In the present study, we investigated co-ordination of breathing with active horizontal head and eye movements and with passive body turnings. The main purpose was to compare co-ordination with active voluntary and with passive movements. Co-ordination was defined according to the relative phases (RPs) between breathing and additional rhythms. It was present when at least five consecutive RPs scattered less than 25% of breath duration or at least three consecutive RPs scattered less than 10%. Additionally, we studied the influence of factors such as size of active muscle groups, mechanical interactions or involuntary motor activities. In consecutive experimental conditions, 17 healthy volunteers performed voluntary rhythmic head (with eyes open and closed) and eye movements and were moved passively (also with eyes open and closed). Co-ordination occurred in all experimental conditions. The highest percentage of co-ordinated breaths was found during active head movements with closed eyes but the difference with other conditions was not significant. However, periods of stable entrainment were observed only in this condition. The results demonstrate that co-ordination is possible even in the absence of voluntary rhythmic movements. Size of active muscle groups or mechanical interactions do not necessarily affect the degree of co-ordination. On the other hand, involuntary motor activities such as reflex eye movements associated with vestibular stimulation may have a considerable influence on co-ordination between other motor processes. These results strongly support the theory of co-ordination as an unconscious nervous interaction that is not based on mechanical or intentional factors.

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