Inclusion Criteria:  (1) age between 20-40 or age over 60; (2) no bilateral shoulder pain or discomfort within six months; (3) no past diagnosis in shoulder or upper extremities.  Exclusion Criteria:  (1) fracture of upper extremities before; (2) be diagnosed as neck radiculopathy, central nervous system lesion of brain, or peripheral nervous system lesion of upper extremities; (3) rheumatoid arthritis or autoimmune disease; (4) have received any shoulder injection or surgery; (5) tendon tear in shoulder sonograms.