Purpose: This study aims to better understand how force is produced during a vertical jump in individuals approximately six months after anterior cruciate ligament reconstruction compared with healthy individuals. After this type of knee surgery, many patients recover well clinically but may still present strength differences between the operated and non-operated leg. These differences are not always visible during routine clinical testing but may influence performance and potentially increase the risk of reinjury. By analyzing how force is generated during a standardized countermovement jump, this study seeks to identify whether biomechanical alterations persist at this stage of recovery and to improve decision-making related to rehabilitation and return to sport.

Study Design: This is an observational comparative study. Participants who have undergone anterior cruciate ligament reconstruction using a hamstring tendon graft will be assessed between five and seven months after surgery. Their results will be compared with those of healthy recreationally active individuals without a history of recent knee injury. No experimental treatment or modification of rehabilitation will be introduced as part of this study.

Procedures: Participants will complete a brief standardized warm-up followed by three maximal vertical countermovement jumps performed on a force platform. During the test, participants will keep their hands on their hips and will be instructed to jump as quickly and as high as possible. The force platform measures ground reaction forces from each leg separately, allowing detailed analysis of the lowering phase (when the body bends before jumping) and the push-off phase (when the body propels upward). The entire testing session lasts only a few minutes and reflects movements commonly used in rehabilitation and sports settings.

Measurements: The primary measurements include jump height and the mean force produced by each leg during both the eccentric (lowering) and concentric (push-off) phases of the jump. The study will also calculate limb symmetry indices to determine the degree of difference between the operated and non-operated legs. These measurements provide objective information about neuromuscular recovery and functional performance after surgery.

Risks: The procedures involve physical activity comparable to exercises already performed during standard rehabilitation. The risks are minimal and similar to those encountered during routine physical training. Participants may stop the test at any time if they experience discomfort.

Potential Benefits: Participants may not receive direct personal benefit from taking part in the study. However, the results may contribute to improving rehabilitation strategies, refining criteria used for return-to-sport decisions, and enhancing long-term functional outcomes after anterior cruciate ligament reconstruction. A better understanding of persistent biomechanical alterations may ultimately help reduce the risk of secondary injury and optimize recovery pathways.

Inclusion Criteria:

• Post-Anterior Cruciate Ligament Reconstruction Cohort:

Age between 18 and 40 years

Primary anterior cruciate ligament reconstruction using a hamstring tendon autograft

Assessment performed between 5 and 7 months post-surgery

Completion of standard postoperative rehabilitation

Medical clearance for functional testing

Ability to perform maximal effort jumping and strength testing

Healthy Control Cohort:

Age between 18 and 40 years

Recreationally active (minimum two sessions of physical activity per week)

No history of anterior cruciate ligament injury

No lower limb surgery

No lower extremity injury within the past 12 months

No current knee pain or functional limitation

Exclusion Criteria:

• Previous anterior cruciate ligament reconstruction on either knee (except the index surgery for the ACL group)

Concomitant ligament reconstruction (e.g., posterior cruciate ligament, medial collateral ligament, lateral collateral ligament)

Symptomatic meniscal repair limiting full participation in testing

Neurological disorders affecting lower limb function

Inflammatory joint disease

Pregnancy

Inability to perform maximal effort testing

The study population includes adults aged 18 to 40 years. The post-anterior cruciate ligament reconstruction cohort consists of individuals who underwent primary reconstruction using a hamstring tendon autograft and are evaluated between 5 and 7 months after surgery, following completion of standard rehabilitation and medical clearance for functional testing. The healthy control cohort includes recreationally active individuals with no history of anterior cruciate ligament injury, no prior lower limb surgery, and no lower extremity injury within the past 12 months. All participants must be able to perform maximal effort jumping and isokinetic strength testing.