BACKGROUND: To compare the effectiveness of 3 different amplitude modulated frequency (AMFs) of interferential current (IFC) on pain, range of motion (ROM), and function in subjects with chronic trigger points (TrPs) of the upper trapezius muscle. METHODS: One hundred twenty subjects, 78 (65%) females and 42 (35%) males completed this study. All of them had latent TrPs in the upper fibers of the trapezius. They were randomly assigned into 4 groups; 3 experimental groups received IFC with different AMFs (4, 80, and 130 Hz) plus standard manual treatment while the 4th group (control group) received the standard treatment only. Pain intensity, cervical lateral flexion ROM, and function were assessed pre- and post-4 weeks of intervention. Average pain episodes and sleeping quality were assessed throughout the 3 months preceding and those following the intervention. RESULTS: Post-intervention measures demonstrated statistical improvement in all outcomes interventions compared to the baseline (p <= 0.02). Pain, ROM, and function were equally improved in the 3 experimental groups compared to the control (p <= 0.04). 4 Hz group showed statistically significant difference compared to the 80 Hz group (p = 0.002) regarding pain, and statistically significant difference compared to 130 Hz and control group regarding the average pain episodes (p = 0.003 and 0.002, respectively). The 4 Hz group demonstrated a statistically significant difference compared to all other groups (p < 0.001) in Favor of the sleeping quality. CONCLUSION: Adding IFC to manual techniques improves pain, ROM, function, average pain episodes, and sleep quality in subjects with upper trapezius TrPs. 4 Hz AMF seems to have superior effects in terms of pain and average pain episodes compared to 80 and 100 Hz AMFs.

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