Unweighted treadmill training in rehabilitation

This card describes unweighted treadmill training, also called lower body positive pressure (LBPP) or "anti-gravity" treadmill training, as used in rehabilitation after surgery or injury. Recovery from any operation, including urinary-tract procedures, often involves a graded return to walking. The information is general, and a person's own clinician decides whether any rehabilitation tool suits them.

What it is

An LBPP treadmill is set inside a waist-high chamber. A systematic review in neurological rehabilitation describes an air compressor that raises the pressure in the chamber, creating an upward buoyant force that lowers the share of body weight the legs carry. The person wears special shorts to use it, and the amount of support can be set precisely. Machines of this type are sold under the brand name AlterG, and a Finnish knee replacement trial protocol uses that name.

The aim is to practice walking, or in some programs running, with less load on joints and muscles than full body weight would bring. It is a rehabilitation tool used alongside, not instead of, a clinician's plan. Wider recovery after surgery is covered in recovery and rehabilitation, and related terms are explained in the glossary.

Why it is done

A systematic review of 15 studies in healthy people found that these treadmills allow a given level of effort with less musculoskeletal loading, but also change muscle activation and movement patterns, which clinicians must account for.

The clinical evidence is mixed and mostly small. A 2023 systematic review of 16 articles in neurological patients found the training safe and feasible, with benefit for motor function when added to traditional rehabilitation, particularly in subacute to chronic stroke and Parkinson disease. It concluded there was insufficient evidence that LBPP is superior to other gait training, and few randomized trials exist.

In orthopedic settings, a 2021 randomized trial of 18 people with mild to moderate knee osteoarthritis found pain and function scores improved in both groups, with larger gains in walking speed, stride length and knee range of motion in the LBPP group. A 2025 cohort study of 80 people after anterior cruciate ligament (ACL) reconstruction reported less thigh muscle atrophy and better balance at six weeks, and more return to sport at one year, in those who trained on an anti-gravity treadmill. It was not randomized, and its authors note a small sample and short follow-up. For knee replacement, the only source found here is a trial protocol with no results.

In a 2025 randomized trial of 72 adults with mild balance impairment, every group improved, including the group on an ordinary treadmill. Some between-group differences disappeared once dropouts were accounted for, and the authors call the evidence for LBPP alone in older adults limited.

How it is done, in outline

The person puts on the required shorts, steps into the chamber and walks while the unloading is set as a percentage of body weight. Speed and, in some programs, incline are adjusted too. Sessions in the knee osteoarthritis and ACL work lasted about 30 minutes. Frequency ranged from three sessions a week over six weeks in the ACL study to six days a week over two weeks in the osteoarthritis trial, so protocols differ.

People in the New York City area who want to see this type of equipment can look up an AlterG Anti-Gravity Treadmill in NYC, while availability, suitability and cost depend on the individual and the referring clinician.

Recovery

The sources describe progression as a gradual build in workload rather than a fixed timetable. In the ACL study, training began in the third week after surgery, and walking speed and incline were raised step by step over six weeks. The Finnish knee replacement protocol schedules ten supervised sessions between weeks 3 and 6, on top of home exercises. The sources do not give one standard route back to full weight-bearing, so that timing is set by the treating team.

Other parts of recovery depend on the procedure, such as pelvic floor muscle training for bladder control or guidance on a urinary catheter after surgery.

Risks

The neurological review reports no adverse events in the included studies. It also lists limits: getting on and off can be hard with severe leg weakness, severe gait limitations may make it unusable because the chamber hinders hands-on help, and some people find the shorts uncomfortable or notice bladder fullness or urgency when unweighted to 50% of body weight.

Trials set their own exclusions, which hint at who may not be suitable. The balance trial excluded people with uncontrolled high blood pressure, arrhythmias, a recent heart attack or severe osteoporosis, and the knee osteoarthritis trial excluded unstable vital signs and serious heart, brain, kidney or liver disease. These are research criteria, not a checklist, and a clinician decides who may train.

Questions to ask your clinician

• Is unweighted treadmill training relevant to my operation or injury, or is ordinary walking enough?
• What does the research say for my condition, and how strong is it?
• When would it start in my recovery, and how would the support be reduced?
• Do my heart, blood pressure, bone density or bladder symptoms affect whether I can use it?
• How will progress be measured so we know whether it is helping?
• Which signs during or after a session should make me stop and contact you?