Introduction: Rotator cuff tears are the main cause of shoulder pain and disability. First line of treatment is conservative; there is evidence regarding the advantage of using eccentric over concentric exercises in tendinopathies, but there are no evidence-based recommendations on starting strengthening exercise during painful phases nor on the effectiveness and advantages of eccentric vs. concentric exercise in rotator cuff tears. Objective: To evaluate the tolerance of a resistance strengthening program and to compare eccentric vs. concentric programs. Design: A pilot, experimental, randomized controlled study. Setting: Outpatient Rehabilitation Center. Patients: Twenty-six patients with a diagnosis of partial rotator cuff tear were randomly assigned to: the experimental group (eccentric, n ¼ 12) and the control (concentric, n ¼ 14). Intervention: The experimental group performed muscle strengthening with eccentric technique directed to shoulder and scapular girdle muscles, while the control group performed the concentric technique. Main outcome measures.: Visual Analogue Scale (VAS), Constant Scale, Strength, Structure (Ultrasound report). Results: The tolerance rate was 96% in both groups. Median age (experimental vs. control) was 54.5 vs. 54 years (p ¼ 0.69