Non-stationarity of the natural frequency of the sympathetic control loop of the heart rhythm

In recent years, heart rate variability biofeedback has rapidly developed and is used to improve the condition and treat a number of diseases in clinical practice. However, despite this, assessing the person’s resonance frequency still remains a complex task, entailing many still unanswered questions. In this work, we investigated the stationarity of the resonance frequency in repeated experiments for young healthy volunteers, as well as the possibility of using rhythmic skeletal muscle tension of the arms and legs instead of the complex requirement of breathing at unusually slow frequencies. The experimental sample consisted of 7 volunteers, for whom a series of 5 experiments were conducted (4 recordings with slow breathing, 1 recording with rhythmic skeletal muscle tension). There were recorded ECG and respiratory signals. During the data analysis, we constructed resonance curves and determined resonance frequency values. It was found that the resonance frequency is not stationary for a particular person. Rhythmic skeletal muscle tension of the arms and legs can be an alternative to breathing at slow frequencies.