# Reliability of a Portable Fixed Dynamometer During Different Isometric Hamstring Assessments
## 4 Discussion
To the authors' knowledge, this is the first study to compare hamstring tests. Our results show significantly higher peak force (35.26-40.71%), mean force (34.67-43.92%), and total impulse (33.42-47.31%) during the 90:20 assessment, compared to the standing hamstring and 90:90 assessment. Only impulse was found to be significantly different between the standing hip assessment and the 90:90 assessment (14.46%), with greater impulse being produced during the 90:90 assessment. These differences in findings are likely due to two key factors. Firstly, the length-tension relationship of muscle will influence how much force can be produced at longer compared to shorter muscle lengths ([PERSON] et al., 2018). Secondly, different angles put emphasis on different muscles of the hamstring complex; for example, the biceps femoris muscle has been shown to be maximally activated between 15\({}^{\circ}\) and 30\({}^{\circ}\) of knee flexion, i.e., the 90:20 assessment, whereas the semi-membranous and semi-tendinous muscles are maximally activated at knee angles between 90\({}^{\circ}\) and 105, i.e., the 90:90 assessment ([PERSON] et al., 2018). Therefore, when choosing a hamstring assessment, practitioners should consider these factors. Given the findings of the current study, it appears that the results of each test are not interchangeable, and therefore, each test needs to be performed in isolation, i.e., comparisons cannot be made across tests. Furthermore, it might be that a short to long muscle length approach to the testing of the hamstrings may better align with early- to late-stage rehab, given the location or the severity of the hamstring strain. Given the prevalence of hamstring injuries in sport ([PERSON] et al., 2017; [PERSON] et al., 2018; [PERSON] et al., 2018) and the modifiable risk factors such as strength and asymmetry, it seems important to have reliable assessments that can assess hamstring strength at a range of different muscle lengths, and during all stages of rehabilitation. As mentioned previously, for the majority of the existing research, peak force is typically the only variable reported. Whilst it is an important variable to measure and monitor, it only represents an athlete's force capability for a small moment in time, i.e., 1/1200 th of a second in this study. Variables such as mean force and impulse are more representative of overall force capability and may better represent hamstring strength. All mean force and impulse measures had acceptable reliability within and between sessions. Based on the results of this study and previous research ([PERSON] et al., 2018; [PERSON] et al., 2018), RFD metrics appear to be unreliable. This finding is consistent with the literature, which states that RFD is typically less reliable than maximum force and substantially so during the early phase of contraction ([PERSON] et al., 2018). Therefore, further instructions in the methodology may be required to improve the consistency of this measure. It needs to be noted that the subjects of this study were self-reported injury-free healthy males; therefore, these results should only be generalized to that specific population. Future research should focus on determining the reliability of this device in females and specific sporting populations.