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BACKGROUND Vibration training is an adjuvant to muscle-strengthening exercises. OBJECTIVE To investigate the short-term effects of direct vibration on the deep trunk muscles of patients with non-specific chronic low back pain (CLBP). METHODS Participants with non-specific CLBP were randomly placed into two groups: the vibration-plus stabilization exercise (VSE) group (n= 31) and the conventional stabilization exercise (CSE) group (n= 31). The groups underwent 12 sessions of an exercise program. The thickness and activity of the trunk muscles were measured using ultrasonography and surface electromyography. RESULTS Ultrasonography revealed that the ratio of muscle thickness to contraction and relaxation was statistically increased after exercise in the bilateral transversus abdominis (TrA) and lumbar multifidus (LM) muscles in the CSE group (p= 0.031), and in the bilateral TrA, LM, and internal oblique (IO) muscles in the VSE group (p< 0.001). The LM/lumbar erector spinae (LES) ratio in the CSE group (p= 0.037), and the IO/rectus abdominis (RA), TrA/RA, and LM/LES ratios in the VSE group (p= 0.019) were statistically increased. Multiple regression analysis showed that symptom improvement was related to increased activity of the deep trunk muscles in the VSE group (p< 0.001). CONCLUSION Direct vibration can increase the selective activity of contracting deep trunk muscles.