ComparativeEffectsofLow-FrequencyHigh-IntensityandHigh-FrequencyLow-IntensityTENSonNeuromuscularActivationinMyofascialPainSyndrome:ARandomizedControlledTrial

Myofascial pain syndrome (MPS) of the upper trapezius is a common condition that causes trigger-point–related pain, limited neck movement, and abnormal muscle activation patterns. Up to 85% of people experience myofascial pain at some point in their lives. Transcutaneous electrical nerve stimulation (TENS) is widely used as a noninvasive treatment, but clinicians face uncertainty about which frequency and intensity combinations work best. Low-frequency high-intensity (LFHI) stimulation works through endorphin release and descending pain inhibition, while high-frequency low-intensity (HFLI) stimulation works through the gate control theory by activating large sensory fibers.

Despite both approaches being common in clinical practice, direct comparisons of their effects on pain, movement, and muscle function in MPS patients remain limited.

The researchers conducted a randomized controlled trial with 30 adults aged 20–40 years who had chronic upper trapezius MPS with active trigger points. Participants were randomly assigned to either the LFHI group (2–8 pulses per second at 25–30 energy units, strong enough to cause visible but non-painful muscle contractions) or the HFLI group (80 pulses per second at 8–10 energy units, set to a comfortable tingling sensation without visible contraction). Both groups received 20-minute TENS sessions twice daily for five consecutive days, with electrodes placed over the most tender trigger points. Before and after the intervention, the researchers measured pain intensity using a 0–10 numeric rating scale, pressure pain threshold with an algometer, cervical range of motion with a goniometer, upper trapezius muscle tone with a myotonometer, and muscle activation patterns using surface electromyography (sEMG) of the upper trapezius, middle deltoid, and levator scapulae during shoulder abduction.

Both TENS protocols produced significant improvements across nearly all measured outcomes. Pain intensity dropped from 4.80 to 1.60 in the LFHI group and from 3.80 to 1.73 in the HFLI group (p < .01 for both), with no difference between groups. Pressure pain threshold increased from 9.40 to 15.74 lbf in the LFHI group and from 7.80 to 15.90 lbf in the HFLI group (p < .01 for both). Cervical range of motion improved significantly in all directions for both groups (p < .01): for example, flexion increased from 42.07° to 49.87° with LFHI and from 41.27° to 47.27° with HFLI.

Upper trapezius muscle tone decreased from 349.54 to 291.21 N/m with LFHI and from 363.74 to 312.63 N/m with HFLI (p < .01 for both). sEMG showed reduced upper trapezius and levator scapulae activity and increased middle deltoid activation in both groups (p < .01). The key difference between protocols emerged in muscle coordination: the deltoid-to-upper trapezius activation ratio increased significantly only in the HFLI group, from 1.06 to 1.22 (p < .05), suggesting better normalization of shoulder muscle recruitment patterns with high-frequency low-intensity stimulation.

For patients with upper trapezius myofascial pain, both LFHI and HFLI TENS provide meaningful short-term relief of pain, improved neck mobility, reduced muscle tightness, and better muscle balance. The comparable pain outcomes mean clinicians can choose either approach based on patient comfort and preference. However, the finding that HFLI stimulation uniquely improved the deltoid-to-trapezius activation ratio suggests it may offer superior benefits for restoring normal shoulder movement patterns, which could be particularly important for patients whose pain is linked to muscle imbalance or repetitive overhead activities. The twice-daily, five-day protocol was practical and well-tolerated, supporting TENS as an accessible home or clinic-based intervention.

Because the study was short-term with a small sample, longer studies are needed to confirm whether these benefits persist and whether one protocol proves superior for long-term management of chronic MPS.