Inclusion Criteria:  Healthy tennis players  Age ranges from 20-45 years old Engaging in tennis for at least 1 year Routine training 3 times a week No history of cervical and/or shoulder injury in the past one year, or injury with VAS (visual analog pain scale) lower than 3 and without cervical-shoulder pain 7 days before testing No operation on the shoulder, no shoulder subluxation, labral tear, and rotator cuff tear. Neer's test, empty can test, and Hawkins Kennedy test are negative.  Exclusion Criteria:  History of cervical and/or shoulder injury in the past one year, or injury with VAS (visual analog pain scale) bigger than 3 and with cervical-shoulder pain 7 days before testing Operation on the shoulder, history of shoulder subluxation, labral tear, and rotator cuff tear. One of the three test results is positive: Neer's test, empty can test, and Hawkins Kennedy test. Perform upper extremity resist training and tennis training 24 hours before testing.