Objective: The aim of this study was to investigate changes in trunk muscle strength 12 months after lumbar spine fusion (LSF) compared to preoperative strength. Methods: A total of 194 patients (mean ± standard deviation [SD] age, 61 ± 21 years) who underwent LSF participated in this prospective longitudinal study. Physical measurements of the participants were made before surgery and 12 months postoperatively. Isometric trunk extension and flexion strength was measured using a strain-gauge dynamometer in the standing position. Strength changes were calculated. Regression analysis was performed to explore which factors predicted strength levels at 12 months postoperatively. Results: The preoperative mean ± SD extension strength was 205 ± 144 N, which increased to 258 ± 142 N (p < 0.001) at the 12-month follow-up. Flexion strength increased from 295 ± 172 N to 364 ± 164 N (p < 0.001). The preoperative extension/flexion strength ratio was 0.75 ± 0.38 and remained similar (0.73 ± 0.26) at 12 months postoperatively (p = 0.39). Conclusion: Although trunk muscle strength increased by 26% for extension and 23% for flexion at the 12-month postoperative follow-up, both values remained objectively low. In addition, flexion strength remained higher than extension strength, which indicates an imbalance between those muscle groups. Age, severe back pain, and low trunk muscle strength before surgery predicted low trunk muscle strength at 1 year after spinal fusion.