Resting-state heart rate variability after stressful events as a measure of stress tolerance among elite performers

Abstract

Introduction: A common trait of elite performers is their ability to perform well when stressed by strong emotions such as fear. Developing objective measures of stress response that reliably predict performance under stress could have far-reaching implications in selection and training of elite individuals and teams. Prior data suggests that (i) Heart rate and heart rate variability (HR/HRV) are associated with stress reaction, (ii) Higher basal sympathetic tone prior to stressful events is associated with higher performance, and (iii) Elite performers tend to exhibit greater increase in parasympathetic tone after a stressful event. Methods: The current study assesses the predictive utility of post-stressful event HR/HRV measures, an under-studied time point in HR/HRV research, in the context of military personnel selection. Specifically, we examined the relationship between a comprehensive set of HR/HRV measures and established questionnaires related to stress tolerance, experimental evaluation of executive function during stress induction, and ecologically valid selection assessment data from a week-long Special Operations Forces selection course (N = 30). Results: We found that post-stressful event HR/HRV measures generally had strong correlations with the neuroticism facet of the NEO personality inventory as well as the general and distress facets of the defensive reactivity questionnaire. HR/HRV measures correlated reliably with a change in executive function measured as a decrease in verbal fluency with exposure to a well-validated stressor. Finally, we observed a divergent pattern of correlation among elite and non-elite SOF candidates. Specifically, among elite candidates, parasympathetic nervous system (PNS) measures correlated positively and sympathetic nervous system (SNS) measures correlated negatively with evaluation of stress tolerance by experts and peers. This pattern was not present in non-elite candidates. Discussion: Our findings demonstrate that post-stressful event HR/HRV data provide an objective non-invasive method to measure the recovery and arousal state in direct reaction to the stressful event and can be used as metrics of stress tolerance that could enhance selection of elite individuals and teams.

Keywords: assessment; elite performance; heart rate variabiity (HRV); individual diffenrences; stress tolerance.

FIGURE 1

Schematic illustrations of how parasympathetic (PNS) and sympathetic (SNS) nervous system change throughout a temporal course of going through a stressful event (A) along with general characteristics of elite performers at each time point (B).

FIGURE 2

Hierarchically clustered correlation matrix among the HR/HRV measures. The color and the size of the circles indicate the strength of correlation with blue shades indicating positive and red shades indicating negative correlations. The black squares on top-left and bottom-right show the grouping based on the similarity and dissimilarity of each variable roughly clustering into the PNS (bottom-right) and the SNS (top-left) groups. Abbreviations: LF.HF, Ratio of low frequency (LF) to high frequency (HF) band powers; SD1.SD2, Ratio of Poincaré Perpendicular Standard Deviation (SD1) to Poincaré Parallel Standard Deviation (SD2); DFAα1, short-term detrended fluctuation analysis; HR, Heart Rate; SI, Stress Index; DFAα2, long-term detrended fluctuation analysis; Max.Min HR, Heart Rate Range; VLF, Very-Low Frequency; RMSSD, Root Mean Square of Successive Differences; SDNN, Standard Deviation between R-R intervals; RR, R to R interval in ECG rhythm; pNN50, percentage of successive R-R intervals that deviate greater than 50 ms; NN50, number of R-R intervals that deviate greater than 50 ms.

FIGURE 3

Correlation between Expert Evaluation-ST and HRV metrics by Selection Status. Selection Status of 0, shown in red with circular data points, indicates a non-selected participant, and Selection Status of 1, shown in green with triangular data points, indicates a selected participant. Abbreviations: ST, stress tolerance; HRV, Heart Rate Variability. Panel descriptions (A) LF.HF, Ratio of low frequency (LF) to high frequency (HF) band powers; (B) SD1.SD2, Ratio of Poincaré Perpendicular Standard Deviation (SD1) to Poincaré Parallel Standard Deviation (SD2); (C) DFAα1, short-term detrended fluctuation analysis; (D) Max Heart Rate; (E) Mean Heart Rate; (F) Minimum Heart Rate; (G) SI, Stress Index; (H) DFAα2, long-term detrended fluctuation analysis; (I) Max.Min HR, Heart Rate Range; (J) logLF, natural log of LF; (K) logVLF, natural log of Very-Low Frequency; (L) Total Power; (M) RMSSD, Root Mean Square of Successive Differences; (N) SD1; (O) SDNN, Standard Deviation between R-R intervals; (P) SD2; (Q) Mean RR, R to R interval in ECG rhythm; (R) logHF, natural log of HF; (S) pNN50, percentage of successive R-R intervals that deviate greater than 50 ms; (T) NN50, number of R-R intervals that deviate greater than 50 ms.

FIGURE 4

Correlation between Peer Perception-ST and HRV metrics by Selection Status. Selection Status of 0, shown in red with circular data points, indicates a non-selected participant, and Selection Status of 1, shown in green with triangular data points, indicates a selected participant. Abbreviations: ST, stress tolerance; HRV, Heart Rate Variability. Panel descriptions (A) LF.HF, Ratio of low frequency (LF) to high frequency (HF) band powers; (B) SD1.SD2, Ratio of Poincaré Perpendicular Standard Deviation (SD1) to Poincaré Parallel Standard Deviation (SD2); (C) DFAα1, short-term detrended fluctuation analysis; (D) Max Heart Rate; (E) Mean Heart Rate; (F) Minimum Heart Rate; (G) SI, Stress Index; (H) DFAα2, long-term detrended fluctuation analysis; (I) Max.Min HR, Heart Rate Range; (J) logLF, natural log of LF; (K) logVLF, natural log of Very-Low Frequency; (L) Total Power; (M) RMSSD, Root Mean Square of Successive Differences; (N) SD1; (O) SDNN, Standard Deviation between R-R intervals; (P) SD2; (Q) Mean RR, R to R interval in ECG rhythm; (R) logHF, natural log of HF; (S) pNN50, percentage of successive R-R intervals that deviate greater than 50 ms; (T) NN50, number of R-R intervals that deviate greater than 50 ms.