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The subjects were 40 male volunteers, with ages ranging from 16- 46 years and with an average of 23.9 months of experience of yoga practice. The autonomic parameters were assessed in 15 subjects (group"s average age was 30.2, SD = 6.1 yr.). In 25 subjects the oxygen consumption, breath rate and volume were measured before and after the two practices (group"s average age was 28.2, SD = 6.6 yr,).

Subjects were studied in two separate relaxation sessions, viz., isometric relaxation technique (IRT), and supine rest (SR) with instructions as a control. The two sessions were on different days, at the same time of the day. For half the subjects, alternately, the IRT session was on the first day with the SR session the next time. The order was reversed for the remaining subjects. Each recording session was of 10 minutes and consisted of 2 periods, viz., before (5 minutes) and after (5 minutes The subjects were sitting at case before and after the practice periods, and supine during the relaxation periods. Both techniques were of 10 minutes duration under the same standard conditions.

The oxygen consumption was recorded with a closed circuit, Benedict-Roth apparatus (INCO, Ambala, India) using the standard method (Mountcastle, 1980). The subject breathed into an oxygen tank, from which exhaled carbon dioxide was excluded by absorption in sodium hydroxide. The subjects were asked to breathe into the mask, which covered their nose and mouth. Recordings were made before and after, but not during test periods.

A 4-channel polygraph (Medicaid Systems, Chandigarh, India) was used to record the electrocardiogram (EKG), respiration, and finger plethysmogram amplitude EKG was recorded using standard Limb lead 1 configuration. The EKG was digitized using a 12 bit analog-todigital convertor (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 (Raghuraj, Ramakrishnan, Nagendra & Telles, 1998). 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.

Respiration was recorded from using a nasal thermistor attached to the more patent nostril. Finger plethysmogram amplitude was recorded placing the photoplethysmograph on the volar surface of the distal phalanx of the index finger of the right hand.

Isometric relaxation technique (IRT)
The isometric relaxation technique lasts for 10 minutes and is done in 5 phases of step-wise relaxation, detailed below (Nagendra & Nagarathna, 1988). (i) Isometric contractions of all the muscle groups from the toes to the facial muscles, mentioning each part of the body specifically, (ii) letting the body collapse" on the ground with a feeling of "letting go", till the changes revert back to normal, (iii) Watching the abdominal movements, (iv) synchronizing the abdominal movements with breath, (v) developing the breath awareness followed by invoking the positive emotions with the breath "This is practiced slowly and with instructions about relaxation and awareness of breath and mental sensations. Throughout the practice the eyes are closed.

Supine rest (SR)
During SA, the subject lies supine with the legs apart, arms away from the sides of the body, and eyes closed. This session lasts 1 0 minutes, as for IRT.

Data extraction
The end expiratory points of the respirogram obtained using the Benedict-Roth apparatus were joined as a slanting line, the slope of which gave the difference between initial and final volumes of oxygen in the tank in a given period, which was approximately 3-4 minutes in most cases. The breath rate and respiratory (tidal breathing) volume were also obtained from the record.

The following data were extracted from the polygraph records: The respiratory rate (in cycles per minute) was calculated by counting the breath cycles in 60 second epochs, continuously. Finger plethysmogram amplitude (in mm) was sampled at 20-second intervals. Values averaged across each of the periods (before, after) of a session, were used for analysis.

Frequency domain analysis of heart rate variability (HRV) data was carried out for the 5- minute recordings before and after the sessions. The mean heart rate was obtained from this record. The mean values were removed from the heart rate series to obtain the HRV values. The HRV power spectrum was obtained using Fast Fourier Transform (FFT). The power in HRV series in the following specific frequency bands was studied, Viz., the very low frequency (VLF) band (0- 0.05 HZ), low frequency (LF) band (0.05- 0.15 HZ), and high frequency (HF) band (0.15- 0.50 Hz). The low frequency and high frequency values were expressed as normalized units, which represent the relative value of each power component in proportion to the total power minus VLF component (LF) norm = LF/ (total power-VLF) X 100; HF norm = HF/(total power-VLF) X 100)" (Task force of the European Society of Cardiology the North American Society of Pacing and Electrophysiology, 1996).

Data analysis
A two factor analysis of variance (ANOVA) was used to check for significant differences between the 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.

Oxygen consumption, breath rate and breath volume
There was a significant difference between values recorded before and after the session of isometric relaxation technique and of supine rest (i.e., Factor B) for breath rate F = 5.22, for df = 1,96, p<.05(2), values for oxygen consumption, breath amplitude were not significant. The difference between two categories of subjects was not significant (Factor A), with no significant interaction between Factors (A X B).

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