II.3.3. Relationship between spectral and temporal study
parameters
Neto et al., (2005) showed that in the
analysis of cardiac variability, many temporal and frequency domain variables
appear strongly correlated. These correlations are in fact a reflection of
their mathematical as well as physiological significance and interdependence.
Unless performing other analyzes than those commonly used in the frequency
domain and which are mentioned above, the variables conventionally used in the
frequency domain are therefore equivalent to those of the time domain.
Table I: Indices of human heart
rate variability in the frequency and time domains and their approximate
matches of 24-hour recordings. (Neto et al.)
Time variable Frequency domain Temporal domain Correspondence
PT Whole frequency scale but approximately <0.4
SDNN
ULF(VLT) 0,0001 à 0,0003 Hz SDNN,
SDANN
VLF(VLT) 0,0001 à 0,0003 Hz SDNN Index
LF(VCT) 0,0001 à 0,0003 Hz SDNN Index
HF(VCT) 0,0001 à 0,0003 Hz RMSSD,
PNN50
PT: total power of the spectrum; ULF: ultra low frequencies;
VLF: very low frequencies; LF: low frequencies; HF: high frequencies; SDNN:
standard deviation of the RR interval over the entire recording period; SDANN:
standard deviation of the mean RR intervals over 5-minute periods over the
entire recording period; SDNN index: mean of standard deviations of the RR
interval over 5-minute periods over the entire recording period; RMSSD: square
root of
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squared differences of successive RR intervals; pNN50: this is
the NN50 divided by the total number of RR intervals.
II.3.4. Physiological interpretation of different
parameters of heart rate variability
The physiological basis for analyzing the short-term
variability of heart rate is based on different mechanisms of action and
control between the parasympathetic and the orthosympathetic systems.
Parasympathetic influences exert a rapid and dynamic control via the release of
acetylcholine and its action on the Muscarinic receptors are mainly reflected
by the high frequency component of heart rate variability.
Moreover, the orthosympathetic system is reflected by the
release of noradrenaline and its action on â-adrenergic receptors, which
exerts a slower influence and is manifested in the low frequency component of
the variability of the heart rate.
The short-term variability of heart rate is therefore an
indirect measure of autonomic nervous system activity. It is a reflection of
autonomous influences on the sinoatrial node, more than on the ventricular
myocardium. The analysis of heart rate variability, however, provides an
insight into the variations in autonomic tone associated with various
conditions. Two major components are studied: low frequencies (LF, from 0.04 to
0.15 Hz) and high frequencies (HF, from 0.15 to 0.4 Hz, synchronized with the
respiratory rhythm). While the high frequency band is clearly attributed to
vagal mechanisms (Akselrod et al., 1981, Malliani et al., 1991, Camm,
1996), several hypotheses have been advanced regarding the low
frequency band. The interpretation of this LF component is considered by some
to be a sympathetic modulation index (Rimoldi et al., 1990, Malliani
1991, Kamath & Fallen, 1993, Montano et al., 1994) and for others
as a parameter. Including both sympathetic and parasympathetic influences
(Akselrod et al., 1981, Appel et al., 1989).
Therefore, the low frequency-high frequency ratio can be
considered as a mirror of the sympathovagal balance or the influence of the
sympathetic system on the heart rate. A circadian rhythm of the sympathovagal
balance has been observed in the population: the LF spectral component
predominates during the day whereas it is the HF spectral component that is
predominant at night. There is therefore a marked decrease in the LF / HF ratio
between day and night. This observation reflects the day / night variation in
the influences of para- and
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orthosympathetic systems (sympathetic predominance during the
day and vagal at night) (Furlan et al., 1991, Malliani et al.,
1991).
Part II : Material and
Methodes
I. 30
Participants
Nine male athletes playing football for at least 5 years in a
"professional league II" club aged 16.2 #177; 0.5 years and a size of 176 #177;
5 cm participated in our study after reading the different modalities of the
experimental protocol. The weight and height of the subjects were measured
using a scale and a height scale. The inclusion criteria consist of keeping
standard meal times (breakfast at 07:00: 00 #177; 1:00, lunch at 12:00: 00
#177; 1:00: 00 and dinner at 20:00: 00 #177; 1: 00) and sleep (sleep between
11:00 pm and 7:00 am + 1:00 am) before the start of the study. This criterion
allowed to provide a sample of participants having the same bedtime (23h: 00
#177; 00h: 30) and to raise (06h: 30 #177; 00h: 30). The subjects are
non-smokers and do not consume caffeine or alcoholic beverages. The first day
of the month of Ramadan of the year 1432 of Hegira corresponds to August 1,
2011 while the last day corresponds to August 30, 2011. The time elapsing from
the beginning of dawn until sunset was from 05h: 24 to 19h: 27 at the beginning
of Ramadan and from 05h: 47 to 18h: 53 at the end of the Holy month. During
this month, participants consume their last meal around 1:00 am and since then
refrain from eating and drinking until sunset.
II. Experimental procedures
The experimental protocol spanned three periods: two weeks
before Ramadan (BR), the end of the second week of Ramadan (R2) and the end of
the fourth week of Ramadan (R4). Before the beginning of the protocol, a
familiarization session is performed in order to avoid the effects of learning
that could occur with the repetition of the test sessions (Pincivero et
al., 1997). During this familiarization session, subjects became aware
of the nature of the test and the constraints of the experiment. Instructions
regarding sleep, diet and physical activity have been provided to the subjects
concerned. During the experimental period, subjects were asked not to perform
intense sporting activities the day before and during the day they were
assessed. During the same experimental period, subjects were reminded that
ingestions of caffeine-based foods and beverages are out of the question, as
anything that could increase their awakening.
The test sessions were conducted in the biology laboratory of
Farhat Hachad Hospital in Sousse at the same time of the day (between 14:00 and
17:00) in order to maintain identical experimental conditions. The subjects are
asked to use the same sports shoes at each session.
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