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European Journal of Applied Physiology
© Springer-Verlag 2005
10.1007/s00421-005-1379-3
Original Article
Cardiorespiratory synchronization during Zen
meditation
Dirk Cysarz
1, 2
and Arndt Büssing
1, 3
(1) Chair of Medical Theory and Complementary Medicine, University of
Witten/Herdecke, 58313 Herdecke, Germany
(2) Gemeinschaftskrankenhaus Herdecke, Gerhard-Kienle-Weg 4,
58313 Herdecke, Germany
(3) Krebsforschung Herdecke e.V., 58313 Herdecke, Germany
Dirk Cysarz
Email: d.cysarz@rhythmen.de
Phone: +49-2330-623637
Fax: +49-2330-624062
Accepted: 11 April 2005 Published online: 7 June 2005
Abstract The impact of meditation on cardiorespiratory synchronization
with respect to breathing oscillations and the modulations of heart rate
induced by respiration (respiratory sinus arrhythmia, RSA) was
investigated in this study. Four different exercises (spontaneous
breathing, mental task, Zen meditation, and Kinhin meditation) were
consecutively performed by nine subjects mainly without any experience
in meditation. An electrocardiogram and a respiratory trace were
recorded simultaneously. On this basis the degree of cardiorespiratory
synchronization was quantified by a technique which has been adopted
from the analysis of weakly coupled chaotic oscillators. Both types of
meditation showed a high degree of synchronization, whereas heartbeat
and respiration were hardly synchronized during spontaneous breathing.
During the mental task exercise the extent of synchronization was
slightly higher than during spontaneous breathing. These results were
largely determined by the breathing frequency because the two types of
meditation induce low breathing frequencies which led to a pronounced
and in-phase RSA. During the meditation the low breathing frequencies
led to a decrease in the high frequency of heart rate variability, whereas
the low frequency and the extent of RSA increased. The heart rate
primarily reflected the degree of physical effort. The high degree of
cardiorespiratory synchronization during meditation in unexperienced
meditators suggests that the physiological implications of meditation
does not require prior experience in meditation.
Keywords Heart rate variability - Respiratory sinus arrhythmia - Respiration -
Synchronization - Bivariate data analysis - Meditation
Introduction
Meditation in its various forms is a traditional exercise with a potential
benefit on well-being and health. On a psychosomatic level these
exercises seem to improve the salutogenetic potential in man
(Antonovsky 1987
). Many studies also focused on the physiological
effects of different meditation techniques to gain insight into the
physiological prerequisites responsible for the improvement of health
(Sudsuang et al. 1991
; Wenneberg et al. 1997; Lehrer et al. 1999; Peng et
al. 1999
, 2004; Lee et al. 2000; Barnes et al. 2001; Travis 2001). Especially
the cardiorespiratory interaction seems to play an important role since
most meditation techniques make use of special low frequency
breathing patterns regardless of whether they result from a deliberate
guidance of breathing or other mechanisms, for example, the recitation
of specific (religious) verse (Bernardi et al. 2001
; Cysarz et al. 2004).
The modulation of the instantaneous heart rate by breathing patterns is
known as respiratory sinus arrhythmia (RSA) (Angelone and Coulter
1964
; Hirsch and Bishop 1981; Berntson et al. 1993). The effects of
different breathing patterns on heart rate variability (HRV) and other
cardiovascular parameters, for example, baroreflex sensitivity or blood
pressure, have been investigated extensively. For example, during
recitation of the rosary prayer and the
OM mantra the breathing
oscillations and the endogenous blood pressure fluctuations adjust their
frequencies (Bernardi et al. 2001
). Furthermore, the arterial oxygen
saturation SaO
2
in patients with chronic heart failure increased strongest
at breathing frequency of 6 breaths/min (Bernardi et al. 1998). These
findings imply that the control of breathing patterns may be used to
maintain conditions for health and seem to be advantageous for
recovery processes.
Different approaches may be used to control a regular breathing
behaviour, like, for example, meditation, recitation of religious verse or
poetry, or biofeedback (Lehrer et al. 2000
). Although the different
techniques and their respective theoretical background vary remarkably
they all make use of a slow–paced breathing pattern or at least produce
low frequency breathing oscillations. The impact of slow–paced
breathing patterns on cardiovascular control has been investigated
extensively. For example, the modulation of heart rate by respiration is
strongest at low breathing frequencies of approximately 0.1 Hz (6
respiratory cycles/min) (Berntson et al. 1993
; Bernardi et al. 2000; Stark
et al. 2000
). In our own investigations we were able to show that a high
degree of cardiorespiratory synchronization occurs during recitation of
hexameter verse (Cysarz et al. 2004
). Although this type of exercise uses
a breathing frequency at approximately 12 min
–1
the therapeutically
guided recitation of hexameter verse produced a low frequency
oscillation in the breathing pattern (at approximately 6 min
–1
) which led
to a pronounced and in-phase RSA. This kind of cardiorespiratory
synchronization may play an important physiological role with respect to
a potential benefit on well-being and health.
During the different exercises of Zen meditation the depth and the
duration of each respiratory cycle is determined only by the process of
breathing. As a consequence, the breathing modalities are produced
according to their own demands, that is, the breathing frequency slows
itself down to a range between 5 min
–1
and 8 min
–1
and the tidal volume
is adjusted appropriately to avoid hypoventilation. This type of exercise
exerts a great influence on heart rate variability and, especially, the
extent of RSA. In this study, we focus on the analysis of
cardiorespiratory synchronization during a succession of different
exercises: two different types of Zen meditation, a mental task and quiet
breathing (i.e. breathing without focussing on the breathing process).
These exercises allow the classification with respect to cardiorespiratory
synchronization because they all make use of different breathing
frequencies and are expressed by different levels of HRV.
Methods
Subjects and experimental procedure
Nine colleagues of the institution took part in the study (4 female, 3
smokers average age 43±7 years). None of the subjects had any history
of cardiovascular diseases, especially no hypo- or hypertension or
antiarrhythmical therapy. One subject had a long experience in Zen
meditation (a teacher of Zen-meditation) and one subject was
experienced with respect to Vipassana meditation (a mindfulness-based
meditation), but had no experience with Zen meditation. All other
subjects did not have any experience with meditation.
The duration of the experimental procedure was approximately 50 min.
The procedure was divided into six different exercises (see Table 1
): (1)
the subject sat quietly on a chair without any restrictions on breathing
and without any extraordinary mental activity (duration: approximately
10 min). There was no instruction to try to get into a relaxed state. (2)
During the
Mental task exercise the subject still sat an a chair and had
to do advanced mental arithmetic for approximately 6 min. (3)
Subsequently, the first session of sitting meditation (Zazen) was
practiced (duration: approximately 10 min) (Metzger 2001
). This kind of
meditation is characterized by breathing at a rate and depth which is
only determined by the breathing process itself, that is, any outward
intention is minimized and the intention towards the breathing process is
increased. The volunteers were advised to avoid any thinking, and to
keep the awareness on
just-sitting and just-breathing . To facilitate
this kind of mediation the subjects sat upright on a cushion and the
hands were held together in front of the navel. (4) During the next 7 min,
a walking meditation (Kinhin) was practiced. With respect to the
breathing modalities it is comparable to Zazen meditation (MZ). In
contrast, this exercise is a distinct form of walking with short steps
(Büssing
2001
): during inspiration the foot is lifted and the whole body is
slightly erected against the gravity. Then, during expiration, the foot is
put down a half-step in front and gravity may slightly pull down the body
again. Thus, the walking speed is unambiguously intertwined with the
breathing process. The hands were held together in front of the sternum
and were slightly pressed against each other and the sternum during
expiration. (5) For the next 7 min Zazen meditation is practiced again to
get back to rest in a calm fashion. (6) At the end of the procedure, the
subject sat in a chair again, similar to the beginning of the experiment
(duration 10 min). It has to be noted that the teacher of Zen-meditation
carried out this procedure alone, whereas all other subjects were
instructed by the teacher (sitting face to face during the Zazen
meditation and walking next to him during the Kinhin meditation). This
way, it was possible to give minor advices (in the sense of clarifications)
to the first-time meditators.
Table 1 Experimental protocol of the study. The experiment was divided
into six consecutive exercises. For further details, see text
Exercise Sitting
Mental
Task
Zazen
Meditation
Kinhin
Meditation
Zazen
Meditation
Sitting
Duration
(min)
8 7 11 7 7 10
Data acquisition
The electrocardiogram (ECG, standard lead) and the uncalibrated
nasal/oral airflow (derived by three thermistors that were placed next to
the nostrils and in front of the mouth) were simultaneously recorded in
all subjects using solid state recorders (Medikorder MK2, Tom-Signal,
Graz, Austria). The sampling rates of the ECG and the nasal/oral airflow
were 3,000 and 100 Hz, respectively. This ensured an accuracy <1 ms for
the times of the identified R-peaks. The local minima and the local
maxima of the nasal/oral airflow were defined as the inspiratory and
expiratory onsets, respectively, since they were due to the change from
exhaling warm air (warmed by the respiratory tract) to inhaling air at the
temperature of the environment (and vice versa). For further analysis the
data were saved to a file and were further processed using Matlab (The
Mathworks, Natick, MA, USA) and C routines. Subsequently, all
automatically identified R-peaks were visually controlled, that is, the
times of the R-waves were marked in the ECG. They were edited if the
identified R-peaks did not match the R-peak in the ECG. The manually
edited R-peaks had an accuracy of 4 ms because the recorded ECG had
a sampling rate of 250 Hz.
Effects of transitions at the beginning of each exercise were reduced by
omitting the first 2 min of each exercise. To avoid a bias due to different
durations of the recordings of each exercise the subsequent 5 min were
used for further analysis.
Analysis of cardiorespiratory synchronization
A heart rate time series with equidistant time steps was constructed as
follows: the times of successive R-peaks were first converted to a RR-
tachogram, that is, the sequence of times between successive R-peaks.
The resulting RR-tachogram was re-sampled at a rate of 5 Hz using
linearly interpolated values. To get a time series for the nasal/oral airflow
at corresponding sampling times, each 20th sample was used. These
two time series share a common time axis and served as the basis for
further calculations.
Figure 1
a and c shows the heart rate time series and the simultaneous
nasal/oral airflow during rest on the chair and during Zazen meditation,
respectively. Obviously, during resting on the chair both time series are
desynchronized, whereas during Zazen meditation a cardiorespiratory
synchronization is present. To quantify the extent of cardiorespiratory
synchronization the phases of each time series were constructed and
subsequently analysed. This procedure was adopted from the
synchronization analysis of weakly coupled chaotic oscillators
(Rosenblum et al. 1996
, 1997) and yielded more consistent results than
the analysis of coherence (Cysarz et al. 2004
). It is based on the
assumption that the coupling leading to the modulation of heart rate by
respiration is essentially linear and non-linear couplings may be
neglected. It is described briefly in the following (for further information,
see e.g. (Cysarz et al. 2004
)).
Fig. 1 a During spontaneous breathing instantaneous heart rate (upper
trace) and simultaneous air flow (lower trace) are desynchronized. c
During Zazen-meditation cardiorespiratory interaction is synchronized. b
and d illustrate the accompanying phase difference
and the distribution of between
heart rate and respiration. In a synchronized state the phase difference is
almost constant and the distribution shows a distinct maximum
First, both time series are low-pass filtered with a cut-off frequency of
0.25 Hz. Next, the so-called phase of each time series, that is,
heart
(t
i
)
and
resp
(t
i
), is constructed with the help of the Hilbert-transformation
(Rosenblum and Kurths 1998
). Both times series are synchronized if the
phase difference
is constant, that is,
( is an offset since the phase difference needs not to
be around zero). The phase difference is quantified by
where brackets denote an
average. Theoretically, 0
1 and =1 if the two time series are
completely synchronized, that is, a constant phase difference, and
=0 if
they are desynchronized. However, for real world data, the data may
always contain some spuriously synchronized patterns although the
systems are completely desynchronized. Thus, the lower bound of
was
estimated with the help of so-called surrogate data, that is, artificial data
without any coupling which were constructed on the basis of the original
data. This estimation yielded =0.14 as an estimation of the lower bound
of
, that is, any value 0.14 may be considered as completely
desynchronized.
The temporal course of the phase difference
(t
i
) (in rads) and the
distribution of
is shown in Fig. 1b and d. In the
desynchronized state the temporal course of the phase difference seems
to be erratic and the values of
are almost equally
distributed. This is also reflected in the low -value ( =0.14) which
denotes a desynchronized state. On the contrary, the synchronized state
the temporal course of the phase difference is almost constant which is
also reflected by the clear maximum in the distribution of
Now, the synchronization gives rise to a -value near 1 ( =0.92).
Heart rate variability
In addition to the quantification of cardiorespiratory synchronization the
HRV was calculated according to the guidelines (Task Force of the
European Society of Cardiology and the North American Society of
Pacing and Electrophysiology 1996
). A power spectrum based on the
fast Fourier transformation (FFT) was calculated for the resampled RR-
tachogram derived from the 5-min-recording of each exercise. Next, the
resulting power spectral density distribution was integrated in the low-
frequency band (0.04–0.15 Hz, LF) and the high-frequency band (0.15–
0.4 Hz, HF). The guidelines suggest to calculate the power in these
frequency bands in square milliseconds (corresponding to the variance
of the RR-intervals in the particular frequency bands). In difference to the
guidelines, we use the square root of LF and HF power to obtain values
in milliseconds that correspond to the standard deviation of the
particular band-pass filtered RR-tachogram because these values allow
to estimate the amplitude of the oscillations in each frequency band.
Furthermore, the balance bal=LF/HF was also calculated as a rough
quantitative estimate of the balance between the sympathetic and
parasympathetic activity of the autonomous nervous system. In addition,
the extent of RSA is expressed as the median of the longest RR-interval
minus the shortest RR-interval of each respiratory cycle.
Statistics
Descriptive methods are used to assess the effects of Zen meditation on
cardiorespiratory synchronization. Since the number of subject is small
(n=9) the distributions of the different parameters are not known. Thus,
the median is used to quantify the distributions of the parameters. The
two sitting periods and the two periods of Zazen meditation,
respectively, were condensed in one quantity by taking the average. In
total, four different exercises were compared: sitting (S), mental task (T),
Zazen meditation (MZ) and Kinhin meditation (MK). The
-value, mean
heart rate, mean respiratory frequency, extent of RSA, HF, LF, and
balance were calculated for each exercise of each subject. Box and
whisker plots were used for visualization of the distribution of heart rate,
respiratory frequency,
-value, extent of RSA, LF, HF, and balance.
The probability of equality between the four different exercises was
quantified by the non-parametric Friedman-test, a non-parametric one-
way ANOVA. A P
Friedman
-value near zero indicates a high probability of
differences between the different exercises with respect to the analysed
parameter. An appropriate post hoc test for multiple comparisons was
used to calculate the probability of equality between two exercises
(Bortz et al. 2000
). If the P
Friedman
-value of a parameter is low, low P-
values of the post hoc test indicate which exercises differ considerably.
Results
In all subjects cardiorespiratory interaction was most desynchronized
during sitting (S) at the beginning of the procedure ( =0.23), c.f. Fig. 2c.
During the mental task exercise (T) the degree of synchronization
increased in all subjects to an intermediate level ( =0.40). Practicing
Zazen meditation (MZ) or Kinhin meditation (MK) increased the extent of
synchronization to a high level (MZ: =0.77, MK: =0.80).
Fig. 2 Heart rate and respiratory frequency during the quiet sitting (S),
mental task (T), Zazen meditation (MZ), and Kinhin meditation (MK). Low
values of P
Friedman
indicate the likely differences between the exercises.
The probability of similar values between the exercises is indicated by
the P-values above the box and whisker plots. The box plots show
median and quartiles (horizontal lines), average (asterisk), and maximum
and minimum values (whiskers)
The heart rate was lowest during the Zazen meditation (68.2 beats/min)
and highest for Kinhin meditation (77.5 beats/min), c.f. Fig.
2
a. Sitting
and the mental task showed intermediate heart rates (S: 72.8 beats/min,
T: 75.9 beats/min). During sitting and mental task the respiratory
frequency was at a normal level (S: 16.2 breaths/min, T:
16.6 breaths/min), whereas the two meditations decreased the
respiratory frequency enormously (MZ: 8.4 breaths/min, MK:
6.1 breaths/min), c.f. Fig.
2
b.
The high-frequency variations of HRV were approximately the same
during sitting, mental task and Zazen meditation (S: 18.9 ms, T: 19.2 ms,
MZ: 19.0 ms), c.f. Fig.
3
a. During Kinhin meditation the high-frequency
component decreased (MK: 13.5 ms). In contrast, the low-frequency
variations were approximately the same during sitting and mental task
(S: 38.1 ms, T: 26.9 ms) and increased considerably during Zazen and
Kinhin meditation (MZ: 61.1 ms, MK: 69.5 ms), c.f. Fig.
3
b. Again, the
balance is approximately the same during sitting and mental task (S: 1.7,
T: 1.5) and increases during Zazen meditation (3.2) and is largest during
Kinhin meditation (4.8), c.f. Fig.
3
c. The extent of RSA is approximately
the same during sitting and mental task (S: 67.9 ms, T: 55.6 ms) and
increases considerably during Zazen and Kinhin meditation (MZ:
134.6 ms, MK: 168.3 ms), c.f. Fig.
3
d.
Fig. 3 Heart rate variability (high frequency HF; low frequency LF;
balance= LF/HF; extent of RSA) during the quiet sitting (S), mental task
(T), Zazen meditation (MZ), and Kinhin meditation (MK). Low values of
P
Friedman
indicate the likely differences between the exercises. The
probability of similar values between the exercises is indicated by the P-
values above the box and whisker plots. The box plots show median and
quartiles (horizontal lines), average (asterisk), and maximum and
minimum values (whiskers)
Discussion
The investigation of different kinds of meditation has gained attention in
recent years. On a psychosomatic level the meditation techniques may
be used to calm down the patient and to direct the awareness to distinct
processes, for example, the process of breathing, that are usually
carried out without much attention, or to the present moment. Thus,
meditation may change the significance of every day activities. This way
the coping with stress and problems (arising e.g. from a specific
disease) may by enhanced, that is, so-called salutogenetic processes
may be improved, and a healthier state may result (Antonovsky 1987).
This may also lead to benefits on a physiological level.
The impact of meditation on physiological parameters may also be
investigated directly because most meditation techniques make use of
specific procedures which also influence the breathing frequency (e.g.
by focussing on breathing during Zen meditation). In this study,
cardiorespiratory interaction has been analysed in nine subjects (one
teacher of Zen meditation and eight first-time meditators) practicing two
different kinds of Zen meditation. Zazen and Kinhin meditation both led
to a highly synchronized interaction between heart rate fluctuations, that
is, RSA, and respiration. Zazen meditation drastically slowed down the
breathing frequency (approximately 8 breaths/min) and, hence, led to a
pronounced and in-phase RSA. Although the heart rate was increased
during Kinhin meditation the extent of synchronization was slightly
higher than during Zazen meditation because this kind of meditation
slowed down the breathing frequency even more (approximately 6
breaths per minute). On the contrary, sitting in a chair with spontaneous
breathing led to an almost completely desynchronized cardiorespiratory
interaction. During the mental task exercise the extent of
cardiorespiratory synchronization was slightly increased compared to
spontaneous breathing. With respect to the extent of cardiorespiratory
synchronization during quiet sitting (spontaneous breathing) and Zazen
meditation our results are in accordance with previous findings (Peng et
al.
2004
).
The origin of this kind of cardiorespiratory synchronization is the RSA,
that is, the modulation of heart rate by respiration (Angelone and Coulter
1964
). The magnitude of RSA depends on the frequency and the
amplitude of the breathing oscillations. Some basic dependencies that
have to be taken into account are described in the following. The extent
of RSA increases as the tidal volume increases and the breathing
frequency is constant (Hirsch and Bishop
1981
). If the tidal volume is
kept constant, breathing oscillations modulate heart rate strongest for
frequencies below 0.15 Hz (Hirsch and Bishop
1981
; Brown et al. 1993;
Hayano et al.
1994; Pitzalis et al. 1998; Bernardi et al. 2000; Stark et al.
2000
). On the other hand, mental effort decreases the extent of RSA
(Bernardi et al.
2000
). Hypercapnia which may result from a low-
breathing frequency, is counterbalanced by adjusting the tidal volume
appropriately and by an improved pulmonary gas exchange resulting
from RSA (Hayano et al.
1996
; Giardino et al. 2003).
These facts have to be taken into account to assess the effects of the
different exercises appropriately. During both kinds of meditation the
breathing frequency was apparently decreased and led to an increase of
low-frequency heart rate variation and the extent of RSA. The low-
frequency breathing pattern produced regular excitatory and inhibitory
effects of the central respiratory generators on vagal and sympathetic
outflow. The local maximum of the cardiorespiratory transfer function at
low frequencies (approx. 0.1 Hz) suggests that this effect is especially
pronounced at these frequencies (Berntson et al.
1993
). As a result,
cardiorespiratory synchronization occurs. During sitting and mental task
the breathing frequency was increased compared to the meditation
exercises. The transfer function decays strongly at these frequencies
and, hence, the regular excitatory and inhibitory effects of the central
respiratory generators on vagal and sympathetic outflow is diminished.
As a consequence, cardiorespiratory interaction was almost fully
desynchronized. Nevertheless, this increase was associated with a slight
increase of the extent of cardiorespiratory interaction.
Interestingly, the HRV reveals a difference between both types of
meditation. During the Kinhin meditation the high-frequency variations
were smallest and the low frequency variations were largest. Thus,
during Kinhin meditation the balance was even higher than during Zazen
meditation, that is, the Kinhin meditation transfers slightly more
oscillatory impact from the high-frequency band to the low-frequency
band than the Zazen meditation. This effect is even more remarkable if
the orthostasis and the slight physical effort of this kind of meditation is
taken into account. Furthermore, it is interesting to note that these
physiological effects appeared in unexperienced meditators. This
suggests that the physiological benefits from practicing meditation
appear regardless of any prior experience in meditation.
The cardiorespiratory synchronization may be associated with findings
of positive signs of recovery: an increase of arterial oxygen saturation
SaO
2
during breathing at a frequency of 6 breaths/min (Bernardi et al.
1998
), an increased arterial baroreflex sensitivity as a favourable long-
term prognostic factor in cardiac patients (Bernardi et al. 2002
), and a
decrease of systolic blood pressure (Grossman et al. 2001
). Although
meditation is often associated with some kind of (mental) relaxation the
different physiological implications of meditation suggest a more
detailed picture: a rather pronounced HRV which is well coordinated with
other oscillations, for example, respiration. The latter characteristic
seems to be essential to make the exercises comfortable (in the sense of
calm) although some physiological functions are at least partially more
active (Peng et al. 2004
). Since many different meditation techniques
make use of breathing patterns with low frequencies, cardiorespiratory
synchronization may also appear during other kinds of meditation. More
generally, this feature may occur whenever the breathing frequency is
low, for example, during recitation of the rosary prayer (Bernardi et al.
2001
) or the OM -mantra (Telles et al. 1995). However, an increased
mental activity may reduce the extent of cardiorespiratory
synchronization.
Limitations
The study design is a balanced design using longitudinal and cross-
sectional elements. Thus, it was possible to achieve reliable results with
a relatively low number of participants and it was not necessary to
introduce any kind of randomization. As pointed out in (Bettermann et al.
2002) this kind of design is especially well suited in the field of creative
arts therapy research
. This statement also applies for the present study
because the creative arts therapy investigated in the cited study also
deals with different breathing modalities.
Another point is concerned with the question whether subjects
practicing Zen meditation for the first time experience the same effects
(and/or benefits) than experienced Zen meditators do. This study
unambiguously reveals that with respect to cardiorespiratory interaction
the Zen meditation shows similar effects in the teacher of Zen meditation
and in first-time meditators. Thus, we conclude that this kind of
physiological impact does not require long time training in Zen
meditation. However, there might be other levels of impact, for example,
different states of mind (or awareness), that probably are not attainable
by first-time meditators (Büssing 2005
). Hence, it may be possible that
other physiological effects, for example, a specific signature in the
electroencephalogram as a physiological representation of the state of
mind (Lutz et al. 2004
), are not attainable by first-time meditators. Such
questions deserve future investigations, but they were not the topics of
this investigation.
In conclusion, Zen meditation synchronizes the cardiorespiratory
interaction with respect to breathing oscillations and the HRV induced
by respiration (RSA). Furthermore, it drastically increased low-frequency
variations of heart rate. Spontaneous breathing patterns hardly showed
any cardiorespiratory synchronization and during mental activity the
cardiorespiratory synchronization was decreased compared to both
types of Zen meditation. It remains to be shown that this kind of
cardiorespiratory synchronization is advantageous for the gas exchange
in the respiratory tract. Furthermore, this study indicates that this kind of
religious practice has immediate physiological effects on
cardiorespiratory interaction without the need of special long-term
training.
Acknowledgments Both authors express their gratitude for the financial
support of the Software AG Stiftung, Darmstadt.
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