Lower-limb muscle strength testing

The lower limb muscle strength assessments were conducted at the Rehasport Clinic, recognized as a FIFA Medical Centre of Excellence, in Poznan, Poland. All measurements were carried out by the same team of researchers. Knee muscle strength was evaluated using the Biodex System 3 dynamometer (Biodex Corp, Shirley, NY, USA), measuring peak torque (PT) and total work (TW). High test-retest reliability was observed (intra-class correlation coefficient [ICC] = 0.93–0.95) [33]. Procedures concerning the alignment of participants’ dynamometer rotation axis, positioning, gravity correction, and stabilization adhered to established guidelines outlined in the literature [34,35]. Prior to the isokinetic assessment, each participant underwent a 10–15-minute warm-up session, including pedaling on a Monark 828E Ergomedic stationary cycle ergometer (Monark, Vansbro, Sweden) at a moderate pace (50–100 watts) and dynamic stretches targeting major lower limb muscle groups [34,36]. Isokinetic torque of the quadriceps and hamstring muscles was assessed through continuous, bidirectional knee extension-flexion movements at angular velocities of 60° ·s^-1 and 240° ·s^-1, spanning a range of motion from 0° (flexion) to 90° (full extension). Testing at angular velocities of 60° ·s^-1 and 240° ·s^-1 has been widely employed in previous studies investigating muscle strength in soccer players [11,12,14,19]. Participants underwent three trials at submaximal efforts, gradually increasing the load to 50%, 75%, and approximately 100% of their maximum capability, followed by one set of three repetitions at maximal concentric contraction at an angular velocity of 60° ·s^-1 and 30 repetitions at an angular velocity of 240° ·s^-1. The same protocol was repeated for the opposite leg. Participants were given a 30-second rest after the third submaximal trial, followed by a one-minute break between angular velocities, and a three-minute break during which machine settings were adjusted for the opposite leg. Participants were instructed to complete the movement through the full range of motion. The order of testing for dominant (DL) and non-dominant (NDL) legs was randomized [34,35]. Dominance was determined based on participants’ preferences when kicking a ball [34]. Only windowed data was analyzed to focus on constant velocity periods. Statistical analysis included relative PTs (normalized by body weight and expressed in Nm·kg^-1) for flexors (PT-Q) and extensors (PT-H) in both legs, unilateral muscle torque ratios for dominant and non-dominant extremities (HDL/QDL and HNDL/QNDL, respectively) at an angular velocity of 60° ·s^-1, and relative TWs (J·kg^-1) for extensors (TW-Q) and flexors (TW-H) for both legs at an angular velocity of 240° ·s^-1. All tests were conducted before 1 pm to minimize inter-day variability and were performed in the same sequence for each participant. Participants refrained from intensive training for 48 hours preceding the testing. Before commencing the test, participants completed a questionnaire regarding any musculoskeletal pain, discomfort, or known lower extremity injuries. Participants with major or moderate lower leg, knee or thigh injuries were excluded from further analysis. None of the participants had a history of significant knee injuries, anterior cruciate ligament (ACL) repairs or rehabilitation, leg fractures, or surgeries within the year preceding the evaluation.