Relationships between internal training intensity, heart rate variability, sleep duration, and neuromuscular performance in professional soccer players

Purpose. This study examined the relationship between internal training intensity and physiological and performance markers, including heart rate variability, resting heart rate, sleep duration, and countermovement jump performance in professional soccer players during a five-week preseason. Methods. This longitudinal design included 10 professional soccer players (age: 20.8 ± 2.3 years) with the following variables measured weekly: ITI, heart rate variability (HRV), resting heart rate (RHR), sleep duration, and countermovement jump (CMJ) performance. Results. Results showed a significant reduction in ITI (p < 0.001) across the preseason, reflecting effective intensity manage-ment. CMJ performance showed an initial decline before significantly increasing after week 2 (p < 0.001), while no significant changes were observed in RHRmean (p = 0.25) or lnRMSSDmean (p = 0.27). However, lnRMSSDCV decreased by 39.9%, indicating improved autonomic stability. A moderate negative correlation was observed between lnRMSSDmean and lnRMSSDCV (r = –0.54, p < 0.001), suggesting that greater parasympathetic tone is associated with reduced daily HRV fluctuations. ITI showed moderate negative correlations with Sleepmean (r = –0.32, p = 0.045) and CMJ performance (r = –0.43, p = 0.005) and a positive correlation with lnRMSSDCV (r = 0.35, p = 0.03). Conclusions. Higher ITI is associated with reduced CMJ performance, likely due to fatigue, emphasising the need to monitor CMJ for optimising training loads and preventing overtraining. Similarly, its link to shorter sleep duration highlights the importance of tracking sleep for effective recovery management. Additionally, the inverse relationship between lnRMSSDmean and lnRMSSDCV suggests that greater parasympathetic activity stabilises HRV fluctuations, reflecting improved autonomic balance and aiding in training adjustments. This study highlights the importance of integrating HRV, sleep, and CMJ into preseason training programs for professional soccer players. A tailored training approach emphasising reduced ITI and monitoring autonomic stability and recovery effectively improves physiological resilience and neuromuscular performance, ensuring players’ readiness for the competitive season.