The sample will be divided into: low back pain group and control (healthy) group. Both groups will receive dry needling to the lumbar multifidus muscles using a deep insertion technique with 4 needles (2 on each side of the spine). The needles will be left in situ for 10 minutes. Ultrasound imaging will be used to measure multifidus muscles' thickness, pre and post- procedure during rest in a prone position and during contralateral active straight leg extension.