When two become one: Spontaneous pattern formation in side-by-side and hand-in-hand walking

When two persons are walking together, their footsteps sometimes spontaneously adjust to one another. Such episodes of entrainment may enhance when holding hands; such mechanical coupling affects arm-leg coordination of each walker and hence between-walker stepping. We examined the effects of detuning (pairs with either similar [n=8] or different [n=8] uncoupled cadences) and coupling (hand-in-hand or side-by-side) on spontaneous pattern formation. Pairs walked 10 minutes alternately hand in hand and side by side on a huge treadmill (1.3km/h). Full-body 3D kinematics were captured with Kinect v2. We observed a greater occurrence of phase locking for hand-in-hand walking. Two interpersonal patterns prevailed: synchronizing of left and right legs (ipsilateral phase-locking) or synchronizing of inner and outer legs (contralateral phase-locking). These two coordination patterns occurred more often in the low-detuning group. We also observed diverse other coordination patterns, one involving an absence of swing in the coupled hands. We conclude that both coupling strength and detuning independently affect the occurrence of spontaneous phase locking in paired walking, a task affording a rich repertoire of coordination patterns.