Horsing around: spontaneous four-legged coordination.

Motivated by previous research suggesting that informational and mechanical interlimb coupling can stabilize rhythmic movement patterns, the authors show that stable 4-legged patterns between 2 individuals, either walking or running, can emerge unintentionally from simple forms of coupling. Specifically, they show that the leg movements of pairs of naive individuals become spontaneously phase locked when visually or mechanically coupled via a foam appendage. Analysis of each of the phase locked trials revealed distinct preferences for particular 4-legged patterns, with interpersonal in- and anti-phase coordination patterns (equitable with quadruped pace and trot, respectively) observed almost exclusively. Preference for either pattern depended on the strength of coupling. The authors discuss these findings in light of previous claims that the patterns of human and animal locomotion-as well as coordinated movements in general-can emerge from lawful coupling relations that exist between the subcomponents of perceptual-motor systems.