The purpose of this study is to determine the advantages of the interferential laser therapy in shoulder pain reduction compared with the conventional low level laser therapy modality.

The spatiotemporal superposition of two independent and opposite beams of laser generates a constructive interference pattern that increases the therapeutic effects in the irradiated tissue. Patients diagnosed of shoulder pain from different musculoskeletal disorders will received conventional or interferential laser therapy. This is a comparative, randomized and double - blind clinical study. Two identical equipments of near infrared laser therapy will be used. Number of sessions: 15. Frequency of the sessions: daily. Irradiation technique: manual, contact and punctual.Pain will be evaluated by visual analog scale (VAS) at rest and during analytical shoulder movements (abduction, adduction, internal rotation, external rotation, flexion, extension) and by means of a pain and disability index (SPADI). Evaluations will take place at pre and post treatment.

• Active Comparator: 100 patients are being treated with a single laser beam over pain area.
• Experimental: 100 patients are being treated with interferential laser from two independent sources

Inclusion Criteria:

• Acute or chronic pain from musculoskeletal origin
• Age between 18 and 70 years

Exclusion Criteria:

• Brachial plexus palsy
• Neurological diseases
• Pacemakers
• Shoulder pain from cervical pathology
• Osteosynthesis material
• Fibromyalgia