Relationships Between Resting Heart Rate Variability, Maximal Heart Rate, And VO2peak Among Active-duty Firefighters

Abstract

Heart rate variability (HRV) is a non-invasive marker of autonomic nervous system (ANS) function based on the beat to beat variation in heart rate measured by the duration of the R-R interval. Clinically, lower resting HRV has been linked to greater risk for cardiovascular disease and higher HRV is thought to be cardioprotective and reflective of a greater fitness. Firefighters represent a unique population where sudden cardiac death is a high risk of the job that demands the ability to perform high intensity tasks. Resting HRV may provide insight into the contributions of the ANS to maximal capacity of a firefighter. PURPOSE: To examine the relationships between resting HRV and heart rate and aerobic capacity (VO2peak) during a maximal treadmill test. METHODS: 37 male career active-duty firefighters (mean ± SD, 39.1 ± 8.9 yrs; 178.8 ± 5.4 cm; 87.9 ± 11.2 kg) participated in this study. All participants completed a 5-minute resting HRV sample followed by a maximal graded exercise test on a treadmill using a modified version of the submaximal Wellness Fitness Initiative test for firefighters. Heart rate and oxygen consumption were monitored throughout the test and maximal HR (MHR) at cessation of the test and VO2peak were recorded for each participant. HRV was determined by measuring the time intervals between R-waves and reflected as the mean RR for the 5-minute resting sample. Bivariate Pearson correlations determined the relationship between resting RR, MHR, and VO2peak. An alpha of 0.05 determined statistical significance. RESULTS: Resting RR (864.8 ± 134.0 ms) had a moderate and significant relationship with VO2peak (44.6 ± 7.1 ml/kg/min; r = 0.458, P = 0.004) and MHR (181.5 ± 10.7 bpm; r = -0.360, P = 0.029), but VO2peak was not related to MHR (r = 0.308, P = 0.064). CONCLUSION: Higher resting RR was associated with greater VO2peak, but lower MHR. The positive relationship between RR and VO2peak is consistent with prior research suggesting that a higher resting HRV is related to higher fitness. The inverse relationship between HRV and MHR suggests that greater parasympathetic nervous system control at rest (i.e., higher RR) may extend into maximal exercise tasks. Thus, for a firefighter, the benefit of a more favorable resting HRV may not only be cardioproective, but also influence the relative intensity of a maximal task.