Background: Low back pain (LBP) is often associated with lumbar spinal instability (LSI). The multifidus muscle is considered a stabilizer of the spine and has been studied extensively with Rehabilitative Ultrasound Imaging (RUSI). There may be a relationshipbetween clinical signs of LSI, decreased cross-sectional area (CSA) of the multifidus and weakness. Having the ability to detect multifidus weakness without the use of RUSI may serve to be invaluable to the clinician in detecting multifidus weakness.Purpose: To investigate the relationship between the modified prone straight leg raise test (MPSLR) and CSA of the multifidusmuscle as measured by RUSI and to investigate the relationship between MPSLR and RUSI findings with the presence of low back pain symptoms that interfere with regular daily activities. Subjects: Participants consisted of two groups of subjects. One group (n=30, 87% male) comprised individuals in general good health, aged 18-55, without history of back pain. The second group (n=36, 56% male) comprised individuals aged 18-55, with history of low back pain within the past 12 months. Methodology: Subjects performed a MPSLR test to identify multifidus weakness. All subjects repeated the same test with concurrent RUSI to visualize the multifidusand measure its CSA. Results: A significant association between a positive MPSLR, asymmetry of the multifidus, and pain was observed (p r = .049, p = .696) was not observed. A sensitivity of 94% and a specificity of 63% was also discovered in the ability of the MPSLR test to detect asymmetry of themultifidus muscle within subjects. A positive MPSLR combined with a high Oswestry score of 25-30 further reinforced the probability of pain (p < .001) Conclusion: The MPSLR test demonstrated a strong association between a positive test and asymmetry of themultifidus muscle within subjects. Clinical Relevance: The MPSLR test can be used to identify patients at risk for LBP symptoms due to asymmetrical changes in the multifidus muscle of the lumbar spine, and aid in directing an appropriate rehabilitation approach to those patients in need of specific multifidus exercise prescription.