Acute exercise impacts heart rate variability but not cognitive flexibility during subsequent simulated firefighter occupational tasks

Abstract

Purpose: Acute exercise can transiently enhance cognitive flexibility. The cognitive demand of firefighters makes it relevant to understand if on-shift exercise could produce similar improvements in cognitive performance during subsequent occupational tasks. Metrics of heart rate variability (HRV), such as time- and frequency-domain outcomes, may shed light upon the influence exercise has on cognition, as they discern information related to cardiac autonomic (sympathetic/parasympathetic) function. We aimed to determine if acute resistance and aerobic exercise impact cognitive flexibility during occupational tasks and its relation to HRV.

Methods: 32 participants completed a baseline Wisconsin Card Sorting Task (WCST) and three experimental trials: resistance exercise (RE), aerobic exercise (AE), or a rested control (CON). An occupational task assessment (OTA) including four rounds of 10 deadlifts and a 0.15-mile sandbag carry in an environmental chamber (35 °C/50% humidity) was completed after each trial. The second round was followed by the WCST. Repeated measures ANOVAs were used to analyze differences by condition.

Results: For the WCST, total, perseverative, and non-perseverative errors did not differ (ps > 0.39). Time-domain HRV metrics were not different (ps > 0.05). All frequency-domain metrics, other than low-frequency power, were not different (ps > 0.24). Low-frequency power was lower based on condition (p = 0.03). Post hoc analysis showed low-frequency power was lower following AE compared to RE and CON.

Conclusion: Results suggest an acute bout of on-shift aerobic or resistance exercise may not impact cognitive flexibility during subsequent simulated occupational tasks, despite depressed metrics of heart rate variability following aerobic exercise.

Keywords: Executive function; Heart rate variability; Heat stress; High-intensity interval training; Resistance training; Tactical athletes.

Fig. 1

Aerobic testing included VO₂peak and lactate threshold assessments; strength testing included 3RM maximum lifts for bench, squat, and deadlift; WCST: Wisconsin Card Sorting Task; familiarization involved exposure to the occupational task assessment (OTA) and environmental conditions; *Pre-OTA exercise condition (aerobic, resistance, or rest) order was randomized. Transition was the time used to replicate response time and involved donning firefighter gear and walking to the environmental chamber. The WCST was completed halfway through the OTA. Heart rate variability was monitored continuously during the WCST

Fig. 2

Wisconsin Card Sorting Task (WCST) outcomes; data are reported as mean ± standard deviation with individual data points overlaid; ns: not significant (p > 0.05)

Fig. 3

Average RR intervals and time-domain HRV outcomes; data are reported as mean ± standard deviation with individual data points overlaid. SDNN: standard deviation of NN intervals (normal RR intervals free of artifact); RMSSD: root mean square of successive RR interval differences; pNN50: percentage of successive RR intervals that differ by more than 50 ms; *denotes p < 0.05

Fig. 4

Frequency-domain HRV outcomes, data are expressed as mean ± standard deviation with individual data points overlaid. LF/HF ratio: low-frequency to high-frequency power ratio; *denotes p < 0.05