This randomized controlled trial investigates the acute effects of transcutaneous electrical nerve stimulation (TENS) on passive cervical muscle stiffness (MyotonPro "dynamic stiffness", N/m) and pain (0-10 VAS) in adults with chronic/mechanical neck pain. Participants will be allocated 1:1 to TENS or control/sham. Outcomes are measured immediately before and immediately after a single 15-minute session.

Background/Rationale: TENS modulates superficial afferent input and may reduce pain and muscle tone. However, its immediate impact on objective passive muscle stiffness measured by a handheld myotonometer (MyotonPro) in neck pain is insufficiently characterized. This trial tests whether a brief, low-risk intervention produces measurable changes in mechanical muscle properties and perceived pain in a short time window.

Objectives/Hypotheses:

Primary objective: Determine whether conventional TENS acutely reduces dynamic stiffness of the upper trapezius compared with control/sham.

Secondary objectives: Evaluate pain (VAS), additional MyotonPro parameters (frequency/tone, logarithmic decrement, relaxation time, creep), and safety/tolerability.

Hypothesis: TENS will show a significant group × time interaction favoring TENS over control/sham.

Design: Single-center (optionally multi-center), parallel-group, randomized (1:1) RCT. Masking: at minimum outcome assessor-blinded; participant blinding if sham TENS is used.

Sample Size: n = 24 (12 per arm), based on a priori power analysis (f≈0.40, α=0.05, power=0.95) for a 2×2 mixed design.

Intervention (TENS): Conventional mode; ~80 Hz, ≤150 µs pulse width; 15 minutes; intensity "strong but tolerable" without visible muscle contraction. Electrodes placed over the most painful/stiff segments (typically upper trapezius and/or suboccipital region), preferably bilateral.

Control/Sham: No intervention or sham TENS (electrodes applied; intensity raised to sensation then reduced below perception/zero; device indicators remain on).

Assessments & Timing:

Time points: T0 (pre-session), T1 (immediately post-session).

Primary outcome: Upper trapezius dynamic stiffness (N/m)-three repetitions averaged at a standardized anatomical landmark.

Secondary outcomes: Pain VAS (0-10); MyotonPro frequency/tone, decrement, relaxation time, creep on upper trapezius ± SCM ± cervical extensors; adverse events/tolerability. (Neck Disability Index may be collected at baseline for descriptive characterization.)

Statistical Plan: Mixed-effects repeated-measures analysis with Group (TENS vs control/sham) × Time (T0 vs T1); assumption checks with robust alternatives/transformations as needed; multiple-comparison controls; effect sizes reported.

Eligibility (summary): Adults 18-50 years with neck pain VAS ≥3/10. Key exclusions include pacemaker, pregnancy, recent invasive procedures (≤6 weeks), significant dermatologic issues at electrode sites, cervical radiculopathy/neurological signs, acute neck injury. (Full list in Eligibility.)

Safety: Low-risk, non-invasive procedure. Expected adverse effects are minor (e.g., transient discomfort or skin irritation). AE/SAE monitoring is prospective; the session will be stopped if intolerable symptoms occur.

Inclusion Criteria:

• Age 18-50 years.

Neck pain VAS ≥ 3/10 at screening and on test day.

Clinical diagnosis of chronic or mechanical neck pain (≥6 weeks).

Able to understand study procedures and provide written informed consent.

Willing to refrain from analgesics/muscle relaxants and vigorous neck/shoulder exercise for 24 hours before testing.

Exclusion Criteria:

• Cervical radiculopathy or neurological signs (myotomal weakness, dermatomal sensory loss, reflex changes).

Recent invasive procedures to the neck/shoulder (e.g., surgery, injections, radiofrequency/neurotomy) within 6 weeks.

Pacemaker/implantable cardioverter, or other active implanted electronic device.

Pregnancy or planning pregnancy.

Skin conditions at electrode sites (open wounds, infection, severe dermatitis) or known allergy to electrode adhesive/gel.

History of epilepsy or unexplained syncope.

Acute neck trauma (<6 weeks) or red flags (fracture, malignancy, infection, inflammatory rheumatic disease).

Systemic disorders likely to affect outcomes (e.g., uncontrolled diabetes with neuropathy, severe cardiovascular disease, fibromyalgia with widespread pain).

Ongoing electrotherapy to the neck region or botulinum toxin injections to cervical muscles within 3 months.

Inability to comply with procedures (e.g., severe communication/cognitive impairment), or insufficient proficiency in Turkish to complete measures.

De-identified IPD (outcomes, baseline demographics, allocation, adverse events) will be shared ~6 months after primary completion for 36 months via an institutional repository or encrypted transfer managed by Erzurum Technical University.

Outcome assessors are blinded to group allocation. Participants are not blinded when the comparator is "no intervention." If a sham TENS comparator is used, participants will also be blinded (double-blind: participant and outcomes assessor). Randomization is concealed using sequentially numbered, opaque, sealed envelopes (SNOSE); assessors do not have access to the allocation list.