Low back pain is frequently associated with increased tone and stiffness of the lumbar erector spinae muscles, which may contribute to pain and functional limitation. Transcutaneous electrical nerve stimulation (TENS) is a non-invasive, safe, and widely used modality for pain reduction, but its very short-term (immediate) effects on lumbar muscle stiffness have not been clearly demonstrated. This randomized controlled trial will compare a single 20-minute session of conventional TENS with a control/rest condition in patients with non-specific low back pain. The primary outcomes will be change in lumbar erector spinae muscle stiffness (Myoton) and change in pain intensity (VAS) from baseline to immediately after the intervention. We hypothesize that TENS will produce a greater immediate reduction in muscle stiffness and pain than control.

Low back pain (LBP) is one of the most common musculoskeletal problems and often presents with increased paraspinal muscle activity and perceived stiffness around the lumbar region. Increased stiffness of the lumbar erector spinae may perpetuate pain and restrict movement. TENS provides afferent stimulation that can modulate pain through gate-control and descending inhibitory mechanisms and may secondarily reduce muscle tone. However, evidence about its acute influence on objective muscle stiffness parameters in LBP is limited.

This single-center, parallel-group, randomized controlled trial will recruit adults with chronic or recurrent non-specific low back pain who meet the eligibility criteria. Participants will be randomly assigned (1:1) to (1) an experimental group receiving conventional TENS applied bilaterally over the lumbar paraspinal area for 20 minutes (frequency ~80-100 Hz, pulse width ~100-150 μs), or (2) a control group undergoing the same positioning and timing without active TENS. All assessments will be performed immediately before and immediately after the intervention by an outcome assessor blinded to group allocation.

Primary outcomes are: (a) lumbar erector spinae muscle stiffness measured with a handheld myotonometer (e.g. Myoton) at standardized lumbar levels, and (b) pain intensity measured with a 10-cm visual analog scale (VAS). Secondary observations may include participant-reported comfort/tolerance of TENS and adverse events. The study is designed as a short-term physiological and symptomatic trial; no long-term follow-up is planned. Ethical approval was obtained from the Erzurum Technical University Ethics Committee (Meeting 13, Decision 04, 18.09.2025)

Inclusion Criteria:

• Aged 18-30 years

Non-specific low back pain persisting or recurring for ≥3 months

Reported tenderness/stiffness on palpation of the lumbar erector spinae muscles

Able to understand the study procedures and provide written informed consent

Able to attend on the same day for TENS application and pre-/post-intervention measurements

Exclusion Criteria:

• Low back pain with a specific cause (e.g. lumbar disc herniation with radiculopathy, infection, tumor, fracture, severe deformity)

Open wound, dermatitis, or any skin condition in the lumbar area that prevents electrode placement

Presence of a cardiac pacemaker or other implanted electronic device

Electrotherapy, injection, or manual therapy applied to the lumbar region within the previous 48 hours

Pregnancy

Severe neurological deficit or sensory loss

Use of analgesics/NSAIDs at a level that may interfere with pain or stiffness assessment at the time of evaluation

Any other condition deemed unsafe or inappropriate by the investigator

Individual participant data (IPD) will not be shared because the study is small, single-center, and includes identifiable clinical measurements that could risk participant privacy despite de-identification.

The assessor who performs Myoton and VAS measurements will be blinded to group allocation.