At approximately 12 months of age, human infants first demonstrate independent bipedal gait in the form of walking. A few months later, the second milestone of upright locomotion emerges as running. The major difference between these gaits is that walking displays a double support phase, while running exhibits an aerial phase. In the present investigations, intralimb coordination patterns for walking and running are obtained by representing the lower limb segments as systems of coupled limit-cycle oscillators and mapping the continuous relative phase of the coupled segment pairs. An initial study of the adult coordination patterns shows strong similarities between the global patterns of walking and running, with velocity and gait-related differences noted within these patterns. The coordination dynamics were constrained most consistently around the common task requirement of foot contact at the transition from the suspended pendulum system of swing to the inverted pendulum system of stance. Intrinsic pendulum-like dynamics also appeared to constrain the coordination during swing phase. The shank-foot coordination had the largest pattern differences, localized in late stance, however these differences were eliminated upon entry into swing phase. The second study, a longitudinal comparison of infants' walking versus their new running patterns, demonstrated similar influences of constraints. The early exemplars of running displayed similar coordination dynamics to those of infant walking and of the adult gaits. The task constraints at the transition between swing and stance and the intrinsic dynamics of the swinging leg were again implicated in shaping the patterns of coordination. Developmental differences were noted in relative phase magnitudes in both gaits, but more so in running. These were interpreted in terms of constraining the available degrees of freedom and/or the lack of exploiting intrinsic constraints that eventually lead to adult-like trajectories. It was concluded that the development of running does not emerge as a qualitative shift away from the coordination for walking, instead, it reflects an adaptation of a highly resilient bipedal form.
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