In physiotherapy, US stands for ultrasound, specifically therapeutic ultrasound. It is a non-invasive treatment that uses high-frequency sound waves, typically between 1 and 3 MHz, which sit above the range of human hearing. A clinician applies a coupling gel to the skin and moves a handheld transducer, also called a sound head, over the treatment area. The sound waves produce two kinds of effect in the tissue underneath: a thermal effect that gently heats deep soft tissue, and a mechanical effect that influences cells and fluid movement without meaningful heating. This article explains what the modality is, how it works, how deep it actually reaches, what it is used for, what the research says about how well it works, what the contraindications are, and where passive modalities of this kind belong in a treatment plan.

What Is US in Physiotherapy

US in physiotherapy is the standard abbreviation for therapeutic ultrasound, a treatment that delivers high-frequency mechanical sound energy into soft tissue through a handheld transducer. It belongs to the group of treatments known as electrophysical agents, though ultrasound is a form of mechanical energy rather than electrical energy.

Mechanical energy is the important technical detail. Sound is vibration passing through a medium, and at frequencies above roughly 20,000 Hz that vibration stops being audible and becomes ultrasound. Therapeutic machines generate it by applying rapidly alternating voltage to a quartz crystal inside the treatment head, which changes shape at the same frequency and produces the wave. That is the piezoelectric effect.

The piezoelectric effect also powers medical imaging machines, which is why the same word covers two very different clinical tools, and why a patient booking physiotherapy sometimes arrives expecting a scan.

Is Therapeutic Ultrasound the Same as Diagnostic Ultrasound?

Therapeutic ultrasound is not the same as diagnostic ultrasound; therapeutic ultrasound delivers energy into tissue to produce a treatment effect, while diagnostic ultrasound sends and receives waves to build an image. Same physics, entirely different purpose and entirely different machine settings.

The machine settings are what separate them. Imaging equipment uses very short bursts at low average intensity and listens for the returning echoes to construct a picture. Therapeutic equipment delivers continuous or pulsed output at higher average intensity and produces no image at all. Nothing appears on a screen during a therapeutic ultrasound session.

Nothing appears on screen because the target is the tissue rather than the picture, and the two effects that tissue experiences are worth separating clearly.

How Does Ultrasound Therapy Work?

Ultrasound therapy works through two mechanisms: a thermal effect, where absorbed sound energy raises the temperature of deep soft tissue, and non-thermal effects, where the vibration produces stable cavitation and acoustic streaming that stimulate cell activity. Which one dominates depends on how the machine is set.

Machine settings decide the balance. Continuous output above roughly 0.5 watts per square centimetre is used when heating is the goal, and the electrotherapy literature notes that raising tissue temperature into the 40 to 45 degree Celsius range produces increased local blood flow and may help resolve chronic inflammatory states. Collagen-rich tissues such as ligament, tendon, and fascia heat most efficiently, along with periosteum and fibrotic muscle.

Fibrotic muscle and collagen respond differently to the non-thermal side. Stable cavitation is the formation and growth of tiny gas bubbles within tissue fluid, and acoustic streaming is the small-scale eddying of fluid around those bubbles and around cell membranes. Together they alter membrane permeability and calcium transport, which effectively up-regulates cell activity. Contemporary accounts treat these non-thermal effects as the more clinically relevant of the two, and the idea of micromassage, once commonly cited, has little supporting evidence behind it.

What Is the Difference Between Pulsed and Continuous Ultrasound?

Continuous ultrasound delivers output without interruption and produces mainly thermal effects, while pulsed ultrasound switches on and off in a set ratio, reducing heating and favouring non-thermal effects. Pulsed mode is generally preferred for more acute presentations.

Acute presentations tolerate less heat, which is the practical reason for the choice. In pulsed mode the machine typically delivers output for 2 milliseconds followed by a rest period, with common ratios of 1 to 1 and 1 to 4. A 1 to 4 ratio means 2 milliseconds of output followed by 8 milliseconds of rest, which some manufacturers express as a 20 percent duty cycle instead.

Duty cycle, intensity, and treatment time together make up the dose, and neither mode achieves anything if the energy never reaches the target. Depth is where that question gets answered.

How Deep Does Ultrasound Therapy Penetrate?

Ultrasound therapy penetrates roughly 4 cm at 1 MHz and roughly 2 cm at 3 MHz, measured as the half value depth, meaning the depth at which only half the surface energy remains. Lower frequency travels deeper; higher frequency concentrates its effect nearer the surface.

Energy loss is exponential rather than linear, so there is no clean cut-off point, only a depth beyond which too little energy remains to do anything useful. The reference below draws on the half value depth figures published by Hoogland and reproduced across the standard electrotherapy texts.

Frequency	Average half value depth	Typical target depth	Best-absorbing tissues	Common applications
1 MHz	About 4.0 cm	Deeper structures under muscle bulk	Ligament, tendon, fascia, joint capsule, scar tissue	Hip, thigh, shoulder girdle, lumbar region
3 MHz	About 2.0 cm	Superficial structures close to the skin	The same collagen-rich tissues, nearer the surface	Hand, wrist, elbow, ankle, forefoot
Tissue absorption	Varies by protein content	Not applicable	High collagen absorbs well; fat and blood absorb poorly	Determines whether a target receives meaningful energy
Energy at depth, 1 MHz	50 percent at 4 cm	25 percent at 8 cm	Not applicable	Deep targets receive far less than the dial suggests
Energy at depth, 3 MHz	50 percent at 2 cm	25 percent at 4 cm	Not applicable	Not suitable for deep structures

Reading across those rows, a deep hip structure beneath a thick layer of soft tissue receives a small fraction of the intensity set on the machine, which is one reason results vary so much between body regions. Before any of that matters, the energy has to cross from the transducer into the skin at all.

Why Is Gel Used in Ultrasound Therapy?

Gel is used in ultrasound therapy because sound energy will not cross an air gap; without a coupling medium, roughly 99.998 percent of the energy reflects straight back off the skin surface. The gel eliminates the air and matches the acoustic impedance well enough for transmission to occur.

Transmission depends on impedance matching between the metal face of the transducer and the skin, and air is the worst possible intermediary. A good coupling medium is fluid enough to fill every space, viscous enough to stay put, and transmits the wave with minimal absorption of its own. Gel-based media currently perform better than oils and creams for this purpose, and the research shows little practical difference between the gels in common clinical use.

Clinical use also has a hygiene dimension, since published work recommends the treatment head be cleaned with an alcohol-based swab between patients rather than simply wiped. Once the energy is inside the body, where it ends up depends on what it meets.

Which Tissues Does Ultrasound Therapy Reach Best?

Ultrasound therapy reaches collagen-rich tissues best, meaning ligament, tendon, fascia, joint capsule, and scar tissue, because absorption rises with protein content and falls with water content. Fat and blood absorb very little, which means they transmit energy onward rather than responding to it.

Responding to energy is the whole point, since energy that is not absorbed produces no effect. This single mechanism explains a great deal about why trial results conflict. Studies applying ultrasound to poorly absorbing tissue, such as work on skeletal muscle regeneration after blunt contusion, have generally shown no benefit, while studies applying it to well absorbing tissue such as ligament have more often shown measurable effects.

Measurable effects in ligament and tendon are why the modality is most often aimed at those structures, and the list of conditions follows directly from the tissue list. Our own rehabilitation services group treatments by the tissue and the problem for the same reason: what a technique can physically reach determines whether it belongs in the plan.

What Conditions Does Ultrasound Therapy Treat?

Ultrasound therapy is used for soft tissue conditions involving tendon, ligament, fascia, and joint capsule, including tendinopathy, ligament sprains, plantar fasciitis, adhesive capsulitis, scar tissue restriction, and knee osteoarthritis. It is also applied to trigger points and, in a low-intensity form, to delayed and non-union fractures.

Fracture applications are a distinct branch. Low intensity pulsed ultrasound uses much smaller doses than standard therapy machines typically deliver, and the research on its role in fracture repair is substantial and separate from the general soft tissue literature.

The general soft tissue literature is where most patients encounter the modality, usually as one component alongside hands-on treatment and exercise rather than on its own. Many clinics pair it with manual work, in the same way that acupuncture or other adjuncts are layered into a plan rather than used in isolation. One claimed effect deserves closer examination than it usually receives.

Does Ultrasound Therapy Reduce Inflammation?

Ultrasound therapy does not reduce inflammation in the way an anti-inflammatory medication does; the evidence indicates it is pro-inflammatory, stimulating the cells that drive the inflammatory phase so that phase completes more efficiently. That is a meaningful distinction rather than a technicality.

The technical picture is that ultrasound stimulates mast cells, platelets, phagocytic white cells, and macrophages, increasing their activity rather than suppressing it. Studies that specifically set out to demonstrate an anti-inflammatory effect have failed to find one. Contemporary accounts describe the modality as an inflammatory optimiser: inflammation is a necessary stage of repair, and helping it run efficiently moves tissue into the proliferative phase sooner.

Moving through phases efficiently is also how the modality is said to influence remodelling, with some evidence that it supports the shift from Type III to Type I collagen and improves fibre orientation in maturing scar tissue. Whether any of that translates into a difference the patient notices is the question the evidence section takes up. First, what a session is actually like.

What Does Ultrasound Therapy Feel Like?

Ultrasound therapy usually feels like nothing at all, or at most a mild warmth and the sensation of a smooth transducer head moving over gel-covered skin. It should never be painful.

Pain during treatment is a signal to stop and adjust. On continuous settings some patients report gentle deep warmth building over the treatment time, while on pulsed settings most feel only the movement of the head. A sharp or aching sensation over bone can indicate the intensity is too high, or in some cases a stress fracture underneath, since higher-intensity ultrasound over a fracture line tends to provoke a strong pain response.

A strong response is uncommon, and most sessions pass without any sensation worth remarking on. Patients who want the more noticeable hands-on experience often prefer massage therapy alongside their rehabilitation for that reason. Session length follows a fairly standard pattern.

How Long Should You Do Ultrasound Therapy?

A single ultrasound treatment usually runs 5 to 10 minutes per area, and a course typically spans 6 to 12 sessions over several weeks depending on the condition and the response. Time is calculated from the size of the treatment area relative to the transducer face rather than chosen arbitrarily.

Arbitrary timing is a common criticism of how the modality is used in practice. Dose depends on frequency, intensity, duty cycle, treatment time, and the size of the area, and standard dose calculation methods exist for exactly this reason. A treatment area much larger than the sound head cannot receive an adequate dose within a realistic session length.

Realistic expectations also mean reassessing rather than continuing indefinitely. If a course has produced no measurable change after several sessions, the modality is not the answer for that presentation, which raises the question the whole topic turns on.

Is Ultrasound Physiotherapy Effective?

The evidence for ultrasound physiotherapy is mixed and, for most musculoskeletal conditions, weaker than its widespread use suggests, with the strongest recent support appearing for knee conditions. This deserves a straight answer rather than a promotional one.

The straight answer starts with how common the treatment is. Published research reported therapeutic ultrasound in use by about 94 percent of physical therapists in Canada and 65 percent in the United States, and the modality has been in clinical use for roughly six decades. A review of therapeutic ultrasound effectiveness studies published in Physical Therapy examined 35 randomized controlled trials and judged 10 to have acceptable methods. Of those 10, two suggested ultrasound outperformed placebo, for carpal tunnel syndrome and calcific tendinitis of the shoulder, and eight suggested it did not.

Later work pointed the same way. A systematic review of ultrasound for musculoskeletal disorders found that the large majority of 13 placebo-controlled trials with adequate methods showed no clinically important difference favouring ultrasound, and a literature review of lower limb conditions found that none of its six placebo-controlled trials produced a statistically significant difference between true and sham treatment. Two earlier studies went further, concluding that the reductions in pain and swelling they measured reflected a major placebo effect.

The picture is not uniformly negative. A 2024 systematic review and meta-analysis published in Therapeutic Advances in Chronic Disease found ultrasound therapy significantly more effective than comparison interventions for knee disorders, while finding both ultrasound and the comparisons ineffective for shoulder disorders. Guidelines recommend against its use for low back pain. Taken together, the sensible reading is that ultrasound may help in specific conditions and specific tissues, and should not be expected to carry a recovery on its own. That is how our physiotherapists weigh any passive modality: useful where the evidence supports it, never as a substitute for the active work.

Is Ultrasound Therapy Good for a Torn Meniscus?

Ultrasound therapy is not an effective treatment for a torn meniscus, because the tear is a mechanical disruption inside the joint that sound energy cannot repair. Reflection at the cartilage surface also means most of the energy never enters the structure.

Entering the structure is the barrier. Although cartilage sits high on the absorption scale by protein content, the impedance difference at its surface causes most of the arriving energy to reflect away. What ultrasound is sometimes used for around a meniscal injury is the surrounding soft tissue irritation rather than the tear itself.

The tear itself is managed through loading progression, quadriceps and hip strengthening, and in some cases surgical opinion. Where joint mechanics elsewhere in the chain are contributing, chiropractic care is sometimes coordinated alongside rehabilitation. Arthritis is the condition where the recent evidence looks different.

Is Ultrasound Therapy Good for Arthritis?

Ultrasound therapy shows the most promising evidence for knee osteoarthritis of any musculoskeletal application, though it remains an adjunct to exercise rather than a primary treatment. The 2024 meta-analysis in Therapeutic Advances in Chronic Disease found it significantly more effective than comparison interventions for knee disorders.

Knee disorders responded where shoulder disorders did not, and the same analysis found both ultrasound and its comparison treatments ineffective for the shoulder. That split is consistent with the tissue absorption picture, since knee structures include accessible collagen-rich tissue relatively close to the surface.

Relatively accessible tissue makes the knee a reasonable target, but arthritis management still rests on strengthening, load management, and activity modification. Where lower limb alignment is loading the joint unevenly, custom orthotics address a mechanical contributor that no modality can reach. Beyond individual conditions, the modality carries some general limitations.

What Are the Limitations of Ultrasound Physiotherapy?

The limitations of ultrasound physiotherapy are shallow effective depth, poor absorption in many target tissues, dose that is difficult to standardize, machine calibration variability, and an evidence base that does not support it as a standalone treatment. None of these makes it useless; together they explain why results are inconsistent.

Inconsistency has a documented technical component. The ultrasound beam is not uniform, and the beam nonuniformity ratio for most applicators sits around 4 to 6, meaning peak intensity within the beam can be four to six times the mean intensity displayed on the machine. Published work has also raised concerns about the calibration accuracy of therapy machines in clinical use, and about variability in effective radiating area between transducers.

Variability between machines means two clinics delivering what appears to be an identical dose may deliver quite different energy. Add the angle sensitivity, where holding the head more than about 15 degrees off the skin plane sends most of the beam along the skin rather than into it, and the scope for unintended under-treatment becomes clear. Separate from effectiveness, there are situations where the modality should not be used at all.

What Are the Contraindications for Ultrasound Therapy?

The contraindications for ultrasound therapy include treatment over malignancy, over the pregnant uterus, over active infection, over implanted electronic devices, over areas with impaired sensation or circulation, and directly over recent deep vein thrombosis. Several site-specific precautions apply alongside these.

• Over or near a known or suspected malignancy. Standard absolute contraindication.
• Over the abdomen, pelvis, or lower back during pregnancy. Applies throughout pregnancy.
• Over active infection or an area of acute sepsis. Treatment risks spreading the process.
• Over a pacemaker, implanted stimulator, or other electronic device.
• Over an area with significantly impaired sensation. The patient cannot report excessive heating.
• Over compromised circulation or a recent deep vein thrombosis.
• Over the eyes, the heart, reproductive organs, or the spinal cord after laminectomy.
• Over active growth plates in children, and over recently irradiated tissue.

Growth plates and irradiated tissue are the two most often overlooked on that list. Any clinician using the modality should screen against all of it before the first application, which is standard practice in any regulated setting. Patients weighing their options also ask how it compares to the alternatives.

Is Ultrasound or Laser Therapy Better?

Neither ultrasound nor laser therapy is universally better; they deliver different forms of energy, reach tissue differently, and carry different evidence bases, so the appropriate choice depends on the tissue and the condition. Both are passive modalities intended to support active rehabilitation rather than replace it.

Replacing active care is the mistake neither modality survives. Ultrasound delivers mechanical sound energy that is absorbed preferentially by collagen-rich tissue, with effective depth limited by frequency. Laser therapy delivers light energy absorbed by cellular structures, with penetration determined by wavelength and power class. At our clinic, the deep-tissue modality we use is K-Laser therapy, which we apply for chronic back and neck pain, sports injuries, and stubborn musculoskeletal conditions alongside hands-on treatment.

Alongside hands-on treatment is the operative phrase for any modality, and it points to the question that matters more than which device is chosen.

Where Passive Modalities Fit in a Treatment Plan

Passive modalities fit as supporting tools inside a plan built on assessment, manual therapy, and progressive exercise, not as the core of treatment. They can reduce pain and irritability enough to let the active work proceed, which is a genuine contribution, but the active work is what produces lasting change.

Lasting change follows a consistent order of priorities:

1. Assessment first. Identify which tissue and which movement pattern is actually producing the problem before anything is applied to it.
2. Address the mechanical cause. Load management, movement correction, and manual therapy where joint or soft tissue restriction is limiting movement.
3. Restore range and control. Regain the movement available before trying to load it.
4. Build capacity with progressive loading. Strength and tolerance for load are what prevent recurrence, and nothing passive substitutes for them.
5. Use modalities to remove obstacles. Where pain or irritability is blocking progress, a passive modality can create the window for loading to continue.
6. Reassess and adjust. Measure against the baseline and change the plan when the numbers stall.

Numbers stalling is the signal to change approach rather than to add another modality on top. Our plans are built around that sequence: a physiotherapy assessment to find the cause, then manual therapy and progressive exercise rehabilitation as the core of the work.

The core of the work is where we put the time at our Markham clinic, which is why physiotherapy in Markham here looks like assessment, hands-on treatment, and a home programme rather than a room full of machines.

Frequently Asked Questions

What Should You Not Do Before Ultrasound Therapy?

Before therapeutic ultrasound, avoid applying lotions, creams, or topical medications to the treatment area, since they interfere with the coupling gel and with energy transmission. Tell your clinician about pregnancy, implanted devices, recent radiation therapy, or any loss of sensation in the area. The fasting and full-bladder instructions people associate with ultrasound apply to diagnostic scans, not to this treatment.

Where Should You Not Use Ultrasound Therapy on the Body?

Ultrasound therapy should not be applied over the eyes, the heart, reproductive organs, the pregnant abdomen or lower back, active growth plates in children, the spinal cord after laminectomy, or over malignancy, infection, or implanted electronic devices. Areas with impaired sensation or circulation are also avoided because the patient cannot reliably report excessive heating. Screening for these is part of standard practice.

Can You Have Ultrasound Therapy at Home?

Home ultrasound units exist but deliver lower intensities than clinical machines and come without the assessment that determines whether the modality suits your condition at all. Dose depends on frequency, intensity, duty cycle, time, and treatment area, and getting that wrong means either no effect or unnecessary heating. Given how mixed the clinical evidence is, home units are a poor substitute for a properly assessed treatment plan.

How Many Ultrasound Therapy Sessions Are Typical?

A typical course runs 6 to 12 sessions over several weeks, though the number depends entirely on the condition and the response. If there has been no measurable change in pain, range of motion, or function after several sessions, continuing is rarely justified. Reassessment at that point should look at whether the diagnosis or the overall approach needs revising.

Does Ultrasound Therapy Hurt?

Ultrasound therapy should not hurt. Most patients feel nothing beyond the movement of the transducer, and some feel mild warmth on continuous settings. Sharp or aching pain during treatment, particularly over bone, means the intensity should be reduced or the treatment stopped, and in some cases it can indicate an underlying stress fracture.

The Bottom Line

US in physiotherapy means therapeutic ultrasound, a treatment that delivers high-frequency mechanical sound energy, typically 1 to 3 MHz, through a coupling gel and a handheld transducer into soft tissue. It works through gentle deep heating on continuous settings and through cavitation and acoustic streaming on pulsed settings, and it reaches collagen-rich tissues such as ligament, tendon, fascia, and joint capsule far better than it reaches fat or muscle bulk. Effective depth is limited, roughly 4 cm at 1 MHz and 2 cm at 3 MHz, and beam uniformity and machine calibration both introduce real variability into what a patient actually receives.

The research, taken honestly, is mixed. Multiple reviews of placebo-controlled trials found no clinically important advantage over sham treatment for most musculoskeletal conditions, while a 2024 meta-analysis did find a significant benefit for knee disorders and none for the shoulder, and guidelines advise against its use for low back pain. The reasonable conclusion is that passive modalities of any kind are supporting tools rather than the core of a recovery. At KC Rehab in Markham, the core is assessment, hands-on treatment, and progressive loading, with modalities used where they genuinely clear an obstacle. Whenever you want to know what is actually driving your problem and what will fix it, you are welcome to book an assessment.