Linear and Non-linear Parameters of Heart Rate Variability in a High Performance World Class Sailor

Tanja Princi^a, Agostino Accardo^b, Larissa Nevierov^a, Daniel Peterec^c

^aPhysiology and Pathology Department; ^bD.E.E.I., University of Trieste, Italy
^cInstitute of Physiology, University of Ljubljana, Slovenia

Correspondence: Tanja Princi, Physiology and Pathology Department, University of Trieste, Via Fleming 22, 34127 Trieste, Italy.
E-mail: princi@dfpmail.units.it, phone +39 040 5587958, fax +39 040 567862

1.  Introduction

Physical activity provokes several modifications in cardiovascular function. The increase of heart rate (HR) during exercise may be mediated by early rapid withdrawal of vagal tone at the beginning of exercise and a more delayed rise in sympathetic activities at higher levels of exercise (Malik, 1995). As instantaneous HR depends on the interaction between sympathetic and parasympathetic efferent activities and pacemakers properties, power spectral analysis of spontaneous oscillations of HR – heart rate variability (HRV) – could assess autonomic nervous control of the cardiac function. The use of linear time and frequency domain measures of HRV to assess the autonomic modulation during exercise has produced different results with different spectral techniques or units. Non-linear methods have therefore been employed to quantify the beat-to-beat dynamics of RR intervals under different physiological conditions. One such method is Poincaré plot analysis of HRV, which consists of each cardiac RR interval plotted as a function of the previous interval (Tulppo et al., 1996). On the other hand, the fractal characteristic of HRV has been investigated as a possible indicator of the complex interaction that might reflect the number of inputs to HR controllers. The fractal dimension (FD), which is considered to be indicative of the complexity of a given time series, was used to measure this characteristic (Nakamura et al., 1993). The physiological demands of sailing are specific, varying with wind and sea conditions, type of craft, and role in the crew (Shephard, 1990). In particular, dinghy sailing requires relatively low energy but high cardiac demand. During upwind sailing different types of muscles are involved with predominant isometric contraction in the special posture of hiking (Felici et al., 1999), which can represent a static exercise.

The sympathetic and the parasympathetic influence on the heart was determined by using previously described linear and non-linear methods during upwind sailing of a high – performance world class sailor at different wind velocities.

2.  Methods

The study was performed in one female dinghy high performance world class sailor (Europe Olympc class), 26 yrs old. HR was recorded continuously for 5 – 10 min at rest and during upwind sailing at wind velocities 2 m/s and 8 m/s by using Polar Vantage NV HR monitor. The series of consecutive intervals (tachogram) in function of beat numbers was extracted. The PSD of the signal was then calculated using the Hanning window on intervals of 512 points. The powers in very-low (VLF: 0.001 – 0.040 Hz), low (LF: 0.040 - 0.150 Hz) and high (HF: 0.150 – 0.400 Hz) spectral bands were evaluated and the LF/HF ratio was derived. From the Poincaré plot analysis of HRV SD₁, SD₂ parameters and SD₁/SD₂ ratio were calculated. Linear regression analysis between log (power) and log (frequency) was performed on the portion of the power spectrum from 0.002 Hz up to 0.200 Hz, and the slope (β) was computed. The fractal dimension (FD) was evaluated by means of Higuchi’s algorithm (Higuchi, 1988), based on the measure of the mean length of the curve using a sample of k segments as a unit of the measure. The FD values were calculated on tracts of 120 consecutive QRS complexes by using an overlap of 100 points. The mean value of FD was considered for the analysis.

3.  Results

In Table 1. RR intervals, VLF (%), LF (%), HF (%) and LF/HF ratio, calculated from FFT spectra, are reported. The LF/HF ratio as well as the VLF % present the lowest value at rest, they increase during upwind sailing at 2 m/s of wind velocity and reach the highest value at 8 m/s of wind velocity. Table 2. represents Poincaré plot parameters SD₁, SD₂ and SD₁/SD₂ ratio, derived from Poincaré plot analysis of HRV, as well as FD and b values, calculated in the same three different conditions.

Table 1.    RR intevals and FFT spectra parameters in a dinghy sailor at different wind velocities.

Table 2.    Poincaré plot parameters, Fractal Dimension (FD) and β in a dinghy sailor at different wind velocities.

4.  Discussion

The linear parameters related to sailing demonstrate an increase of HR and LF/HF ratio (Table 1.) as a function of the wind velocity, compared to rest values. In particular, the increase of HR and LF/HF at 2 m/s of wind could be correlated to the early rapid withdrawal of vagal tone at moderate level of physical activity, whereas the highest value of LF/HF at 8 m/s of wind is likely due to the different sympatho-vagal balance at a higher level of exercise, when the sympathetic outflows arise. In any case, during this type of prevalent static exercise in hiking posture the LF and HF spectral components decrease in comparison to rest values as an expression of a decreased HRV during physical activity. b values increase at both exercise conditions. FD values, derived directly from the tachograms, show the same tendency. It could be expected that a lower HRV present at a high exercise level would be produced by a lower complexity of the system, corresponding to low FD values. Even if the amplitude of the variability around the mean RR interval value decreases with the exercise, the higher frequency of these oscillations could justify a higher complexity level, not pointed out by the spectral analysis. In conclusion, this study indicates a decrease of HRV during upwind sailing at different wind velocities, whereas the overall complexity of HR dynamics, measured by FD, increased in particular during high level of isometric exercise, according to Tulppo et al. (2001).

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