Electrical Therapy Principles (Lecture Slides)

Overview

This lecture explores the foundational physics and clinical principles behind therapeutic electrical stimulating currents in athletic training, emphasizing how to select appropriate parameters for targeted physiological responses and safe application.

Basic Electrical Concepts in Therapy

• Therapeutic currents modify wall outlet power to achieve physiological effects in tissues.
• Electrical current is the net movement of charged particles, measured as flow (amperes) driven by voltage (volts) and opposed by resistance (ohms).
• Ohm's Law: Current = Voltage / Resistance; more voltage means more current, more resistance means less current.
• The skin is the primary resistor to electrical current in the body.

Tissue Conductivity Hierarchy

• Blood conducts electricity best due to high water/ion content, followed by peripheral nerves, muscle, tendon, fat, and bone (worst conductor).
• Current flows easiest through tissues with least resistance, affecting targeting strategies.

Types of Therapeutic Currents

• Direct Current (DC): Unidirectional, used for iontophoresis, creates chemical tissue effects.
• Alternating Current (AC): Bidirectional, continuous reversal, no net ionic movement, safer for long use.
• Pulsatile Current (PC): Brief pulses separated by off periods, can be uni- or bidirectional; includes HVPC and Russian currents.

Waveforms & Parameters

• DC is monophasic (one phase); AC and PC are biphasic (two phases).
• Symmetric biphasic means no net charge; asymmetric/unbalanced can create net DC effects.
• Key parameters: intensity (amplitude), pulse duration (width), and frequency (Hz or PPS).

The Strength-Duration (SD) Curve

• SD curve describes nerve excitability: Shorter pulse durations require higher intensity for activation.
• Different nerve fibers (sensory, motor, pain) have unique SD curves, enabling selective targeting.
• Typical recruitment order: sensory (A-beta), then motor, then pain (A-delta/C fibers).

Pulse Frequency & Contraction Type

• Low frequency (<50 PPS): Individual muscle twitches.
• High frequency (≥50 PPS): Fused, sustained (tetanic) contractions, suitable for strengthening but increase fatigue.

Modulation and Comfort

• Burst modulation: Brief AC/PC bursts (e.g., Russian current).
• Beat modulation: Two ACs interact (e.g., IFC) to create a therapeutic beat frequency.
• Ramping modulation: Gradually increases current for comfort during muscle contraction.
• Polarity: Cathode (negative electrode) is more effective for depolarizing nerves; anode hyperpolarizes.

Electrode Setup & Current Density

• Current density is greatest under smaller electrodes; monopolar setups focus effect under active electrode.
• Wider electrode spacing increases depth of current penetration.

Clinical Applications

• Muscle strengthening: High intensity, high frequency (50-85 PPS), long off-times, using HVPC or Russian current.
• Edema reduction: Moderate frequency (35-50 PPS), equal on/off time, limb elevation.
• Sensory pain modulation (conventional TENS): High frequency (80-125 PPS), low intensity for tingling.
• Motor TENS/low-frequency: Low frequency (<20 PPS), high intensity for muscle contraction/pain relief via endogenous opioids.
• Noxious TENS: Very low frequency, high intensity for long-lasting opioid-mediated relief.

Evidence & Specialized Currents

• Mixed evidence for TENS and IFC in chronic pain; usually higher for acute or in addition to other therapies.
• Russian current: Medium frequency for muscle strengthening, designed to overcome skin resistance.
• Microcurrent therapy: Sub-sensory intensity for biostimulation; mostly theorized for tissue healing, with weak clinical evidence.

Safety Considerations

• Use grounded (three-prong) equipment, especially near water.
• Water reduces skin resistance, increasing shock risk; GFIs (ground fault interrupters) are mandatory in wet areas.
• Avoid extension cords and ensure all equipment is regularly checked for faults.

Key Terms & Definitions

• Ohm’s Law — Current = Voltage ÷ Resistance.
• Strength-Duration Curve (SD curve) — Graph shows threshold intensity vs. pulse duration for nerve activation.
• Tetanic Contraction — Sustained muscle contraction due to high-frequency stimulation.
• Gate Control Theory — Stimulation of sensory fibers can inhibit transmission of pain signals.
• Ground Fault Interrupter (GFI) — Device that cuts power to prevent electrical shock in wet areas.
• Current Density — Current per unit area under an electrode.

Action Items / Next Steps

• Review the strength-duration curve for major nerve types.
• Practice selecting appropriate parameters for different clinical goals.
• Review safety protocols for electrotherapy equipment, especially in hydrotherapy settings.