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The purpose of this study was to investigate the effect of grade IV lumbar mobilization on the activity/contraction of erector spinae (ES) and lumbar multifidus (LM) muscles in healthy people.A randomized, repeated-measures design was used. Sixteen healthy subjects attended 3 testing sessions with a different intervention in each session (no intervention, grade IV central lumbar mobilization at L4, and placebo/light touch). Lying in a prone position, subjects lifted a light weight with their right arm while ultrasound images of LM and surface electromyography signals of ES were captured before and immediately after application of the intervention in the session. The contraction of LM was calculated from US images, and the root mean square was calculated from the electromyography signals of ES and used as outcome measures.A significant difference was found in LM contraction between the placebo and mobilization intervention (difference = 0.04, P = .02). There was no difference for the root mean square of electromyography signals between the interventions.The significant difference in LM contraction was small and may not have clinical significance. Lumbar mobilization did not change the activity of ES in healthy people. Future studies with larger samples are needed to confirm our findings and to investigate the effect of mobilization on back muscles in people with low back pain.