Summer Heat, Physical Activity, and Sedentary Behavior of Children Outdoors and Indoors in an Urban Setting.

Only one-quarter and one-fifth of US children achieve physical activity and screen time guidelines, with children often least active during summer. This seasonal slump may be partly attributed to uncomfortable heat conditions. We examined how summer temperatures associate with children's moderate-to-vigorous physical activity (MVPA) and sedentary behavior outdoors and indoors in a warm urban climate. Children ages 9-10 (n = 25) in Austin, Texas, wore wGT3X-BT accelerometers and GPS devices for seven days in summer of 2023. Accelerometers recorded activity counts and ambient light levels in 15-s epochs, allowing classification of activity intensity using Evenson cut points and identification of indoor versus outdoor time. Air temperature and relative humidity data originated from a nearby weather station. To test how temperatures associated with physical activity and sedentary behavior, we fit two-level, cross-classified, mixed-effects negative binomial regression models with 15-s observations nested within child and date. During study days, waking-hour temperatures averaged 36.3 °C (97 °F). Children averaged 36 min of MVPA daily (4 min outdoors, 32 min indoors) and spent 55% of waking hours sedentary indoors. Higher temperatures were associated with lower MVPA outdoors (RR = 0.79, 95% CI = 0.72-0.87) and indoors (RR = 0.92, 95% CI = 0.89-0.95), and higher sedentary behavior indoors (RR = 1.02, 95% CI = 1.01-1.02). Models with heat index instead of air temperature produced similar estimates. Findings suggest that heat may constrain children's healthy movement behaviors. With projected future urban warming, heat-safe physical activity interventions may become increasingly important for children's health.