Familiarization and Reliability of the Isometric Knee Extension Test for Rapid Force Production Assessment  
[PERSON]  
1 Faculty of Sport Sciences, Human Performance and Sports Science Laboratory, University of Murcia, 30720 Murcia, Spain; [EMAIL_ADDRESS] (A.H.-B.); [EMAIL_ADDRESS] (A.C.-M.); [EMAIL_ADDRESS] (J.G.P.)  
1 Faculty of Sport Sciences, Human Performance and Sports Science Laboratory, University of Murcia, 30720 Murcia, Spain; alejandro.hernandez7@um.  
Current resistance training practices involve the use of technology to obtain real-time data about neural and muscular determinants of performance [11; 12]. Muscular determinants during isometric exercises are conventionally assessed by the maximal voluntary concentric force (MVC), which refers to the highest force the individual is able to produce during the test [7; 13]. The MVC is demonstrated as a reliable and easy-to-obtain variable during the isometric knee extension test with portable force sensors [14; 15]. However, while most of sport-related movements (i.e., jumping, sprinting, kicking) involves rapid contraction times (50-250 ms), the MVC is typically reached at later phases \(\geq\)300 ms, which may limits its ability to explain performance for rapid actions [13; 16].  
In turn, the ability to produce force rapidly (\(<\)250 ms) mostly relies on neural determinants such as motor unit recruitment, discharge rates and force twitches [8; 12]. Neural factors are commonly assessed during isometric tests by the rate of force development (RFD) exerted within the early phase of rising muscle force, and the contractile impulse that can be produced within a given contraction time [16; 17]. Despite the rising interest in the use of isometric testing to determine improvements in ballistic sport-specific movements [6; 7], the design of practical field-based methods to obtain reliable RFD and impulse measures at different phases of the contraction is an ongoing challenge [18; 19; 20; 21]. This is further confounded by the fact that the measurement of both RFD and impulse is highly dependent to the testing procedures [9; 10], the signal filtering [19] and the warm-up [22]. Hence, a further examination of the quality and consistency of the RFD and impulse measures is of critical importance.  
To provide these insights would have practical implications for coaches and therapists, since nowadays one can easily collect automatically RFD and impulse data while providing real-time visual feedback using low cost, portable force sensors attached to a bench or table [9]. To the best of our knowledge, only one previous study has examined the reliability of rapid force production variables during the knee extension [21]. Albeit showing promising results, the fact that they conduced time-consuming testing procedures (separate tests for MVC and RFD which doubled the time required) makes difficult to transfer their results into the sport daily practice thus requires a field-based replication. Furthermore, despite side-to-side asymmetry in quadriceps RFD has gained interest as a screening tool for injury management [23], little is known about the differences between dominant and non-dominant limbs when testing the rapid force production during the knee extension test in athletes. Besides, there is no quantitative data describing the influence of a familiarization session to maximise the reliability of RFD during the knee extension strength test [9].  
Therefore, the current study aimed to determine the intra-session and test-retest reliability of a rapid, isometric knee extension test to evaluate the MVC, RFD and impulse on both limbs in young athletes unfamiliar with the test, following a field-based approach. Based on the existing literature, we hypothesize that late contractions \(>\)250 ms would reach sufficient reliability since the first session, whilst earlier phases would require a previous familiarization.