Back to Life - Early Rehabilitation Therapy for Patients Undergoing Lumbar Spinal Fusion - A Single-case Experimental Study

The investigators will in total include 12 patients undergoing an LSF from the Centre for Rheumatology and Spine Diseases at Rigshospitalet, Denmark. Before inclusion, a pilot of three participants will be performed. Inclusion Criteria for all included: * ≥ 18 years of age * Low back pain (LBP) \> 3 months * Read and understand the Danish language * The participants should be a part of Frederiksberg Municipality and Copenhagen Municipality (in Denmark) Exclusion Criteria: * Post-traumatic vertebral compression/deformity * Patients with the following conditions (infection, neoplasm, metastasis, metabolic bone disease, fractures or other known autoimmune arthropathies) * Patients with patient-related circumstances (physical or mental status) that impede their ability to give informed consent and adhere to the study program (e.g., dementia, developmental disorders, or substance-induced cognitive impairment) * Other special conditions, where the patient is judged by the surgeon as inappropriate to participate