Inclusion Criteria for CLBP group:  Patients age between 20-50yrs Patients with chronic low back pain (pain > 3 months). Patients with unilateral symptoms (facet, disc, SIJ dysfunction) will be referred by an orthopedic physician with or without referral to the leg.  Inclusion Criteria for the healthy control group:  1. matched controls in terms of age, weight, height, and BMI.  Exclusion Criteria for both groups:  History of shoulder pain 3 months ago. History of any shoulder pathology. Congenital pathology affecting the spine or shoulder. Any neurological disorder. Non-mechanical LBP (e.g., fracture, malignancy, infection) Difficulty performing the PHE task because of weakness in the GM or the test provokes pain. BMI 30 or higher as fat tissue may decrease the ability to measure surface EMG activity