Introduction

Childhood is a critical period in human development characterized by rapid growth, maturation, and the establishment of lifelong habits. Recently, research has emphasized the importance of adequate outdoor play for the mental and physical development of children [1–3]. Outdoor play gives children the opportunity to be physically active and exposes them to the natural environment, influencing their overall health. The World Health Organization recommends at least 1 hour of moderate-to-vigorous daily physical activity for children [4]. However, reports on the relationship between children’s specific physical performance and outdoor play habits are rare.

The phase angle (PhA) shows the geometric relationship between resistance and reactance, constituting the raw data for bioelectrical impedance (BIA) [5]. BIA is a body composition analysis method that uses low-frequency currents to estimate body composition. The theoretical assumption is that the human body is a network of resistors (body fluids) and capacitors (cell membranes) [6]. Resistance is caused by body fluids and electrolytes, whereas reactance is the ability of membranes to maintain their potential. Several reports have recently indicated that BIA is useful for the qualitative assessment of muscles [7–10]. Although several reports have been published on various physical abilities and BIA in adults [11–13], few reports exist on children’s physical performance and body composition, including BIA.

Understanding the relationship between children’s outdoor play, body composition (including BIA), and detailed physical performance is crucial for comprehending which aspects of body composition are influenced by outdoor play and how they relate to motor performance. This provides a more concrete understanding of the significance of outdoor play in children’s motor development.

This study aimed to investigate how children’s outdoor playtime affects their body composition and physical performance, examining the correlation between body composition and physical performance.

Materials and methods

Results

Table 1 summarizes each survey item for all participants. The sex differences in PhA were significant in LU and RL, whereas not in other items. The MW was significantly greater in boys than in girls, mainly in the lower limbs. Sports test results were superior in boys except for the HG.

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Table 1. Examined items and their sex differences.

https://doi.org/10.1371/journal.pone.0317925.t001

Table 2 shows the differences in body composition and sports test results based on whether the participants played outside for at least 1 hour each day. In girls, PhA did not differ by time spent playing outside; however, the MW of the RU, RL, and LL were significantly higher in the group that played more outside, and the MW of the trunk and LU was greater in the group, although this was not significant. Sports test results showed that the 50S and SLJ were significantly better in the group with more outdoor play. In boys, unlike girls, the lower limb PhA was greater in the group with more outdoor playtime. Moreover, MW did not differ significantly between the two groups. Sports test results were significantly better in the group with more outdoor playtime in all categories.

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Table 2. Each study item divided by outdoor playtime.

https://doi.org/10.1371/journal.pone.0317925.t002

Table 3 shows the correlations between body composition and the sports test results. HG showed a significantly moderate correlation with MW and a weak correlation with PhA in both sexes. Other sports test results showed no correlation with MW; however, they showed a weak but significant correlation with PhA in the lower limbs, particularly in boys.

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Table 3. Correlations between sports test results and body composition.

https://doi.org/10.1371/journal.pone.0317925.t003

Discussion

This is the first study to report the relationship between outdoor play, body composition, and physical performance in children.

Overall, the group with more outdoor play had better results on the sports tests. Significant differences were found for girls in the 50S and SLJ and boys in all disciplines. Regarding body composition, MW in girls and PhA in boys were greater in the group that played more outside. HG correlated with PhA and MW; however, other sports test results did not correlate with either PhA of the upper limb or MW. However, they did correlate with PhA of the lower limb, with this correlation being stronger in boys.

Although many reports exist on outdoor play and the health and physical development of children, few have explored the relationship between specific physical functions and outdoor play. Kwon et al. reported that the more time children spent playing outside, the better their gross motor development, especially in object control (throwing and kicking a ball); however, no differences were observed in locomotor items such as running and jumping. The current study found significant differences in running and jumping, in contrast to the findings of Kwon et al. The Test of Gross Motor Development-Second Edition [15], the method used by Kwon et al. to assess motor skills, roughly assesses running and ball throwing and does not numerically record actual abilities such as running, jumping, throwing, and kicking. This divergence in assessment methods might account for the differing results between the two studies. Additionally, this study included a larger number of participants and a more uniform age range; therefore, it might have projected better real-world results than Kwon’s report.

Several reports exist on the relationship between outdoor play and BF [16–18]; however, no previous reports have explored the relationship with MW or PhA. As mentioned in the introduction, PhA has been reported to be related to muscle quality [7–10], representing the geometric relationship between resistance and reactance and serving as raw data for BIA. In this study, PhA did not significantly differ according to the amount of time spent playing outside in girls. However, in boys, the PhA of the lower limbs was significantly greater in the group with more outdoor play. This suggests that in boys, more outdoor play improves the muscle quality of the lower limbs and not the upper limbs, and in girls, more outdoor play does not improve the muscle quality of the whole body. The causes of these sex differences are unclear; however, we speculate that sex differences in the content of outdoor play may be related. Girls who played outside had more MW, whereas no difference was observed between the two groups of boys. This trend contrasts with the pattern observed in PhA, and the reason for this trend is unclear. However, it might be attributable to girls who play more outside having a greater HT and BW, as indicated in Table 1. Like PhA, future considerations should account for the content of outdoor play.

Various reports exist on physical performance and body composition, and the relationship between HG and muscle mass has long been pointed out [19–21]; however, correlations between other detailed physical performance and body composition are unknown. In this study, the PhA of the lower limb was significantly correlated with 50S, SLJ, and 20SR besides HG, with the correlation coefficients being greater in boys. HG was also significantly correlated with MW besides PhA. These results suggest that MW is related to muscle strength, similar to HG, whereas PhA correlates more strongly with movements that require complex coordinated movements, such as running and jumping. Previous studies have reported that PhA is associated with muscle quality [7–10], and the results of this study indicate that the ability to run or jump may be more related to muscle quality than to muscle strength alone. In a report on sex differences in the effects of outdoor play on physical activity, Kwon et al. reported that outdoor activity contributed more to physical activity in girls in their study of preschool children; however, the causes were not considered in detail [3]. Like sex differences and their effects on body composition on outdoor playtime, further investigation is needed regarding sex differences and the relationship between outdoor play, body composition, and physical function. This includes exploring external factors, like sex differences in the content of outdoor play, and internal factors, such as physiological differences between sexes.

This study had a few limitations. First, uniformity was lacking in the conditions during which body composition was measured. PhA is an evaluation index based on information obtained when a 50 kHz electric current is applied to the inside and outside of cells, and it varies with the amount of water in the body and the time elapsed after a meal. Participants in this study were not subjected to restrictions regarding fasting duration or water intake before the examination, and the potential impact of this factor on the results cannot be dismissed. Secondly, only children aged 8 years were examined. PhA increases and decreases with age in children [22,23]. Similar studies in different age groups may not yield significant results, and the results of this study may not be universally applicable to all age groups. We plan to conduct similar studies in different age groups to compare the results with those of the present study. Finally, complete data were available for only 494 out of the 1,340 participants in the cohort. The primary reason for the exclusion of so many participants was the inability to collect the school sports test results. The sports test results were collected by asking each participant to voluntarily submit the sports test results conducted at their elementary school. Therefore, it is believed that many participants forgot to record the results of the sports test or did not submit the results because it was inconvenient. However, the relatively low inclusion rate of participants had minimal impact on the overall results, given the absence of significant differences in body composition or time spent playing outside when comparing the groups with and without sports test results.

Conclusions

Outdoor play habits improve muscle quality, particularly in boys, and this improvement may be related to enhanced athletic performance in children. The effect of outdoor play on girls differed from that of boys in that it affected muscle mass. Further research examining the sex differences in the results obtained in this study will clarify the relationship between physical activity and body composition, contributing to children’s health.

The conclusions of this article are solely the responsibility of the authors and do not represent the official views of the above government.

Supporting Information

Acknowledgments

We are grateful to all the participants of the JECS-Y and all individuals involved in the data collection. We also thank the following members of the JECS-Y as of 2020: Zentaro Yamagata (Principal investigator, e-mail: zenymgt@yamanashi.ac.jp), Ryoji Shinohara, Sanae Otawa, Anna Kobayashi, Sayaka Horiuchi, and Megumi Kushima (Center for Birth Cohort Studies, Interdisciplinary Graduate School of medicine, University of Yamanashi, Chuo, Japan); Takeshi Inukai and Emi Sawanobori (Department of Pediatrics, School of Medicine, University of Yamanashi, Chuo, Japan); Kyoichiro Tsuchiya (Third Department of Internal Medicine, University of Yamanashi, Chuo, Japan); Takahiko Mitsui (Department of Urology, Interdisciplinary Graduate School of Medicine, University of Yamanashi, Chuo, Japan); Kenji Kashiwagi (Department of Ophthalmology, University of Yamanashi, Chuo, Japan); Daijyu Sakurai and Hiroyuki Watanabe (Department of Otorhinolaryngology-Head and Neck Surgery, School of Medicine, University of Yamanashi, Chuo, Japan); Koichiro Ueki and Naana Baba,

(Department of Oral and Maxillofacial Surgery, Interdisciplinary Graduate School of Medicine, University of Yamanashi, Chuo, Japan); and Hiroshi Yokomichi, Kunio Miyake, Yuka Akiyama, Tadao Ooka, and Reiji Kojima (Department of Health Sciences, School of Medicine, University of Yamanashi, Chuo, Japan).

We would like to thank Editage (www.editage.jp) for English language editing.