Spontaneous play in the early infantile period is important for the development of motor and sensory systems, but its actual status remains unclear. The present study focused on Japanese infants and aimed to investigate the developmental sequence of rolling over, as well as the subsequent acquirement of a sitting posture when supported by an adult. To examine the developmental process and gross skill acquisition of infants, a questionnaire was administered to 671 Japanese parents. According to the survey results, of the 671 infants, 89.7% (n = 602) started to sit up after rolling over, with rolling over beginning at 4.5 (±1.1) months and sitting up at 5.9 (±1.2) months of age. In contrast, the other 10.3% (69 infants) started to roll over after sitting up, with rolling over and sitting up starting at 6.6 (± 1.2) and 5.2 (± 1.0) months of age, respectively. Infants who acquired a sitting position after turning over were more likely to experience pivoting than those who acquired a sitting position before turning over. The order of acquisition of gross motor development in early infancy was shown to be related to the experience of playing on the floor. Furthermore, the motor development process of infants was seen to be influenced by the efforts of parents and caregivers. The findings of this study emphasize the importance of experiencing spontaneous and varied motor play in infancy to ensure gross motor skill acquisition, thereby enhancing both the physical and mental development of infants.
