This is a prospective, randomized controlled trial aimed at evaluating whether a structured multimodal rehabilitation program can delay or reduce the occurrence of adjacent segment degeneration (ASDeg) in frail elderly patients after short-segment lumbar fusion surgery.

1. Rationale and Scientific Background Adjacent segment degeneration (ASDeg) represents a major determinant of long-term clinical success after lumbar fusion. In frail older adults (≥75 years, Fried phenotype), the risk is theorized to be amplified due to age-related sarcopenia, osteopenia, diminished healing capacity, and compensatory biomechanical overload. While Enhanced Recovery After Surgery (ERAS) protocols address acute perioperative stress, they do not provide a targeted, long-term strategy to modify the underlying biological and biomechanical progression toward ASD. This trial is grounded on the hypothesis that a structured, multimodal rehabilitation program, initiated after the initial bone-soft tissue healing phase (12 weeks), can enhance paraspinal muscular endurance, improve global sagittal alignment, increase bone density, and mitigate systemic frailty. This multisystem approach aims to decelerate the degenerative cascade at adjacent segments compared to standard care alone.

2. Intervention Design and Theoretical Framework Timing Rationale: The intervention commencement at 12 weeks postoperatively is deliberately chosen to avoid interference with initial osseous fusion and wound healing, while capitalizing on the phase of maximal functional plasticity and patient engagement in recovery.

   Multimodal Components: The program integrates three evidence-based components supervised by a physiotherapist:

   Progressive Resistance & Stabilization Training: Targets deep spinal stabilizers and global movers to restore functional strength and reduce segmental strain.

   Neuromotor & Balance Re-education: Addresses age-related proprioceptive decline to improve movement efficiency and reduce fall risk.

   Pain Neuroscience Education & Activity Pacing: Aims to modify maladaptive pain behaviors and fear-avoidance beliefs, promoting sustained engagement.

   Adherence Strategy & Measurement: Adherence is not merely recorded but is actively managed as a modifiable process variable. The weekly Likert-scale tracking (via phone/app) serves as both a measurement tool and a behavioral prompt. The >70% adherence threshold (cumulative score >135/196) is based on prior rehabilitation efficacy studies, defining a minimum "dose" for expected biological effect. Illustrated manuals and periodic in-person sessions ensure correct technique and gradual progression, addressing a key limitation of unsupervised home exercise.

3. Outcome Selection and Measurement Rationale Primary Outcome Justification: Time to ASD (in days) was selected over a simple binary incidence rate because it is a time-to-event measure that captures both the occurrence and the timing of degeneration, providing greater statistical power and clinical relevance regarding the delay of disease.

   Secondary Outcome Triangulation: The combination of patient-reported outcomes (VAS, ODI), physician-assessed function (JOA), objective physiological measures (BMD, Fried score), and radiographic biomechanics (PI-LL, SVA) allows for a comprehensive evaluation of treatment effect across the domains of symptoms, function, biology, and structure. This aligns with the multifactorial nature of both frailty and ASD.

4. Methodological Rigor and Design Considerations Randomization & Blinding: 1:1 block randomization will be performed by an independent statistician using sealed envelopes. While participants and treating therapists cannot be blinded to group assignment, all outcome assessors (radiologists evaluating ASD, researchers collecting PROs) will remain blinded to minimize assessment bias.

   Handling Expected Challenges in a Frail Population: The protocol includes explicit strategies for managing common issues: 1) Flexible follow-up modes (clinic, phone, app) to reduce dropout; 2) LOCF imputation for missing data specified a priori in the statistical plan; and 3) Adverse event monitoring specifically for rehabilitation-related musculoskeletal complaints.

   Sample Size Consideration: The 20% inflation for attrition is conservative but justified for this older, frail population over a 2-year follow-up. Sensitivity analyses (e.g., per-protocol, complier-average causal effect models) are pre-planned to assess the robustness of findings to deviations from the protocol and variable adherence.

5. Anticipated Impact and Mechanistic Exploration This trial is designed to establish not only whether the intervention works (efficacy) but also to generate hypotheses on how it works. By correlating changes in muscular performance (via functional tests), frailty status, and sagittal alignment with the primary ASD outcome, the study aims to explore potential mediators of treatment effect. The findings are expected to inform the development of a standardized, reproducible rehabilitation protocol tailored to the high-risk frail elderly spine surgery population.

Inclusion Criteria:

• Age ≥ 75 years.
• Classified as "frail" according to the Fried frailty phenotype.
• Scheduled for first-time, posterior short-segment (1-2 levels) lumbar instrumented fusion surgery.
• Willing and able to provide written informed consent and complete the 2-year follow-up.

Exclusion Criteria:

• History of previous lumbar spine surgery.
• Lumbar pathology due to tumor, trauma, infection, or congenital deformity.
• Comorbidities with a severe impact on prognosis or ability to participate in rehabilitation (e.g., severe cardiopulmonary disease, advanced dementia).