The investigators will ask the participants to attend the laboratory in two sessions. They will seek the tender spots on the trapezius muscle. Then the participants will be asked to roll a Myostorm ball ( a small hard ball made of plastic with a layer of soft material cover) on their affected muscles. They will be tested for pain-pressure threshold (PPT), shoulder range of motion, electromyography (EMG), and force before and after the manipulation.
The current study aims to examine the potential effects of rolling a Myostorm ball on the trigger points in the upper back muscles.