Reliability of Voluntary Response Index Analysis Using Surface Electromyography in Professional Football Players

Background: Surface electromyography (sEMG) represents a widely utilized technique in medical research. The voluntary response index (VRI) introduces a novel method for analyzing the motor control of voluntary movements via sEMG. This study aimed to assess the reliability of VRI and breakpoint angles (BPA) among professional football players during the Nordic Hamstring test (NHT). Methods: This study included 24 healthy professional football players. Through the use of sEMG, two aspects of VRI [magnitude (MAG) and similarity index (SI)] were evaluated during the NHT. Additionally, BPAs were determined by capturing slow-motion videos of functional tasks. The test-retest reliability across three trials was determined using the intra-class correlation coefficient (ICC). Results: The reliability of the VRI components during the NHT was found to be excellent. The ICC for MAG was 0.9 and 0.81 for the SI. Breakpoint angles exhibited moderate reliability (ICC = 0.65) during a functional task. The SI displayed a lower standard error of measurement (SEM = 0.07). The MAG’s SEM was 0.25, while BPA’s SEM was 17.8. Conclusions: The analysis of the VRI via sEMG showed nearly perfect reliability in professional football players during the NHT, a functional task. Breakpoint angles demonstrated significant reliability. The findings of this study indicate that both VRI and BPA are reliable methods for estimating motor control during dynamic movement tasks in professional athletes.