Cardiorespiratory Coordination in Repeated Maximal Exercise

Abstract

Increases in cardiorespiratory coordination (CRC) after training with no differences in performance and physiological variables have recently been reported using a principal component analysis approach. However, no research has yet evaluated the short-term effects of exercise on CRC. The aim of this study was to delineate the behavior of CRC under different physiological initial conditions produced by repeated maximal exercises. Fifteen participants performed 2 consecutive graded and maximal cycling tests. Test 1 was performed without any previous exercise, and Test 2 6 min after Test 1. Both tests started at 0 W and the workload was increased by 25 W/min in males and 20 W/min in females, until they were not able to maintain the prescribed cycling frequency of 70 rpm for more than 5 consecutive seconds. A principal component (PC) analysis of selected cardiovascular and cardiorespiratory variables (expired fraction of O₂, expired fraction of CO₂, ventilation, systolic blood pressure, diastolic blood pressure, and heart rate) was performed to evaluate the CRC defined by the number of PCs in both tests. In order to quantify the degree of coordination, the information entropy was calculated and the eigenvalues of the first PC (PC1) were compared between tests. Although no significant differences were found between the tests with respect to the performed maximal workload (Wmax), maximal oxygen consumption (VO₂ max), or ventilatory threshold (VT), an increase in the number of PCs and/or a decrease of eigenvalues of PC₁ (t = 2.95; p = 0.01; d = 1.08) was found in Test 2 compared to Test 1. Moreover, entropy was significantly higher (Z = 2.33; p = 0.02; d = 1.43) in the last test. In conclusion, despite the fact that no significant differences were observed in the conventionally explored maximal performance and physiological variables (Wmax, VO₂ max, and VT) between tests, a reduction of CRC was observed in Test 2. These results emphasize the interest of CRC evaluation in the assessment and interpretation of cardiorespiratory exercise testing.

Keywords: cardiorespiratory exercise testing; coordinative variables; exercise-induced fatigue; information entropy; principal components analysis.

Figure 1

Typical example of the reduction of cardiorespiratory variables to time series of cardiorespiratory coordination variables (PCs) in Test 1 and Test 2. Top graphs: original time series of the six selected cardiorespiratory variables in Test 1 and Test 2. Middle graphs: time series of PC scores (standardized z-values in the space spanned by PCs) in both tests. The six time series are collapsed to one time series (Test 1) or two time series (Test 2) as a consequence of the PC dimension reduction. The black and the red line show the average trend of both processes as calculated by weighted least squares method. Data points of the x-axis of both graphs refers to the number of measurements recorded along the cardiorespiratory test. Bottom graphs: position of the six original cardiorespiratory vectors within the coordinate system of PCs.