(1) PERFORMANCE AND AUTONOMIC NERVOUS SYSTEM CHANGES IN REPEATED THREE-MINUTE ALL-OUT TESTS IN FIREFIGHTERS

Purpose: Critical velocity (CV) and anaerobic distance capacity (D’) have a growing research interest as fitness measures for occupational performance (4,6). CV represents the highest work rate that can be sustained for long durations, yet firefighters (FFs) perform repeated short duration, high-intensity tasks (3). The three-minute all-out test (3MT) to determine CV and D’ may be meaningful to quantify performance capacity in repeated high-intensity efforts. Measurement of autonomic nervous system (ANS) function can provide insight to recovery after repeated efforts. The purpose of this study was to examine differences in performance capacity and ANS recovery in consecutive 3MT bouts in FFs. Methods: Twelve male active duty FFs (34.83 ± 8.63 yrs, 1.79 ± 0.02 m, 88.85 ± 16.33 kg) from a Midwestern metropolitan fire department completed two, 30-meter shuttle sprint 3MT bouts separated by ten minutes of passive rest. Hand-timed sprints allowed for velocity to be calculated for each sprint (m·s^-1), with the highest value representing maximal sprint speed (S_MAX, m·s^-1). Average velocity of the final 30 seconds of each bout represented CV (m·s^-1). D’ (m) was calculated using the average velocity from 0-150 seconds. Heart rate variability (HRV, lnRMSSD) was recorded using chest strap heart rate monitors during a five-minute pre-test seated rest (HRV_REST) and in three-minute samples post-Bout 1 (HRV_B1) and Bout 2 (HRV_B2). Paired t-tests examined differences in CV, D’, and S_MAX between each 3MT bout. A repeated measures ANOVA determined significant differences between HRV_REST, HRV_B1, and HRV_B2 with follow-up pairwise comparisons using a Bonferroni correction. An alpha of 0.05 determined statistical significance. Results: CV did not significantly differ (t = 0.04, p = 0.97) between bouts, yet D’ (t = 2.52, p = 0.03) and S_MAX (t = 2.97, p = 0.01) were significantly lower in Bout 2. Significant differences were detected in HRV (F = 35.51, p < 0.001) where HRV_B1 and HRV_B2 were lower than HRV_REST (Figure 1, p < 0.001). Conclusions: A ten-minute passive rest was not adequate for ANS recovery to baseline, indicating high sympathetic nervous system (SNS) influence after each bout. Prior work has suggested that all-out efforts like the 3MT result in metabolite accumulation from anaerobic mechanisms (1,5,7), and it is possible that this contributed to an inability for the ANS to return to baseline. As a result of the increased SNS influence there was a decrease in central drive, resulting in an inability to repeat maximal sprint ability. Practical Applications: Current literature suggests long duration, low-intensity training may improve aerobic capacity and CV. Long duration training combined with short duration, high-intensity sprint intervals have been indicated to improve muscle reoxygenation after high-intensity efforts, which may be beneficial for recovery and repeatability of S_MAX and D’ (2). Future research should determine strategies for improving performance repeatability in high-intensity efforts in FFs. Acknowledgements: None

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