Hydrotherapy Systems for Pain Relief: Comparing the Evidence for Each Type

Key Takeaways

• Aquatic exercise in warm pools (33–36 °C) has the strongest evidence for chronic pain — a 2023 meta-analysis of 32 RCTs with 2,200 participants found moderate improvements in pain, function, and quality of life (Shi et al., 2023).
• Warm water immersion alone reduces pain perception through multiple mechanisms: buoyancy (up to 90% body weight offloading), hydrostatic pressure, and heat-induced muscle relaxation.
• Whirlpool baths add mechanical massage via water jets but how much benefit the jets add beyond warm water immersion alone is not well quantified.
• Contrast therapy (alternating hot/cold) improves perceived soreness after exercise but does not reduce inflammatory biomarkers (Bieuzen et al., 2013).
• Steam baths and saunas have some evidence for blood pressure and cardiovascular benefits but limited controlled evidence for pain specifically.

How Water-Based Therapy Reduces Pain

Before comparing specific systems, it helps to understand why water works for pain at all. Three physical properties of water do most of the therapeutic work:

Buoyancy. Water supports body weight proportionally to immersion depth. At chest level, buoyancy offloads approximately 80–90% of your weight from your joints. This is why people with arthritis, back pain, or post-surgical stiffness can often move in water when land-based movement is too painful.

Hydrostatic pressure. Water exerts pressure on submerged tissues, which compresses surface blood vessels and reduces oedema (swelling). This is a passive, physical effect — it happens simply by being in the water, regardless of what you do there.

Heat transfer. Water conducts heat roughly 25 times more efficiently than air. Warm water (38–41 °C) raises skin and superficial muscle temperature quickly, causing vasodilation, reduced muscle tone, and decreased nerve conduction velocity — all of which reduce pain perception.

These three mechanisms work simultaneously during any warm water immersion. The specific “system” you use matters less than whether you’re in warm water, at sufficient depth, for enough time.

System-by-System Evidence Review

1. Warm Water Pools (Aquatic Exercise)

Evidence strength: Good — this is the gold standard

Warm water exercise pools (typically 33–36 °C) are the most studied hydrotherapy system. This is where the best evidence sits:

• Chronic musculoskeletal pain: Shi et al. (2023) reviewed 32 RCTs with 2,200 participants and found moderate improvements in pain, physical function, and quality of life across osteoarthritis, chronic low back pain, and fibromyalgia
• Knee osteoarthritis: A 2024 meta-analysis of RCTs found significant pain reduction at 1, 4, and 8 weeks compared with controls, with no serious adverse events reported (Lu et al., 2024)
• Chronic low back pain: Hydrotherapy improved pain scores and functional measures at both 10 and 20 sessions compared with baseline and control groups (Baena-Beato et al., 2021)

The key finding across these reviews: aquatic exercise is consistently as effective as land-based exercise for pain relief, and sometimes better — primarily because the buoyancy allows people to exercise who otherwise couldn’t tolerate it. The exercise itself, not just the water, drives much of the benefit.

Cost: Access via NHS physiotherapy (free with referral), private hydrotherapy sessions (£40–£80 per session), or public pools with warm water lanes (£5–£10 per visit).

2. Whirlpool Baths and Hot Tubs

Evidence strength: Moderate

Whirlpool baths combine warm water immersion with pressurised water jets that provide mechanical massage. They are widely available in spas, physiotherapy clinics, and as home installations.

The evidence for whirlpools specifically (separate from warm water immersion generally) is harder to isolate. Most studies comparing whirlpool therapy with control conditions don’t separate the effect of the jets from the effect of the warm water. What we can say:

• A systematic review by McVeigh et al. (2008) included whirlpool-based hydrotherapy among modalities that improved pain, health status, and tender point count in fibromyalgia patients
• Whirlpool combined with supervised exercise improved peripheral circulation in patients with peripheral artery disease (Mika et al., 2024)
• For wound care, whirlpool debridement was historically standard but has been largely replaced by more targeted wound care methods in modern practice

Honest assessment: Most of the pain relief from a whirlpool bath comes from the warm water immersion. The jets likely provide some additional benefit through mechanical stimulation and increased local blood flow, but the magnitude of this additional effect is not well quantified. If you already have access to a warm bath, the extra cost of a whirlpool may not be justified purely on evidence grounds.

Cost: Home hot tubs £3,000–£15,000; portable inflatable hot tubs £300–£800; professional sessions £30–£60. See our year-round cost guide for full running expenses.

3. Underwater Treadmills

Evidence strength: Moderate, primarily for rehabilitation populations

Underwater treadmills allow walking or running with reduced body weight (typically 50–80% offloading depending on water depth). They are primarily used in clinical rehabilitation settings.

• A meta-analysis by Villalta & Peiris (2013) found that early aquatic therapy (including underwater treadmill work) was safe to begin as early as 4–14 days post-surgery for lower limb procedures
• Studies in post-knee-replacement patients show improvements in range of motion and reduced oedema when aquatic therapy was added to land-based rehabilitation
• For spinal cord injury patients, Jung et al. (2014) found improved walking ability and balance with underwater treadmill training

Practical limitation: Underwater treadmills are almost exclusively found in clinical settings. They cost £30,000–£100,000+, making them impractical for home use. Access is through NHS physiotherapy, private clinics, or specialist rehabilitation centres.

4. Contrast Therapy (Hot/Cold Alternation)

Evidence strength: Moderate for recovery, limited for chronic pain

Contrast therapy alternates between warm (38–42 °C) and cold (10–15 °C) water, typically 1–2 minutes cold, 3–4 minutes warm, repeated 3–5 times.

• A PLOS ONE meta-analysis (Bieuzen et al., 2013) found contrast therapy reduced perceived muscle soreness and strength loss versus passive recovery after exercise
• However, it did not reduce CRP or IL-6 — meaning the anti-inflammatory claims are overstated
• For chronic pain conditions, there is limited controlled evidence. Most contrast therapy research focuses on acute post-exercise recovery

Cost: Free at home (shower hot/cold alternation), or using two buckets/basins. No special equipment needed.

5. Steam Baths and Saunas

Evidence strength: Moderate for cardiovascular health, low for pain specifically

Steam baths (wet heat, 40–50 °C with near-100% humidity) and saunas (dry heat, 80–100 °C with 10–20% humidity) provide thermal therapy without water immersion, so they lack the buoyancy and hydrostatic pressure benefits of pool-based hydrotherapy.

The best evidence for saunas relates to cardiovascular health rather than pain management. The Finnish KIHD cohort study (Laukkanen et al., 2015) — a 20-year follow-up of 2,315 men — found that frequent sauna use (4–7 times per week) was associated with a 40% lower risk of all-cause mortality compared with once-weekly use. However, this was observational and from a country where sauna use is culturally embedded.

For pain specifically, the evidence is thin. Heat generally reduces pain perception, but whether the specific conditions of a sauna or steam room offer advantages over a warm bath is unclear.

What saunas cannot do: The “detoxification through sweating” claim is not supported. Sweat is 99% water with trace minerals — it is not a significant excretory pathway for metabolic waste or toxins.

Cost: Public sauna/steam access £5–£15; home infrared sauna £500–£5,000; traditional sauna installation £3,000–£20,000.

Comparison Table: Systems at a Glance

System	Pain Evidence	Key Mechanism	Home Access	Typical Cost
Warm water pool (aquatic exercise)	Good (32 RCTs)	Buoyancy + heat + exercise	Public pool / NHS referral	£5–£80/session
Whirlpool / hot tub	Moderate	Heat + hydrostatic pressure + jets	Home installation possible	£300–£15,000
Underwater treadmill	Moderate (clinical only)	Buoyancy + gait training	Not practical for home	£30,000–£100,000+
Contrast therapy	Moderate (acute recovery)	Vasoconstriction/dilation cycling	Free (shower or buckets)	Free
Steam bath / sauna	Low for pain; moderate for cardiovascular	Heat (no buoyancy)	Home options available	£500–£20,000
Warm bath (standard)	Good	Heat + partial buoyancy	Already in most homes	~£1/bath

Choosing the Right System for Your Situation

If you have osteoarthritis or chronic joint pain: Start with warm water aquatic exercise. Ask your GP for a physiotherapy referral that includes hydrotherapy pool access. This has the strongest evidence and is often available on the NHS.

If you have fibromyalgia: Both warm water exercise and passive warm water immersion show benefits. A regular programme (2–3 times per week) is more important than the specific equipment used.

If you’re recovering from surgery: Discuss aquatic therapy timing with your surgeon. For knee replacements, early aquatic therapy (from day 14) shows benefits. For hip replacements, later introduction may be better. Wound healing must be complete before pool immersion.

If you have acute post-exercise soreness: Cold water immersion (10–15 °C, 10–15 minutes) is the most evidence-supported approach. Reserve it for intense training phases rather than daily use.

If your budget is limited: A standard warm bath provides the core therapeutic mechanisms — heat and partial buoyancy. You don’t need specialised equipment to benefit from hydrotherapy for pain. The research shows the water matters more than the technology around it.

Safety Considerations

• Heart conditions: Full immersion increases venous return to the heart. Consult your cardiologist before hydrotherapy if you have heart failure or uncontrolled hypertension.
• Pregnancy: Keep water temperature below 38 °C, particularly in the first trimester.
• Post-surgical wounds: Do not enter shared pools until wounds are fully closed and dry. Surgical site infections from pool bacteria are a real risk.
• Neuropathy: If you have reduced sensation (common in diabetes), always check water temperature with a thermometer — not your hand or foot.
• Epilepsy: Water-based therapy should always be supervised if you have a seizure disorder.

The Bottom Line

The most important finding across the hydrotherapy research is this: the warm water and the movement it enables are the primary therapeutic agents. The specific system — whirlpool, pool, hot tub — matters less than you’d expect from the marketing around these products.

For chronic pain, the best evidence supports regular warm water exercise in a pool, supervised by a physiotherapist for the first few sessions. For home use, a warm bath delivers the core mechanisms at minimal cost. More expensive systems may add convenience and comfort, but the evidence does not show dramatically better pain outcomes with more expensive equipment. Our equipment decision framework provides a structured approach to making this choice.

Related Reading

• How Hydrotherapy Machines Actually Work: The Physics
• A Beginner’s Guide to Underwater Treadmills
• Extreme Hydrotherapy Techniques for Fast Recovery
• Choosing the Right Hydrotherapy Equipment

References

• Shi, Z. et al. (2023). Efficacy of aquatic exercise in chronic musculoskeletal disorders: a systematic review and meta-analysis of 32 RCTs. Journal of Orthopaedic Surgery and Research, 18, 895. PMC10704680
• Lu, M. et al. (2024). The efficacy and safety of hydrotherapy in patients with knee osteoarthritis: a meta-analysis of randomized controlled trials. BMC Musculoskeletal Disorders. PMC10942168
• Baena-Beato, P.A. et al. (2021). Efficacy of hydrotherapy treatment for the management of chronic low back pain. Clinical Rehabilitation. PubMed
• McVeigh, J.G. et al. (2008). The effectiveness of hydrotherapy in the management of fibromyalgia syndrome: a systematic review. Rheumatology International, 29, 119–130. PubMed
• Mika, P. et al. (2024). Evaluation of peripheral circulatory changes following hydrotherapy and controlled physical training. Life, 14(12), 1578. MDPI
• Bieuzen, F. et al. (2013). Contrast water therapy and exercise induced muscle damage: a systematic review and meta-analysis. PLOS ONE, 8(4), e62356. PMC3633882
• Laukkanen, T. et al. (2015). Association between sauna bathing and fatal cardiovascular and all-cause mortality events. JAMA Internal Medicine, 175(4), 542–548. PubMed
• Rahmann, A.E. et al. (2009). A specific inpatient aquatic physiotherapy program improves strength after total hip or knee replacement surgery: a randomized controlled trial. Archives of Physical Medicine and Rehabilitation, 90(5), 745–755. PubMed