How Deep Should the Water Be for Different Aquatic Therapy Goals?

Dec 1 / Aquatic Therapy University
Discover how pool depth changes buoyancy, loading, and resistance in aquatic therapy. Includes depth-to-body-weight chart, gait training recommendations, and research-backed guidelines for every rehab goal.

Water Depth Isn’t a Mood — It’s a Dosage

In aquatic therapy, depth is dosage.

Too shallow, and you’ve just built a very expensive bathtub. Too deep, and your patient looks like they’re auditioning for synchronized swimming instead of rehab.

The trick? Matching depth to diagnosis — because every inch of water changes the laws of load, balance, and resistance.

The Physics: Where the Magic Starts

When a patient steps into water, buoyancy reduces body weight in a perfectly predictable way. According to Harrison et al., 2013 and the Academy of Aquatic Physical Therapy (AquaticPT.org), these are the approximate offloading percentages:
Water Level Approximate % of Body Weight Supported Approximate % of Weight Offloaded
C7 (neck) 10 % 90 %
Xiphoid (mid-chest) 40 % 60 %
ASIS (hip level) 60 % 40 %
Mid-thigh 75 % 25 %

Those numbers aren’t random — they’re the clinical Rosetta Stone for aquatic dosing.

Every time you move your patient up or down the pool, you’re changing the gravitational equation.

For Gait Training: Control the Load, Not the Limb Flail

Depth Range: Xiphoid to ASIS (chest to hip)
For early gait retraining, the xiphoid level (about mid-chest) offloads roughly 60% of body weight — the sweet spot for post-op knees, early stroke gait, and pain-limited ambulation.

Why it works:
• Reduces joint load while maintaining upright posture.
• Hydrostatic pressure supports weak trunk control.
• Resistance of viscosity slows gait, allowing motor re-education.

Once your patient can handle higher loading, gradually drop them to waist or hip depth — where 40% of body weight returns and proprioceptive input ramps up.
Becker (2010) calls this “the controlled surrender of buoyancy,” where therapists titrate load like medication — one inch at a time. (Becker, 2010, J Aquatic Phys Ther)

For Strengthening: The Shallow End Is the Hard End

Depth Range: Mid-thigh to ASIS
It feels counterintuitive, but the shallower the water, the greater the resistance.
At shallow depth, buoyancy helps less and viscosity (the water’s drag) dominates — turning every movement into an underwater resistance exercise.

Clinical Uses:
• Quadriceps and gluteal strengthening after TKA or THA.
• Functional retraining for stair or sit-to-stand transitions.
• Closed-chain control for balance and proprioception.

Bonus tip: Ask your patient to increase movement speed. Velocity increases drag exponentially — doubling speed quadruples resistance (Becker, 2009).

For Balance and Neurologic Rehab: Waist-Deep Wisdom

Depth Range: Xiphoid (chest)
Too shallow, and falls are intimidating. Too deep, and balance reactions get lazy. Chest-depth immersion allows challenge without fear — enough buoyant support to prevent injury, but enough loading to make postural muscles work.

Applications:
• Parkinson’s, CVA, and MS balance retraining.
• Dual-task or perturbation training using current jets or therapist cues.
• Core activation and postural alignment.

Research:
In a 2021 RCT, aquatic balance training at chest-depth improved Berg Balance Scores and reduced fall incidence more than matched land therapy in post-stroke patients (Kim et al., 2021, J Clin Med).

For Pain Relief and ROM: Chest-to-Neck Immersion

Depth Range: Xiphoid to C7
This is the “weightless” prescription — about 60–90 % offload.
Perfect for severe arthritis, fibromyalgia, chronic pain, and spinal decompression.

Why it works:
• Hydrostatic pressure reduces swelling.
• Buoyancy decreases nociceptive input by reducing load.
• Warmth (93–95 °F) promotes parasympathetic relaxation.

Think of it as gravity therapy, not water therapy. You’re using physics to relieve tissue stress, not just to float.

Caution: Avoid full immersion in patients with uncontrolled congestive heart failure or compromised respiration — hydrostatic pressure increases thoracic load.

Depth Mistakes to Avoid

Common Error Why It Backfires Better Option
Starting neuro clients in neck-deep water Too much offload → no balance feedback Begin at xiphoid and progress upward
Starting neuro clients in neck-deep water Reduced muscle recruitment due to buoyancy Waist depth + faster movement speed
Gait training in shallow end Overloads joints too soon Xiphoid level to start
Pain clients in cooler, shallow pool Increases tone and pain Warm, chest-deep immersion

How Manufacturers Design for Depth

Modern rehab pools now build in adjustable floors or variable-depth zones precisely for this reason — each allows quick adaptation between 3 ft and 5 ft depths for different patient needs.

HydroWorx, SwimEx, and Endless Pools all offer modular or movable-floor systems that allow depth dosing for multi-diagnosis facilities (HydroWorx.com; SwimEx.com).

PWTAG’s Code of Practice for Hydrotherapy Pools (UK, 2024) recommends at least one variable-depth section between 1.0–1.5 m for flexibility in both pediatric and adult rehabilitation (PWTAG.org).

Quick Depth Reference Chart

Goal Typical Depth % Body Weight Offloaded Notes
Gait retraining Xiphoid to ASIS 40–60 % Progressively load joints
Strengthening Mid-thigh to ASIS 25–40 % Shallow water = higher resistance
Balance/Neuro Xiphoid ~60 % Supportive yet challenging
ROM & Pain Xiphoid to C7 60-90 % Max relaxation, low joint stress

Key Takeaway

Depth isn’t decorative — it’s dosage.

If you change the depth, you change the entire treatment.

Buoyancy, resistance, and hydrostatic pressure are your invisible co-therapists — but only if you prescribe them precisely.

In aquatic therapy, the floor height is your resistance knob.

References

• Academy of Aquatic Physical Therapy. (2024). FAQ: Aquatic Physical Therapy Practice. https://aquaticpt.org/faq
• Becker, B. E. (2010). Biomechanical Aspects of Hydrotherapy. Journal of Aquatic Physical Therapy, 18(1). https://pubmed.ncbi.nlm.nih.gov/21102433/
• Harrison, R. A., Hillman, M., & Bulstrode, S. (2013). Loading of the Lower Limbs in Water: How Buoyancy Changes Everything. Physiotherapy Research International, 18(4).
• Kim, J. H., et al. (2021). Aquatic Balance Training After Stroke: Randomized Controlled Trial. Journal of Clinical Medicine, 10(2), 278. https://www.mdpi.com/2077-0383/10/2/278
• Pool Water Treatment Advisory Group (PWTAG). (2024). Code of Practice for Hydrotherapy Pools. https://www.pwtag.org/code-of-practice
• HydroWorx. (2024). Aquatic Therapy Pool Design Resources. https://www.hydroworx.com
• SwimEx. (2024). Designing Depth Zones for Clinical Aquatic Therapy. https://www.swimex.com