Anterior cruciate ligament (ACL) injury is a common sports-related injury, particularly in sports that require extensive jumping and cutting movements. Although anterior cruciate ligament reconstruction (ACLR) surgery can restore knee stability, many athletes still face the risk of re-injury after returning to competition.Current return-to-play assessment relies primarily on single-plane horizontal hop tests and symmetry indices (LSI ≥ 90%); however, an increasing body of research suggests that these tests may not comprehensively reflect functional deficits in the vertical plane, repeated hopping, or high-intensity sport-specific activities.This study hypothesizes that incorporating multiple vertical jump tests-including single-leg vertical jumps and 10-second repeated vertical jumps-combined with advanced force plate analytics (such as Reactive Strength Index [RSI] and Time to Stabilization [TTS])-can more sensitively reveal residual neuromuscular control deficits following surgery. This approach would provide evidence-based guidance for return-to-play decision-making, thereby improving athletes' long-term athletic performance and safety.

This cross-sectional study compares biomechanical performance between ACLR athletes (>6 months post-op) and healthy controls to evaluate whether vertical jump protocols reveal greater limb asymmetries compared to traditional horizontal hop tests. Following a 5-minute stationary bicycle warm-up and familiarization trials, participants perform four unilateral tasks on both limbs: Single-Legged Hop for Distance (SHD), Crossover Hop for Distance (CHD), Single-Legged Vertical Countermovement Jump (SLVJ), and 10-Second Repeated Single-Legged Vertical Jump (SLVJs). All tasks are performed with hands on hips to minimize compensatory motion. Three successful trials are recorded per limb per task with 3-minute rest intervals between sets. Trials are discarded if participants land on the contralateral limb, lose balance, or fail to land on the force plate. Vertical ground reaction forces are collected using a dual-force plate system (Hawkin Dynamics, 3rd Generation) at 1000 Hz. Data processing calculates Limb Symmetry Index (LSI), Modified Reactive Strength Index (RSI) from SLVJs as Jump Height divided by Movement Time, Time to Stabilization (TTS) defined as the duration for vGRF to stabilize within ±5% of body weight, and eccentric/concentric impulse.

Inclusion Criteria:

• 1.Age 18-35 years

  2.Unilateral anterior cruciate ligament (ACL) rupture confirmed by magnetic resonance imaging (MRI)

  3.Positive Lachman test grade ≥2 pre-operatively

  4.Anterior cruciate ligament reconstruction (ACLR) using hamstring or gracilis autograft

  5.Post-operative time ≥6 months

  6.Completed rehabilitation program with clearance from orthopedic surgeon or physical therapist

  7.Quadriceps strength symmetry index ≥90%

  .Elite athlete status (prior participation in national-level competitions)

Exclusion Criteria:

1. Other lower extremity injuries or pathologies (including other knee ligament injuries)
2. Contralateral lower limb injury within the past 3 months
3. Quadriceps strength symmetry index <90%
4. Non-elite athlete status (no prior national-level competition participation)
5. Incomplete rehabilitation or lack of medical clearance

The study population consists of two distinct groups: (1) ACLR athletes: elite athletes aged 18-35 years who have undergone unilateral anterior cruciate ligament reconstruction using hamstring or gracilis autograft with a minimum of 6 months post-operative. These participants will be recruited from orthopedic clinics, sports medicine centers, and rehabilitation facilities in the region. (2) Healthy controls: age, sex, and athletic-level matched individuals aged 18-35 years with no history of lower extremity injuries, recruited from local athletic facilities, university sports programs, and community sports organizations. Both groups must have elite athlete status, defined as prior participation in national-level sports competitions. The ACLR group represents athletes in the late rehabilitation phase preparing for return-to-play, while the healthy control group serves as a reference for normal biomechanical performance.

De-identified individual participant data (IPD) that underlie the main published results, including demographic characteristics and biomechanical outcome measures (e.g., jump height, limb symmetry index, RSI, TTS, and impulse), will be shared with qualified researchers upon reasonable request, after approval of a data use agreement and in accordance with applicable ethical and regulatory requirements.

De-identified individual participate data (IPD) and supporting information will be available starting 2 years after the publication of the primary study results and will remain accessible for 5 years.

The related supporting documents will only be made available to qualified researchers affiliated with academic institutions, medical organizations, or public health agencies. Applicants must submit a detailed research proposal and ethical approval documents, and access will be granted only upon approval.