The association between the health-related physical fitness and inhibitory control in preschool children

The basic information of the participants was obtained through a questionnaire filled out by the participant's parents or legal guardians, including the participants’ age, sex, mother’s education (Below high school, Junior high school, Senior high school, College/Associate degree, Bachelor’s degree, and Master’s degree/Doctor’s degree), and household income (< 9,000RMB, 9,000-30,000RMB, 30,000-100,000RMB, > 100,000RMB), etc.

The measurement of HPF was according to the Chinese National Physical Fitness Measurement Standards Manual-preschool children version (CPFS-preschool) [13], including cardiorespiratory fitness (20 m shuttle run test, 20mSRT), musculoskeletal fitness (handgrip strength and standing long jump), motor fitness (one-leg balance and 2 × 10 m SRT), and body composition (BMI, kg/m²). The specific test methods are as follows:

Cardiorespiratory fitness reflects the integrated ability to transport oxygen to muscles for sustained physical activity(PA) in the cardiovascular and respiratory systems [14]. The prevailing test for cardiorespiratory fitness is 20 m SRT [6]. During the test, an adult tester led children to jog back and forth at an increasing speed (starting with a speed of 8.5 km/h and increasing 0.5 km/h every minute) with the music rhythm between the lines 20 m apart until they were too tired to reach the end line. The test was conducted once, and the total number of round trips was recorded.

Musculoskeletal fitness is the ability to maintain movement through muscle contraction against resistance, including muscle strength, muscle endurance, explosive strength and muscle flexibility. [15]. It is commonly assessed by handgrip strength and standing long jump (upper and lower limbs, respectively) in preschoolers [16]. As for the handgrip strength test, children were naturally standing, and their arms on the test side were stretched out straightly about 10° apart from the body. T.K.K.5401 (Takei, Niigata, Japan) was used for the test, and the handgrip length was adjusted to the best position according to children’s hand length [17]. As for the Standing long jump, children’s feet open to the same width as shoulders, standing behind the marking line, bending their knees and swinging their arms to jump forward. The two tests were measured twice, and the maximum values were recorded in kilograms (precision of 0.1 kg) and centimeters (precision of 0.1 cm), respectively.

Motor fitness refers to physical fitness related to sports performance and motor skills, which is composed of speed/agility and balance [9]. One-leg balance is a reliable indicator of balance for preschool children [9]. The tester held the stopwatch, gave the starting order, and timed from the time the tested child lifted one foot from the ground till the lifted foot touched the ground. The speed/agile quality of preschool children was evaluated by 2 × 10 m SRT, which has been adopted in China since 2003[13]. Children were led by an adult tester, who ran as fast as possible from the starting point to the end point (10 m apart) back and forth. Testers used a stopwatch to time. The test was measured twice, the shortest time was recorded as a final result (precision of 0.1 s).

Firstly, the height and weight of the participants were measured by the CPFS-preschool, and BMI (BMI= kg/m²) and BMI scores (see Additional file 1) were calculated [13]. Second, BMI was graded according to the cut-off point established by the Chinese Working Group on Obesity for Children (CWGOC) [18]. The BMI cut-off points for overweight and obesity in Chinese adults recommended by CWGOC are 24 kg/m² and 28 kg/m² ( BMI₂₄, BMI₂₈ ). BMI cut-off points for preschoolers were inferred through the method of ‘cut-off point screening’ (see Additional file 2), and BMI were graded as normal, overweight and obesity.

Flanker Task can objectively evaluate inhibitory control in preschool children [19]. This study used the Fish flanker task (FFT)in E-Prime software to investigate the inhibitory control.

Before the formal test, there was a practice aimed to familiarize the participants with the test process, and require them to achieve more than 80% accuracy(ACC) before entering the formal test. The formal test was repeated 3 times, 40 judgments each time. Considering that the participants were younger (3–6 years old), they were guided by professionals during the test. The instructions were as follows: “There are five small fishes swimming in the water on the screen, but the middle fish is hungry. You can feed them with the button in your hand. When the middle fish swims to the left, you click the left button. When the middle fish swims to the right, you click the right button’’. At the beginning of the test, a set of stimulus pictures (Fig. 1) would appear randomly on the screen; pictures were divided into congruent conditions (the direction of all the fishes were the same) and incongruent conditions (the direction of the middle fish is different from the other fish). Participants immediately pressed the button after judging. If participants press the wrong button, or reaction time (RT) was less than 200 ms or more than 3 s, it would be judged as an invalid reaction, not included in statistical analysis. The formal test included 120 conditions (60 congruent conditions and 60 incongruent conditions). Two types of stimuli appeared at random with the same probability. The participants had a 10 s rest after every 40 judgments. The tests were completed one-on-one by an adult tester and participant in a quiet kindergarten classroom, and RT and ACC of each test were directly recorded on the software.

Continuous variables and categorical variables are presented as mean, standard deviation (SD), or percentages. The values beyond Mean ± 3SD are considered as outliers and removed from the final analysis. The scores of each index are converted into standardized scores with a mean of 50 and SD of 10. BMI was scored according to the CPFS-preschool. Because 2 × 10 m SRT is negatively associated with speed/agility, the score was multiplied by −1 and then standardized. The HPF standard score formula is T=[(X-M)/SD] × 10 + 50(X: personal performance, M: mean). The sum of each standard score was recorded as HPFT (T _{standing long jump} + T _{handgrip strength} + T _{One−leg balance} + T _{2 × 10 m SRT} + T _{20 m SRT} + T _BMI).

Firstly, the relationship between HPFT and its four components with inhibitory control (ACC and RT) was tested by a multiple linear regression model. HPFT, T _{standing long jump}, T _{handgrip strength}, T _{One−leg balance}, T _{2 × 10 m SRT}, T _{20 m SRT} and T _BMI were categorized into tertiles ( T1 -T3, T1 defined as lowest tertiles ). With continuous variables and categorical variables as independent variables and inhibitory control as dependent variables, HPFT and its four components showed the relationship between ACC and RT after adjusting age, sex, mother’s education, and household income. Secondly, the piecewise linear regression was used to test whether there is a non-linear relationship and threshold effect between HPFT and inhibitory control. The R language-based Empower Stats software was used for statistical analysis P < 0.05 was considered statistically significant.