**Background:** Physical activity (PA) development in toddlers (age 1–2 years) is poorly understood, largely due to a lack of validated accelerometer-based analytic tools for this age group. Existing evidence shows that physical inactivity habits begin before age 5 and persist, and that preschool-age females may be less active than males. Parents—particularly mothers—play a critical role in shaping early PA behaviors, but the influence of maternal PA, parenting stress, parenting practices around PA, and child developmental/behavioral problems on toddler PA trajectories has not been prospectively examined. Gross motor competence is cross-sectionally associated with PA in early childhood, but temporal relationships remain unclear.
**Methods:** The Child and Mother Physical Activity Study (CAMPAS) is a longitudinal study planning to recruit 124 mother–toddler dyads (child age 10–15 months at baseline) in the Chicago area. Five waves of assessments will occur at 6-month intervals (approximate child ages 12, 18, 24, 30, and 36 months). For Aim 1, a subsample of 62 children will complete a validation study across waves 1–4, wearing a hip accelerometer (ActiGraph wGT3X-BT, ±6 g, 100 Hz) during 20-minute free-play sessions in four settings (home, indoor play space, childcare class, outdoor playground) while being video recorded. Video data will be coded using BORIS software to establish ground truth activity labels across five classes: walking/running, non-walking/running PA, light PA, sedentary, and being carried. A random forest classifier will be trained and validated using leave-one-subject-out cross-validation, with performance assessed via precision, recall, F1 score, weighted kappa, Bland-Altman plots, and root mean squared error. For Aims 2 and 3, all children will wear a hip accelerometer for 7 consecutive days (24 hours/day) at each wave. Sufficient wear time is defined as ≥500 minutes/day for ≥3 days. Gross motor skills will be assessed via the Ages & Stages Questionnaire gross motor subscale (6 items) at all waves. Parenting practices around PA will be measured using the Preschooler Physical Activity Parenting Practices (PPAPP) questionnaire (30 items, 5 subscales) at waves 1, 3, and 5. Maternal PA will be self-reported using a questionnaire adapted from NHANES and FLASHE at waves 1, 3, and 5. Parenting stress will be measured using the Parental Stress Scale (18 items) and Parenting Daily Hassles Scale (20 items) at waves 2 and 4. Child externalizing behavioral problems will be assessed using the Conduct Problem and Hyperactivity subscales of the Strengths and Difficulties Questionnaire (10 items) at waves 2 and 4.
**Key Results:** This is a protocol paper; no results are presented. The sample size calculation for Aim 1 is based on detecting a ≥4% improvement in accuracy from the pilot algorithm's 79%, requiring 62 participants (31 training, 31 testing). For Aims 2 and 3, 100 participants (50 males, 50 females) are needed to detect an 8-minute sex difference in moderate-to-vigorous PA (SD=14 min, effect size 0.57) at 80% power and α=0.05. Accounting for an anticipated 81% retention at wave 3, 124 participants will be recruited at baseline. The retention goal is ≥90% per subsequent assessment, with strategies including assigned primary staff, regular communication every 3 months, incremental compensation ($40–$80 per wave, plus $10 extra per validation wave), and remote assessment options.
**Clinical Implications:** This study will be one of the first to use longitudinal free-living data to validate a machine learning activity recognition algorithm for toddlers and apply it to quantify ambulatory movement from age 12 to 36 months. The algorithm will be made publicly available to facilitate dissemination. Findings will describe PA trajectories by sex during toddlerhood and elucidate the roles of gross motor development, maternal PA, parenting practices, parenting stress, and child behavioral problems in shaping early PA. This epidemiologic data is critical for informing future Physical Activity Guidelines for children under age 3 and identifying targets for early intervention to promote PA and prevent physical inactivity from the earliest stages of life.