The effect of stabilization of the degenerative segment on changes in the pattern of paraspinal muscle activity was investigated using a previously developed musculoskeletal model. Muscle activity change depending on L4-L5 segment stabilization with and without taking into account the presence of multifidus atrophy according to direct invasion of the back muscle during surgery (MADIBM) was analysed in erect standing and 20° flexed postures. For the stabilization of the degenerative segment, a fusion or non-fusion stabilization with a pedicle-based dynamic stabilization system (PBDS) was applied. During erect standing, fusion generated a 12% reduction in the total multifidus muscle force, while its reduction was 6.6% with PBDS application. The presence of MADIBM produced 23.0% and 22.5% reductions in fusion and with PBDS application, respectively. During 20° flexion, 10.5% and 9.3% reductions were produced for fusion and PBDS application, respectively, and the corresponding values were 23.4% and 23.0%, respectively, in the presence of MADIBM. Increased facet joint contact forces were produced at the non-stabilized levels after stabilization while in erect standing posture. Alterations in muscle activity, which could be regarded as adaptions to altered spinal stability, may generate unexpected secondary problems in the spine.

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