Introduction: Both breathing and skeletal muscle activation affect the autonomic nervous system and heart rate. Recently, the body relaxation technique has been introduced, in which we combine resonant breathing and isometric muscle contraction, which is reflected in increased heart rate variability (HRV). Studying the efficacy of techniques to increase HRV in individuals after acute myocardial infarction (AMI) is clinically important, as higher HRV is associated with a higher survival rate in individuals after AMI. Purpose: To investigate whether the size of active muscle mass during the implementation of the combined paced breathing and dynamic muscle contraction technique significantly affects HRV among patients after AMI. Methods: 20 patients after AMI were included, who attended the outpatient cardiac rehabilitation program at the Ljublana University Medical Center. Patients performed a three-minute combined paced breathing and dynamic muscle contraction. Resonant breathing of 0.1 Hz was performed. In the inhalation phase, the knee extensors were isometrically tensioned at ~ 80 ° flexion, and the muscles were relaxed in the exhalation phase. Each patient performed the technique with both one-legged and two-legged muscle contraction, followed by performing the Stroop test. Heart rate was measured with an ECG. We calculated the averages of the frequency values of HRV (low-frequency (LF) and high-frequency (HF) component of the power spectrum and the ratio between them (LF / HF)) in the observed time intervals. Differences in averages were tested with 2 × 3 factor ANOVA and Tukey’s HSD post hoc test. Results: During the implementation of both one-legged and two-legged combined paced breathing and dynamic muscle contraction technique, there were statistically significant changes in HRV (p < 0.05), namely an increase in LF and LF/HF values and a decrease in HF values. No significant differences (p > 0.05) were detected between the one-legged and two-legged combined paced breathing and dynamic muscle contraction technique for any of the HRV frequency values. We also did not detect significant differences (p > 0.05) for any of the HRV frequency values during the Stroop test after the one-legged and two-legged combined paced breathing and dynamic muscle contraction technique. Discussion and Conclusion: The combined paced breathing and dynamic muscle contraction causes an acute increase in HRV in individuals after AMI and therefore has potential for clinical use. In our study, we did not detect a significant effect of active muscle mass size during the implementation of the technique on HRV change. Due to the small size and variability of the sample, the power of statistical analyzes was small, so additional research is needed on a larger number of patients.
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