Transcranial Electrical Stimulation in Subacromial Pain Syndrome

The goal of this clinical trial is to analyze whether a physiotherapy protocol based on strengthening exercises to the shoulder girdle muscles carried on during anodal transcranial electrical stimulation (a-TES) is more effective on the symptoms and functionality of subjects with subacromial pain syndrome than an isolated strengthening program. The main questions it aims to answer are: * Is the strengthening + a-TES protocol effective on functionality? * Is the strengthening + a-TES protocol effective on quality of life? * Is the strengthening + a-TES protocol effective on pain? * Is the strengthening + a-TES protocol effective on muscle strength? * Is the strengthening + a-TES protocol effective on active range of movement? * Is the strengthening + a-TES protocol effective on proprioception? * Is the strengthening + a-TES protocol effective on posture? * Is the strengthening + a-TES protocol effective on glenohumeral and scapulothoracic kinematics? Participants will perform an 8 weeks-3 days/week shoulder strengthening protocol for the rotator cuff and scapulothoracic muscles using elastic bands and dumbbells. The rotator cuff exercises will be performed while they receive a-TES using a neoprene helmet with the electrodes placed in the opposite hemisphere to the affected side. The scapulothoracic exercises will be performed without the transcranial stimulation. The experimental group will receive real a-TES, and will be compared with a control group that will perform the same therapeutic exercises but with placebo a-TES to see the effectiveness of the a-TES on the outcomes cited above.