One cause of muscle strain is tendinitis which is an inflammation of the tendons that surround the joint. Shoulder pain could also be caused by a pulled muscle or injury to the muscle. Shoulder pain is also a symptom of a heart attack.

Reference:

www.mayoclinic.com

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Resource:

  1. What Causes Shoulder Pain? – http://answers.ask.com/Reference/Dictionaries/what_causes_shoulder_pain
  2. What Causes Neck and Shoulder Pain? – http://answers.ask.com/Health/Diseases/what_causes_neck_and_shoulder_pain
  3. what are the causes of shoulder & knee pain? – http://www.ehow.com/about_6587524_causes-shoulder-knee-pain_.html
  4. what are the causes of pain under the right shoulder blade? – http://www.ehow.com/about_6563283_causes-under-right-shoulder-blade_.html