Participants will receive one session of Transcutaneous Electrical Nerve Stimulation (TENS) applied over the radial nerve using surface electrodes. Stimulation parameters: 2 Hertz (Hz) frequency, 250 microseconds (µs) pulse width, and intensity set at the individual tolerance threshold for 20 minutes.

Participants will receive one session of Percutaneous Electrical Nerve Stimulation (PENS). A sterile 0.30 × 50 millimeter (mm) needle will be inserted near the radial nerve under ultrasound guidance. Stimulation parameters: 2 Hertz (Hz) frequency, 250 microseconds (µs) pulse width, and intensity set at the individual tolerance threshold for 20 minutes.

Participants will undergo a sham Transcutaneous Electrical Nerve Stimulation (TENS) procedure identical in setup and duration to the active TENS group. Electrodes will be placed in the same positions, and the current intensity will be increased for about 30 seconds until a strong but comfortable tingling is felt, then gradually reduced to 0 milliamperes (mA). The electrodes will remain in place for 20 minutes to mimic the active procedure. Neither participants nor evaluators will have visual access to the device display.

Participants will also receive a placebo needle intervention, consisting of a non-penetrating sham needle applied to the same anatomical region without skin insertion. This procedure mimics the sensation of needling but does not break the skin or produce physiological effects.

Participants will receive one 20-minute session of Transcutaneous Electrical Nerve Stimulation (TENS) applied over the radial nerve of the non-dominant forearm. Two surface electrodes will be placed under ultrasound guidance (General Electric (GE) Logiq P9), longitudinally along the nerve path, approximately 5-8 centimeters (cm) proximal and distal to the lateral epicondyle. Stimulation parameters will consist of a biphasic symmetrical current at 2 Hertz (Hz) frequency, 250 microseconds (µs) pulse width, and an intensity adjusted to the maximum tolerable level without pain. The stimulator used will be an ENRAF NONIUS Endomed 484 (Madrid, Spain). Participants will remain in a supine position with the forearm supported in a prone posture. In addition, participants will receive a placebo needle intervention, consisting of a non-penetrating sham needle applied to the same anatomical region without skin insertion. This procedure mimics the sensation of needling but does not break the skin o

Description: It will be assessed as an indirect measure of the effect on the Aδ sensory fibers of the radial nerve. To record the mechanical pain threshold, a point will be marked on the dorsal surface of the trapeziometacarpal joint. A digital algometer with an increment scale of 0.1 Newtons (N) (Wagner Instruments, model FDIX) and a circular probe of 1 centimeter (cm) in diameter will be used. Once the probe is placed perpendicular to the skin, pressure will be increased at a rate of approximately 5 Newtons per second (N/s). Three measurements will be taken, with 15-second intervals between them, and the mean of the three values will be recorded as the pressure pain threshold. The reproducibility and reliability of pressure algometry to measure pressure pain threshold have been demonstrated in previous studies.

Thermal pain threshold will be assessed as an indirect measure of the effect on unmyelinated C-type sensory fibers of the radial nerve. Hot stimuli will be applied to the dorsal area of the second metacarpal (radial nerve dermatome) using a 9 square centimeters (cm²) Peltier thermode (TSA 2, MEDOC, Israel). Three ascending temperature ramps will be performed with one-minute intervals. The heat pain threshold will be determined by the method of limits, and the average of the three measurements will be recorded. Baseline temperature: 32 degrees Celsius (°C); increase rate: 1 degree Celsius per second (°C/s); decrease rate: 3 °C/s. Participants will press a button with the contralateral hand when the heat becomes painful, triggering automatic cooling. For safety, a 50 °C cutoff temperature will be used. Prior to testing, a familiarization trial will be conducted on the dominant hand over the first and second metacarpals.

It will be assessed as an indirect measure of the blockade of motor fibers (Aα motoneurons) of the radial nerve. Maximal isometric wrist extensor strength will be measured using a hand-held dynamometer (MicroFET 2, Hoggan Scientific, USA) following Méndez-Rebolledo et al. protocol. Subject position: supine, shoulder at 0° abduction, elbow at 0° flexion, forearm fully pronated, and wrist at 30° flexion with fingers extended. Evaluator position: standing, thighs touching the table; upper hand stabilizes the forearm above the styloid processes, lower hand holds the dynamometer over the distal third of the metacarpals. Stabilization: provided by the upper hand. Adapter: large flat type. Dynamometer placement: over metacarpal heads. Execution: three maximal isometric contractions (kilogram-force (kgf)) of at least 5 seconds each, with 30 seconds rest between them. The mean of the three trials will be used as the final strength value.

Questionnaire for adverse events

The unpleasantness of the intervention will also be assessed using a 10 centimeters (cm) visual analogue scale (VAS), where 0 represents 'not unpleasant at all' and 10 represents 'the most unpleasant experience imaginable'.