Stability and variability of acoustically specified coordination patterns while walking side-by-side on a treadmill: Does the seagull effect hold?

To examine whether the Haken–Kelso–Bunz model for rhythmic interlimb coordination applies to walking side-by-side on a treadmill, we invited six pairs of participants to coordinate their stepping movements at seven prescribed relative phases (between 0° and 180°) to scan the attractor layout governing their coordination. Two auditory metronomes, one for each participant, specified the required relative phase. For each trial participants were instructed to synchronize their left heel strikes with the beeps of the metronome (2 min) and to continue walking after the metronome stopped (1 min). If the Haken–Kelso–Bunz model applies to interpersonal coordination during treadmill walking, then (1) the variability of in- and antiphase should be minimal, (2) intermediate relative phases should be attracted to either in- or antiphase, and (3) the absolute shift away from the required relative phase should be greatest for a required relative phase of 90°. Only the third of these hypotheses was confirmed, indicating that the dynamical model for rhythmic interlimb coordination does not readily apply, at least not generically or robustly, to interpersonal coordination during walking side-by-side on a treadmill.