Acute Effects of TENS on Cervical Muscle Stiffness and Pain in Neck Pain

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.