**2. Materials and Methods**

## *2.1. Patient Recruitment*

From January 2021 to the end of December 2021, 37 consecutive patients who underwent ACL reconstruction at the Department of Sport Traumatology and Minimally Invasive Surgery of our Institution were prospectively enrolled and evaluated pre-operatively and 6 months after surgery. Diagnosis was done with clinical examination and knee magnetic resonance imaging, and further verified arthroscopically. All surgeries were performed by the same senior surgeon.

Inclusion criteria were: primary unilateral ACL reconstruction; age ≥ 18 years and ≤45 years at surgery; time from injury to surgery ≥ 2 weeks and ≤12 months; participation in sporting activity; same postoperative rehabilitation protocol. Exclusion criteria were: past history of ligamental surgery on the same or contralateral knee; concomitant surgical procedures with the exception of treatment for meniscal pathology; pregnancy; inability to complete clinical and functional testing.

The study received Institutional Review Board approval (IRB number: 57/INT/2020, released from IRCCS San Raffaele Hospital, Milan, Italy). All participants signed informed consent.

## *2.2. Surgical Technique and Rehabilitation Protocol*

All patients underwent arthroscopic-assisted ACL reconstruction using doubled autologous hamstring graft [6]. Tibial tunnel was drilled using a 55◦ guide (Acufex; Smith & Nephew, Andover, MA, USA) using as reference the posterior cruciate ligament, while the femoral half-tunnel was prepared either through the medial portal or with a trans-tibial technique. Fixation was achieved proximally with a cortical suspension device (Retrobutton; Arthrex Inc., Naples, FL, USA) and distally through a bioadsorbable interference screw (Milagro; DePuy Mitek, Raynham, MA, USA). A brace-free rehabilitation protocol starting the day after surgery was adopted in all patients, with immediate regaining of extension, isometric exercises, and walking with crutches with partial weight bearing for the first 3 weeks. Swimming and indoor cycling were allowed after 12 weeks, while after 5 months a protocol of jump technique training and plyometric exercises was started.
