ey words :inverted posture, skin conductance,
level heart rate, variability, finger blood flow
head stand (sirsasana) has been described
as the king of all yoga postures.
It is believed that mastering it gives one physical and
mental balance and poise (1).
on the physiological effects of this practice have been contradictory,
with some results suggesting physiological activation, while others
suggested the reverse effect.
An early study described an increase in brachial blood
pressure in eleven subjects during five minutes of the head stand
(2). A 68.6 percent increase
in oxygen consumption during the head stand was also reported
in six subjects (3). In
contrast to these reports suggesting physiological activation,
in another study, the heart rate decreased during five minutes
of this practice, compared to standing erect before, with a further
reduction while supine after it (4). While a decrease in heart rate has been associated
with reduced mental arousal, the reduction in heart rate during
this inverted yoga posture, compared to standing erect before
it, may have been reflexly brought about by baroreceptor
stimulation. The change
in brachial blood pressure during the head stand may be related
to better left ventricular filling and cardiac output because
of the inverted posture with better blood flow to the 'dependent'
upper parts of the body. These changes during the head stand were comparable,
to the effects of 70 degrees head down tilt, which resulted in
an increase in BP despite peripheral vasodilation,
along with increased cardiac output and left ventricular filling
(5). Hundred degrees head down tilt brought about
a decrease in forearm vascular resistance, and in the absolute
and relative low frequency component of the heart rate variability
studies on the effects of the head stand reported changes whose
mechanisms (in terms of sympathetic activation or withdrawal)
were inferred, the present study aimed at understanding sympathetic
and vagal activity, following the head
stand, using the heart rate variability spectrum.
Also, since the head stand is practiced both with the support
of a wall and without support (7), the present study assessed
subjects practicing both methods to see whether the effects were
Forty adult male volunteers who practiced
the head stand in two ways (i.e., with the support of a wall and
without), were studied, with 20 subjects practicing each method. For the 'With support
group’, the group average age ± SD was 26.0 ± 4.8 years. For the 'Without support group' the group average
age ± SD was 24.0 ± 3.2 years.
Design of the
The subjects of both groups were assessed
in a single session of 12 minutes.
Recordings were made for 5 minutes each, both before and
after 2 minutes of the head stand, but not during the practice,
as artifacts related to muscular contraction due to the inverted
posture interfered with the recording.
Recordings were taken both before and after the practice
while subjects were seated, to provide a contrast to the inverted
A 4-channel polygraph (Medicaid Systems,
Chandigarh, India) was used to record the electrocardiogram
(EKG), respiration, finger plethysmogram
amplitude and skin conductance level.
EKG was recorded using standard limb lead I configuration. The EKG was digitized using a 12 bit analog-to-digital
converter (ADC) at a sampling rate of 500 Hz. The data recorded were visually inspected off-line
and only noise free data were included for analysis. The HRV analysis software was developed at the
Dept. of Electrical Engineering, Indian Institute of Science, Bangalore (by A.G. Ramakrisnan,
Ph.D.). The R waves were detected to obtain a point event series
of successive R-R intervals, from which the beat to beat heart
rate series was computed (8).
Simultaneously, polygraph recordings of the variables mentioned
below were also carried out. Skin conductance was recorded using Ag/AgCl disc electrodes with electrode gel (Medicon, Madras, India), placed in contact with the volar surfaces of the
distal phalanges of the index and middle fingers of the left hand. A low-level DC preamplifier was used and a constant
voltage of 0.5V was passed between the electrodes. Respiration was recorded using a nasal thermistor clipped to the more patent nostril. Finger plethysmogram
was recorded placing the photoplethysmograph
on the volar surface of the distal phalanx of the index finger of
the right hand. The amplitude
was sampled from the ascending portion of the wave (9). The blood pressure was recorded with a sphygmomanometer
by auscultation over the right brachial artery.
The following data were extracted
from the polygraph records : The breath
rate (in cycles per minute) was calculated by counting the breath
cycles in 60 second epochs, continuously.
The skin conductance level (SCL in micro siemens) and finger
plethysmogram amplitude (in mm) were sampled at 20 second
intervals. For each subject,
the average of the values obtained during the 5-minute session
was used for analysis. Frequency domain analysis of heart rate variability
(HRV) data was carried out for the 5-minute recordings before
and after the head stand. The
mean heart rate was obtained from this record.
The HRV power spectrum was obtained using Fast Fourier
Transform (FFT). The energy in HRV series in the following specific
frequency bands was studied, viz. the very low frequency band
(0.0-0.05 Hz), low frequency band (0.05-0.15 Hz), and high frequency
band (0.15-0.50 Hz). According
to guidelines of the Task force of the European Society of Cardiology
and the North American Society of Pacing and Electrophysiology,
the low frequency and high frequency values were expressed as
normalized units (10).
two factor analysis of variance (ANOVA) was used to check for
significant differences between the two categories of subjects,
i.e., factor A, and for differences between recordings before
and after, i.e., factor B. The Tukey test for the least significant difference between means
was used for multiple comparisons.
The 't' test for paired data was
used to assess the significance between after and before values
of each group, separately, to detect changes which were not significant
with the Tukey test.
The head stand (sirsasana)
At the start of the practice, subjects
kept the crown of their head on a firm surface supported with
interlocked fingers behind and forearms on either side. With the knees flexed, the legs were gradually
raised, till the body was stretched upwards, resting on the head
(7). During this practice one group of subjects were
supported by a wall, while the other group was not.
The data before and after the practice
were found to be (a) normally distributed and (b) not of unequal
variance. Hence parametric
statistics were used for analysis.
Two factor analysis of variance (ANOVA)
was a significant difference between the recordings before and
after the head stand (i.e., factor B) in the power of the low
frequency component of HRV [F = 6.79, since F .05 (2) 1, 76 =
5.23, hence P<.05], in the high frequency power value [F =
(6.74, since F .05 (2) 1, 76 = 5.23, hence P<.05], and in low
frequency/high frequency ratio [F = 9.84, since F .005 (2) 1,
76 = 9.79, hence P<.005]. The heart rate, skin conductance
level, breath rate, finger plethysmogram amplitude and both systolic and diastolic blood
pressure were not significantly different between categories or
recordings, with interaction between the factors being not significant,
i.e., A X B (P>.50). Here, the F values for the degrees of
freedom (1, 76) have been derived by linear interpolation from
the values for denominator equal to 70, 80 in the standard table,
according, to the method described (11).
Tukey test for multiple comparisons between
None of the values were significantly
different after the practice compared to before, for both categories
of subjects. Also, the
baseline values of the two categories of subjects did not differ
(P>.10, for all comparisons).
't' test for paired data
the head stand compared to before, there were the following changes:
(i) In both groups, there was a significant increase in the
low frequency and reduction in high frequency power values of
the, heart rate variability (HRV) spectrum expressed in normalized
units (in subjects who practiced without wall support, for both
LF and HF, P<.02) and in subjects who practiced with wall support,
for both LF and HF, P<.05). This contributed to a significant
increase in low frequency/high frequency power ratio (P<.02
for 'Without support' subjects; P<.05 for 'With support' subjects).
The heart rate was significantly reduced following the
head stand (P<.02 for 'Without support' subjects; P<.05
for 'With support' subjects). (ii) The 'Without support' subjects
showed a significant increase in skin conductance level (P<.05)
and (iii) the 'With support' subjects showed a reduction in the
amplitude of the finger plethysmogram
(P<.05). No significant changes were observed in respiratory
rate or systolic and diastolic blood pressure values in either
group following the head stand.
group average values ± S.E.M are given in Table I.
Sample tracings of the HRV spectra and polygraph recordings
in two subjects each belonging to one of the two groups, are given
in Fig. 1, 2 and 3, 4 respectively.
click for full view
I: Components of heart rate variability spectrum and other
autonomic variables before and after the head stand in two
categories of subjects (n=20) values are group mean ± SEM.
click for full view
1: Sample records of heart rate variability spectrum made
before (upper record) and after (lower record) two minutes
of the head stand practiced with wall support in a single
subject (RAV/24/M). The vertical axis gives the power values
in BPM2/Hz. The two vertical dotted lines separate
the three frequency components, viz. very low frequency
(VLF), low frequency (LF) and high frequency (HF). The present
record shown an increase in the
low frequency power (stippled portion) and a decrease in
high frequency power (hatched portion) following two minutes
of the head stand.
click for full view
2: Sample records of heart rate variability spectrum made
before (upper record) and after (lower record) two minutes
of the head stand practiced without wall support in a single
subject (SRI/27/M). The rest of the details and the findings
are as for Figure 1. However note the magnitude of changes
is less in the subject who practiced the head stand with
wall support (RAV; Fig.1) compared to this subject (SRI)
who practiced it without wall support.
click for full view
3: Polygraphic tracing of subject
(RV/27/M) as a representative of the ‘With Support’ group.
Four variables are shown here: (a) volar
skin conductance, (b) EKG (lead I), (c) respiration with
nasal thermistor, and (d) finger
plethysmogram. For each variable the upper trace was taken
before and the lower trace was after the practice of sirsasana. Note the decrease in finger plethysmograpm amplitude after sirsasana
(tracing 4b) compared to before (4a).
click for full view
4: Polygraph tracing of subject (SRI/27/M) as a representative
of the ‘Without Support’ group. The rest of the details
are the same as for Figure 3. Note the increase in skin
conductance after sirsasana (tracing
1b) compared to before (1a).
Following two minutes of the head-stand
compared to the preceding period while sitting erect, there was
a significant increase in the power of the low frequency component
of the heart variability spectrum, a decrease in the high frequency
power value and decrease in heart rate in both categories of subjects
(i.e., those who practiced without support and those who used
support). Also, the 'Without
support' group showed a significant increase in skin conductance
level. The 'With support'
group showed a significant reduction in finger plethysmogram
The low frequency component (0.05- 0.15
Hz) of the heart rate variability spectrum when expressed in normalized
units, is a quantitative marker of sympathetic modulations; the
high frequency component (0.15-0.50 Hz) has been correlated with
parasympathetic activity and the LF/HF ratio reflects the sympathovagal
balance (10). From the
present study it appears that in both categories of subjects the
cardiac sympathetic activity has increased, vagal
activity has reduced and sympathovagal balance has changed.
activity recorded from the distal phalanges is a sensitive index
of changes in sympathetic tone (12).
The skin resistance is the reciprocal of skin conductance
(13) and is a measure of arousal.
In a subject at rest, an increase in skin resistance is
an index of relaxation (14), while reduced skin resistance has
the reverse interpretation. The
'Without support' subjects in the present study showed increased
skin conductance level, while the finger plethysmogram
amplitude was significantly lower in the 'With support' group
subjects. The finger photoplethysmograph
measures the peripheral cutaneous blood
flow and in turn the sympathetic vasomotor changes.
Decreased amplitude of the plethysmogram
record as observed in the present study indicates peripheral vasoconstriction
due to sympathetic arousal (15).
A previous study reported a decrease
in heart rate during the head stand, which was more in magnitude
after the practice, while lying supine (4).
In the present study also, the heart rate significantly
decreased in both groups after the practice.
While a decrease in heart rate during the head stand may
be a reflex response due to baroreceptor
activation, a decrease in heart rate while seated erect after
the practice, is less easy to explain and may be due to a continuation
of changes which occurred during the head stand.
The inverted posture increases left
ventricular filling and cardiac output (5).
This can be expected to activate the baroreceptors. When resuming the erect posture, the arterial
pressure in the head and upper part of the body tends to fall
and the falling pressure at the baroreceptors
elicits an immediate reflex, resulting in sympathetic discharge
throughout the body (16).
In the present study, this mechanism
may explain the following changes : increased
power of the low frequency component of the HRV spectrum, increased
skin conductance level, and decreased finger plethysmogram
The two categories of subjects ('With support', 'Without support') showed activation of different
subdivisions of the sympathetic nervous system selectively, viz.,
sudomotor and vasomotor respectively. In attempting to explain this, a previous study
reported that during hypothermia, static hand grip caused sustained
increase in skin sympathetic nerve activity (SNA) and in electrodermal
activity along with a transient increase in skin vascular resistance
(17). There were directionally opposite changes in
estimated skin vascular resistance :
with exercise-induced vasodilatation during hyperthermia, and
exercise induced vasoconstriction during hypothermia.
It was concluded that static exercise markedly increases
sympathetic out-flow to skin; the increases in skin SNA appear
to be caused mainly by central command rather than by muscle afferent
reflexes and this cutaneous sympathetic
activation appears to be targeted both to sweat glands and to
vascular smooth muscle, with relative targeting being temperature
Hence we concluded that sirsasana was a form of 'static exercise' which
produced both of the reported effects (increased electrodermal
(sudomotor) activity and increased cutaneous vasomotor activity). The presence of differential effects in the
two groups was not easy to explain, however an early report showed
that different stimuli generate activity in different divisions
of the sympathetic nervous system (18).
This shows that under different types of exercise the different
subdivisions get selectively and differentially activated.
It is possible that the two methods of practicing the postures
(i.e., with support and without) may activate different mechanisms,
and even induce different, minor temperature changes, according
to the method of practice. Further studies are needed for better understanding.
The findings of the present study have
to be interpreted with discretion, as recordings were not made
during the practice of the headstand.
However, the present results suggest that the practice
causes sympathetic activation, which required further investigation
to understand applications and precautions related to this posture.
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