In physical therapy, ultrasound is a treatment that uses high-frequency sound waves to deliver heat and mechanical energy deep into soft tissues. A therapist applies a handheld device to your skin, and the sound waves penetrate below the surface to warm muscles, tendons, and ligaments, or to stimulate cellular repair processes depending on the settings used. It’s one of the most common modality treatments in outpatient physical therapy, though its effectiveness varies by condition.
How Ultrasound Works on Your Tissues
Therapeutic ultrasound produces two categories of effects: thermal and non-thermal. Which one your therapist is targeting determines how the machine is set up.
The thermal effect is straightforward. Sound waves vibrate tissue molecules rapidly, generating friction and heat deep below the skin. This raises the temperature in targeted muscles, tendons, or joint capsules, which increases blood flow to the area, improves the stretchability of collagen-rich structures like scar tissue, and can temporarily reduce pain and stiffness. The heating happens at depths that a hot pack simply can’t reach.
The non-thermal effects are more complex. As sound waves travel through tissue, they create tiny pressure changes that push fluid along and around cell membranes, a process called acoustic streaming. These same pressure waves cause microscopic gas bubbles naturally present in tissue fluid to expand and contract rhythmically. This gentle pulsation, known as stable cavitation, appears to stimulate cells at a mechanical level. Early research suggests this process mildly disrupts cells, triggering a recovery response that includes increased protein production, which is part of how damaged tissue rebuilds itself.
Thermal vs. Pulsed Settings
When the goal is deep heating, your therapist will use continuous mode, where the sound energy runs at 100% duty cycle with no breaks. This is common for chronic tightness, joint stiffness, or scar tissue that needs to be stretched.
When the goal is tissue repair rather than heating, pulsed mode is used. The machine cycles the sound energy on and off, typically at a 20% duty cycle (on for one unit of time, off for five). This dramatically reduces heat buildup while still delivering the mechanical, cell-level stimulation that supports healing. A 50% duty cycle sits in the middle, producing some warmth but less than continuous mode.
Frequency also matters. A 1 MHz setting penetrates deeper, reaching tissues roughly 2.3 to 5 centimeters below the skin surface. This is the setting for larger joints, deeper muscles, and thicker tissue layers. A 3 MHz setting targets more superficial structures, from about 0.8 to 1.6 centimeters deep, making it better suited for tendons, ligaments, and tissues closer to the surface. Research published in the Journal of Athletic Training found that 3 MHz ultrasound can heat tissues at 2.5 centimeters deep more effectively than previously believed, which means there’s some overlap between the two frequencies for medium-depth targets.
Conditions Commonly Treated
Therapists use ultrasound for a range of musculoskeletal problems. It’s frequently applied to myofascial pain syndrome (tight, painful knots in muscles), back pain, knee osteoarthritis, acute ankle sprains, and hip pain. It also sees regular use for tendinitis, frozen shoulder, and areas where scar tissue limits movement after surgery or injury.
One specialized application is phonophoresis, where ultrasound is used to push anti-inflammatory or pain-relieving medication through the skin and into deeper tissues. Instead of regular coupling gel, the therapist applies a medicated cream to the treatment area and then uses the ultrasound transducer over it. The sound waves help drive the drug into the subcutaneous tissue. This technique has been studied with several anti-inflammatory medications and local anesthetics, with variable results depending on the drug and condition being treated.
What a Session Feels Like
Before treatment, a water-based coupling gel is applied to your skin. This gel fills any tiny air pockets between the transducer and your body, since ultrasound waves reflect off air and wouldn’t reach the tissue without it. The therapist then moves the transducer head slowly in small circles or strokes over the treatment area, keeping it in constant motion to prevent hot spots from forming in one location.
Sessions typically last 5 to 10 minutes per area, with intensity set between 0.5 and 2 watts per square centimeter depending on the condition, the depth of the target tissue, and whether the goal is heating or repair. During continuous mode, you’ll feel a gentle warmth that builds gradually. It should never feel hot or sharp. During pulsed treatments, you may feel very little or nothing at all. The treatment is painless, and no adverse events were identified in a large study of 225 participants.
What the Evidence Says About Effectiveness
This is where the picture gets more nuanced. Therapeutic ultrasound has been a staple in physical therapy clinics for decades, but the research supporting it is mixed, particularly for certain conditions.
For chronic low back pain, a Cochrane review found low-certainty evidence that ultrasound has a small effect on back-specific function compared to a sham (placebo) treatment in the short term, but the effect was not considered clinically meaningful. The same review found that ultrasound probably makes little to no difference to overall well-being, patient satisfaction, or the likelihood of achieving a 30% or greater reduction in pain within three months. The reviewers concluded there is little to suggest ultrasound is an effective standalone treatment for nonspecific chronic low back pain.
That doesn’t mean ultrasound is useless across the board. Its evidence base is stronger for some applications than others, and many therapists use it as one component of a broader treatment plan rather than as a standalone fix. The deep heating effect, for instance, can make a subsequent stretching or manual therapy session more effective by warming stiff tissues first. Used this way, ultrasound serves as a preparation tool rather than the primary treatment.
Who Should Not Receive Ultrasound
Therapeutic ultrasound is considered low-risk, but several situations rule it out entirely. It should not be applied over areas of active cancer or tumors, since increased blood flow and cellular stimulation could theoretically promote tumor growth. Other contraindications include:
- Pregnancy: ultrasound is not applied to the abdomen or low back of pregnant women
- Pacemakers or implanted electrical devices: the energy could interfere with device function
- Growth plates in children: this is one of the most frequently cited contraindications in the clinical literature, since disrupting growth plates could affect bone development
- Deep vein thrombosis: increased circulation could dislodge a blood clot
- Burns, open wounds, or healing skin injuries: these are considered absolute contraindications
- Impaired sensation: if you can’t feel heat in the area, you can’t provide feedback about whether the intensity is too high
Ultrasound is also generally avoided during the acute inflammatory phase of an injury when using continuous (thermal) mode, since adding heat to freshly inflamed tissue can make swelling worse. Pulsed mode at low intensity may still be appropriate in early healing stages, but that’s a clinical judgment your therapist makes based on your specific situation.