4.1 HIIT Analysis of The Reasons for The Change of Athletic Performance In Middle-Distance Running 800 Meters
This study focuses on utilizing heart rate variability indicators to discern the disparate effects of two distinct training methodologies, HIIT and MICT, and whether they yield significant enhancements in sports performance. The findings revealed that post-HIIT intervention, exercise performance markedly surpassed that of the MICT group, with significant disparities evident in the sample t-test conducted pre and post intervention.
Upon delving into the internal milieu of the body, it's apparent that the transfer rate of oxygen to working muscles has been enhanced. Notably, maximum heart rate remains unaffected by external factors. The elevated oxygen demand during high-intensity exercise can be ascribed to augmented ejection volume per unit time, primarily attributed to heightened blood volume at the conclusion of diastole and ventricular contraction. Reinforcing these pathways [20], research has substantiated that short-term HIIT training augments atrial volume [22]. Increased blood volume plays a pivotal role in sustaining cardiovascular equilibrium and body temperature, emblematic of athletes' superior athletic prowess. Another potential mechanism entails the body's adaptation to increased skin blood flow and sweat rate amid prolonged high-intensity exercise, culminating in a degree of heat adaptation. Studies indicate that body temperature can reach approximately 40°C during such exertion[23], underscoring the imperative of enhancing students' heat resistance to bolster sports performance [24].
Externally, protracted exercise training undoubtedly enhances muscular development[25]. Hence, refining and optimizing training methodologies atop traditional approaches is imperative to elicit greater efficacy. Following extended HIIT training among the general populace, the performance capacity of type I muscle fibers significantly escalates, facilitating the body's resynthesis of creatine phosphate and enhancing lactate oxidation capacity, thereby positively impacting subjects' aerobic work ability [26]. Additionally, studies indicate that alterations in training duration and intensity can influence the participation ratio of various muscle fibers and energy metabolism in subjects[27], both significantly correlated with athletic performance. Given that 800 meters constitutes a middle-distance running event predominantly reliant on glycolysis for energy, mirroring the energy mobilization characteristic of HIIT, it is evident that the convergence of internal and external factors substantiates the favorable impact of high-intensity interval training on middle- and long-distance running performance. An integrative approach targeting physiological adaptations and muscle function underscores the efficacy of HIIT in optimizing athletic performance, specifically in the domain of middle-distance running.
4.2. Analysis of Heart Rate Variability
Adolescents and adults with exercise experience or individuals with obesity should ideally exhibit stable heart rate variability (HRV) indicators[28]. Upon reviewing pertinent literature, this study emerges as the pioneering work to integrate HRV indicators with exercise performance assessment, exploring the differential impacts of HIIT and MICT on adolescents with exercise experience. Both methodologies significantly enhanced the time domain variable RMSSD and frequency domain variable HF of HRV, with HIIT demonstrating superior improvement in various indicators, including exercise performance.
RMSSD, which reflects the variance in heart rate beats and serves as the primary time domain indicator for gauging vagal changes in HRV [29], is closely associated with the short-term regulatory function of the cardiac autonomic nervous system. Higher RMSSD values typically denote favorable autonomic nervous system function, signifying cardiac autonomic regulation flexibility and robust health. Conversely, lower RMSSD values may correlate with adverse health conditions such as autonomic nervous system dysfunction, inflammation, and cardiovascular ailments[30]. Diminished RMSSD values may elevate the risk of seizures and sudden death[31], likely stemming from compromised vagus nerve function, leading to diminished heart rate regulation. Thus, higher RMSSD values generally indicate active and well-functioning vagal activity, highlighting robust vagal inhibition over heart rate and enhanced regulatory flexibility.
SDNN, reflective of overall HRV magnitude, assumes paramount importance in HRV analysis, representing the standard deviation amid consecutive heartbeats. Elevated HRV signifies greater variability between successive heartbeats, whereas decreased HRV indicates diminished variability over time. Studies[32]suggest that an SDNN value below 50ms denotes an unhealthy state, while values between 50-100ms imply a suboptimal health status. Only when the SDNN value surpasses 100ms is the body deemed relatively healthy. The intervention effectively bolstered SDNN values, positively correlating with endurance event performance. Consequently, it can be tentatively concluded that SDNN enhancement during training exerts a beneficial influence on sports performance, warranting its application in routine monitoring.
The LF frequency band, ranging from 0.04 to 0.15 Hz, encompasses the energy within the low-frequency spectrum of heart rate variability. Historically termed the "baroreceptor range," this band primarily reflects baroreflex activity during the resting state[33], intimately linked with respiratory rhythms. The prevailing consensus suggests that the low-frequency band is predominantly influenced by the sympathetic nervous system and intricately associated with blood pressure and body fluid regulation [34]. Higher LF values are typically indicative of heightened sympathetic nerve activity, whereas lower LF values suggest suppressed sympathetic nerve activity. In certain instances, elevated LF levels may coincide with increased blood pressure, particularly during transient stress responses. In endurance sports, heightened sympathetic modulation of the heart may bolster cardiac function and adaptability, thereby enhancing athletic performance. However, there exists contention regarding this perspective, with some proposing that, during the resting state, the LF frequency band primarily reflects baroreflex activity rather than sympathetic nerve activity within the heart [35].
The HF frequency band, ranging from 0.15–0.4 HZ, regulated by the vagus nerve, predominantly reflects the interplay between respiration and vagal activity. This frequency band, integral to respiratory sinus arrhythmia, underscores the rhythmic heart rate regulation by respiratory activity[29]. Prolonged regular physical training enhances vagal nerve activity within the heart's autonomic nervous system, likely accounting for the observed improvements in RMSSD and HF indicators, predominantly regulated by the vagus nerve.
LF/HF denotes the relative activity levels of sympathetic and parasympathetic nerves in heart rate regulation, along with their equilibrium state[21]. Elevated LF/HF values signify heightened sympathetic nerve activity or diminished parasympathetic nerve activity relative to the sympathetic nervous system. Alterations in LF/HF indicators may be attributed to diverse physiological and psychological factors, including stress, tension, and pathological conditions like cardiovascular diseases and autonomic nervous system disorders. Both sympathetic and parasympathetic nerves play pivotal roles in various physiological processes, necessitating a delicate balance and coordination between them to uphold bodily homeostasis and health. Thus, while circumstances may render one nervous system more active than the other, acknowledging the complexity and dynamism of their interactions underscores the indispensability of balanced regulation and coordination between both systems.
4.3. Suggestions and implications for future research
When exercising with consistent content, heart rate typically escalates alongside increasing exercise intensity loads. Conversely, if subjects experience decreased heart rates and subjective fatigue sensations despite unchanged exercise content, it indicates enhanced anti-fatigue capabilities and endurance levels. This improvement enables them to endure greater loads comfortably, facilitating ongoing enhancements in sports performance throughout the intervention period. Real-time intensity monitoring becomes invaluable at this juncture, enabling coaches to promptly identify and address issues during the intervention process. As exercise intensity increases, heart rate naturally rises. Therefore, during exercise training, heart rate should be maintained within a reasonable range, often termed the target heart rate. A direct linear relationship exists between exercise intensity and heart rate. Predefining heart rate zones enhances training specificity, allowing trainers and coaches to conduct more targeted sessions, ensuring that training consistently yields positive physiological responses.
Maintaining intensity levels above 80% of maximum heart rate underscores the high-intensity nature of HIIT training. Subjects acclimated to such conditions necessitate heightened self-control to sustain exercise efforts. Over time, through continuous self-control endeavors, individuals develop adaptive capabilities conducive to enduring long-term, high-intensity training. This enhanced capacity aids in sustaining sports performance, delaying fatigue onset, and achieving peak performance during competitions. Volitional fatigue serves as a catalyst for muscle anabolism, and bolstering tolerance to volitional fatigue is crucial for seamless training progression[6, 36]. It equips individuals to confront diverse pressures and challenges, fostering an optimistic mindset amidst training adversities. Enhanced stress resilience enables individuals to sustain stable and efficient performance within highly competitive and demanding training environments.
One limitation of this study was its exclusive focus on male participants, potentially limiting the generalizability of findings to women. Previous research has highlighted sex-specific differences in exercise responses, suggesting that outcomes observed in men may not directly translate to women undergoing similar training interventions. Future studies incorporating a more diverse participant pool, including both men and women, will yield a comprehensive understanding of training program effects across genders.
Adolescents may exhibit cardiovascular responses to exercise and stimuli distinct from fully matured adults. Understanding these age-specific responses is pivotal for tailoring interventions to different age groups and ensuring study results' relevance to target populations. Future research endeavors could delve into the evolution of these responses across various age brackets, furnishing a nuanced comprehension of training intervention impacts.