Inattention and hyperactivity symptoms in childhood predict physical activity in adolescence

Physical activity is an important life style factor for many health outcomes throughout life. While the most commonly known health effects include cardiovascular health and metabolic health, a number of recent studies have suggested that physical activity might also benefit brain health. WHO recommends that children and adolescents should do at least an average of 60 min per day of moderate-to vigorous-intensity, mostly aerobic, physical activity, across the week, and that vigorous-intensity aerobic activities, as well as those that strengthen muscle and bone should be incorporated at least 3 days per week [1].

In cross-sectional studies, physical activity and aerobic fitness have been shown to be positively associated to children’s cognitive function and academic achievements [2, 3]. In Sweden schools, parents and other parts of society have showed a growing interest in long-term health consequences of symptoms of inattention and hyperactivity / impulsivity. The prevalence of ADHD among Swedish school-aged children has been estimated by the National Board of Health and Welfare to be around 3 to 5%, based on a review of Swedish and international studies in 2002 [4]. However, since then another study has concluded that increasing numbers of children in Sweden are being diagnosed with ADHD and that clinically diagnosed ADHD increased more than fivefold from 2004 to 2014 [5]. Furthermore, other coexisting neurodevelopmental problems are also quite common among children with ADHD [6].

Children with ADHD in ages 6–17 years tend to be less involved in physical activity and organized sports than their peers [7], and are almost twice as likely to have fewer healthy behaviors than their peers [8].

While in cross sectional data, a cause and effect relationship between ADHD and associated factors cannot be deduced, longitudinal studies can be used to study how symptoms and lifestyles develop over time.

In an attempt to study the causality between physical activity and symptoms of inattention and hyperactivity / impulsivity, Rommel et al. used twin-analysis in a longitudinal study between adolescence and young adulthood. They found that physical activity in adolescence was inversely associated with ADHD symptoms in young adulthood, even after adjusting for unmeasured confounding, suggesting that physical activity might indeed mitigate future symptoms of inattention and hyperactivity / impulsivity [9]. When studying a younger age group, less sedentary behavior at age 7 has on the other hand been shown to predict ADHD diagnosis at age 14 [10]. The findings that more activity in adolescence is associated with fewer symptoms of ADHD in adulthood does not necessarily contradict the finding that less sedentary behaviour in childhood predicts higher ADHD symptoms in adolescence. These studies involve two different age groups, which can explain differing results over time.

Since ADHD is highly heritable [11], it is not uncommon that a child with ADHD has a parent who also has ADHD. It has been shown that adult ADHD is associated with functional impairment with consequences such as lower educational attainment and lower level of employment [12]. This may influence the parent’s capacity to support a child’s daily life in an optimal way to create healthy behaviours and habits.

Several studies have shown associations between ADHD and poor health outcomes [13, 14], but less is known about how the specific symptoms of inattention, hyperactivity, and impulsivity might differ in influencing health enhancing behaviors such as physical activity.

Khalife et al., provided evidence that teacher-reports of ADHD symptoms or conduct disorder symptoms in 8 year old children were associated with an increased risk for being less physically active in adolescence [15]. The findings of Khalife et al., provides initial support for the notion that children with such symptoms might be of increased risk of being less physically active, but such findings could be specific to the Finnish society, and hence need to be confirmed using validated questionnaires in more populations. In Sweden, the government supports the Swedish Sports federation so that they will be able to provide young individuals with good and equal opportunities to participate in sports. A better understanding of how childhood symptoms of inattention and hyperactivity may predict physical activity in adolescence will be important information for health care, schools, parents and sport organizations aiming to provide children with equal opportunities to health promoting lifestyles.

We aimed to explore how symptoms of inattention and hyperactivity/impulsivity in childhood are associated with self-rated physical activity in adolescence in a nationally representative sample of Swedish adolescent twins, while controlling for sex, age, parental education, and comorbidity with other neurodevelopmental problems (NDPs). Based on the findings of Khalife et al., we expected to find that more symptoms of inattention and hyperactivity / impulsivity in childhood would be positively associated with self-rating of less physical activity in adolescence [15].

The present study aimed to assess the associations between symptoms of inattention and hyperactivity / impulsivity in childhood and physical activity in adolescence. We explored this relationship in a nationally representative sample of Swedish adolescent twins of both sexes with childhood symptoms of inattention and hyperactivity / impulsivity while controlling for sex, parental education, parent-rated physical activity at age 9/12 and NDP comorbidity.

The main findings were that inattentive symptoms in childhood were associated with being less likely to be physically active in adolescence, and that hyperactivity/impulsivity symptoms in childhood were associated with a higher likelihood of being physically active in adolescence. These associations remained after adjusting for sex, age, parental education level and parent-rated physical activity at age 9/12, and coexisting NDPs and NDP related problems at age 9/12. Nonetheless, our results also indicated a potential floor effect of the symptom scales in relation to physical activity, as most participants who were physically active scored 0 on both scales.

Overall, the expected results were that more symptoms of ADHD in childhood would be positively associated with self-rating of less physical activity in adolescence. Khalife et al. had earlier found such an association [15]. Furthermore, when a child with high levels of symptoms of inattention enters into adolescence, it seems more likely that he/she doesn’t want to adapt, or is incapable of adapting, to regular physical training routines in a group setting or by him−/herself [25]. The impairment that the influence of ADHD symptoms leads to, can also manifest more powerfully during adolescence and result in comorbidity with depression, anxiety, behavioral problems, and bullying. These co-occurring problems would lessen the likelihood of being physically active even more.

The prevalence and social consequences of being diagnosed with ADHD might differ over time and between countries [26]. Recent cross-sectional investigations reported that American children with ADHD were less involved in physical activity than their peers [7], and that children with ADHD were almost twice as likely to have fewer healthy behaviors than peers [8]. However, these studies did not have a longitudinal design.

Our findings of a prospective association between childhood symptoms of inattention and hyperactivity / impulsivity and adolescent physical activity among Swedish twins are supported by results from a longitudinal study by Khalife et al. They found that in a Finnish context, childhood ADHD symptoms and conduct disorder symptoms were linked to being less physically active in adolescence [15]. In these two Nordic contexts therefore, children with inattention symptoms appear to be at increased risk of becoming less physically active in youth. These findings are of great importance for schools, sports clubs, parents and health care workers trying to promote health enhancing physical activity and participation in sports for all.

In the analysis, we controlled for sex. This suggests that despite ADHD being more commonly seen in boys [27, 28], and despite that girls have been shown to be less physically active [29, 30], the association found here between childhood symptoms of inattention and hyperactivity / impulsivity and adolescence physical activity cannot be attributed to sex differences.

As expected, physical activity at age 9/12 in the form of ‘spare time physical activity’ was associated with higher odds of being physically active at age 15. This finding concurs with earlier studies concluding that a physically active lifestyle starts to develop very early in childhood and that stability of physical activity is moderate to high along the life course from youth to adulthood [31]. It has also been shown that persistent participation in sport increases the probability of a higher level of physical activity in later life [32].

Interestingly, inattention was associated with lower odds of self-rating as being physically active in adolescence whereas the opposite was true for hyperactivity/impulsivity, even after taking possible confounders such as parent-rated physical activity at age 9/12, sex, parental education level and coexisting NDPs into account. This is in line with earlier findings in a cross-sectional study [33] and might imply that children with inattention but not hyperactivity/impulsivity are less attracted to physical activity contexts such as organized sports. Some of them are overweight and they might also have developmental coordination disorder (DCD) [15, 34]. These co-existing problems may increase difficulties to get involved in physical activity. While for some adolescents the hyperactive symptoms diminish and internalize, creating more of an inner sense of restlessness, one might speculate that for others a hyperactive personality profile at age 9/12 could persist into adolescence and even adulthood. Moreover, one might speculate that children with mainly hyperactivity at age 9/12, are less impaired in daily life concerning for example social interaction and aggression, and thus might have the capacity to ‘canalize’ or express their hyperactivity through engaging in different kinds of sports and physical activity. These children are less impaired and have more social skills to be able to cope with others in group activities.

A future line of research could be to carry out studies with our design, but in which the participants belong to clinical populations of children with ADHD. Future studies could also be directed towards exploring more specifically how physical activity can moderate the associations between symptoms of inattention and hyperactivity / impulsivity and mental health.

Based on our and others’ findings, children with more inattentive symptoms are at risk of being less physically active in adolescence compared to children with less inattentive symptoms. Since insufficient physical activity might drive inequalities in health and wellbeing, research efforts should be directed towards investigating attractive and effective ways to promote life-long physical activity for children and youths with inattention symptoms in particular. In Sweden, the financial governmental support promoting health enhancing physical activity, is primarily directed to the Swedish sports federation and its clubs, who typically do not work proactively to include children with ADHD symptoms in their activities.

In a clinical setting, it is valuable to be able to distinguish differences between patients with different symptom profiles to be able to better adjust and individualize treatments. Children with mainly inattention can be quite different from those with mainly hyperactivity/impulsivity. Our results showing that children with inattention symptoms in childhood are less likely to be physically active in adolescence, can help clinicians to be more observant concerning physical activity among those patients.

There are several unique aspects of this study. The CATSS-study is one of the most comprehensive twin studies ever performed including childhood mental and somatic health. All Swedish twins were invited to participate and the response rate was high. Moreover, this study adds to previous literature by assessing population-data on a large cohort, including NDP comorbidity, looking at sex differences, and analyzing the different subtypes of ADHD symptoms.

At age 9/12 symptoms of ADHD are most certainly best rated by parents compared to the children, and at age 15 it can be easier for the adolescent to evaluate the amount of physical activity. The questions assessing physical activity were designed by the CATSS authors and are not yet validated, although they have substantial face validity. While self-rated measures of physical activity often overestimate levels of physical activity, categorical answer modes, as used in the current study at age 15, have been shown to have superior validity as compared to more open answer modes [35]. While this self-assessment item has not been validated against device-based measures of physical activity, a similar single item showed similar correlation to devise-based physical activity (r = 0.44 and 95% CI = 0.24–0.63) as a longer questionnaire (r = 0.50 and 95% CI = 0.30–0.65) [36]. This variable was dichotomized based on the WHO recommendations, so that being active enough to fulfill the recommendations was compared to not fulfilling the recommendations. When evaluating a child’s physical and mental health at age 9, the parents are usually in a better position to judge this. Symptoms of inattention, hyperactivity/impulsivity and other neuropsychiatric problems are difficult for a child in that age to describe, and this is better assessed by adults in the child’s environment. However, at age 15 the situation is different. A teenager has more insight into his or her feelings and daily life, than the adults around. That is especially true concerning symptoms of anxiety, depression and physical activity throughout the day. Another strength of the study is that we adjusted for physical activity at age 9/12. There might still, however, be residual confounding, and future investigations might consider investigating also how childhood ADHD symptoms are related to sedentary behaviors such as screen time. However, future longitudinal investigations including device-based measures as a complement to the subjective measures of physical activity will be needed to confirm the assumption that the predictors of self-rated and device-based measures of physical activity are the same.

Our study has several limitations. First, changes over time can occur under the influence of many different factors. We have not been able to control for medication, as this type of data was not available for this cohort. Second, as this is a longitudinal study, our results only include cases where the parent (at baseline) as well as the child (at follow-up) participated. This limits the number of participants compared to a cross-sectional study. Previous research using the CATSS population has demonstrated that participants have a higher socioeconomic status compared to non-participants [16]. Hypothetically, a family with lower socioeconomic status may be less motivated to participate, which could explain some of the attrition. The results need to be interpreted with this possible bias in mind. Third, we use data from ages 9 and 12 at baseline. Twelve-year-olds are on the verge of puberty. This might possibly influence the expression of ADHD symptoms. However, age differences in ADHD level at baseline were small, why possible age effects should be small. To further investigate possible age differences, we included age as a covariate in our models. According to earlier analyses though, there are no differences between the age groups. Fourth, there was a lack of information on which parent that did the reporting, the exact age of the participants (year/month/day) and it was not possible to control for income in the analyses due to lack of this data.

Generalizability from a twin-study may be questioned, despite the fact that several studies have suggested that twins are representative of the population at large [37, 38], and also that monozygotic and dizygotic twins are similar in personality variation [39]. Using a cluster robust sandwich estimator is an often applied tool in analyses to adjust the standard errors for the nested twin data when computing regression models.

The finding that children with inattention symptoms are at increased risk of being less physically active in adolescence, illustrates the importance of encouraging children and adolescents with inattention to engage in physical activity. It also highlights the importance of adapting forms of physical activity to better suit children with symptoms of inattention. Children with hyperactivity symptoms were at decreased risk of self-rating as being less physically active in adolescence. Further research beyond the scope of this study is needed to challenge these findings, also using device-based measures. Furthermore, research in clinical settings including children diagnosed with ADHD, could be of great value in designing and evaluating suitable models of supporting health enhancing physical activity for children and adolescents with ADHD.