It has been reported that the human postural sway is significantly reduced if one touches a stationary object (i.e., "light touch effect"). This reduction is observed also when one touches another person's body. Interestingly, moreover, postural sways of two persons in contact show significant correlation, suggesting the existence of mechanical coupling of body dynamics and haptic communication between the two persons (Johannsen et al., 2009, 2012).
In the present study, we hypothesized that such haptic communication plays an essential role in Japanese martial arts (especially in "Jujutsu") because martial artists presumably try to collapse the opponent postural control through physical and sensorimotor interactions. In order to examine the nature of these interactions, we analyzed how the temporal behavior of COPs (center of pressures) of two persons in touch changes when one took a particular body operation of martial arts. Major findings are as follows. First, the fluctuation of COP of participants standing alone increased when they stood with the martial body operation, compared with the case when they stood normally. Second, COPs of two participants became correlated when one held the other's arm, replicating the previous finding. Third, most importantly, this correlation was diminished when one stood with the martial arts' body operation even though their physical contact was kept steady. Fourth, shift of weights (i.e., vertical component of floor reaction force) between two participants caused by arm hold was gradually canceled when one performed the martial arts' body operation. These suggest that interpersonal dependency/coupling of postural control might be dissolved by martial arts' body operation, presumably through haptic communication.