The new study used a baby ECG machine and video cameras to systematically compare changes in heartrate and behavior as mothers acted out activities that are commonly used to calm infants, including carrying, being pushed in a stroller, and holding while sitting.
Data during these activities were recorded from babies that were crying, awake and calm, or sleeping. At each heartbeat, behavior was assessed as asleep, alert, or crying, and scored accordingly. This way the researchers could track changes in both behavior and physiology with sub-second precision.
The experiment led to a few important findings. First, as Kuroda explains, “walking for five minutes promoted sleep, but only for crying infants. Surprisingly, this effect was absent when babies were already calm beforehand.” Among the babies studied, all had stopped crying by the end of the five-minute walk and had reduced heart rates, and about half were asleep. Second, sitting and holding crying babies was not calming; heart rates tended to go up and crying persisted.