We collected several different kinds of information. Anthropometric and fitness measurements were conducted. Participants self-completed two questionnaires: firstly, about dietary assessment, and secondly, regarding habitual physical activity and sedentary behaviors. The participants were provided with additional clarifications if needed. The completion of the questionnaires took approximately 25 min. The fitness test procedures took approximately 2 h. The testing procedure was completed during obligatory classes (including physical education) on two consecutive days according to the teachers’ suggestions.
2.4.3. Fitness Tests
The fitness variables were measured using the American Alliance for Health, Physical Education, Recreation and Dance (AAPHERD) youth fitness assessment [
36], the EuroFIT Physical Fitness Test Battery (EuroFIT) [
37] and the Fitnessgram [
38] test batteries as appropriate. These are widely recognized tests of physical and health-related fitness assessment in children.
Flexibility (Sit and Reach Test)
Flexibility was measured using a standard sit and reach the box. This test indicates lower back and hamstring flexibility. Participants’ sat barefoot with their knees and feet placed firmly on a floor mat. They were instructed to gradually push the indicator on the ruler forward as far as possible with hands stretched, without jerking and knees fully extended. Fingers of both hands had to reach the same distance simultaneously and any bouncing movement was disallowed. The best of three trials was recorded as each participant’s criterion score, i.e., the farthest distance (in cm) reached.
Muscular Endurance (Sit-Up Test)
Muscular endurance was assessed based on the maximum number of sit-ups performed in 60 s. In a starting position, the subjects were seated on a floor mat, back straight, keeping the hands clasped behind their neck, and the knees flexed at 90 degrees with heels and feet flat on a floor mat. They were then asked to lie down on their backs, so that the shoulders touched the floor mat and returned to the sitting position with elbows pointed out in front, touching their knees, whilst keeping their hands clasped behind their neck for the whole time. The test was terminated when participants were too tired to complete a full sit-up. The total number of correctly completed movements was recorded as the number of successfully executed sit-ups.
Muscular Strength (Push-Up Test)
Muscular strength test was assessed using the push-up test. In the starting position, a participant’s abdomen was parallel to the floor with elbows extended on both sides to support the trunk, head facing forward. Both the feet were plantar flexed balancing on the flat hard surface next to each other. During the movement, the upper body was lifted from above the floor at an angle of about 60° using the shoulder muscles supported by both arms. The chest and chin were only allowed to touch the floor slightly. The test was terminated when participants were exhausted or no longer able to perform a complete push-up. The result was recorded as the number of correctly executed push-ups per minute.
Standing Long Jump
This test measured the explosive power of leg extensors. Using a double-foot take off from a half-squat posture, the participant jumped forward as far as possible. The best of three trials was recorded in centimeters.
Grip Strength
In a standing position, a participant held a handgrip dynamometer with the hand hanging downward naturally at the side at any angle between 90° and 180°. The participant then exerted maximum strength to squeeze the spring of the dynamometer as prompted by the tester. Three separate trials on the left and right hands were recorded in kg. The mean value of the best results for the left and right hands was used for further analysis.
2.4.4. Questionnaires
Participants completed two self-report questionnaires: the Previous Day Physical Activity Recall (PDPAR) [
39] and the Food Frequency Questionnaire (FFQ) on dietary habits. The PDPAR was translated from English into Polish, and after re-translation to English, both versions were compared. The PDPAR was adapted and modified for Polish context. The coefficient of reliability for section A (physical activity level) of the PDPAR was: Cronbach’s alpha = 0.77, for the section b (sedentary behaviors) Cronbach’s alpha = 0.58. The dietary questionnaire was constructed as the Food Frequency Questionnaire [
40]. The FFQ consisted of a list of foods and a selection of options relating to the frequency of consumption of each of the foods listed. We constructed an FFQ as it is designed to collect dietary information from large numbers of individuals (100 individuals or more) and it usually is a self-administered questionnaire [
40]. The coefficient of reliability for the FFQ was: Cronbach’s alpha = 0.88.
Personal Dietary Questionnaire
Although the questionnaire may consist up to 150 foods, in general, as few as 11 food items may be sufficiently informative since the major sources of the nutrients of interest are found in relatively few food types [
40]. Our personal dietary assessment was completed with the use of a questionnaire consisting of forty closed-ended items, thus making it a relatively simple instrument that could be completed in approximately 15 min.
Participants were asked to indicate how frequently they consume the food listed in the questionnaire. They could mark the number(s) of their choice with an X on the following six-point Likert scale: 6 (always), 5 (usually), 4 (often), 3 (sometimes), 2 (never), 1 (do not know).
The children indicated the food they consume from the following list of items: chips (as snacks); milk and dairy products; meat, fish; eggs; vegetables; fast food (for example, from McDonald’s, Burger King, knish, pizzas, burgers, etc.), fruits, sugar, fruit juices, soft drinks, such as Coca Cola, Pepsi Cola, Fanta, Mirinda, tonic, Sprite, Nestea, Jupik, orangeade; ice creams; chocolate; hamburger; crisps; cake; yoghurt; brown bread (wholegrain); white bread; cereals; corn flakes; sweets; maize; pizza; crackers; potatoes; rice; hot-dogs; sweet rolls; margarine on bread; butter on bread; peanuts; cheese; beef; pork; chicken; nuts; olive oil (e.g., for baking); sunflower/rape oil for baking; sausage.
Habitual Physical Activity and Sedentary Behavior Questionnaire
The aim of the questionnaire was to determine the level of physical activity (PA) participation and sedentary behavior of the children.
Participation in Physical Activities
The questionnaire consisted of three questions. In the first one, children indicated all the physical activities they participated in the previous day. They were asked to recall how often they participated in the following activities: running, play ball, walking, mixed walking-running, cycling, outdoor games and plays, climbing on the playground equipment, basketball, roller-skating, volleyball, swimming, soccer, jump rope, dancing. Children could choose one of three possible answers: “not at all”, “sometimes”, “always (every day)”. Children could also specify other sports; however, most of their answers indicated activities not related to physical activities, such as “computer” (4 kids), or “singing” (1 kid). Answers related to PA, e.g., “ice hockey” (2 kids), “horse riding” (3 kids), “wrestling” (3 kids), “tennis” (3), “paintball” (2 kids) and so on were too few to be included for further analysis. Given the number of other sports indicated and their popularity among children, we also confirmed that sports listed in the survey were the most popular at this age group.
Sedentary Behavior
We asked participants about their sedentary behavior. Children were asked to indicate how many hours per day they spend on the nine following activities: watching TV/video/movies on computer; using a computer or playing video games; reading books or magazines; playing board games; washing dishes or cleaning the kitchen; talking on the phone, cleaning a room/house; arts; listening and playing music. Children could indicate their choice by marking an X on the five-point scale: 1 (less than 30 min), 2 (30 min – 1 h), 3 (1–2 h), 4 (2–3 h), and 5 (more than 3 h).
As it can be noticed, we decided to include questions about washing dishes or cleaning the kitchen separately from those about cleaning a room/house. We decided to do so because, in Poland, there are still regions and places where the traditional division between so-called “women’s duties” and “men’s duties” can be observed. Therefore, some girls could have more physically demanding domestic duties than boys.