## 4 Discussion
The demand of a competitive week itself may be sufficient to alter muscle activation, mediated by the motor neuron discharge rate and the recruitment threshold [38; 39] influencing the greater variability of RFD measurements during the early stages. On the other hand, although stabilization straps were used for the knee during the execution of the tests, the athlete's torso was not stabilized. The participant was instructed to keep his torso upright, and repetitions that were not maintained during the whole effort were discarded. However, there is still the possibility of hip and pelvic compensations, as they were not totally controlled. This may have modified the degree of muscle-tendon stiffness and have had a major influence on the time to reach a given force [40]. One last finding of this study was the calculation of the H:Q muscle strength ratio to check whether MVIC and RFD metrics can yield reliable values using a field-based protocol. Previous studies demonstrated that a reduced H:Q muscle strength ratio could increase four to five times the risk of sustaining hamstring injuries [27]. This is explained by the fact that weaknesses of hamstring muscles are associated with an imbalance in the propulsive quadriceps maximal torque during extended knee movements, and that hamstring muscles are responsible for deceleration actions that frequently occur before an injury situation [8]. According to our results, MVIC records can be used to calculate the H:Q ration with sufficient reliability (ICC > 0.75, [95% CI: 0.48, 0.95]; CV < 15%, [95% CI: 8.9, 22.4]), using a portable force sensor at 80 Hz. This seems important because coaches are now provided with a tool to easily screen the H:Q ratio, well-documented to be related to the risk of injuries in football [27; 41]. In contrast to our expectations, the current protocol and equipment were not good enough to provide reliable RFD ratio metrics. Given the increasing interest in these outcomes, future studies are encouraged to further investigate RFD measures with field-based devices to obtain additional information on neuromuscular determinants which can be critical in the injury management and return-to-play process. The current experiment and the field-based test presented here possess some limitations that should be discussed. First, the custom-built bench used for this study does not have a backrest to fix the torso to ultimately avoid trunk flexion and extension during the isometric effort. This is a difference from the more traditional hamstring and quadriceps strength measurements, such as the isokinetic dynamometer. Although we acknowledge the importance of trunk control when applying lower extremity strength, we designed the bench without a backseat to facilitate the adoption of 90\({}^{\circ}\) hip flexion in participants with different body characteristics. However, it is important to note that researchers interested in this field test have to observe each repetition and discard those in which the participant flexes/extends his/her trunk visibly. The second limitation is that this test-retest study was developed with juvenile football players, and its reliability should be tested in other football populations such as professional football players and female football players before its application as a field test. The strengths of the work include the simplicity and cost of the load cell-based devices, which improve the applicability of the test over the use of laboratory-based equipment. In addition, our device features a rigid bar that allows the evaluation of the hamstring and quadriceps muscles sequentially, ensuring the mechanical rigidity needed for a correct isometric strength evaluation.