This research aimed to study the relationship between PA, fatness, and fitness in the Preschool Education stage. The major findings were: 1) There were differences in fitness test between boys and girls; 2) There were not significant differences in fitness tests between normal-weight and over-weight children, except in handgrip strength in which over-weight children had better results; 3) There were no significant associations between body composition and PA subcomponents; 4) Higher PA levels were related with better physical fitness values.
Differences in physical fitness tests were more related to sex than fatness. Boys had significant better results in standing long jump, 4x10 m shuttle run and 20 m shuttle run tests, but their performance in the sit and reach test was worse. This finding is in concordance with Cadenas-Sánchez et al.  study that found significant better results in the standing long jump test in boys than girls. Latorre et al.  found that boys had better performance in speed and lower body strength tests and girls had higher values in the cardiorespiratory fitness test. This last difference could be because a different test was used to measure cardiorespiratory fitness.
However, significant differences were not found in fitness tests means between normal-weight and over-weight children, except in handgrip strength, in which over-weight children had higher results. Thus, higher BMI was associated with better results in handgrip strength. These results agree with Cadenas-Sánchez et al.  study that found this result in both sex and Riso et al.  research that found that the mean in handgrip strength of normal-weight children was 10.5 kg, while the mean of over-weight participants was 12.2 kg. In addition, higher BMI was associated with worse performance in 4x10 m shuttle run. This finding does not agree with Henriksson et al.  and Riso et al. , that both did not find significant association (p < 0.05) between BMI and 4x10 m shuttle run. This discrepancy in the results could be due to the different age of the participants in these studies. However, no significant association with standing long jump, sit and reach and 20 m shuttle run test was found. Henriksson et al.  aimed that BMI could not detect the associations between body composition and physical fitness in Preschool children. This could be because Preschool children are so young  due to the fact that different research with older participants (6-7-year-old) observed significant associations between higher BMI, greater standing long jump and lower cardiorespiratory fitness . Thus, for example, Carson et al.  found some research with mixed or null findings because of their infants and toddlers’ samples.
Furthermore, fat mass percentage was related with lower results in 4x10 m shuttle run. This concordats with Henriksson et al.  study that found a significant negatively association (p < 0.001) among both variables, but not with Riso et al.  research that did not find a significant association (p > 0.05). This difference could be because of the different age of the participants in each study. Moreover, our results showed that waist circumference was associated with better performance in handgrip strength and triceps skinfold was related with worse results in 4x10 m shuttle run test. As far as we know, no study has analysed the relationship of these two components of body composition with these fitness tests in Preschool children, so we cannot compare these findings with others. So, there are some studies that have studied the relationship between body composition and physical fitness in Preschool children  and their results have been controversial [5, 21, 23]. These differences might be caused by different reasons: different tests, age of the participants, sample size, study design, etc. were used [5, 23].
Body composition values were not significantly related to PA levels, although waist circumference was associated positively with sedentary time. These results coincide with those found by Carson et al.  review that observed a lot of null, unfavorable and mixed associations between PA and adiposity. Previous studies have shown BMI was not related to PA in some studies . For this reason, Leppänen et al.  aimed future research should include different body composition variables instead of just BMI. The results in this research were in agreement with this fact because BMI is the only value that had no significant associations, while waist circumference was related to sedentary time and fat mass percentage and triceps skinfold were associated negatively with the number of counts per minute. Moreover, Fang et al.  found that MVPA was associated with triceps skinfold, but the results of this study showed no association between both. So, there were discordant results among the studies. Regarding sedentary time, our results were in concordance with Riso et al.  study. Both found that sedentary time did not relate to BMI and fat mass.
Finally, higher PA levels, specially MVPA, vigorous PA and total PA, were associated with better performance in physical fitness tests. This finding is in accordance with Carson et al. ; Fang et al.  and Riso et al.  research. Thus, like Carson et al.  and Riso et al.  studies, a greater performance in standing long jump, 4x10 m shuttle run and 20 m shuttle run were associated with PA, while more time spent in sedentary was related to lower results standing long jump and 4x10 m shuttle run tests. However, according with Fang et al. , associations between PA levels and handgrip strength and sit and reach tests were not found. Nevertheless, Riso et al.  found significant associations (p. 0.045) between MVPA and handgrip strength. The difference might be caused by the age of the participants.
Nevertheless, this research has some limitations. Firstly, sample size was not too big, especially the over-weight population (n = 36). So, we could not separate over-weight and obese children into different groups. This could influence the significant differences that were found as Niederer et al.  found associations between normal-weight and obese children which were not found in over-weight participants. In addition, 79 children did not accomplish the inclusion criteria. Therefore, PA associations could be influenced by the relatively small final sample (n = 151). Another limitation was that a cross-sectional design was used, so we could not conclude causality and direction of associations found. Moreover, the research did not analyse some variables that could be influential like socioeconomic status or participation in sports clubs. Regarding the body composition, fat free mass was not studied, which Riso et al.  found was associated with all physical fitness and PA levels. For this reason, considering that it is an important value, it should have been analysed. However, we used waist circumference and triceps skinfold that few research has studied, so our study revealed new information about the relationship of these body composition variables with fitness and PA levels.
The strength of this study is that the measurements used were objective. PA was measured by accelerometer and physical fitness was measured by PREFIT battery, which is validated for 3-5-year-old children . In addition, we analysed some variables that only some research studied such as: flexibility, waist circumference and cpm. Moreover, we used a model that was adjusted for age and sex of the participants to analyse data. Differences among the variables in unadjusted model usually disappeared in the adjusted model because these variables are influenced by age and sex. So, our results showed the real relationship between fatness, fitness, and PA regardless of age or sex. Finally, our sample included 3-year-old children, who were not included in the research that has studied Preschool children health.