1 3 Exp Brain Res DOI 10.1007/s00221-015-4235-7 RESEARCH ARTICLE Coupling of postural activity with motion of a ship at sea Manuel Varlet · Benoît G. Bardy · Fu-Chen Chen · Cristina Alcantara · Thomas A. Stoffregen Received: 28 October 2014 / Accepted: 16 February 2015 © Springer-Verlag Berlin Heidelberg 2015 not. Coupling of postural activity with ship motion differed between body axes as a function of body orientation relative to the ship. In addition, coupling differed between partici- pants who had been seasick at the beginning of the voyage and those who had not. We discuss the results in terms of implications for general theories of postural control, and for prediction of susceptibility to seasickness in individuals. Keywords Postural coordination · Vehicular travel · Spontaneous coordination · Seasickness Introduction Control actions that are used to maintain upright stance often become coupled to oscillatory motion of the environment. One well-known example is the tendency of postural activity in upright stance to become coupled with externally gener- ated (i.e., experimentally imposed) oscillations in optic flow, widely known as the moving room paradigm (e.g., Dijkstra et al. 1994). Postural activity becomes coupled with imposed optic flow when people are instructed to couple with it (Bardy et al. 1999, 2002), when participants are not instructed to cou- ple with it but only to look at it (e.g., Lee and Lishman 1975; Oullier et al. 2002; Stoffregen 1985; Warren et al. 1987), and even when participants are instructed actively to resist cou- pling (Barela et al. 2009; Stoffregen et al. 2006). In the pre- sent study, we focused on coupling of postural activity with oscillatory motion of the inertial environment, that is, oscilla- tion of the surface of support. Postural responses to laboratory devices Many studies have examined the control of upright stance on moving surfaces. The principal example is moving Abstract On land, body sway during stance becomes coupled with imposed oscillations of the illuminated envi- ronment or of the support surface. This coupling appears to have the function of stabilizing the body relative to the illu- minated or inertial environment. In previous research, the stimulus has been limited to motion in a single axis. Little is known about our ability to couple postural activity with complex, multi-axis oscillations. On a ship at sea, we evalu- ated postural activity using measures of body movement, as such, and we separately evaluated a direct measure of cou- pling between body movement and ship motion. Participants were tested while facing fore-aft and athwartship. We com- pared postural activity between participants who had been seasick at the beginning of the voyage and those who had M. Varlet · B. G. Bardy Movement to Health Laboratory, EuroMov, Montpellier-1 University, Montpellier, France M. Varlet The MARCS Institute, University of Western Sydney, Sydney, Australia B. G. Bardy Institut Universitaire de France, Paris, France F.-C. Chen Department of Recreation Sport and Health Promotion, National Pingtung University of Science and Technology, Pingtung, Taiwan C. Alcantara Escola de Educação Física e Esporte, Universidade de São Paulo, São Paulo, Brazil T. A. Stoffregen (*) School of Kinesiology, University of Minnesota, Minneapolis, MN, USA e-mail: tas@umn.edu