Red Light Therapy vs Therapeutic Ultrasound for Horses: The Established Tool Is Being Questioned While the Newer One Is Being Validated
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Therapeutic ultrasound was a standard tool in equine rehabilitation clinics before most horse owners had heard of red light therapy. It has been used for decades on tendons, muscles, joints, and scar tissue — and for most of that time, it was accepted as effective without much scrutiny. That scrutiny has now arrived, and the results are not flattering. A 2022 systematic review of therapeutic ultrasound in sport and companion animals, published in Animals, concluded that there is "insufficient scientific evidence for favourable effects in conditions affecting tendons, ligaments, muscles, and joints" in horses and dogs. A 2024 review in Equine Veterinary Education went further, describing the evidence for therapeutic ultrasound, shockwave, electrical stimulation, and laser as having "no substantial evidence for their efficacy" and noting they "potentially can be damaging." The established tool is being questioned. The question for horse owners is: what do we use instead — or alongside?
That is where the comparison with photobiomodulation becomes relevant. PBM has a more precisely characterised mechanism than therapeutic ultrasound — the primary chromophore (cytochrome c oxidase) has been identified, the downstream pathways are mapped, and the dose-response relationship is well-established across over 6,000 peer-reviewed publications. Therapeutic ultrasound's mechanism is described in broader, less specific terms — thermal effects (heating) and non-thermal effects (cavitation, microstreaming) — without the same level of molecular target identification. For the full cellular science, our guide to how PBM works covers the pathway. But honesty requires saying clearly: PBM's equine-specific clinical evidence is also still developing. The mechanism is better characterised, the overall evidence base is larger, and the safety profile is cleaner — but the depth of equine-specific controlled clinical trials for PBM is not yet at the level that would constitute definitive proof for most conditions either.
The third point is practical: therapeutic ultrasound requires a trained practitioner, coupling gel, and continuous probe movement — and improper use can thermally damage tissue, particularly tendons. PBM is owner-applied daily with a wearable or handheld device, requires no coupling gel, no continuous movement, and carries no risk of thermal tissue damage at consumer LED power densities. For owners building a daily therapy routine, this access difference determines whether the therapy actually happens 300 times a year or three times a year.
The Short Answer
Therapeutic ultrasound has been used for decades, but systematic reviews have found insufficient evidence for most equine soft-tissue applications. PBM has a better-characterised mechanism and a larger overall evidence base — though equine-specific clinical evidence for both is still developing.
Therapeutic ultrasound: thermal + mechanical energy. Requires practitioner, coupling gel, continuous probe movement. Risk of thermal tissue damage if improperly used. Evidence questioned by 2022 and 2024 systematic reviews.
PBM: photochemical energy. Owner-applied daily. No coupling gel, no movement required. No thermal damage risk. Mechanism well-characterised (CCO, 6,000+ publications). Growing evidence trajectory.
The honest bottom line: neither has the depth of equine-specific RCTs to be called "proven" for most conditions. But PBM has the stronger mechanistic foundation, the cleaner safety profile, and the access model that allows daily owner application.
The Mechanism Comparison: Heat and Vibration vs Light and Chemistry
| Therapeutic ultrasound (TUS) | Red light therapy (PBM) | |
|---|---|---|
| Physics | Acoustic — high-frequency sound waves (1–3 MHz) transmitted into tissue via coupling gel | Photonic — photons at 660 nm and 810–850 nm from LED arrays |
| Thermal mode | Yes (continuous mode). Sound waves generate friction in tissue → deep heating (1–4°C temperature rise). Increases blood flow, tissue extensibility, and metabolic rate | No significant thermal effect. LED devices at consumer power densities produce negligible heat. The therapeutic mechanism is photochemical, not thermal |
| Non-thermal mode | Yes (pulsed mode). Mechanical vibration creates cavitation (microscopic gas bubbles) and microstreaming (fluid movement around cells). May stimulate cellular activity | N/A. PBM does not produce mechanical vibration. Its non-thermal effects come from photon-CCO interaction |
| Molecular target | Not precisely identified. Effects are described broadly: thermal effects on tissue extensibility, mechanical effects on cell membrane permeability. No single molecular target comparable to PBM's CCO | Well-identified: cytochrome c oxidase (CCO) in the mitochondrial electron transport chain. Action spectrum matches absorption spectrum |
| Downstream effects | Increased local blood flow (thermal), increased tissue extensibility, potential cellular stimulation (pulsed), pain relief | ATP production increase, NO release (vasodilation), inflammatory mediator modulation, collagen synthesis support |
| Penetration | Good. 1 MHz penetrates approximately 3–5 cm; 3 MHz penetrates 1–2 cm. Sound waves pass through soft tissue effectively | Variable. Red (660 nm) penetrates 1–2 cm; NIR (810–850 nm) penetrates 3–5 cm. Attenuated by coat, skin, and subcutaneous tissue |
| Coupling requirement | Yes. Coupling gel mandatory — air gaps block sound wave transmission. Probe must be moved continuously to prevent hot spots | No. Photons pass through air. Contact improves delivery but no coupling medium is required |
The mechanism clarity gap
This is the most consequential difference between the two modalities from a scientific standpoint. PBM's mechanism has a specific, identified molecular target — cytochrome c oxidase. The action spectrum (which wavelengths produce effects) precisely matches CCO's absorption spectrum. The downstream pathways (ATP, NO, ROS, inflammatory mediators) are mapped. The dose-response relationship is established through the biphasic (Arndt-Schulz) curve. This level of molecular specificity — from photon to enzyme to cellular outcome — does not exist for therapeutic ultrasound.
Therapeutic ultrasound's mechanism is described in broader, less specific terms: heat increases blood flow and extensibility; mechanical vibration stimulates cells through cavitation and microstreaming. But which specific molecular targets are activated, through which downstream pathways, at what precise dose — these are less clearly defined. As the 2022 systematic review noted, the evidence for favourable clinical effects has not held up under scrutiny for most applications. A therapy with a vaguely defined mechanism and insufficient clinical evidence is harder to trust than a therapy with a precisely defined mechanism and growing clinical evidence — even when neither has reached definitive proof for every condition.
What the Systematic Reviews Actually Found
This is the section that matters most for horse owners choosing between the two — because the evidence picture for therapeutic ultrasound has changed significantly from what most practitioners learned in their training.
Boström et al., 2022 (Animals) — Systematic review of therapeutic ultrasound in sport and companion animals
- Finding: "insufficient scientific evidence for favourable effects in conditions affecting tendons, ligaments, muscles, and joints" in horses and dogs.
- The studies were few, most involved small numbers of animals, many had methodological problems, and when beneficial results were reported, they had not been repeated independently.
- Exception: bone healing — several experimental studies with low risk of bias reported consistent positive results, suggesting ultrasound may have value for fracture healing.
Smith, 2024 (Equine Veterinary Education) — Treatment of tendinopathies
- Finding: "Additional strategies for modulating repair using physical means include shockwave, electrical stimulation, therapeutic ultrasound and laser. None of these modalities have any substantial evidence for their efficacy and potentially can be damaging."
- This assessment groups therapeutic ultrasound with other physical modalities as lacking substantial evidence for equine tendon applications — the condition it is most commonly used for.
What this means practically
Therapeutic ultrasound has been used clinically for equine rehabilitation far beyond what the evidence supports. This does not mean it definitively does not work — absence of evidence is not evidence of absence. It means the modality has not been validated through the rigorous, controlled, adequately powered equine studies that would justify the confidence practitioners and owners have placed in it. The gap between clinical use and clinical evidence is wide — and systematic reviews have now explicitly identified it.
Where Does PBM's Evidence Stand by Comparison?
Honesty demands that we position PBM's evidence with the same transparency we applied to ultrasound.
| Evidence category | Therapeutic ultrasound | PBM |
|---|---|---|
| Overall publication volume | Moderate — decades of use, but fewer peer-reviewed studies than the clinical prevalence would suggest | Large — 6,000+ peer-reviewed publications across species and conditions |
| Mechanism characterisation | Broad — thermal and mechanical effects described, but no single identified molecular target | Specific — CCO identified as primary chromophore. Downstream pathways mapped. Action spectrum matches absorption spectrum |
| Dose-response | Less well-defined — optimal intensity, frequency, and duration not established for most equine conditions | Better defined — biphasic (Arndt-Schulz) curve established. Irradiance × time = fluence. Therapeutic window approximately 4–50 J/cm² |
| Equine soft-tissue evidence | Insufficient — per 2022 systematic review | Promising but limited — in vitro equine cell studies positive; controlled in vivo studies show some positive outcomes; large equine RCTs still lacking |
| Equine bone-healing evidence | Positive — consistent results in experimental bone-healing studies | Supportive — mechanism supports circulation and cellular energy, but less specific bone-healing evidence than ultrasound |
| Systematic review assessment | "Insufficient evidence" for most equine soft-tissue applications (Boström 2022) | No comparable systematic review has concluded "insufficient evidence" for PBM overall — though equine-specific reviews note the need for more controlled trials |
| Safety profile | Can cause thermal tissue damage if improperly used. Requires trained operator | No tissue damage risk at consumer LED power densities. Owner-applied safely |
The honest position
PBM's mechanistic foundation is stronger than therapeutic ultrasound's. PBM's overall evidence base is larger. PBM's safety profile is cleaner. And PBM's access model is far more practical for daily owner use.
But PBM's equine-specific clinical evidence is also still developing. The mechanism is well-characterised, the in vitro results are positive, the controlled animal studies are promising — but large-scale, multi-centre equine RCTs for specific conditions remain limited.
The difference between the two is not "proven vs unproven." It is "questioned and weakening vs characterised and growing." One modality's evidence is being dismantled by systematic review. The other's is being built by ongoing research. The trajectories matter as much as the current state.
The Thermal Damage Risk: A Critical Practical Difference
This is the safety concern that most directly separates the two modalities in practical use.
Therapeutic ultrasound in continuous (thermal) mode
Sound waves at 1–3 MHz generate friction as they pass through tissue, producing a deep heating effect. This is intentional — the heat increases blood flow, tissue extensibility, and metabolic rate. But it comes with a real risk: if the probe is held stationary, or if the intensity is too high, the concentrated acoustic energy can overheat the tissue.
This is particularly dangerous for tendons — the structures most commonly treated with therapeutic ultrasound in equine practice. Tendons are dense, poorly vascularised tissues that do not dissipate heat efficiently. Overheating a healing tendon can worsen the injury, damage the collagen matrix, and set back recovery. This is why therapeutic ultrasound requires a trained operator who keeps the probe moving continuously and monitors the intensity carefully.
The 2024 EVE review explicitly noted that physical modalities including therapeutic ultrasound "potentially can be damaging." This is not a theoretical concern — it is a recognised clinical risk that limits who can administer the therapy and under what conditions.
PBM: no thermal damage risk
LED-based PBM devices at consumer power densities produce negligible heat. The therapeutic mechanism is photochemical — photon absorption by CCO — not thermal. The device can be held stationary without risk. No coupling gel is needed. No continuous movement is required. There is no scenario in normal use where a consumer LED device can overheat tissue, burn skin, or thermally damage a tendon. This fundamental safety difference is why PBM is owner-applied and therapeutic ultrasound requires a trained practitioner.
Condition-by-Condition: Where Each Stands
| Condition | Therapeutic ultrasound | PBM |
|---|---|---|
| Tendon injuries (SDFT, DDFT) | "Insufficient evidence" per systematic review. Can potentially damage healing tendon if improperly applied | In vitro equine tenocyte studies positive. Controlled studies promising for collagen organisation. No damage risk. Daily owner application during months-long healing. For deep tendons, see our DDFT guide |
| Ligament injuries (suspensory) | "Insufficient evidence" per systematic review. Used clinically but not validated | Limited direct equine ligament evidence. Mechanism supports repair (ATP, collagen, circulation). No damage risk |
| Muscle soreness / spasm | Thermal mode may relax muscle and increase extensibility. Evidence insufficient per review | Mechanism well-suited (ATP for recovery, vasodilation for waste clearance, inflammatory modulation). Extensive human DOMS evidence. Daily application practical |
| Osteoarthritis | Some clinical use for deep heating of joints. Evidence insufficient per review | Mechanism well-suited for accessible joints (inflammation modulation, circulation). Daily maintenance practical. Equine OA-specific studies still limited |
| Bone healing / fractures | Positive evidence. The one area where the 2022 systematic review found consistent, low-risk-of-bias results in experimental studies | Supportive — mechanism supports cellular energy and circulation. But less specific bone-healing evidence than ultrasound |
| Wound healing | Some use, but not a primary application. Coupling gel on a wound is impractical | Strong mechanism fit. Non-contact treatment. Fibroblast stimulation, collagen synthesis, epithelialisation. Broader evidence base across species |
| Scar tissue / adhesions | Traditional application — thermal heating to increase extensibility before stretching. Clinical use widespread but evidence insufficient per review | May support tissue remodelling through cellular stimulation, but mechanical stretching (not light) is what physically breaks adhesions |
| Post-exercise recovery | Not a typical application — impractical for daily use (requires practitioner) | Strong fit. Daily, owner-applied. Extensive human DOMS evidence. ATP, vasodilation, inflammatory modulation all well-suited |
The Access Model: Why It Determines Real-World Outcomes
| Therapeutic ultrasound | PBM | |
|---|---|---|
| Administered by | Trained practitioner (vet, physiotherapist, or certified rehab therapist) | Owner — no specialised training required |
| Equipment requirement | Ultrasound unit (professional grade), coupling gel, trained hands for continuous probe movement | LED device (blanket, pad, handheld). Place on body, press start |
| Session frequency | 2–6 sessions per week for 2–4 weeks (typical clinic protocol). Requires scheduling with practitioner | Daily or near-daily, ongoing. Owner-applied whenever the schedule allows |
| Practical sessions per treatment course | 8–24 (over 2–4 weeks of clinic visits) | 60–120+ (daily for 2–4 months) |
| Cost model | Per-session practitioner fee + travel. Ongoing cost throughout treatment | One-time device purchase, then $0/session |
| Risk of improper use | Real — thermal tissue damage if probe is stationary, intensity too high, or treatment too long | Negligible — no thermal risk at consumer LED power densities. Biphasic response means over-dosing produces diminishing returns, not damage |
The consistency advantage
Tendon healing takes months. Arthritis management is lifelong. Post-exercise recovery is a daily need. Any therapy that requires a practitioner visit for each session is limited to the frequency the owner can schedule and afford — typically two to four sessions per week for a few weeks. PBM's owner-applied model allows daily application for as long as the condition warrants, providing consistent cellular support throughout the entire healing or management period. The cumulative cellular stimulus from 90 daily PBM sessions over three months is something a practitioner-dependent modality cannot replicate.
What Therapeutic Ultrasound Still Does That PBM Does Not
Honest comparison requires acknowledging where ultrasound provides something PBM cannot:
- Deep tissue heating. Therapeutic ultrasound in continuous mode raises tissue temperature 1–4°C at depth. This thermal effect increases tissue extensibility — making tight fascia, scar tissue, and stiff joints more pliable before stretching or manual therapy. PBM does not heat tissue and does not increase extensibility through thermal means.
- Mechanical tissue effects. Pulsed ultrasound produces cavitation and microstreaming — mechanical events at the cellular level that may stimulate cell membranes and fluid dynamics. PBM's mechanism is photochemical, not mechanical.
- Preparation for manual therapy. The deep heating effect makes therapeutic ultrasound a useful warm-up tool before massage, stretching, or joint mobilisation — softening the tissue before mechanical intervention. PBM supports circulation (vasodilation), which provides some pre-treatment benefit, but not the direct tissue-softening that thermal ultrasound achieves.
- Bone healing evidence. The 2022 systematic review found consistent positive results for bone healing in experimental studies — an evidence strength that PBM does not match for bone-specific applications.
If your vet or rehab practitioner uses therapeutic ultrasound as part of a structured rehabilitation programme — particularly for bone healing, pre-stretching tissue preparation, or deep heating of specific structures — the modality has legitimate applications supported by clinical experience even where the systematic review evidence is insufficient. The reviews do not say ultrasound does not work. They say the evidence does not support the breadth of claims made for it. That distinction matters — and your practitioner's clinical judgment and experience with specific conditions carries weight even when published evidence is lacking.
Conclusion: The Evidence Has Shifted — and the Access Model Has Changed
Therapeutic ultrasound was the standard equine rehabilitation tool for decades. It was accepted as effective based on clinical experience, physiological reasoning, and limited studies. Systematic reviews in 2022 and 2024 have now found that the evidence for most equine soft-tissue applications is insufficient — the studies were few, small, methodologically flawed, and unreplicated. The one area of consistent positive evidence is bone healing.
PBM has a different evidence trajectory. Its mechanism is more precisely characterised (CCO identified, pathways mapped, dose-response established). Its overall evidence base is larger (6,000+ publications). Its equine-specific clinical evidence is still developing — but the foundation is stronger, the trajectory is upward, and the safety profile is cleaner. No thermal damage risk. No coupling gel. No requirement for a trained operator.
The access model is the practical deciding factor for most owners. Therapeutic ultrasound requires a practitioner, limits session frequency to what the schedule and budget allow, and carries a real risk of thermal tissue damage if improperly administered. PBM is owner-applied daily with a consumer device, provides consistent cellular support throughout months-long healing processes, and poses no tissue risk at standard power densities.
Neither modality has reached the depth of equine-specific clinical evidence that would constitute definitive proof for most conditions. But when you compare a modality whose evidence is being questioned and weakened by systematic review against a modality whose mechanism is precisely characterised and whose evidence is growing — the trajectory tells you where the field is heading. The established tool is being scrutinised. The newer tool is being validated. And the one you can use every day, safely, at home, without a practitioner — is the one that delivers 300 sessions a year instead of 20.
Frequently Asked Questions
Is red light therapy or therapeutic ultrasound better for horses?
The evidence picture has shifted significantly. A 2022 systematic review found insufficient scientific evidence for favourable effects of therapeutic ultrasound in conditions affecting tendons, ligaments, muscles, and joints in horses and dogs. A 2024 review described the evidence for therapeutic ultrasound as lacking substantial evidence and potentially damaging. PBM has a larger overall evidence base (6,000+ publications), a more precisely characterised mechanism (cytochrome c oxidase absorption), and a cleaner safety profile. PBM's equine-specific clinical evidence is also still developing, but the mechanistic foundation is substantially stronger, and the evidence trajectory is upward rather than under scrutiny. Neither has definitive proof for most equine conditions, but PBM's foundation positions it more strongly.
What is the difference between therapeutic ultrasound and red light therapy for horses?
They use different physics and target tissue differently. Therapeutic ultrasound delivers high-frequency sound waves (1–3 MHz) producing thermal (deep heating) and non-thermal (cavitation, microstreaming) effects. PBM delivers photons at 660 nm and 810–850 nm absorbed by cytochrome c oxidase in the mitochondria, stimulating ATP production, NO release, and inflammatory modulation without significant heat. Ultrasound requires coupling gel and continuous probe movement by a trained operator. PBM is applied with a wearable or handheld device requiring no gel, no movement, and no specialised training.
Can therapeutic ultrasound damage a horse's tendon?
Yes — therapeutic ultrasound in continuous (thermal) mode generates deep tissue heating. If the probe is held stationary or the intensity is too high, the concentrated heat can overheat the tissue — particularly tendons, which are dense, poorly vascularised structures that do not dissipate heat efficiently. Overheating a healing tendon can worsen the injury. This risk is why therapeutic ultrasound should only be administered by a trained practitioner. PBM does not carry this thermal damage risk — LED devices at consumer power densities produce negligible heat regardless of whether the device is held stationary.
Why has therapeutic ultrasound evidence been questioned for horses?
A 2022 systematic review (Boström et al., published in Animals) examined the scientific literature on therapeutic ultrasound in sport and companion animals and found that the studies have been few, most involved small numbers of animals, many had methodological problems, beneficial results were not independently replicated, and overall there was insufficient evidence for favourable effects on tendons, ligaments, muscles, and joints. A 2024 review (Smith, Equine Veterinary Education) described the evidence as lacking substantial evidence and potentially damaging. This does not mean therapeutic ultrasound definitively does not work — it means the evidence supporting its widespread use has not held up under systematic scrutiny.
Can you use red light therapy and ultrasound together on a horse?
They work through different mechanisms and there is no direct conflict. If your vet or rehab practitioner uses therapeutic ultrasound as part of the treatment plan, adding daily PBM between sessions is a reasonable complementary approach — different energy types targeting different aspects of healing. Apply PBM after the ultrasound session, once the tissue has returned to normal temperature if thermal mode was used. However, given the questions surrounding ultrasound's evidence base, discuss with your vet whether ultrasound is contributing meaningfully or whether PBM alone — potentially combined with other evidence-supported modalities — would serve the horse's needs more effectively.
