Heart rate variability associated with different modes of lower abdominal muscle tension during Zen meditation

To understand the effect of lower abdominal muscle (LAM) tension modes on hemodynamic events elicited by deep Zen meditation during Zazen, we studied heart rate in 2 intermediate Zen meditators. The study took place over 9 sets of Zen meditations in a quiet, Zen practice hall. Each set of Zazen lasted at least for 25 minutes. The first sets were used for habituation, and the data obtained from the following eight sets were used for analysis, where 2 modes of LAM tension, i.e., intentional LAM contraction (LAMC) and LAM relaxation (LAMR), were allocated alternately. Power spectrum analysis showed distinctive change in frequency components. High frequency (HF) components increased for LAMC (p=0.05). Most notably, detrended fluctuations analysis (DFA) of HRV were around 1/2 for LAMC and 0.82 for LAMR. During Zen practice, we seat ourselves in a lotus posture with LAMC, practice lower abdominal (Tanden) breathing, and keep mind free from specific state of consciousness. We attribute the increase of HF components and decrease of DFA exponent in LAMC to the parasympathetic stimulation, and to the minimal use of antigravity muscles in the optimal posture generated by LAMC, as contrasted with the active use of antigravity muscles in the unstable posture during LAMR.