## 2 Materials and Methods
### Procedures  
Evaluations were performed independently on two separate occasions one week apart, at the same time to produce a test-retest experimental design. On each occasion, participants performed 2 s maximal isometric efforts on a custom-built bench with 30\({}^{\circ}\) of knee flexion and 90\({}^{\circ}\) of hip flexion while the force was measured with a portable load cell. Participants performed two maximal isometric repetitions intending to flex and extend the knee in each lower limb to assess isometric hamstring and quadriceps strength, respectively. Leg dominance was determined in a self-reported manner by asking for the leg used to kick a ball toward a target [29]. Before the onset of the testing, football players performed a standardized and specific warm-up consisting of 2 sets of 6 s of three rapid response neuromuscular activation exercises (base rotations, side to side over a line and 2-inch run in place) [30] and 2 submaximal contractions for each movement (flexion and extension) in both legs (dominant and non-dominant). After the warm-up, the test was performed on a custom-built bench (75 cm high) while participants were in a seated position at 30\({}^{\circ}\) of knee flexion and 90\({}^{\circ}\) of hip flexion with stabilization straps around the thighs (Figure 1). A portable S-Type Stainless Steel Load Cell (Model 620 Tedea-Huntleigh, Vishay Precision Group Inc., Holon, Israel) was attached to a rigid bar anchored, and the rigid bar was linked to the ankle by using a strap placed above the malleolus. The length of the bar was adjusted to maintain the 30\({}^{\circ}\) of the knee flexion and to be perpendicular to the shank, allowing the strain gauge to collect linear force generated by the hamstrings and quadriceps. The bench and the bar were reinforced to guarantee mechanical rigidity and minimize joint movement. The load cell was connected to an A/D converter to obtain force data at 80 Hz, and the software Chronojump (Chronojump Bosco System, Barcelona, Spain) was used to filter and smooth the signal using the settings recommended by the manufacturer.  
Data for MVIC and RFD metrics at different time phases from the onset of contraction (RFD\({}_{\text{max}}\), RFD\({}_{\text{b-50}}\), RFD\({}_{\text{b-150}}\) and RFD\({}_{\text{b-250}}\)) were automatically collected during each repetition. All players were familiarized with the procedures one week before evaluations to avoid the influence of the learning effect [16]. Participants first performed two executions to assess hamstring strength with a 20 s rest period between repetitions. Then, after a 30 s rest, participants consecutively performed two executions to assess quadriceps strength with the same leg used to assess hamstring strength. After 5 min of recovery, the other leg was evaluated following the same process. Participants were instructed to contract knee muscles \"as fast and hard as you can while keeping your torso upright\" with the emphasis on the explosive/ballistic phase of contraction for 2 s. There was a standardized verbal encouragement to produce maximal efforts [16]. Quadriceps strength was evaluated with a traction isometric effort and hamstring strength with an isometric compression effort. The starting leg (dominant or non-dominant) was counterbalanced. Attempts in which the participant flexed or extended his trunk or hips were considered invalid. Two researchers were present for the assessment and determined the validity of each repetition visually. The mean value from both attempts was used for the analysis. The strain gauge was calibrated before each session using a 5 kg weight, according to the manufacturer's specifications.