## 4 Discussion  
This study examines the reliability of a field-based test to assess isometric hamstring and quadriceps strength at 30\({}^{\circ}\) of knee flexion and 90\({}^{\circ}\) of hip flexion in football players. Results suggested an acceptable level of reliability for testing MVIC strength of both isometric hamstring and quadriceps muscles and for the calculation of the H:Q muscle strength ratio. However, although some RFD metrics yielded reliable results, especially for larger phases (>150 ms after contraction onset), caution is needed when interpreting these values due to the large errors of measurement. These findings contribute to the existing knowledge on field-based tests to examine lower limb strength in football by providing results from a new tool to easily assess isometric hamstring and quadriceps strength. Another important contribution of the present field-based protocol using a strain gauge is the allowance for testing consecutively the strength of both flexor and extensor muscles from the same position, which may save time during evaluations. Information about errors of measurement for each metric may have practical implications to assist physicians and coaches in a more accurate description of players' strength status and their evolution during the injury management and return-to-play processes. In line with our results, MVIC has been suggested as the more reliable measurement during isometric extension [15; 17; 25] and flexion tests [15; 26] at angulations close to full extension (0-45\({}^{\circ}\)). However, because changes in the knee angle may lead to different mechanical outputs, the reliability of the MVIC can be compromised if conducting tests at different knee and hip positions [35]. It is important to note that hamstring strain injuries are the most common injury in several sprint team sports such as football [36]. It is well recognized that most (83%) of hamstring strain injury occurs to the biceps femoris [22] during a terminal swing. Several studies have suggested that inadequate levels of hamstring strength and imbalances between the hamstring and quadriceps strength are potential risks factors for acute hamstring strain injury and re-injury [5; 6; 7]. Furthermore, the anterior cruciate ligament is often injured in a position where athletes have a relatively straight knee [23]. A recent review highlighted the importance of monitoring hamstring strength and its relationship with the quadriceps assisting in the function of the anterior cruciate ligament [8]. Thus, MVIC measurements of the isometric hamstring and quadriceps muscles strength at 30\({}^{\circ}\) of knee flexion and 90\({}^{\circ}\) of hip flexion could provide reliable knowledge of the functional status of the knee and assist in the prevention of hamstring strain and anterior cruciate ligament injuries, as they are in a key position during the injury [22; 23]. The RFD metrics can provide significantly different information than MVIC assessment on neuromuscular determinants [20], being more sensitive to fatigue [37]. However, the present results show a high variability of the outcomes which may limit its reliability in the practice. Although previous studies have evaluated the RFD reliability of knee extension [16; 25] and flexion [26], it is difficult to make comparisons as there are no reports about the reliability of isometric hamstring and quadriceps muscle strength tests at 30\({}^{\circ}\) of knee flexion and 90\({}^{\circ}\) of hip flexion. The observed variability in RFD measurements may be due to some methodological considerations made to adapt to the context of a team. The test-retest was carried out in 7 days, despite the recommendations to perform it in an interval close to 48 h [20]. In addition, it was only possible to leave 24 h without vigorous exercise because the athletes were in a competitive period. RFD has been suggested as a more sensitive measure for acute and chronic changes in neuromuscular function than MVIC [20].