Author(s): ZHOU, Y., SHI, X., LEONG, K.M., KONG, Z., SHI, Q., Institution: MACAO POLYTECHNIC UNIVERSITY, Country: MACAU, Abstract-ID: 486

INTRODUCTION:
High-density sprint interval exercise (e.g., 6-s sprints/15-s recovery) elicits accumulated maximal aerobic demand (≥90% VO2max) significantly greater than traditional long-sprint protocols (e.g., 30-s sprints/4-min recovery), despite shorter individual work durations. However, the autonomic regulatory burden of sustaining such rapid metabolic switching remains obscure. Traditional linear heart rate variability metrics (e.g., RMSSD) often succumb to a "floor effect" (saturation) under near-maximal loads. This study utilized non-linear dynamics (Sample Entropy, SampEn) to quantify cardiac complexity in endurance versus team-sport athletes during strictly work-matched protocols, aiming to differentiate autonomic regulation across varying sprint densities where linear metrics fail.
METHODS:
A cohort of 28 young male athletes (13 elite endurance, 15 team-sport) performed four randomized protocols standardized to a total work duration of 120 s. Protocols comprised 20 repetitions of 6-s sprints with 15-s (6-15), 30-s (6-30), or 60-s (6-60) recovery, versus a control of 4 repetitions of 30-s sprints with 4-min recovery (30-4m). R-R intervals were recorded continuously, and HR, linear (RMSSD), and non-linear (SampEn) metrics were computed. Data were aggregated into four sequential analytical blocks, and metrics were calculated using a fixed window of the first 200 beats per block to standardize signal length.
RESULTS:
Mean HR increased stepwise: 30-4m (135 ± 18) < 6-60 (143 ± 17) < 6-30 (158 ± 15) < 6-15 (167 ± 12 bpm) (all p < 0.05). Linear HRV (RMSSD) exhibited a ubiquitous saturation (< 10 ms) across all protocols (p > 0.05). In contrast, SampEn revealed a significant, density-dependent gradient. The 30-4m control showed the lowest complexity (SampEn: 0.3 ± 0.2). As recovery shortened, SampEn increased stepwise: 6-60 (0.6 ± 0.4) < 6-30 (1.2 ± 0.6) < 6-15 (1.9 ± 0.4) (all p < 0.05, Cohen's d = 0.7–2.2). No significant group or interaction effects were observed (p > 0.05).
CONCLUSION:
While linear metrics saturate, SampEn mirrors external duty cycle density in a graded manner. The paradoxical entropy surge in the 6-15 protocol signifies a state of "Dynamic Instability"—where the cardiac rhythm is forced into stochastic transitions, preventing coherent regulation. Unlike the low-entropy recovery in traditional protocols, high-frequency loading induces regulatory disarray. The absence of group differences further suggests this instability is a fundamental physiological constraint independent of athlete phenotype. SampEn thus serves as a superior, saturation-resistant biomarker for monitoring the autonomic strain of rapid work-rest transitions.