Too much stress -- or too many fastballs -- can cause partial tears and swelling in the tendons of the rotator cuff. Abrupt stress may even cause one of the tendons to pull away from the bone or tear in the middle of the tendon.o diagnose a rotator cuff tear, your doctor will give you a thorough physical exam. He or she will want you to move your arm in different directions to see what causes pain. In addition, your doctor might want to order the following tests: 1  X-ray of the shoulder with some special views.
In rare cases, generalized muscle weakness may be caused by another health problem, such as: 1  A problem with the minerals (electrolytes) found naturally in the body, such as low levels of potassium or sodium. 2  Infections, such as a urinary tract infection or a respiratory infection.
The causes are many and can be divided into conditions that have either true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis. Muscle weakness can also be caused by low levels of potassium and other electrolytes within muscle cells.