15.2 Electrotherapy & Therapeutic Ultrasound

Quick Answer: Conventional TENS (high rate, sensory) uses gate control. Low-rate TENS (about 1–10 Hz, motor twitch) uses endogenous opioids. Dexamethasone is negative—put it on the cathode. 1 MHz ultrasound is deeper (2–5 cm); 3 MHz is superficial (1–2 cm). Trap: TENS on a demand pacemaker, or a stationary high-intensity 1 MHz head on bone.

PA8 task 0404 includes electrical stimulating currents and acoustic energy. The exam tests parameters you can defend and contraindications you never skip. It does not require you to claim that ultrasound heals every tendon.

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TENS: two mechanisms, two recipes

Transcutaneous electrical nerve stimulation (TENS) is for pain, not for a 10-repetition-maximum quad set.

Gate control (Melzack and Wall, as commonly taught): large-diameter A-beta (touch/pressure) input closes the substantia gelatinosa 'gate' and reduces nociceptive traffic to the brain. That is conventional / high-rate / sensory TENS:

Burst TENS packages low-rate-like bursts into a more comfortable waveform; think opioid-style effects with less sting. Electrode placement: around the painful site, along the dermatome, or at related trigger/acupuncture points. Bipolar is common; interferential current (IFC) is the 'bigger, deeper TENS' cousin.

IFC: two medium-frequency alternating currents (classically ~4000 Hz and ~4100 Hz) interfere in the tissue. The beat frequency is the difference (1–150 Hz) and is what you dose like a TENS frequency. Medium frequency meets less skin impedance, so athletes often tolerate it over a larger area (low back, thigh). Quadripolar setup produces a cloverleaf field; vector scan can 'stir' the field. Use IFC for pain (and, in some protocols, edema) with the same systemic contraindications as TENS.

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NMES and Russian current: make muscle fire

Neuromuscular electrical stimulation (NMES) is for re-education and strengthening when the athlete cannot volitionally recruit (arthrogenic inhibition after ACL reconstruction or TKA is the classic AT example). You need a tetanic contraction, not a sensory tingle.

Commonly taught NMES parameters:

• Frequency about 35–80 Hz (50 Hz is a workhorse) for tetany
• Pulse duration about 200–400 μs for large muscles
• On:off about 1:5 for strength (for example 10 s on / 50 s off) so the muscle can recover; shorter rest if the goal is endurance or a pump, accepting more fatigue
• Ramp 1–2 s so the contraction is not a slap
• Amplitude to a strong, tolerated contraction in the target muscle

Russian current (Kots, as taught): 2500 Hz carrier burst-modulated at 50 bursts/s, classically 10/50/10 (10 s contraction, 50 s rest, 10 minutes). Use it when the unit offers it for quad re-ed. It is still electricity across the chest and neck—pacemaker and carotid rules still apply.

Place electrodes over the motor point and along the muscle belly, not over the carotid sinus, not across the heart, and not on broken skin. Combine NMES with voluntary effort when the athlete can—the goal is a brain-muscle loop, not a passive twitch show.

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Iontophoresis: polarity is the exam item

Iontophoresis uses direct current (DC) to repel a charged drug through the skin. Like charges repel. The electrode that holds the drug is the active electrode.

Dexamethasone sodium phosphate is negatively charged at physiologic pH. Place it under the cathode (−). Delivery from the anode is the historical error (Glass mixed competing ions and used the anode); modern teaching and in-vitro data (Kalia and colleagues) favor the cathode. Lidocaine is positive (anode). Acetic acid is negative (calcium deposits, as taught).

Dosage is mA × minutes. Commonly taught target: about 40–80 mA·min (example: 4 mA × 20 min = 80). Current is usually 1–4 mA, limited by electrode size (current density) and sting. DC causes electrochemical burns if you over-dose, leave a dry electrode, or treat broken skin. Do not mix dexamethasone and lidocaine in the same electrode 'to cancel charges'—you add competing ions and wreck delivery.

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Therapeutic ultrasound: frequency, duty cycle, BNR, ERA

Ultrasound is high-frequency sound (in AT units, 1 MHz or 3 MHz). The crystal vibrates; the beam is absorbed more in protein-rich tissue (tendon, ligament, muscle) than in fat, and bone and nerve absorb still more—which is why a stationary head over the malleolus is a periosteal-pain machine.

Draper/Lehmann-style heating-rate teaching (textbook-conventional, not a BOC-stamped law): at 1 MHz, about 0.2°C/min per 1.0 W/cm²; at 3 MHz, about 0.6°C/min per 1.0 W/cm². Thermal goals often taught: +1°C metabolism, +2–3°C blood flow/spasm, +4°C collagen extensibility.

Coupling is mandatory. Air reflects almost all of the beam. Use gel, or underwater treatment about 1 cm from irregular bony contours (wipe bubbles). Move the soundhead in overlapping circles at about 4 cm/s. A stationary head, especially 1 MHz, high intensity, over bone, creates a standing-wave hot spot.

Precautions / do-not-treat fields: eyes, heart, pregnant uterus, malignancy, DVT, pacemaker/electronics in the field, exposed CNS after laminectomy, acute fracture (thermal US; low-intensity pulsed US for fractures is a different, device-specific prescription), and thermal dose over open growth plates (precaution—do not cook an epiphysis). Cemented or plastic implants can overheat with thermal US—know what is in the limb.

Evidence honesty: therapeutic ultrasound parameters are still taught because units are everywhere and the physics is testable. For many musculoskeletal conditions, clinical-outcome evidence is mixed or modest (Cochrane and subsequent reviews). Do not claim US 'heals' a chronic rotator cuff. Phonophoresis (US plus hydrocortisone in the gel) has limited evidence that meaningful drug reaches the target—do not treat it as a steroid injection. Low-level laser therapy (LLLT) evidence for common AT diagnoses is mixed; if you use it, protect the eyes and do not claim FDA-untrue cures. Shortwave diathermy heats large volumes (27.12 MHz is the classic ISM frequency); metal, pregnancy, and pacemakers are hard stops. Many AT clinics no longer own a diathermy unit; know the contraindications anyway.

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Shared electrical and US contraindications, and the traps

Do not electrically stimulate:

• Demand pacemaker / implanted defibrillator (and generally implanted stimulators in the field)
• Carotid sinus (dysrhythmia, hypotension)
• Pregnancy over the abdomen or low back
• DVT / thrombophlebitis
• Malignancy in the field
• Active hemorrhage, over the eyes, through the chest in unstable cardiac disease, and over broken skin for DC (iontophoresis)

Exam traps: (1) TENS on a patient with a demand pacemaker—sensory intensity does not make it safe. (2) Stationary 1 MHz at high intensity over a bony prominence. (3) Putting dexamethasone on the anode. (4) Treating phonophoresis as proven transdermal steroid delivery.