Effect of Stress on Cardiorespiratory Synchronization of Ironman Athletes

Abstract

The aim of this paper is to investigate the cardiorespiratory synchronization in athletes subjected to extreme physical stress combined with a cognitive stress tasks. ECG and respiration were measured in 14 athletes before and after the Ironman competition. Stroop test was applied between the measurements before and after the Ironman competition to induce cognitive stress. Synchrogram and empirical mode decomposition analysis were used for the first time to investigate the effects of physical stress, induced by the Ironman competition, on the phase synchronization of the cardiac and respiratory systems of Ironman athletes before and after the competition. A cognitive stress task (Stroop test) was performed both pre- and post-Ironman event in order to prevent the athletes from cognitively controlling their breathing rates. Our analysis showed that cardiorespiratory synchronization increased post-Ironman race compared to pre-Ironman. The results suggest that the amount of stress the athletes are recovering from post-competition is greater than the effects of the Stroop test. This indicates that the recovery phase after the competition is more important for restoring and maintaining homeostasis, which could be another reason for stronger synchronization.

Keywords: ECG; athletes; cardiorespiratory; empirical mode decomposed; respiratory signal; synchronization.

Figure 1

Example of a Stroop test. Participants were required to state the color of the word instead of reading the word. For example the first line would be red, blue, red.

Figure 2

Histograms of (A) Race and (B) Recovery time, (C) Fitness and (D) BMI for the athletes.

Figure 3

An example of how the cardiorespiratory synchrogram works. On the top is the respiration, in the middle the corresponding ECG signal and at the bottom, the formation of the synchrogram. The position of each heartbeat in relation to its appearance in the phase of the respiratory cycles can be clearly seen. Red broken vertical lines indicate picks in the heart beats and black vertical broken lines indicate 1 s interval. This example illustrates n:1 synchronization.

Figure 4

Illustration of EMD decomposition into IMFs for athlete #14.

Figure 5

Phase differences between one IMF (IMF5) from respiration decomposition and several IMFs from the ECG decomposition (athlete # 14 post-race).

Figure 6

The cardiorespiratory synchrogram for an athlete #14 completing a Stroop test before Ironman Competition, 3:1 phase locking.

Figure 7

The cardiorespiratory synchrogram for an athlete #14 completing a Stroop test after Ironman Competition, with 4:1 phase locking.

Figure 8

The cardiorespiratory synchrogram for an athlete #3 completing a Stroop test before Ironman Competition, with 4:1 phase locking.

Figure 9

The cardiorespiratory synchrogram for an athlete #3 completing a Stroop test after Ironman Competition, with 4:1 phase locking but longer time.

Figure 10

EMD decomposition with data for athlete #14 showing the plateau in pink for (A) Pre-race upper panel and (B) Post-race upper panel. The length of synchronizations, given by the length of the plateaus (in pink) is larger in the post-race indicating that the athlete is more relaxed after the competition and shows a better synchronization between cardiac and respiratory systems. The lower panel in (A,B) represents the variance in phase difference.

Figure 11

Box plots displaying the times the cardiorespiratory systems spent synchronized pre- and post-Ironman race. Outliers are presented by star (*). The length of synchronizations, given by the length of the plateaus is larger in the post-race indicating that the athlete is more relaxed after the competition and shows a better synchronization between cardiac and respiratory systems.