Relationships Between Performance in the Frequency Speed Kick Test, Heart Rate Variability, and Body Composition in Amateur Muay Thai Fighters

In martial arts, heart rate variability (HRV) measurements can potentially be used to assess injury risk and susceptibility to fatigue. Muay Thai uses the Frequency Speed of Kick Test (FSKT) to measure kick speed, but it is unknown whether better fitness results in improved autonomic function and body composition, which may affect athletic performance and recovery. We aimed to evaluate the correlations between FSKT, HRV, and body composition measures in amateur Muay Thai fighters. This is a cross-sectional study with 37 Muay Thai fighters. They underwent FSKT-10s and multiple FSKT to measure the kick fatigue index (KFI). In addition, body composition was assessed using bioelectrical impedance analysis and HRV using a heart rate monitor. When comparing the values before and during the FSKT-10s, there was a decrease in the percentage of interval differences of consecutive successive NN intervals greater than 50 ms (pNN50), approximate entropy, and parasympathetic nervous system (PNS) index, with an increase in the sympathetic nervous system (SNS) index. The number of kicks assessed by FSKT-10s positively correlated with fat-free mass (FFM) and low frequency (LF) of the HRV assessment. The KFI correlated negatively with the FFM and PNS index, and positively with the LF and SNS index. When amateur Muay Thai fighters perform the FSKT-10s, there is an acute effect on autonomic modulation with increased sympathetic activation and vagal withdrawal. FFM and sympathetic activation are related to performance and susceptibility to fatigue. These results indicate the importance of assessing HRV in Muay Thai fighters to improve performance.