Objective: To clarify the progression of core flexibility and core muscle strength in fresh lumbar spondylolysis before and after the introduction of rehabilitation treatment. Methods: We enrolled 160 patients diagnosed with fresh lumbar spondylolysis based on magnetic resonance imaging findings from September 2019 to December 2022. Posterior Lumbar Flexibility test and Lumbar Locked Rotation test were performed to assess core flexibility. The Kraus-Weber test and Sahrmann Core Stability test were performed to assess core muscle strength. The initial and final evaluations of all tests were compared.Results: All tests showed significant improvement at the final evaluation compared to the initial evaluation.Conclusion: Patients with lumbar spondylolysis often had decreased core flexibility and core muscle strength at the initial evaluation; these parameters improved at the final evaluation owing to rehabilitation treatment.