This prospective observational study will compare recovery after anterior cruciate ligament reconstruction using quadriceps tendon autograft or hamstring tendon autograft. The main goal is to examine differences in knee muscle strength recovery during the early and mid-term postoperative period. Participants aged 13 to 45 years who undergo unilateral ACL reconstruction and meet the eligibility criteria will be followed at Hacettepe University. Knee extensor and flexor strength will be assessed using isometric testing at postoperative weeks 4, 8, and 12 and month 6, and with isokinetic concentric testing at months 3 and 6. Functional outcomes and kinesiophobia will also be evaluated at month 6 using IKDC, KOOS, and the Tampa Scale for Kinesiophobia. All participants will receive the same standard postoperative rehabilitation program under physiotherapist supervision. The study aims to clarify whether graft type is associated with different patterns of muscle strength recovery and functional improvement after ACL reconstruction

Anterior cruciate ligament reconstruction is commonly performed in physically active individuals, and graft choice may influence postoperative muscle strength recovery, donor-site morbidity, and return to function. Although quadriceps tendon and hamstring tendon autografts are both widely used, previous studies have mainly focused on longer-term functional outcomes, while direct serial comparisons of early postoperative strength recovery remain limited. This study was designed to address that gap by prospectively comparing early and mid-term muscle strength profiles and 6-month clinical outcomes between these two autograft groups.

This is a prospective clinical observational study conducted at the Department of Sports Physiotherapy and Rehabilitation, Hacettepe University Faculty of Physical Therapy and Rehabilitation. The planned sample size is 34 participants, with 17 participants in each graft group. Eligible participants are individuals aged 13 to 45 years who undergo unilateral anterior cruciate ligament reconstruction with either a quadriceps tendon autograft or a hamstring tendon autograft, have a Tegner Activity Score of 5 or higher, and consent to participate. Key exclusion criteria include patellar tendon or allograft reconstruction, revision surgery, multiple ligament injuries, major collateral ligament injury, marked cartilage lesions, major systemic or neurologic disease, and inability to adhere to rehabilitation.

All participants will be followed prospectively and will receive a standardized postoperative rehabilitation program supervised by a physiotherapist twice weekly from the first postoperative week through week 12, together with a home exercise program. The rehabilitation protocol is phase- and criterion-based. Early rehabilitation focuses on pain and edema reduction, restoration of knee range of motion, quadriceps activation, and gait recovery. Later phases progressively add neuromuscular, balance, lumbopelvic stabilization, closed-chain and open-chain strengthening, and sport-specific exercises. Participants with hamstring tendon grafts begin resisted hamstring open-chain exercises from postoperative week 8, while resisted quadriceps open-chain exercises begin from postoperative week 4 within a limited range and later progress to full range as tolerated.

Outcome assessment includes demographic and clinical data collection, pain ratings, knee muscle strength testing, and patient-reported outcomes. Isometric quadriceps and hamstring strength will be measured at postoperative weeks 4, 8, and 12 and month 6 using an IsoMed 2000 dynamometer with the knee positioned at 60 degrees of flexion. After submaximal familiarization trials, participants will perform three 5-second maximal contractions, and the highest value normalized to body weight will be recorded. Isokinetic concentric quadriceps and hamstring strength will be assessed at months 3 and 6 at angular velocities of 60°/s for 5 repetitions and 180°/s for 15 repetitions. Peak torque normalized to body weight and limb symmetry index will be calculated. Functional outcomes will be assessed at month 6 using the International Knee Documentation Committee Subjective Knee Form and the Knee Injury and Osteoarthritis Outcome Score, and kinesiophobia will be evaluated using the Tampa Scale for Kinesiophobia.

The primary aim is to compare postoperative quadriceps and hamstring isometric strength recovery and functional outcomes between quadriceps tendon and hamstring tendon autograft groups across postoperative weeks 4, 8, and 12 and month 6. The secondary aim is to compare knee symptoms, function, and fear of movement at month 6 and to interpret these findings together with strength asymmetries. The study is considered low risk; possible discomfort is limited mainly to temporary pain, fatigue, or muscle strain during rehabilitation or strength testing. To minimize risk, all procedures are supervised by experienced physiotherapists, warm-up and stretching are performed before testing, symptoms are monitored, and testing is stopped if clinically significant pain occurs. Participant data will be coded and analyzed in anonymized form

Inclusion Criteria:

• Age 13 to 45 years
• Undergoing anterior cruciate ligament reconstruction with either hamstring tendon autograft or quadriceps tendon autograft
• Unilateral anterior cruciate ligament injury
• Complete ACL tear with accompanying minimal collateral ligament injury and first-degree meniscal tear
• Tegner Activity Score of 5 or higher
• Willingness to participate in the study

Exclusion Criteria:

• Anterior cruciate ligament reconstruction with patellar tendon autograft or allograft
• Revision surgery
• Systemic or neurological disease
• Multiple ligament injuries
• Grade III lateral or medial collateral ligament tear or significant articular cartilage lesion
• Failure to regularly attend the rehabilitation program during the first 3 postoperative months
• History of lower extremity injury in the contralateral limb within the last 12 months
• Hypertension, cardiovascular disease, peripheral vascular disease, history of deep vein thrombosis, neurological disease, systemic inflammation, diabetes, cancer, or obesity

Participants will be patients with anterior cruciate ligament reconstruction using either quadriceps tendon autograft or hamstring tendon autograft who are referred by an orthopedist to the Hacettepe University Sports Health Unit, meet the eligibility criteria, and provide voluntary consent