6.2 Electrical Stimulation Modalities

Electrical Stimulation Modalities

Electrical stimulation (e-stim) items on the NPTE-PTA reward two skills: (1) selecting the correct modality and parameter set for a stated goal, and (2) recognizing contraindications. The same physical hardware can deliver pain control, muscle contraction, edema reduction, or drug delivery depending on settings. The three settings that change everything are pulse rate (frequency, in Hertz), phase duration (in microseconds), and waveform.

TENS: Transcutaneous Electrical Nerve Stimulation

TENS targets pain. Two parameter families dominate the exam.

If the POC says "chronic low back pain, wearable unit for home use," low-rate TENS is a strong fit because the endorphin-mediated relief carries over after the unit is off. If the POC says "acute postoperative pain, immediate sensory relief," high-rate TENS is the better choice because gate-control analgesia begins within minutes.

NMES: Neuromuscular Electrical Stimulation

NMES produces a tetanic muscle contraction to address weakness, atrophy, motor re-education, or spasticity management.

• Waveform: symmetrical or asymmetrical biphasic pulsed current; alternatively, 2500 Hz medium-frequency burst-modulated current ("Russian stimulation").
• Pulse rate: 35-80 Hz to achieve fused tetanus.
• Phase duration: 200-400 microsec for large muscles such as the quadriceps.
• On/off ratio: start at 1:5 (e.g., 10 sec on / 50 sec off) to limit fatigue; progress toward 1:1 with strengthening tolerance.
• Ramp: 1-3 second ramp up and down to make contractions comfortable.
• Intensity: to a strong, visible contraction the patient tolerates.

Classic NMES applications include post-operative quadriceps activation after Total Knee Arthroplasty (TKA) or Anterior Cruciate Ligament (ACL) reconstruction, shoulder subluxation after stroke, and dorsiflexor activation for foot drop. Functional electrical stimulation (FES) is NMES timed to a functional task, such as triggering dorsiflexion during the swing phase of gait.

IFC: Interferential Current

IFC crosses two medium-frequency channels to generate a therapeutic low-frequency "beat" in the tissue where the channels intersect. Because the carrier frequency is high (around 4000 Hz), skin resistance (impedance) is lower and patients tolerate higher intensities than with direct low-frequency stimulation. IFC is commonly used for deep pain control and edema management, with four electrodes arranged so the target tissue lies at the crossing point of the two circuits. The beat frequency equals the difference between the two channels: a 4000 Hz channel and a 4100 Hz channel produce a 100 Hz beat.

A beat near 100-150 Hz favors sensory pain control; a low beat of 1-10 Hz can drive a pumping muscle contraction for edema. Premodulated IFC delivers the same crossing internally and needs only two electrodes, useful for small areas.

High-Volt Pulsed Current (HVPC) and Microcurrent

High-volt pulsed current (HVPC) uses twin monophasic pulses at high voltage (over 100 V) with a very short phase duration. It is favored for wound healing and edema control. Polarity matters: the negative (cathode) electrode is often used early to promote autolytic debridement and bacteriostasis, while the positive (anode) supports later epithelialization. For acute edema, the cathode placed over the swollen tissue creates a like-charge repulsion of negatively charged plasma proteins. Microcurrent (sub-sensory, in microamps) is sometimes used for tissue healing but has weaker evidence.

Iontophoresis

Iontophoresis uses continuous direct current to push ionized medications across the skin. The key NPTE-PTA points:

• The drug ion and the delivery electrode must share the same polarity ("like repels like").
• Dexamethasone phosphate is negatively charged and is therefore placed under the negative (cathode) electrode for delivery.
• Lidocaine is positively charged and is delivered from the positive (anode).
• Typical dose is 40 milliamp-minutes (e.g., 4 mA for 10 minutes, or 2 mA for 20 minutes).
• Skin irritation and chemical burns are more common under the cathode; inspect skin before and after treatment.

Electrode Placement Principles

• Use the largest electrodes the area allows; current density (and therefore intensity of sensation) rises as electrode size falls.
• Maintain at least one electrode-width of skin between electrodes to prevent the current from concentrating at the surface.
• For NMES strengthening, place electrodes over the muscle belly and motor point, parallel to muscle fibers.
• For TENS pain control, options include over the painful area, surrounding it, on the dermatome, on the contralateral side, or at trigger points.

Contraindications

A pacemaker is an absolute contraindication for most clinical settings; some current guidelines allow distal-extremity TENS in selected patients with cardiology clearance, but on the NPTE-PTA, pick the option that withholds e-stim and contacts the supervising PT.