Underwater Treadmills: How They Work, Who They Help, and What the Research Shows

Key Takeaways

• An underwater treadmill is a treadmill built into the floor of a shallow pool or tank. You walk or run on it while submerged to chest or waist depth, which reduces your body weight by 40–80% depending on water level.
• People with knee osteoarthritis reported less pain on an underwater treadmill than on a land treadmill — while getting an equivalent workout. The water allows a normal walking pattern without the joint impact.
• Research shows underwater treadmill training improved leg strength, balance, and walking performance in people with incomplete spinal cord injury over 8 weeks.
• ACL reconstruction patients who trained on underwater treadmills gained more knee flexibility and muscle size than those who used stationary bikes.
• Underwater treadmills are found in physiotherapy clinics, hospitals, and sports rehab centres. Home versions exist but cost £25,000–£80,000+. For most people, accessing one through a professional is more realistic.

What an Underwater Treadmill Actually Is

An underwater treadmill is exactly what it sounds like: a motorised treadmill belt set into the floor of a small heated pool or tank. You step in, the water fills to somewhere between your waist and chest, and you walk or run on the belt — just like a normal treadmill, except you are partially submerged.

The water does two things at once. It takes weight off your joints (so walking hurts less) and provides resistance against your legs (so your muscles work harder than they would moving through air). This combination — less joint stress, more muscle work — is what makes underwater treadmills so useful for rehabilitation.

Most clinical units heat the water to 30–34°C and let you adjust the speed, water depth, and sometimes add water jets for extra resistance. Sessions typically last 20–30 minutes.

How It Differs from Walking in a Pool

You might wonder: why not just walk in a pool? You can, and pool walking is beneficial. But an underwater treadmill offers specific advantages:

• Controlled speed. The belt moves at a set pace, so you maintain consistent effort. In a pool, most people walk too slowly to get a meaningful workout.
• Normal gait pattern. The flat belt lets you do a proper heel-to-toe walking pattern, which closely mimics land-based walking. Pool walking on a pool floor often leads to shuffling because the surface is slippery and there is no forward momentum.
• Measurable progress. Your therapist can track speed, duration, and water depth precisely. This makes it easy to gradually increase difficulty as you get stronger.
• Adjustable water depth. Some units have adjustable floors or fill levels, letting the therapist control exactly how much of your weight the water supports.

Who Benefits Most

People with knee or hip osteoarthritis

This is the most common use. Research consistently shows that people with knee osteoarthritis report less pain during underwater treadmill walking than during the same exercise on land — while getting an equivalent or better workout.

A study of obese patients with knee osteoarthritis found that four weeks of underwater treadmill exercise was as effective as home exercise for pain relief and functional improvement — but patients in the underwater group reported significantly higher satisfaction and better perceived improvement.

Another study found that high-intensity interval training on an aquatic treadmill improved pain, balance, function, and mobility in adults with osteoarthritis — showing that you can train hard in the water without the joint damage that hard training on land would cause.

People recovering from ACL or knee surgery

After ACL reconstruction, getting the knee moving early is critical — but land-based walking is often too painful in the first weeks. Underwater treadmill training lets patients start walking sooner. One study found that ACL patients using underwater treadmills gained more knee flexion range and greater muscle girth than those using stationary bikes, suggesting the underwater treadmill may be more effective at preventing the muscle wasting that commonly follows knee surgery.

People with spinal cord injuries

An 8-week underwater treadmill training programme for adults with incomplete spinal cord injury produced measurable improvements in leg strength, balance, and walking performance. The water supports enough body weight to allow walking practice even when leg strength is significantly reduced — something that would require complex harness systems on land.

People recovering from stroke

A study of subacute stroke patients found that aquatic treadmill training improved gait symmetry and balance. Walking after a stroke is often uneven — one leg does more work than the other. The water’s resistance evens out the effort, helping retrain a more balanced walking pattern.

People with obesity

For people carrying significant extra weight, walking on land puts enormous stress on knees, hips, and ankles. Water buoyancy removes most of that stress. A person weighing 120 kg at chest depth effectively weighs only 30–40 kg. This makes regular exercise possible — and sustainable — when land-based walking would cause pain or injury.

Athletes returning from injury

Professional sports teams use underwater treadmills to keep injured athletes training while injuries heal. A Texas A&M study found that 10 minutes of aquatic treadmill running at 5 mph significantly reduced perceived muscle soreness after intense sprint exercise. The same study found that aquatic treadmill training — but not land treadmill training — significantly reduced resting blood pressure.

What to Expect in a Session

If your physiotherapist recommends underwater treadmill training, here is what a typical session looks like:

1. You change into swimwear and enter the tank or pool via steps or a ramp.
2. The water fills or you adjust depth — usually to waist or chest level depending on how much weight support you need.
3. You warm up with slow walking (1–2 km/h) for 3–5 minutes.
4. The therapist increases speed gradually. A typical rehabilitation walking speed is 2–4 km/h. Athletes may train at 6–10 km/h or more.
5. You walk for 15–25 minutes, with the therapist monitoring your gait, adjusting speed, and sometimes adding water jets for resistance.
6. You cool down with slow walking for 3–5 minutes.
7. Total session: about 30 minutes in the water, plus time to change.

Most programmes run 2–3 sessions per week for 4–8 weeks.

How to Access an Underwater Treadmill

Through the NHS (UK): Some NHS physiotherapy departments have underwater treadmills, usually in larger hospitals or specialist rehabilitation centres. Ask your GP for a referral to a physiotherapist with aquatic therapy access.

Private physiotherapy: Many private rehab clinics and sports medicine centres have them. Sessions typically cost £50–£100. Some health insurance plans cover aquatic physiotherapy with a medical referral.

Home use: Home underwater treadmill units exist (brands like HydroWorx and Endless Pools offer residential models) but cost £25,000–£80,000+ installed. They also require dedicated space, plumbing, electrical work, and ongoing maintenance. This is only practical if you need daily access and have the budget and space for it.

The practical alternative: If you cannot access an underwater treadmill, walking in a warm pool at chest depth gives you many of the same benefits — buoyancy, resistance, and thermal effects. You lose the controlled speed and precise gait pattern, but you gain a significant portion of the therapeutic value at a fraction of the cost.

Limitations to Be Honest About

• Bone loading is reduced. Buoyancy is great for joint protection, but bones need impact to stay strong. People with osteoporosis should combine underwater treadmill work with some weight-bearing exercise on land.
• It does not replicate all land conditions. The water changes how your muscles fire and how your balance works. If your goal is to return to running or sports on land, you will eventually need to transition back to land-based training.
• Availability is limited. Not every physiotherapy clinic has one. You may need to travel or go private.
• It is not a magic fix. Underwater treadmill training is one tool in a rehabilitation programme. It works best combined with land-based exercises, manual therapy, and — where needed — medication or surgery.

The Bottom Line

Underwater treadmills are one of the most useful tools in rehabilitation. They let you walk with a normal gait, at a controlled speed, with most of your body weight taken off your joints. For people with arthritis, recovering from surgery, living with neurological conditions, or managing obesity, this combination is hard to replicate any other way.

You do not need to own one. Access through a physiotherapist is enough for most people. If underwater treadmill training helps, you can continue many of the same benefits by walking in a warm pool on your own.

Related Reading

• How Hydrotherapy Machines Work: The Physics Behind the Relief
• How Hydrotherapy Helps You Recover Faster from Injuries
• Hydrotherapy Pools for Elderly Care
• Choosing the Right Hydrotherapy Equipment

References

• Jung, T. et al. (2014). Effects of underwater treadmill training on leg strength, balance, and walking performance in adults with incomplete spinal cord injury. Journal of Rehabilitation Medicine. PMC4293539
• Kuptniratsaikul, V. et al. (2022). Pain management and strength gains compared between pneumatic partial weight support treadmill and underwater treadmill in overweight patients with knee osteoarthritis. International Journal of Rehabilitation Research. DOI link
• Valtonen, A. et al. (2018). Is four-week underwater treadmill exercise regimen compared to home exercise efficacious for pain relief and functional improvement in obese patients with knee osteoarthritis? PubMed
• Bressel, E. et al. (2012). Acute aquatic treadmill exercise improves gait and pain in people with knee osteoarthritis. Archives of Physical Medicine and Rehabilitation. PubMed
• Park, J. et al. (2017). Efficacy of aquatic treadmill training on gait symmetry and balance in subacute stroke patients. Journal of Physical Therapy Science. PMC5532342
• Mooventhan, A. & Nivethitha, L. (2014). Scientific evidence-based effects of hydrotherapy on various systems of the body. North American Journal of Medical Sciences, 6(5), 199–209. PMC4049052

Last reviewed: February 2026. This article is for informational purposes and does not replace medical advice. Consult a physiotherapist to determine if underwater treadmill training is appropriate for your condition.