Within‐Day Variations in Infant Body Position Predict Caregiver Speech Input

Infants' time spent in different body positions varies substantially within a day: lying supine on their backs, crawling or playing while prone on their bellies, sitting on the floor or in seating devices, standing or walking upright on two feet, and held by an adult. Infants may spend long periods in one position, such as sitting in a highchair, followed by periods of switching positions (e.g., prone and upright), such as during play. Speech input has been linked with infants' placement and motor development. Here, we extend past work to ask whether adults speak more frequently depending on infants' time spent in different body positions throughout the day. We gathered home recordings from 64 infants in two age groups, 4-7 months and 11-14 months, using movement sensors to measure body position and a LENA audio recorder to estimate adult word count. Data were analyzed in 10-min bins throughout the day to determine whether within-day differences in infants' time spent in each position were associated with the frequency of adult's speech. Indeed, increased time spent supine and prone predicted fewer adult words, whereas increased time sitting and held predicted increases in word count. Upright was age dependent, with more time spent upright predicting an increase in word count for younger infants, but a decrease for older infants. These findings reveal the importance of considering infants' body position in understanding speech input and subsequent language development. SUMMARY: Tested how infants' body positions (supine, prone, sitting, upright, and held) relate to adults' speech input in naturalistic home settings. Machine-learning algorithms predicted infants' body positions from motion sensors embedded in custom leggings. Adults spoke relatively more when holding infants and while infants sat, but less frequently when infants were prone and supine. Results provide partial support for theorized links between motor skill acquisition and language development.