Longitudinal Associations Between Physical Activity and Sedentary Time and Cardiorespiratory and Muscular Fitness in Preschoolers.

Background/Objectives: The impact of physical activity (PA) and sedentary time (ST) during preschool years on the physical fitness (PF) levels of school-aged children remains unaddressed. This study aimed to investigate the associations of objectively measured vigorous physical activity (VPA), moderate-to-vigorous physical activity (MVPA), total physical activity (TPA), and ST in the last year of preschool (age of 6-7 years; n = 77; 51% boys) with cardiorespiratory fitness (CRF) and muscular fitness (MF) in the first grade of school among Estonian children. Methods: We assessed PA (accelerometers), CRF (20 m shuttle run), and MF (z-score of relative upper- and lower-limb muscular strength). Results: In the unadjusted analysis, higher VPA, MVPA, and TPA in preschool were associated with a higher MF in school among boys, while a higher VPA in preschool was related to a higher CRF in school among girls. However, VPA, MVPA, TPA, and ST in preschool were unrelated to CRF and MF among boys and girls after adjustment for baseline age, accelerometer wear time, the corresponding PF item, and parent's education. In addition, a higher PF level in preschool was frequently related to a higher corresponding PF item in school among both genders. Conclusions: Moderate-to-vigorous and vigorous type of activities during final year of preschool, as well the amount of TPA that preschoolers are involved in, are not sufficient to affect their CRF and MF longitudinally. In addition, ST in preschool did not impact the CRF and MF of boys and girls in the first grade.