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The level of physical fitness in children aged 6–7 years with low birthweight
Early Human Development 111 (2017) 23–29
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The level of physical fitness in children aged 6–7 years with low birthweight☆
MARK
Elżbieta Cieślaa, Monika Zarębab, Sławomir Koziełc,⁎ a b c
Faculty of Health Sciences, Jan Kochanowski University, 19 IX Wieków Kielc Street, 25-317 Kielce, Poland Faculty of Pedagogy and Arts, Jan Kochanowski University, 11 Krakowska Street, 25-029 Kielce, Poland Department of Anthropology, Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, 75 Podwale Street, 50-449 Wroclaw, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Physical fitness Eurofit Low birth weight Children
Background: Level of physical fitness is related to the functional status of most of the bodily functions and so it appears to be very important to identify perinatal factors influencing physical fitness. Aim: The purpose of this study was to determine the influence of birth weight on the level of physical fitness in children 6–7 years of age. Subjects and method: Physical fitness was assessed using EUROFIT tests in 28,623 children, aged 6–7 years, from rural areas in Poland. Children below the 10th percentile for birth weight for gestational age were defined as small for gestational age (SGA). The influence of birth weight on parameters of fitness was assessed by means of covariance analysis. Results: With the controls of age, sex and body size, children of low birth weight have shown significantly lower levels of body flexibility and running speed. The leg strength of children with SGA turned out to be significantly lower only in 7-year-old boys. Conclusion: This study has revealed the significant influence of birth weight on physical fitness. The results suggest the importance of early intervention and its possible benefits for developing and maintaining the proper level of physical fitness further in life.
1. Introduction Since the level of physical fitness is related to the functional status of most of the bodily functions, it is considered one the most important health indicators for the general population, regardless of age [1]. It is a good predictor of morbidity and mortality for cardiovascular diseases and for mortality from all causes, as well as increased occurrence of metabolic syndrome among adolescents [2–5]. Thus, it seems that a priority for public health should be the monitoring of fitness, especially among the youngest, popularizing recommendations concerning daily exercise, and identifying risk factors which could hamper/harm/interfere with its natural development. Physical fitness is partly genetically determined, but it is also strongly influenced by various environmental factors, among which physical activity is considered to be the most significant [6]. However, the given level of fitness of a child at a certain developmental stage is the result of the interaction between genetic and environmental factors, which might be influenced by perinatal factors,
such as the weight of a child at birth [7–9]. Birth weight largely depends on intrauterine growth rate and is a commonly used indicator of the state of fetal growth and development [10,11]. Low birth weight is correlated with a significantly more frequent (compared to children with normal birth weight) occurrence of several short- and long-term health problems in the postnatal period of life [12–14]. Research shows that children with low birth weight more often suffer from behavioral and emotional problems related to attention deficiency and social development, and equally often, related to physical health [15–17]. Many studies frequently relate their results to respiratory incapacity and cardiorespiratory insufficiency. They are less frequently associated with other components of fitness [8,11]. It has been shown that birth weight indicates a relationship with physical fitness, motor agility and also neuromotor and motor skills (Fundamental Motor Skills). Some studies have reported a significant correlation between birth weight and cardiorespiratory fitness, exercise capacity, flexibility and strength [9,18]. Kilbride et al. [19] established
☆ This data is a part of the project “A six-year-old child at school” run by the Ministry previously known as the Ministry of National Education and Sport, partially financed by the European Union and partially by the state budget within European Social Funds (nr 5/2.1a/2004). ⁎ Corresponding author at: Department of Anthropology, Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Podwale 75, 50-449 Wroclaw, Poland. E-mail address: [email protected] (S. Kozieł).
http://dx.doi.org/10.1016/j.earlhumdev.2017.05.008 Received 12 December 2016; Received in revised form 16 May 2017; Accepted 16 May 2017 0378-3782/ © 2017 Published by Elsevier Ireland Ltd.
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of the legs. The subject stands behind a line marked on the ground with feet slightly apart. A two foot take-off and landing is used, with swinging of the arms and bending of the knees to provide forward drive. The subject attempts to jump as far as possible, landing on both feet without falling backwards. 3. Bent Arm Hang Test (modified). The test measures arm strength. The subject is assisted into position, the body lifted to a height so that the chin is level with the horizontal bar. The bar is grasped using an overhand grip (palms are facing away from body), with the hands shoulder - width apart. The timing starts when the subject is released. They should attempt to hold this position for as long as possible. This test was modified since 67% of children were not able to do this test. Thus, the duration of a straight arm hang was measured. 4. Sit and Reach Test. This test measures flexibility and involves sitting on the floor with legs out straight ahead. The feet (shoes off) are placed with the soles flat against a box, shoulder-width apart. Both knees are held flat against the floor by the tester. With hands on top of each other and palms facing down, the subject reaches forward along the measuring line as far as possible. After one practice reach, the second reach is held for at least 2 s while the distance is recorded. Since the results of this test varied from −31 to 30 cm, to avoid negative values, we added 32 cm to all records. 5. 10 × 5 m Shuttle Run Test. This test measures speed. Marker cones and/or lines are placed five meters apart. Start with a foot at one marker. When instructed by the timer, the subject runs to the opposite marker, turns and returns to the starting line. This is repeated five times without stopping (covering 50 m total). At each marker, both feet must fully cross the line.
a connection between exercise capacity and low birth weight, which cause children and adolescents to be incapable of extended strenuous physical effort. Other studies have shown relationship between birth weight and childhood aerobic fitness, even in a normal range of birth weight [8,20]. In a recent study Ridgway et al. (2011) have pointed out that the association between birth weight and aerobic fitness could be mediated by fat-free mass, which is also associated with birth weight, independently of height [21]. However there are very few complex studies on relationship between birth weight and different aspect of physical fitness like muscular strength, flexibility, agility, speed and aerobic fitness, especially in childhood [8,18,19,22]. The aim of the study is to determine the influence of birth weight adjusted for gestational age on the level of different aspects of physical fitness, controlling for body size and relative weight in children aged 6 and 7 years. 2. Material and methods Data of 28,623 6 and 7-year-old children (14,633 boys and 14,020 girls) were collected as a part of the project “A six-year-old child at school” run by the Ministry previously known as the Ministry of National Education and Sport, partially financed by the European Union and partially by the state budget within European Social Funds (nr 5/2.1a/2004) [23]. The project was conducted between April–June and September–November in 2006. The project was carried out by the Jan Kochanowski University of Humanities and Sciences (JKUHS) in Kielce (Poland). Prior to the study, parents or legal caretakers were informed regarding the aim and procedure of the project. Parents or caretakers provided written consent of the study and were assured that participants could leave the study at any time. At the time of the study ethical approval was not required. The Bioethical Committee of the Faculty of Health Sciences of JKUHS approved the ethical and methodological aspects of the project after it had been finished. The study population was randomly selected through multiple sampling, including voivodeships (administrative unit comparable to province) and type of institution (pre-school and primary school). In this way, 32 layers of sampling were distinguished 16 voivodeships and two types of institution, regardless of the level of urbanisation of the place of residence (rural and urban areas). At each stage of sampling, 10% of institutions of each type were selected. Information for sampling was provided by the Bureau of Educational Information and the Central Statistical Bureau. Such random sampling allowed to control for environmental and social influences on outcomes. The present analysis includes 6 and 7-year-old children from rural areas, which constitutes 3.74% and 3.80% of all children at age 6 and 7 in Poland, for boys and girls respectively. The average age was 6.85 (SD = 0.298), and did not differ between boys and girls. In Poland compulsory education starts at 7 years of age, but children at 6 years of age, based on the parents' decision, are accepted in schools. This meant that only a selected group of 6-year olds attended school and therefore it was decided to include also 6-year-olds from pre-school, who were not sent to school by their parents. All the participants underwent the European Test of Physical Fitness (EUROFIT) [24] which included the measurements of different aspect of general physical fitness, defined as general state of health and wellbeing and ability to perform sports, like muscle strength, flexibility, agility, speed and aerobic fitness. The following tests were carried out in order of realisation:
All tests were conducted under the observation of two trained teachers of physical education. All the tests were chosen in accordance with Health related Fitness construct [1]. During the tests, height and weight were measured using a stadiometer and digital scales, according to Martin's instructions [25]. The BMI was calculated using height and weight (kg/m2). Parents were asked to complete two questionnaires, one concerning the general health of the child, including the birth parameters – this was carried out with the assistance of the school nurse, and the second concerned the socio-economic situation of family. Birth weight showed a significant correlation (r = 0.424; p < 0.001) with gestational age (measured in weeks that had passed since the last menstruation), which varied in our sample between 21 and 44 weeks. Thus, birth weight was adjusted to gestational age by linear regression. Then, based on the standardized residuals, we used the 10th percentile (− 1.2816 standard deviation score (SDS)) as a cutoff value to define subjects born small for gestational age (SGA) or appropriate for gestational age (AGA), separately for boys and girls. This procedure removed the influence of gestational age on birth weight and SGA was used to identify the most vulnerable low birth weight babies. Sex differences in parameters were assessed by the tStudent test separately, for each parameter. An analysis of covariance implemented by the Generalised Linear Model was used to assess the effect of the birth weight category on each parameter, controlling for sex, age, and body size (height and BMI), separately for each parameter. Additionally, the model included the effects of 2 second order interactions: between birth weight and age, and birth weight and sex. Significant interactions are presented on the graphs. 3. Results
1. Sit - Up Test. The test measures the strength of the abdominal muscles. The aim of this test is to perform as many sit-ups as you can in 30 s. The fingers are to be interlocked behind the head. On the command ‘Go’, raise the chest so that the upper body is vertical, then return to the floor. Continue for 30 s. For each sit-up the back must return to touch the floor. 2. Standing Long Jump Test. The test measures the explosive strength
The statistical means of specific components of physical fitness, depending on the sex, separately for each age category, are presented in Table 1. In each boys' age group in comparison with the girls', a significantly higher level of strength of abdominal muscles has been noted. The difference between the means increases along with age, and 24
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weight and BMI, we have found that low birth weight significantly affected only three out of five fitness parameters: the explosive power of lower limbs, running speed and flexibility in the lumbar region. The 7year-old SGA boys, in comparison with individuals with normal birth weight, achieved significantly worse results in the standing broad jump test, which evaluated the explosive power of the lower limbs. However, running speed and body flexibility turned out to be significantly worse in SGA children in each age category and sex. No significant correlations were observed for the abdominal muscles, arms and shoulders. In infants and pre-school children, researchers most frequently focused on neuromotor functioning, whereas in adolescents and adults taking part in studies on the long-term effects of low birth weight, the focus on motor performance and physical fitness were more common, stressing their close relationships with health. Very few studies have taken into account the variation of ethnic or social environment, as well as differences in family income of the subject children and regions of their residence [26]. It was established that the explosive power of the lower limbs was higher in children with normal birth weight, and differences widened with age. The standing board jump (SBJ) test, used in the present study, is considered to be one of the most valid methods of strength evaluation due to significant correlations with isokinetic parameters [27]. Moreover, the correlation with other strength tests of different muscle groups is significant (R2 = 0.69–0.86). Thus, it could be considered a general proxy of lower body muscular fitness [28]. Most of the previous studies have indicated deficits of different aspects of strength in children of low birth weight [22]. Results published by Ortega et al. [29], showed a close, positive relationship between strength and birth weight. The heavier the individual at birth, the higher level of strength is presented during childhood and adolescence. Controlling for the factors of age, pregnancy duration and breastfeeding, the authors observed stronger correlations between birth weight and strength in girls. Body size seemed to have an important influence on the this relationship. When body height was added to the model, the relationship with birth weight became weaker, while including fat-free mass caused the relationships to become insignificant. It probably points to a stronger correlation of abdominal strength with present body-build than birth weight [30]. The significant role of low birth weight on strength was confirmed by studies on younger children (7–10 years old) born prematurely between 5 and 10 weeks (mean birth weight for subjects was 1884 g; range: 1248–2460 g) [31]. Falk et al. [32] have reported a strong deficit of other strength components, such as the vertical jump in 5–8 year old children born prematurely (range of birth weight from 535 g to1760 g) in comparison with a control group. However, the level of maximum strength and power calculated per kilogram of body mass, was not significantly different between low birth weight and the control groups. It only becomes significant between the control group and extremely low birth weight group. In other studies it has been concluded that low birth weight not only impaired muscle strength, but also increased metabolic risk due to hand - muscle strength [33]. Contrary to this, Moura-dos-Santos et al. [34], have reported no relationship between abdominal muscle strength and the explosive power of the lower limbs and low birth weight. Studies concerning speed have shown that running speed and
Table 1 Descriptive statistics of measured parameters in 6 and 7-year-old boys and girls. Differences in means were assessed by t-Student test (t). Parameters
Boys
Girls
t
N
mean
SD
N
mean
SD
6-year olds Trunk strength Leg strength Arm strength Flexibility Speed
1999 1991 1994 1999 1991
8.62 99.03 26.45 0.83 27.47
5.40 19.71 20.07 5.50 3.93
1942 1936 1943 1942 1943
8.23 91.45 23.90 2.20 28.14
5.11 17.90 18.96 5.45 4.18
2.19⁎ 7.93⁎⁎⁎ 12.59⁎⁎⁎ 4.09⁎⁎⁎ 5.14⁎⁎⁎
7-year olds Trunk strength Leg strength Arm strength Flexibility Speed
12,751 12,701 12,734 12,751 12,701
9.46 102.74 29.34 0.58 26.91
5.36 19.87 21.81 5.73 5.27
12,175 12,130 12,162 12,175 12,130
8.93 94.90 25.66 2.11 27.53
5.24 18.36 19.66 5.55 4.74
7.81⁎⁎⁎ 21.50⁎⁎⁎ 32.31⁎⁎⁎ 13.98⁎⁎⁎ 9.76⁎⁎⁎
⁎
p < 0.05. p < 0.001.
⁎⁎⁎
is respectively: 0.39 n/30 s (p ˂ 0.05) at the age of 6, and 0.52 n/30 s. at the age of 7 (p ˂ 0.001). Also, the explosive power of the lower limbs, arms and shoulder strength are significantly stronger in boys than the girls. In both age groups boys show better results in the standing broad jump and bent arm hang test respectively: 7.58 cm (p ˂ 0.001), 2.55 s (p ˂ 0.001) and 7.85 cm (p ˂ 0.001), 3.67 s (p ˂ 0.001), respectively. The boys were found to be also faster than the girls. The average time necessary to cover a distance of 10 × 5 m was shorter at the age of 6 by 0.66 s (p ˂ 0.001), and among 7 year-olds, 0.62 s (p ˂ 0.001) in comparison with girls. The boys achieved a lower level of body flexibility, measured by means of the sit-and-reach test in comparison with girls. The difference in the level of average results was found to increase with age (6 years: 1.37 cm, p ˂ 0.001; 7 years: 1.53 cm, p ˂ 0.001). Table 2 presents the descriptive statistics for AGA and SGA boys and girls. The birth weight of both AGA and SGA children showed significant sex differences, favoring boys in AGA, and favoring girls in SGA children. Results of the influence of birth weight on physical fitness parameters, allowing for sex, age, height and BMI index, are presented in Table 3. The explosive power of the lower limbs, flexibility and running speed have been shown to be dependent on birth weight (Table 3). Only in the 7-year-old boys, was a significant difference observed, i.e. boys with low birth weight have a significantly lower explosive power of the lower limbs. There is a lack of such dependencies in both age groups in girls and 6-year-old boys (Fig. 1). Among boys and girls in both age groups, SGA children show significantly lower flexibility (Fig. 2). The results for speed present similar, both among the girls and boys (Fig. 3). Children with AGA covered the distance of 10 × 5 m significantly faster, these differences are particularly great in 7-year-old boys. 4. Discussion In our study, we assessed the effect of low birth weight on a level of components of physical fitness of children aged 6 and 7 years, from a rural environment. Controlling for age, sex, as well as height, body
Table 2 Descriptive statistics of small for gestational age (SGA) and appropriate for gestational age (AGA) for boys and girls. Differences in means were assessed by t-Student test (t). Parameters
AGA SGA ⁎⁎⁎
Boys
Girls
t
N
mean
SD
N
mean
SD
14,575 1189
3513.80 2332.81
475.83 462.46
13,433 1600
3406.84 2405.60
436.47 439.66
p < 0.001.
25
− 19.55⁎⁎⁎ 4.23⁎⁎⁎
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Table 3 Results of analysis of covariance where physical fitness parameters were dependent variables (separately) and birth weight (SGA vs. AGA) was an independent variable where sex, age, height, and BMI were covariates.
Sex Age AGA/SGA Height BMI AGA/SGA × age AGA/SGA × sex ⁎⁎ ⁎⁎⁎
Trunk strength χ2 Wald's
Leg strength χ2 Wald's
Arm strength χ2 Wald's
Flexibility χ2 Wald's
Speed χ2 Wald's
13.73⁎⁎⁎ 10.84⁎⁎⁎ 0.21 104.41⁎⁎⁎ 263.38⁎⁎⁎ 0.68 1.05
282.61⁎⁎⁎ 19.18⁎⁎⁎ 6.72⁎⁎ 292.47⁎⁎⁎ 652.84⁎⁎⁎ 0.21 9.12⁎⁎
87.44⁎⁎⁎ 21.58⁎⁎⁎ 0.001 72.91⁎⁎⁎ 651.94⁎⁎⁎ 0.50 1.01
157.92⁎⁎⁎ 0.15 14.45⁎⁎⁎ 177.55⁎⁎⁎ 1.61 1.26 0.21
29.88⁎⁎⁎ 13.87⁎⁎⁎ 10.38⁎⁎ 12.16⁎⁎⁎ 165.18⁎⁎⁎ 0.04 0.93
p < 0.01. p < 0.001.
muscle fibers are important for removing blood glucose. Slow-oxidative muscle fibers appear to be more active in this process. Hence, low birth weight, which is a sign of undernourishment and poor intrauterine conditions, becomes a possible risk factor related to a lower level of muscle bulk, insulin resistance and related overweight and obesity [39]. Inappropriate proportions of muscle fibers, with the tendency of dominance of fast-glycolytic fibers, and also biomechanical properties of skeletal muscles, which is accompanied by a lack of physical activity largely determines the development of overweight and obesity and lowers the potential for physical fitness [9,38]. It has been suggested that pregnancy length highly affects the development of joint mobility. Shorter gestational age causes joint laxity and the lack of firmness of collagen in the neonate. That is why children from shorter pregnancies, with low birth weight, show greater collagen stretch ability and joint laxity, which can persist until school age [40], however those findings need further support. In low and extremely low birth weight children, the prenatal process of the nervous system development is impaired leading to weaker neuromotor potential [7,16,35,36]. It seems that there is an association between the growth processes in the postnatal period and the level of physical fitness achieved at the particular stage of development. After a birth with low weight, the catch-up process commonly occurs, thus it probably leads to achieving
agility, regardless of the test used (running for a distance: 20 m distance), were significantly weaker in children with low birth weight [31,34]. In 5–7 year old children with low birth weight, a slower reaction time has been observed [35]. In addition, a lower speed in children with extremely low birth weight assessed by means of the BOTMP test (Bruininks-Oseretsky Test of Motor Proficiency) showed developmental coordination disorder [36]. Considering body flexibility, Svien [31] did not find any significant correlation between the results of the sit-and-reach test and the joint laxity index and low birth weight in a group of 7–10 year-old children [29]. However, children with extremely low birth weight have achieved significantly lower results compared to the control group, in measurements of trunk flexion and hamstring length [8]. It is widely known that the intrauterine environment shapes numerous processes related to postnatal growth and the development of newborns. It especially determines the physical birth parameters such as body length and weight. These, in turn, are considered to be significant predictors of physical growth and development, including body weight components: for instance the thickness of adipose tissue and fat free mass [37]. During intrauterine development, constituted muscle fiber composition and its size influences individual insulin resistance in adult life in individuals with LBW [38]. In turn, developing mutual proportions of muscle fibers: slow-oxidative fast-glycolytic
Fig. 1. Mean (95%CI) of leg strength by age, sex and birth weight categories.
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Fig. 2. Mean (95%CI) of flexibility (modified by addition value 32 to avoid value 0) by age, sex and birth weight categories.
has not been completely understood. It could be supposed that low birth weight children showed neuromotor deficits, lower skeletal mineralization, and were also overprotected preventing them from taking up physical activity [14,32,45]. There are important sex differences in intensity and forms of physical activity. Thus, boys more often than girls undertake moderate-to-vigorous activity. They also prefer exercise related to team sports, where development of persistence, speed and strength training is possible, and where psychological aspects, including perseverance and competition, are practiced. Girls take up moderate physical activity more often, engaging in individual physical exercises, such as skipping rope, running, playing ball, dancing or doing gymnastics. These forms allow them to shape other compo-
higher parameters of lean body mass and fat mass further in life [41]. However, some studies have not confirmed this, showing a strong positive relationship with lean body mass, and a negative one with high general body mass [42]. The above dependencies are stronger in men [43]. Physical activity should be considered in the interpretation of variation of the physical fitness level in relation to birth weight. It is widely known that pre-school age is characterized by spontaneous physical activity, the levels of which and preferred forms are sex dependent from the early years [44]. The potential for physical activity in children with low birth weight, compared to their peers with normal birth weight, is significantly lower. The mechanism of the correlation
Fig. 3. Mean (95%CI) of speed by age, sex and birth weight categories.
27
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nents of physical fitness compared to in boys: speed, balance, coordination, flexibility and agility [46]. The differences in the level of physical activity become more significant with age, at the same time differentiating according to sex, the level of physical fitness later in life. What is more, dependencies observed between physical activity and physical fitness are stronger in boys [44]. It is probable that stronger relations between low birth weight and physical fitness in 7-year-olds are a consequence of the mechanism signaled above. It should also be assumed, that even though low birth weight children generally present a lower physical activity level, the correlation between its level and low birth weight among boys and girls is maintained. Finally, some limitations of the present study should be mentioned. Firstly, we used only the gestational age and birth weight as indicators of intrauterine growth, and did not consider the aetiology of low birth weight. Therefore, we do not know whether different factors, which cause LBW, equally affected the analyzed parameters of physical fitness. However, in epidemiological studies the birth weight is commonly used as a simple and reliable measure of fetal development, and perhaps is a single most useful health indicator [47]. Secondly, we do not have any information about lifestyle, which could affect the level of physical activity, and hence physical fitness. We also did not examine any maternal anthropometry, for example body size and factors of lifestyle, which might have affected both birth weight and behavior related to the physical activity of the children. In summary, it should be stressed that children small for gestational age shows a relationship with components of physical fitness. So, it is justified to introduce early intervention in groups at risk of decreased fitness due to low birth weight, and also to competently apply several other health behaviors, including the promotion of physical activity. The rationale being that when a child is susceptible to the suggestions of others, and the brain plasticity being so high, the creation of a healthy lifestyle inclined toward physical activity will bring notable benefits in the future.
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Early Human Development journal homepage: www.elsevier.com/locate/earlhumdev
The level of physical fitness in children aged 6–7 years with low birthweight☆
MARK
Elżbieta Cieślaa, Monika Zarębab, Sławomir Koziełc,⁎ a b c
Faculty of Health Sciences, Jan Kochanowski University, 19 IX Wieków Kielc Street, 25-317 Kielce, Poland Faculty of Pedagogy and Arts, Jan Kochanowski University, 11 Krakowska Street, 25-029 Kielce, Poland Department of Anthropology, Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, 75 Podwale Street, 50-449 Wroclaw, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Physical fitness Eurofit Low birth weight Children
Background: Level of physical fitness is related to the functional status of most of the bodily functions and so it appears to be very important to identify perinatal factors influencing physical fitness. Aim: The purpose of this study was to determine the influence of birth weight on the level of physical fitness in children 6–7 years of age. Subjects and method: Physical fitness was assessed using EUROFIT tests in 28,623 children, aged 6–7 years, from rural areas in Poland. Children below the 10th percentile for birth weight for gestational age were defined as small for gestational age (SGA). The influence of birth weight on parameters of fitness was assessed by means of covariance analysis. Results: With the controls of age, sex and body size, children of low birth weight have shown significantly lower levels of body flexibility and running speed. The leg strength of children with SGA turned out to be significantly lower only in 7-year-old boys. Conclusion: This study has revealed the significant influence of birth weight on physical fitness. The results suggest the importance of early intervention and its possible benefits for developing and maintaining the proper level of physical fitness further in life.
1. Introduction Since the level of physical fitness is related to the functional status of most of the bodily functions, it is considered one the most important health indicators for the general population, regardless of age [1]. It is a good predictor of morbidity and mortality for cardiovascular diseases and for mortality from all causes, as well as increased occurrence of metabolic syndrome among adolescents [2–5]. Thus, it seems that a priority for public health should be the monitoring of fitness, especially among the youngest, popularizing recommendations concerning daily exercise, and identifying risk factors which could hamper/harm/interfere with its natural development. Physical fitness is partly genetically determined, but it is also strongly influenced by various environmental factors, among which physical activity is considered to be the most significant [6]. However, the given level of fitness of a child at a certain developmental stage is the result of the interaction between genetic and environmental factors, which might be influenced by perinatal factors,
such as the weight of a child at birth [7–9]. Birth weight largely depends on intrauterine growth rate and is a commonly used indicator of the state of fetal growth and development [10,11]. Low birth weight is correlated with a significantly more frequent (compared to children with normal birth weight) occurrence of several short- and long-term health problems in the postnatal period of life [12–14]. Research shows that children with low birth weight more often suffer from behavioral and emotional problems related to attention deficiency and social development, and equally often, related to physical health [15–17]. Many studies frequently relate their results to respiratory incapacity and cardiorespiratory insufficiency. They are less frequently associated with other components of fitness [8,11]. It has been shown that birth weight indicates a relationship with physical fitness, motor agility and also neuromotor and motor skills (Fundamental Motor Skills). Some studies have reported a significant correlation between birth weight and cardiorespiratory fitness, exercise capacity, flexibility and strength [9,18]. Kilbride et al. [19] established
☆ This data is a part of the project “A six-year-old child at school” run by the Ministry previously known as the Ministry of National Education and Sport, partially financed by the European Union and partially by the state budget within European Social Funds (nr 5/2.1a/2004). ⁎ Corresponding author at: Department of Anthropology, Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Podwale 75, 50-449 Wroclaw, Poland. E-mail address: [email protected] (S. Kozieł).
http://dx.doi.org/10.1016/j.earlhumdev.2017.05.008 Received 12 December 2016; Received in revised form 16 May 2017; Accepted 16 May 2017 0378-3782/ © 2017 Published by Elsevier Ireland Ltd.
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of the legs. The subject stands behind a line marked on the ground with feet slightly apart. A two foot take-off and landing is used, with swinging of the arms and bending of the knees to provide forward drive. The subject attempts to jump as far as possible, landing on both feet without falling backwards. 3. Bent Arm Hang Test (modified). The test measures arm strength. The subject is assisted into position, the body lifted to a height so that the chin is level with the horizontal bar. The bar is grasped using an overhand grip (palms are facing away from body), with the hands shoulder - width apart. The timing starts when the subject is released. They should attempt to hold this position for as long as possible. This test was modified since 67% of children were not able to do this test. Thus, the duration of a straight arm hang was measured. 4. Sit and Reach Test. This test measures flexibility and involves sitting on the floor with legs out straight ahead. The feet (shoes off) are placed with the soles flat against a box, shoulder-width apart. Both knees are held flat against the floor by the tester. With hands on top of each other and palms facing down, the subject reaches forward along the measuring line as far as possible. After one practice reach, the second reach is held for at least 2 s while the distance is recorded. Since the results of this test varied from −31 to 30 cm, to avoid negative values, we added 32 cm to all records. 5. 10 × 5 m Shuttle Run Test. This test measures speed. Marker cones and/or lines are placed five meters apart. Start with a foot at one marker. When instructed by the timer, the subject runs to the opposite marker, turns and returns to the starting line. This is repeated five times without stopping (covering 50 m total). At each marker, both feet must fully cross the line.
a connection between exercise capacity and low birth weight, which cause children and adolescents to be incapable of extended strenuous physical effort. Other studies have shown relationship between birth weight and childhood aerobic fitness, even in a normal range of birth weight [8,20]. In a recent study Ridgway et al. (2011) have pointed out that the association between birth weight and aerobic fitness could be mediated by fat-free mass, which is also associated with birth weight, independently of height [21]. However there are very few complex studies on relationship between birth weight and different aspect of physical fitness like muscular strength, flexibility, agility, speed and aerobic fitness, especially in childhood [8,18,19,22]. The aim of the study is to determine the influence of birth weight adjusted for gestational age on the level of different aspects of physical fitness, controlling for body size and relative weight in children aged 6 and 7 years. 2. Material and methods Data of 28,623 6 and 7-year-old children (14,633 boys and 14,020 girls) were collected as a part of the project “A six-year-old child at school” run by the Ministry previously known as the Ministry of National Education and Sport, partially financed by the European Union and partially by the state budget within European Social Funds (nr 5/2.1a/2004) [23]. The project was conducted between April–June and September–November in 2006. The project was carried out by the Jan Kochanowski University of Humanities and Sciences (JKUHS) in Kielce (Poland). Prior to the study, parents or legal caretakers were informed regarding the aim and procedure of the project. Parents or caretakers provided written consent of the study and were assured that participants could leave the study at any time. At the time of the study ethical approval was not required. The Bioethical Committee of the Faculty of Health Sciences of JKUHS approved the ethical and methodological aspects of the project after it had been finished. The study population was randomly selected through multiple sampling, including voivodeships (administrative unit comparable to province) and type of institution (pre-school and primary school). In this way, 32 layers of sampling were distinguished 16 voivodeships and two types of institution, regardless of the level of urbanisation of the place of residence (rural and urban areas). At each stage of sampling, 10% of institutions of each type were selected. Information for sampling was provided by the Bureau of Educational Information and the Central Statistical Bureau. Such random sampling allowed to control for environmental and social influences on outcomes. The present analysis includes 6 and 7-year-old children from rural areas, which constitutes 3.74% and 3.80% of all children at age 6 and 7 in Poland, for boys and girls respectively. The average age was 6.85 (SD = 0.298), and did not differ between boys and girls. In Poland compulsory education starts at 7 years of age, but children at 6 years of age, based on the parents' decision, are accepted in schools. This meant that only a selected group of 6-year olds attended school and therefore it was decided to include also 6-year-olds from pre-school, who were not sent to school by their parents. All the participants underwent the European Test of Physical Fitness (EUROFIT) [24] which included the measurements of different aspect of general physical fitness, defined as general state of health and wellbeing and ability to perform sports, like muscle strength, flexibility, agility, speed and aerobic fitness. The following tests were carried out in order of realisation:
All tests were conducted under the observation of two trained teachers of physical education. All the tests were chosen in accordance with Health related Fitness construct [1]. During the tests, height and weight were measured using a stadiometer and digital scales, according to Martin's instructions [25]. The BMI was calculated using height and weight (kg/m2). Parents were asked to complete two questionnaires, one concerning the general health of the child, including the birth parameters – this was carried out with the assistance of the school nurse, and the second concerned the socio-economic situation of family. Birth weight showed a significant correlation (r = 0.424; p < 0.001) with gestational age (measured in weeks that had passed since the last menstruation), which varied in our sample between 21 and 44 weeks. Thus, birth weight was adjusted to gestational age by linear regression. Then, based on the standardized residuals, we used the 10th percentile (− 1.2816 standard deviation score (SDS)) as a cutoff value to define subjects born small for gestational age (SGA) or appropriate for gestational age (AGA), separately for boys and girls. This procedure removed the influence of gestational age on birth weight and SGA was used to identify the most vulnerable low birth weight babies. Sex differences in parameters were assessed by the tStudent test separately, for each parameter. An analysis of covariance implemented by the Generalised Linear Model was used to assess the effect of the birth weight category on each parameter, controlling for sex, age, and body size (height and BMI), separately for each parameter. Additionally, the model included the effects of 2 second order interactions: between birth weight and age, and birth weight and sex. Significant interactions are presented on the graphs. 3. Results
1. Sit - Up Test. The test measures the strength of the abdominal muscles. The aim of this test is to perform as many sit-ups as you can in 30 s. The fingers are to be interlocked behind the head. On the command ‘Go’, raise the chest so that the upper body is vertical, then return to the floor. Continue for 30 s. For each sit-up the back must return to touch the floor. 2. Standing Long Jump Test. The test measures the explosive strength
The statistical means of specific components of physical fitness, depending on the sex, separately for each age category, are presented in Table 1. In each boys' age group in comparison with the girls', a significantly higher level of strength of abdominal muscles has been noted. The difference between the means increases along with age, and 24
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weight and BMI, we have found that low birth weight significantly affected only three out of five fitness parameters: the explosive power of lower limbs, running speed and flexibility in the lumbar region. The 7year-old SGA boys, in comparison with individuals with normal birth weight, achieved significantly worse results in the standing broad jump test, which evaluated the explosive power of the lower limbs. However, running speed and body flexibility turned out to be significantly worse in SGA children in each age category and sex. No significant correlations were observed for the abdominal muscles, arms and shoulders. In infants and pre-school children, researchers most frequently focused on neuromotor functioning, whereas in adolescents and adults taking part in studies on the long-term effects of low birth weight, the focus on motor performance and physical fitness were more common, stressing their close relationships with health. Very few studies have taken into account the variation of ethnic or social environment, as well as differences in family income of the subject children and regions of their residence [26]. It was established that the explosive power of the lower limbs was higher in children with normal birth weight, and differences widened with age. The standing board jump (SBJ) test, used in the present study, is considered to be one of the most valid methods of strength evaluation due to significant correlations with isokinetic parameters [27]. Moreover, the correlation with other strength tests of different muscle groups is significant (R2 = 0.69–0.86). Thus, it could be considered a general proxy of lower body muscular fitness [28]. Most of the previous studies have indicated deficits of different aspects of strength in children of low birth weight [22]. Results published by Ortega et al. [29], showed a close, positive relationship between strength and birth weight. The heavier the individual at birth, the higher level of strength is presented during childhood and adolescence. Controlling for the factors of age, pregnancy duration and breastfeeding, the authors observed stronger correlations between birth weight and strength in girls. Body size seemed to have an important influence on the this relationship. When body height was added to the model, the relationship with birth weight became weaker, while including fat-free mass caused the relationships to become insignificant. It probably points to a stronger correlation of abdominal strength with present body-build than birth weight [30]. The significant role of low birth weight on strength was confirmed by studies on younger children (7–10 years old) born prematurely between 5 and 10 weeks (mean birth weight for subjects was 1884 g; range: 1248–2460 g) [31]. Falk et al. [32] have reported a strong deficit of other strength components, such as the vertical jump in 5–8 year old children born prematurely (range of birth weight from 535 g to1760 g) in comparison with a control group. However, the level of maximum strength and power calculated per kilogram of body mass, was not significantly different between low birth weight and the control groups. It only becomes significant between the control group and extremely low birth weight group. In other studies it has been concluded that low birth weight not only impaired muscle strength, but also increased metabolic risk due to hand - muscle strength [33]. Contrary to this, Moura-dos-Santos et al. [34], have reported no relationship between abdominal muscle strength and the explosive power of the lower limbs and low birth weight. Studies concerning speed have shown that running speed and
Table 1 Descriptive statistics of measured parameters in 6 and 7-year-old boys and girls. Differences in means were assessed by t-Student test (t). Parameters
Boys
Girls
t
N
mean
SD
N
mean
SD
6-year olds Trunk strength Leg strength Arm strength Flexibility Speed
1999 1991 1994 1999 1991
8.62 99.03 26.45 0.83 27.47
5.40 19.71 20.07 5.50 3.93
1942 1936 1943 1942 1943
8.23 91.45 23.90 2.20 28.14
5.11 17.90 18.96 5.45 4.18
2.19⁎ 7.93⁎⁎⁎ 12.59⁎⁎⁎ 4.09⁎⁎⁎ 5.14⁎⁎⁎
7-year olds Trunk strength Leg strength Arm strength Flexibility Speed
12,751 12,701 12,734 12,751 12,701
9.46 102.74 29.34 0.58 26.91
5.36 19.87 21.81 5.73 5.27
12,175 12,130 12,162 12,175 12,130
8.93 94.90 25.66 2.11 27.53
5.24 18.36 19.66 5.55 4.74
7.81⁎⁎⁎ 21.50⁎⁎⁎ 32.31⁎⁎⁎ 13.98⁎⁎⁎ 9.76⁎⁎⁎
⁎
p < 0.05. p < 0.001.
⁎⁎⁎
is respectively: 0.39 n/30 s (p ˂ 0.05) at the age of 6, and 0.52 n/30 s. at the age of 7 (p ˂ 0.001). Also, the explosive power of the lower limbs, arms and shoulder strength are significantly stronger in boys than the girls. In both age groups boys show better results in the standing broad jump and bent arm hang test respectively: 7.58 cm (p ˂ 0.001), 2.55 s (p ˂ 0.001) and 7.85 cm (p ˂ 0.001), 3.67 s (p ˂ 0.001), respectively. The boys were found to be also faster than the girls. The average time necessary to cover a distance of 10 × 5 m was shorter at the age of 6 by 0.66 s (p ˂ 0.001), and among 7 year-olds, 0.62 s (p ˂ 0.001) in comparison with girls. The boys achieved a lower level of body flexibility, measured by means of the sit-and-reach test in comparison with girls. The difference in the level of average results was found to increase with age (6 years: 1.37 cm, p ˂ 0.001; 7 years: 1.53 cm, p ˂ 0.001). Table 2 presents the descriptive statistics for AGA and SGA boys and girls. The birth weight of both AGA and SGA children showed significant sex differences, favoring boys in AGA, and favoring girls in SGA children. Results of the influence of birth weight on physical fitness parameters, allowing for sex, age, height and BMI index, are presented in Table 3. The explosive power of the lower limbs, flexibility and running speed have been shown to be dependent on birth weight (Table 3). Only in the 7-year-old boys, was a significant difference observed, i.e. boys with low birth weight have a significantly lower explosive power of the lower limbs. There is a lack of such dependencies in both age groups in girls and 6-year-old boys (Fig. 1). Among boys and girls in both age groups, SGA children show significantly lower flexibility (Fig. 2). The results for speed present similar, both among the girls and boys (Fig. 3). Children with AGA covered the distance of 10 × 5 m significantly faster, these differences are particularly great in 7-year-old boys. 4. Discussion In our study, we assessed the effect of low birth weight on a level of components of physical fitness of children aged 6 and 7 years, from a rural environment. Controlling for age, sex, as well as height, body
Table 2 Descriptive statistics of small for gestational age (SGA) and appropriate for gestational age (AGA) for boys and girls. Differences in means were assessed by t-Student test (t). Parameters
AGA SGA ⁎⁎⁎
Boys
Girls
t
N
mean
SD
N
mean
SD
14,575 1189
3513.80 2332.81
475.83 462.46
13,433 1600
3406.84 2405.60
436.47 439.66
p < 0.001.
25
− 19.55⁎⁎⁎ 4.23⁎⁎⁎
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Table 3 Results of analysis of covariance where physical fitness parameters were dependent variables (separately) and birth weight (SGA vs. AGA) was an independent variable where sex, age, height, and BMI were covariates.
Sex Age AGA/SGA Height BMI AGA/SGA × age AGA/SGA × sex ⁎⁎ ⁎⁎⁎
Trunk strength χ2 Wald's
Leg strength χ2 Wald's
Arm strength χ2 Wald's
Flexibility χ2 Wald's
Speed χ2 Wald's
13.73⁎⁎⁎ 10.84⁎⁎⁎ 0.21 104.41⁎⁎⁎ 263.38⁎⁎⁎ 0.68 1.05
282.61⁎⁎⁎ 19.18⁎⁎⁎ 6.72⁎⁎ 292.47⁎⁎⁎ 652.84⁎⁎⁎ 0.21 9.12⁎⁎
87.44⁎⁎⁎ 21.58⁎⁎⁎ 0.001 72.91⁎⁎⁎ 651.94⁎⁎⁎ 0.50 1.01
157.92⁎⁎⁎ 0.15 14.45⁎⁎⁎ 177.55⁎⁎⁎ 1.61 1.26 0.21
29.88⁎⁎⁎ 13.87⁎⁎⁎ 10.38⁎⁎ 12.16⁎⁎⁎ 165.18⁎⁎⁎ 0.04 0.93
p < 0.01. p < 0.001.
muscle fibers are important for removing blood glucose. Slow-oxidative muscle fibers appear to be more active in this process. Hence, low birth weight, which is a sign of undernourishment and poor intrauterine conditions, becomes a possible risk factor related to a lower level of muscle bulk, insulin resistance and related overweight and obesity [39]. Inappropriate proportions of muscle fibers, with the tendency of dominance of fast-glycolytic fibers, and also biomechanical properties of skeletal muscles, which is accompanied by a lack of physical activity largely determines the development of overweight and obesity and lowers the potential for physical fitness [9,38]. It has been suggested that pregnancy length highly affects the development of joint mobility. Shorter gestational age causes joint laxity and the lack of firmness of collagen in the neonate. That is why children from shorter pregnancies, with low birth weight, show greater collagen stretch ability and joint laxity, which can persist until school age [40], however those findings need further support. In low and extremely low birth weight children, the prenatal process of the nervous system development is impaired leading to weaker neuromotor potential [7,16,35,36]. It seems that there is an association between the growth processes in the postnatal period and the level of physical fitness achieved at the particular stage of development. After a birth with low weight, the catch-up process commonly occurs, thus it probably leads to achieving
agility, regardless of the test used (running for a distance: 20 m distance), were significantly weaker in children with low birth weight [31,34]. In 5–7 year old children with low birth weight, a slower reaction time has been observed [35]. In addition, a lower speed in children with extremely low birth weight assessed by means of the BOTMP test (Bruininks-Oseretsky Test of Motor Proficiency) showed developmental coordination disorder [36]. Considering body flexibility, Svien [31] did not find any significant correlation between the results of the sit-and-reach test and the joint laxity index and low birth weight in a group of 7–10 year-old children [29]. However, children with extremely low birth weight have achieved significantly lower results compared to the control group, in measurements of trunk flexion and hamstring length [8]. It is widely known that the intrauterine environment shapes numerous processes related to postnatal growth and the development of newborns. It especially determines the physical birth parameters such as body length and weight. These, in turn, are considered to be significant predictors of physical growth and development, including body weight components: for instance the thickness of adipose tissue and fat free mass [37]. During intrauterine development, constituted muscle fiber composition and its size influences individual insulin resistance in adult life in individuals with LBW [38]. In turn, developing mutual proportions of muscle fibers: slow-oxidative fast-glycolytic
Fig. 1. Mean (95%CI) of leg strength by age, sex and birth weight categories.
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Fig. 2. Mean (95%CI) of flexibility (modified by addition value 32 to avoid value 0) by age, sex and birth weight categories.
has not been completely understood. It could be supposed that low birth weight children showed neuromotor deficits, lower skeletal mineralization, and were also overprotected preventing them from taking up physical activity [14,32,45]. There are important sex differences in intensity and forms of physical activity. Thus, boys more often than girls undertake moderate-to-vigorous activity. They also prefer exercise related to team sports, where development of persistence, speed and strength training is possible, and where psychological aspects, including perseverance and competition, are practiced. Girls take up moderate physical activity more often, engaging in individual physical exercises, such as skipping rope, running, playing ball, dancing or doing gymnastics. These forms allow them to shape other compo-
higher parameters of lean body mass and fat mass further in life [41]. However, some studies have not confirmed this, showing a strong positive relationship with lean body mass, and a negative one with high general body mass [42]. The above dependencies are stronger in men [43]. Physical activity should be considered in the interpretation of variation of the physical fitness level in relation to birth weight. It is widely known that pre-school age is characterized by spontaneous physical activity, the levels of which and preferred forms are sex dependent from the early years [44]. The potential for physical activity in children with low birth weight, compared to their peers with normal birth weight, is significantly lower. The mechanism of the correlation
Fig. 3. Mean (95%CI) of speed by age, sex and birth weight categories.
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nents of physical fitness compared to in boys: speed, balance, coordination, flexibility and agility [46]. The differences in the level of physical activity become more significant with age, at the same time differentiating according to sex, the level of physical fitness later in life. What is more, dependencies observed between physical activity and physical fitness are stronger in boys [44]. It is probable that stronger relations between low birth weight and physical fitness in 7-year-olds are a consequence of the mechanism signaled above. It should also be assumed, that even though low birth weight children generally present a lower physical activity level, the correlation between its level and low birth weight among boys and girls is maintained. Finally, some limitations of the present study should be mentioned. Firstly, we used only the gestational age and birth weight as indicators of intrauterine growth, and did not consider the aetiology of low birth weight. Therefore, we do not know whether different factors, which cause LBW, equally affected the analyzed parameters of physical fitness. However, in epidemiological studies the birth weight is commonly used as a simple and reliable measure of fetal development, and perhaps is a single most useful health indicator [47]. Secondly, we do not have any information about lifestyle, which could affect the level of physical activity, and hence physical fitness. We also did not examine any maternal anthropometry, for example body size and factors of lifestyle, which might have affected both birth weight and behavior related to the physical activity of the children. In summary, it should be stressed that children small for gestational age shows a relationship with components of physical fitness. So, it is justified to introduce early intervention in groups at risk of decreased fitness due to low birth weight, and also to competently apply several other health behaviors, including the promotion of physical activity. The rationale being that when a child is susceptible to the suggestions of others, and the brain plasticity being so high, the creation of a healthy lifestyle inclined toward physical activity will bring notable benefits in the future.
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