The development of the speech-gesture system was examined by evaluating the relationship between rhythmic arm activity (RAA) and reduplicated babble (RB). Using a novel longitudinal observational design, infants were observed in-home weekly, for twelve weeks playing with a rattle (22 to 34 weeks old). Video and audio-recordings were submitted by caregivers via a
secure file-sharing service. The design was an effective alternative for longitudinal data collection in infant studies. RAA and RB were positively correlated, and infants exhibited greater amounts of tightly synchronized vocal-manual coordination (VMC) over time. Infant threshold was not a significant predictor of RB or VMC, and babble onset did not significantly predict the frequency or the type of VMC. Trajectory analyses revealed synchronous change across RAA, RB, and VMC. Findings suggest the linkage between RB and RAA is not sequential; but is a simultaneous process representing a moment of re-organization to the maturing speech-gesture system.