# Reliability and Validity of the Portable KForce Plates for Measuring Countermovement Jump (CMJ)
## 4 Discussion
In our study, athletes derived from various sports backgrounds (such as football, volleyball, gymnastics, etc.) implicated physiological and biomechanical variables such as muscle force and flexibility, anaerobic characteristics, and optimal neuromuscular control. Therefore, CMJ performance might also involve the stimulation of these mechanisms related to sports performance [25; 36; 37]. As a matter of fact, an improvement in the jump height variable is directly associated with greater sports performance, mainly in athletes with explosive activities [38]. Furthermore, CMJ training protocols are progressively used in various types of sports [39; 40]. Thus, the portable KForce plates might be useful in evaluating athletes competing in explosive-type sports and also for athletes deriving from other types of sports. The reasons to choose the KForce Plates system are that it is portable, lightweight, and low-cost for use by sports coaches and sports practitioners. Its small size allows the assessment of a plethora of variables, such as external forces exerted by each limb separately when using instrumented systems with more than one force plate [20]. Our findings provide a reliable and valid perspective for using the KForce plates in sports and clinical biomechanics, such as in the analysis of functional tests and pylemetric jump training. Moreover, the portability of the instrument allows practitioners to evaluate complex parameters, such as jumping height, leg stiffness, velocity, power, and ground reaction forces in either professional or amateur athletes with accuracy, thus allowing the possibility to improve sports performance, reduce the risk of injury, and manage reliable assessments either in a laboratory environment or in the sports field [3; 4; 5; 6]. The use of the portable KForce plates might help to implement improved training-induced performance adaptations, providing accurate feedback to practitioners and athletes, respectively. The main limitation of our study is that participants were collegiate professional athletes deriving from various sports backgrounds (e.g., football, basketball, and tennis), possibly affecting our results. Future research could explore the reliability and validity parameters in amateur athletes or in a specific sports activity (e.g., basketball). Another possible limitation of our study is the dimensions of the KForce plates. Athletes with a larger foot imprint compared to the size of the KForce plates would need an extension of the surface of the portable plates. This alteration could affect athletes' comfort and their execution of the CMJ and, therefore, the results of the study. Moreover, professional athletes frequently experience alterations to their training schedule due to training decisions or competitions. These parameters could affect the availability for testing and the athletic fitness of each athlete, especially when they come from different sports backgrounds. In our study, we applied the standardized execution of CMJ (athletes had to keep their hands on the hips, and they had to execute a quick downward movement at an estimated 90\({}^{\circ}\) knee flexion and then a quick upward movement in order to perform the highest possible jump) in order to analyze the test--retest reliability. Future studies could apply a variation of CMJ, including arm swing, in order to explore if the jump technique affects the reliability and validity variables of the instruments. Furthermore, the main limitation of our study is that the My Jump 2 application requires the precise manual selection of the video frames in which the athlete executes the take-off and the landing action and, therefore, makes the procedure challenging and subjective. Future studies could include in their methodology design a pilot study prior to the actual execution of CMJ in order to overcome the obstacles of the familiarization procedure for both athletes and researchers.