Infant bouncing: the assembly and tuning of action systems.

We outline a theory of infant skill acquisition characterized by an assembly phase, during which a task-specific, low-dimensional action pattern emerges from spontaneous movement in the context of task constraints, and a tuning phase, during which adjustment of the system parameters yields a more energetically efficient and more stable movement. 8 infants were observed longitudinally when bouncing while supported by a harness attached to a spring. We found an initial assembly phase in which kicking was irregular and variable in period, and a tuning phase with more periodic kicking, followed by the sudden appearance of long bouts of sustained bouncing. This "peak" behavior was characterized by oscillation at the resonant frequency of the mass-spring system, an increase in amplitude, and a decrease in period variability. The data are consistent with a forced mass-spring operating at resonance.