Associations Between Paraspinal Muscle Degeneration, Trunk Isokinetic Strengths, and Spinopelvic Alignment in Patients Undergoing Lumbar Fusion: A Cross-Sectional Study.

Background/Objectives: Spinopelvic alignment represents the sagittal relationship between the lumbar spine and pelvis and is closely related to compensatory posture and disability in degenerative lumbar diseases. Previous studies have mainly focused on magnetic resonance imaging (MRI)-derived paraspinal muscle morphology, whereas evidence linking objective dynamic trunk strength to spinopelvic alignment remains limited. The present study aims to investigate the associations between spinopelvic alignment and muscle degeneration as assessed by MRI-based morphology and trunk isokinetic dynamometry. Methods: In this prospective cross-sectional study, patients undergoing lumbar fusion surgery for degenerative lumbar diseases at a tertiary center were consecutively enrolled. Peak trunk isokinetic torques were measured using the BioniX Sim3 Pro system. Preoperative paraspinal muscle degeneration and spinopelvic alignment were measured. Results: A total of 119 patients were analyzed (52.1% female). In the total sample, paraspinal muscle degeneration, isokinetic torques, and spinopelvic alignment showed statistically significant but generally weak associations. After adjustment for age, sex, and body mass index (BMI), rotation and flexion torques were associated with pelvic tilt, and multifidus functional cross-sectional area (MF-fCSA) was associated with lumbar lordosis (LL), pelvic tilt (PT), pelvic incidence-lumbar lordosis (PI-LL), and PI-LL mismatch. In the hypothesis-generating sex-stratified analyses, trunk isokinetic torques were associated with spinopelvic alignment in males but not females, whereas MF-fCSA was associated with spinopelvic alignment in both sexes. Conclusions: Paraspinal muscle morphology and trunk isokinetic strength were significantly associated with spinopelvic alignment, although the magnitudes of these associations were weak. These findings suggest that imaging-based muscle morphology and dynamic trunk muscle performance are associated with spinopelvic alignment but their implications should be interpreted with caution. Paraspinal muscle imaging and dynamic isokinetic strengths may provide complementary information in the assessment of patients with degenerative lumbar disease. Longitudinal studies are needed to determine their prognostic and clinical value.