Reliability and Reproducibility of the Kinvent K-push Dynamometer for Assessing Quadriceps Strength and Force Development in Athletes and Untrained Individuals

Robert Trybulski^1,2*Jakub Więckowski³Jarosław Muracki^4*Filip Matuszczyk²Kamil Gałęziok²Michał Wilk⁵Adrian Kużdżał⁶

• ¹Medical Department, Wojciech Korfanty Upper Silesian Academy, Katowice, Silesian, Poland
• ²Provita Medical Center, Żory, Poland
• ³National Telecommunications Institute, Warsaw, Masovian, Poland
• ⁴Institute of Physical Culture Sciences, Department of Physical Culture and Health, University of Szczecin, Szczecin, Poland, Szczecin, West Pomeranian, Poland
• ⁵Institute of Sport Sciences, Jerzy Kukuczka Academy of Physical Education in Katowice, Katowice, Silesian, Poland
• ⁶Institute of Health Sciences, College of Medical Sciences, University of Rzeszów, Rzeszów, Poland

Objectives: This study aimed to (1) analyze the reliability of the Kinvent K-push handheld dynamometer for assessing quadriceps muscular voluntary contraction (MVC) and rate of force development (RFD), and (2) evaluate inter-rater reliability by assessing the reproducibility of measurements between examiners. Methods: A blinded, randomized, comparative study evaluated the reliability of quadriceps femoris MVC and RFD measurements obtained by two independent researchers. Forty-four male volunteers participated, divided into three groups based on motor skill level: elite mixed martial arts (MMA) athletes (n=15), amateur football players (n=15), and untrained healthy volunteers (n=14). Three measurements per leg were taken during the experimental session, with the Kinvent K-push handheld dynamometer automatically calculating the average MVC and RFD. Results: Pearson correlations revealed that MVC and RFD measurements were highly consistent, with the left quadriceps showing nearly perfect correlations (r = 0.96 for MVC, r = 0.97 for RFD), while the right quadriceps displayed more variability, particularly in MVC. Inter-examiner comparisons showed high reproducibility, with minimal differences between the two measurers (p>0.05). The ICC further supported this, with very high intraclass correlation values, especially for RFD (ICC = 0.999), confirming almost perfect agreement between the measurers. Group comparisons revealed that athletes, particularly MMA fighters and footballers, had significantly higher muscle strength and RFD compared to the general non-training group (p<0.05), with athletes showing similar values for both MVC and RFD, while the general non-training group exhibited greater variability in both parameters. Bland-Altman analysis revealed strong agreement between measurers across all quadriceps measurements, with minimal systematic bias and acceptable variability, particularly in MVC assessments. Conclusions: MVC and RFD measurements in the quadriceps using the K-push handheld dynamometer are highly reliable and consistent, with minimal inter-examiner variability. These findings, in conjunction with high ICC and low MAE/MSE values, underscore the reliability of the measurement protocol used in this study.The tested instrument provides consistent and accurate results, ensuring reliable measurements across different examiners.