# Reliability and Validity of the Portable KForce Plates for Measuring Countermovement Jump (CMJ)
###### Abstract  
Physical fitness is of great significance to athletes in both single-player and team sports. The countermovement jump (CMJ) is one of the most commonly applied jump tests for assessing the mechanical capacities of the lower extremities. The KForce Plates system is a portable force platform that sends action-time audio and visual biofeedback to a smartphone or tablet through the KForce application, making it a suitable instrument for assessing the CMJ. The aim of the present study was to evaluate the test-retest reliability and validity of the portable force platform (KForce Plates) in the evaluation of the CMJ in collegiate athletes compared to a validated application, My Jump 2. Thirty-four collegiate professional athletes, deriving from various sports backgrounds, participated in the present study. The CMJs were reported with the portable KForce Plates and the simultaneous use of the 'My Jump 2' application using an iPhone 13 between days 1 and 7. Our findings revealed high test-retest reliability (ICC = 1.00 and ICC = 0.99) in-between measurements. High correlations were monitored amongst the portable KForce plates and the My Jump 2 application for measuring the CMJ (r = 0.999, \(p=0.001\)). The Bland-Altman plot exhibits the limits of agreement amongst the portable KForce plates and the My Jump 2 application, where the bulk of the data are within the 95% CIs with an agreement of \(\approx\)1 cm. Our findings suggest that the portable KForce Plates system is a reliable and valid instrument and, therefore, can be used by experts in the sports field. reliability; validity; KForce plates; My Jump 2; countermovement jump +
Footnote †: journal: _Applied Sciences_  
time, mean power, peak power, etc.) [10; 12]. Jump height is the mainstay of performance factors, as it has a direct association with performance in sports-specific tasks and athletes' mobility [10; 13; 15]. Several different methods exist for measuring jump height due to the simplicity of the testing procedure and the eco-friendly nature of the impact in comparison to other conventional methods (e.g., isokinetic tests) [10]. However, despite the technological advancements for measuring biomechanics and sports performance, force platforms are still deemed the gold standard for measuring CMJ performance [10; 11; 13; 15; 16]. They provide sports practitioners with highly reliable equipment, helping them to assess a profound number of variables. Force platforms are typically stabilized on the ground, thus preventing any unrelated vibrations from raw force assessments [17]. One obvious limitation of the in-ground force platforms is that they are not portable, and therefore, measurements can only take place in a laboratory environment. However, several instruments provide a valid and reliable jump height evaluation, such as camera-based methods [15], accelerometers [18], and infrared platforms [19]. Nowadays, a number of portable force platforms (e.g., Kinvent, Kistler, and Biodex) are on the market, giving clinicians new opportunities to explore aspects and variables of the applications of CMJ. The KForce Plates system (by Kinvent) is a portable force platform that is composed of a unidirectional strain gauge (vertical axis) and an electronic pressure transducer that connects through Bluetooth to a smartphone or tablet application (KForce, Kinvent, Montpellier, France) and, therefore, is an interesting substitute for the in-ground force platforms [20]. Validation of portable devices such as KForce plates is limited. [PERSON], [PERSON], and [PERSON] [20] found in their study that plates were valid and reliable for assessing unipodal static and dynamic balance in the laboratory or in the sports field, suggesting that CMJ measurements and their factors (e.g., jump height and flight time) will also prove the KForce plates' reliability and validity.