OBJECTIVES: To test the safety of the diode light therapy and evaluate the advantages of the interferential effect of two light probes versus a conventional light probe in the relief of shoulder pain and disability caused by shoulder tendinopathies. DESIGN: Randomized single-blind pilot study. SETTING: Clinical electrotherapy unit. PARTICIPANTS: A total of 30 patients with shoulder pain from tendinopathies. INTERVENTIONS: The patients were randomly assigned into two groups. Group 1 (n = 15) received interferential light therapy generated by two independent and identical cluster probes composed of light emitting and superluminescent diodes. Similarly, two applicators were applied in group 2 (n = 15), but only one was active, as in conventional clinical therapy. Each multi-diode cluster probe was composed of seven light-emitting diodes at 600 nm and 12 superluminescent diodes at 950 nm. MAIN OUTCOME MEASURES: Pain was evaluated by visual analogue scale (VAS) at day, at night and during several shoulder movements. Shoulder functional status was measured by means of the University California Los Angeles scale (UCLA). RESULTS: Comparison between both treatments using the Mann-Whitney U-test showed better results for the interferential treatment. There were significant differences in pain reduction during abduction (p < 0.05) and external rotation (p < 0.05), with pain reductions in abduction and external rotation of 1.5 (+/- 1.3) and 0.5 (+/- 1.0) respectively. CONCLUSION: Interferential light therapy was safe and effective regarding the shoulder pain reduction during abduction and external rotation movements. The estimated size sample needed for future two-treatment parallel-design studies will require about 60 patients.

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