Autonomic responses and internal load analysis through acute assessment of heart rate variability after a high-intensity functional training session

Backgroud: Heart rate variability (HRV) measurement is an important tool that may help indicate possible physiological changes, and HRV monitorization could be a great strategy for clinical analysis (autonomic control) and performance (internal load). Objective: The aim of the present study was to evaluate autonomic responses and internal load through HRV during a high-intensity functional training (HIFT) session. Material and method: Thirty-three individuals (22 men and 11 women) participated in the study (Age: Mean = 34.9 ± 7.2 years; Weight: Mean = 72.3 ± 13.7 kg; Height: Mean = 1.72 ± 0.1 m; BMI: Mean = 24.4 ± 3.0 kg/m^2). All participants engaged in a 60-to-90-minute HIFT session. HRV analysis was performed during the specific warm-up period (targeted warm-up or skill training that followed the general mobility and light cardiovascular warm-up), during exercise (approximately 50 minutes), and in the recovery phase (10 minutes post-training). A Polar H10 heart rate monitor chest strap (Kempele, Finland®) was used to collect HRV and was connected to the Elite HRV mobile application. The data were further transferred to Kubios HRV Standart software, version 3.3.1, in order to process the acquired data. Results: For isolated analyzes (pre- and post-), differences were found for SDNN (P<0.001), RMSSD (P<0.001) and HF (P=0.041), yet not for LF / HF (P=0.483). In the analysis of HRV kinetics, significant results were found between moments for RR, SDNN, RMSSD, LF and HF (P<0.05). In the analysis of the internal load, the highest level of stress was identified in 40 (P=0.010) and 50 minutes of exercise (P=0.001), as well as in recovery (P<0.001), this assessment being carried out through HRV through the LnRMSSD index. A negative correlation was observed between maximum heart rate (HRmax) and LnRMSSD at 40 (r=-0.51) and 50 minutes of exercise (r=-0.58). In recovery, the correlation was positive, yet insignificant (r=0.032). Conclusion: The present study observed that HIFT could alter HRV and thus cause changes in autonomic behavior. In addition, this type of modality can offer significant levels of training loads, thus affecting the physiological responses and consequently the individual’s functional efficiency.