In martial arts, heart rate variability (HRV) measurements can be used to assess injury risk and susceptibility to fatigue. In Muay Thai, the frequency speed of kick test (FSKT) is used to measure kick speed, which may be influenced by autonomic modulation.

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-10 and multiple FSKT to measure the kick fatigue index (KFI). In addition, body composition was assessed using bioimpedance analysis and HRV using a heart rate monitor before and during the FSKT-10s. Inferential analysis consisted of the following methods: (1) the variation between the two moments (before and during the FSKT) was evaluated using the Wilcoxon signed rank test, and (2) correlations between the FSKT-10s or KFI derived from the FSKT-mult and all other variables were evaluated using Spearman's coefficient.

The mean age was 27.9 ± 7.7 years, and all participants were male. Based on body mass index (BMI), 15 (44%) were eutrophic, 12 (35%) were overweight and 7 (21%) were obese. The median number of kicks in the FSKT-10 and the KFI were 20 (17–26) and 20 (14–29) %, respectively. 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 in the FSKT-10s correlated positively with fat-free mass (FFM) and low frequency range (LF). 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, vagal withdrawal, and decreased system complexity. There are relationships between FFM and sympathetic activation with performance and susceptibility to fatigue.

The ability to generate force in the lower limbs with the least amount of fatigue is critical to the success of many sporting events, including amateur Muay Thai fights. In this sense, we used the FSKT-10s technique and its multiple version, as they are among the most used techniques during an official competition. Thus, the results of this study can be used by coaches as guidelines for developing strength and conditioning programs for their Muay Thai athletes in preparation for martial arts competitions.