# Reliability of a Portable Fixed Dynamometer During Different Isometric Hamstring Assessments
## 1 Introduction  
Hamstring strains are one of the most common non-contact injuries in sports such as soccer [1], sprinting [2], rugby union [3], and many other team sports that require high-intensity bouts of maximal sprinting [4]. There are two main non-modifiable risk factors for hamstring strain injuries, such as older age and previous hamstring injuries, with reinjury rates reported at 12-13% [5; 6]. However, there are several modifiable risk factors for hamstring strain injuries, such as fatigue, biceps femoris fascicle length [7], high-speed running loads [4; 8], strength [9], interlimb asymmetry--the difference between left and right leg [9; 10]--and intralimb asymmetry--the difference between quadriceps and hamstring strength within the same limb [2; 11]. Given the importance of strength and asymmetry for minimizing hamstring strain injury risk, it seems important to screen and track hamstring strength on a regular basis. Isokinetic dynamometry is considered the 'gold standard' screening tool for the assessment of hamstring strength [12; 13]; however, the use of this equipment comes with an array of limitations, such as high cost, lack of portability and considerable training required by the assessor [14]. There have been numerous studies that have used cheaper and more portable pieces of equipment for measuring hamstring strength, such as force plates [15; 16], strain gauge/load cells [17; 18] and handheld dynamometers [19; 20]. Force plates have been used across a range of different hamstring assessments, such as the supine 150:30\({}^{\circ}\)[15; 16; 21], supine 90:90\({}^{\circ}\)[16; 21; 22] and standing 90:20\({}^{\circ}\) (hip angle/knee angle) tests [22]. Research groups have reported acceptable within- (CV's = 4.85-8.61%, ICC = 0.83-0.93) and between-session (CV's = 4.34-8.72%, ICC = [PERSON]) reliability for peak force, as confirmed by a force plate (500-1000 Hz) measuring isometric hamstring strength during the 150:30\({}^{\circ}\)[15; 16; 21]. Additionally, [PERSON] et al. [15] also reported on the within-session reliability of rate of force development (RFD) over time intervals of 50-100 ms and 100-150 ms relative to the onset of contraction; however, despite the strict methodological procedures, RFD was reported as not reliable (CV = 18.42-28.77%, ICC = [PERSON]). In terms of the supine 90:90\({}^{\circ}\) test, researchers have also reported acceptable within- (ICC = [PERSON], CV = 5.32-7.72%) and between-session (ICC = 0.70-0.95, CV = 2.84-10.23%) reliability for peak force [16; 18; 21]. [PERSON] et al. [18] also reported acceptable reliability for average RFD over 100 ms and 200 ms (CV = 8.33%, ICC = 0.64, CV = 4.40%, ICC = 0.61, respectively). To the authors' knowledge, the standing 90:20\({}^{\circ}\) test has only been used previously by one group of researchers [22], who only reported on absolute consistency (CV = 7.3-11.0%)[PERSON] Though force plates have been reported to be reliable during these isometric hamstring assessments, they still pose some limitations around transportability and cost. For many years, practitioners have utilized handheld dynamometers (HHDs) as cost-effective alternatives for isometric assessments during rehabilitation. Difficulties have been documented regarding the use of HHD, as tester strength can significantly impact the results of the test [23]. Researchers have reported acceptable inter-rater reliability for HHDs among different hamstring isometric assessments, such as prone 15\({}^{\circ}\) (knee flexion) (ICC = 0.74-0.83, SEM = 29-31), 90\({}^{\circ}\) (knee flexion) (ICC = 0.71-0.76, SEM = 23-26) [19], supine 90\({}^{\circ}\) (ICC = 0.71-0.83, SEM = 42-32) [24] and seated hamstring curl (ICC = 0.91) [25]. To the authors' knowledge, there is no research determining the between-session reliability of HHDs during isometric hamstring assessments. In more recent times, the use of strain gauge and load cell technology has been implemented by practitioners.