress injury
• Late-stage calf strains and hamstring strains, where straight-line running needs to restart without full impact
• Achilles, patellar and gluteal tendon problems that flare with outdoor volume
• After ankle fracture or ankle surgery, when walking gait still needs work
• Runners with knee or hip osteoarthritis who want to keep running some of the week
• Postnatal return to running, and return to running after a long layoff

It is also used simply to hold aerobic volume during a training block when one tissue is irritable and the rest of the body is fine.

What it does not do

Unweighted running does not build tolerance for full body weight running on its own. Bone, tendon and muscle adapt to the load you give them, so the plan always has to walk the percentage back up towards 100% and out onto the road. Treating the AlterG as the destination is the most common way it gets misused.

It also does not replace strength work. Calf raise capacity, hip and quadriceps strength and single-leg control are usually part of why a problem appeared, and they need direct loading in the gym. We pair treadmill sessions with home programs or strength and conditioning classes for that reason.

Some situations rule it out or need clearance first: undiagnosed leg pain, an unstable fracture, open wounds or skin problems where the shorts sit, recent abdominal or pelvic surgery, and any pain that is significant with walking. Tell us about these at the assessment so the plan accounts for them.

What a session involves

The first appointment is an assessment rather than a run. That means your history in specifics: what you play or run, weekly volume, when symptoms started, what aggravates and eases them, previous injuries, surgery dates where relevant. Then physical testing, usually including palpation, range of motion, single-leg calf raise counts, hop and squat testing, and video of your walking or running gait.

If treadmill work fits, we set a starting body weight percentage, speed and duration, and you run with a physiotherapist beside the machine adjusting the unweighting based on symptoms and what the gait looks like. Sessions commonly run 20 to 40 minutes. Progression is written down: which percentage, which speed, how long, and the symptom rules that decide whether the next session goes up, holds or comes back a step.

Most people work through a series of sessions over several weeks, with the percentage climbing and outdoor running gradually taking over the load. Along the way we keep checking the strength numbers, because those often tell us more about readiness than how the running feels on the day.

If you are in the Inner West and want to know whether unweighted running suits your injury, you can book an assessment to have it looked at.