Purpose: Rehabilitation and physiotherapy have a major role in reducing the pain and increasing the functional ability in patients with shoulder joint pain. This study aimed to evaluate the effect of Action Potential Simulation (APS) and Transcutaneous Electrical Nerve Stimulation (TENS) in patients with Chronic Mechanical Shoulder Pain (CMSP). 
Methods: In this quasi-experimental study, 32 patients with CMSP were enrolled. Patients were randomly assigned into APS group and TENS group, both of them received 6 sessions of intervention. The study instruments included pain intensity and shoulder Range Of Motion (ROM), America Surgeons Standardized Shoulder Assessment Form, and Western Ontario Rotator Cuff Index. The stability of scapula and shoulder function (shoulder dyskinesia) were measured before and after the intervention. The obtained data were analyzed using t tests by SPSS19 (P<0.05).
Results: Pain intensity, shoulder ROM, and stability of scapula decreased in both groups. However, there were no significant differences between groups (P>0.05). In addition, shoulder function did not change after intervention in both groups (P>0.05). 
Conclusion: With regard to study findings, APS and TENS are effective in pain reduction but none has preference over the other in treatment approaches.