Promoting health-related cardiorespiratory fitness in physical education: A systematic review

Background This article aimed to systematically review the contribution of physical education (PE) classes to improve cardiorespiratory fitness (CRF) in children and adolescents; and to define potentially relevant factors for promoting CRF in PE classes. Methods Studies were identified from searches in ERIC, PubMed, SPORTDiscus, and Web of Science databases. Primary source articles, relating PE classes and CRF, published up to July 2019 in peer-reviewed journals were eligible for inclusion. Specific inclusion criteria were: (a) having cross-sectional or longitudinal and observational or interventional study designs; (b) targeting school-aged children or adolescents; (c) measuring CRF, heart rate or CRF test results as an outcome; (d) having statistical analyses of the CRF, heart rate or CRF test results outcomes reported; (e) focusing on PE classes or PE interventions that did not extended time or frequency of the classes; and (f) published in English, French, Portuguese, or Spanish. Results A total of 24 studies met the inclusion criteria. Overall, 10 studies have found a neutral effect of PE classes in students’ CRF, eight studies found that PE indeed contributed to the improvement of CRF and six studies revealed mixed findings, when PE classes where controlled for others variables (e.g. body mass index, intensity). Higher intensity PE classes consistently demonstrated contributions to improving students’ CRF. Conclusion Review findings suggest that PE classes can contribute to the improvement of students’ CRF. Intensity, age and weight status were identified as potentially relevant factors for promoting CRF in PE classes. To improve CRF, higher intensity classes should be provided.

Introduction Cardiorespiratory fitness (CRF) mirrors the overall capacity of the cardiovascular and respiratory systems [1]. It is considered as an important health variable, which is associated with several risk factors for cardiovascular diseases independent of socio-demographic factors, diet, and physical activity [2,3]. Furthermore, CRF is suggested to be a significant risk factor to include in the assessment of the metabolic syndrome for children and adolescents [4]. Hereby, the study of this variable and its associations to health is widely recognized as essential both among adults and youth.
The school setting gives youth the opportunity to be physically active, mainly through physical education (PE) classes [5]. For this reason, the school system is viewed as an important means of promoting physical activity and health among children and adolescents. When performed appropriately and incorporated as one component of a broad and holistic health education programme, fitness monitoring in PE serve as a valuable part of the curriculum and play a role in supporting healthy lifestyles and physical activity [6].
It has been suggested that PE classes may play a significant role in CRF development [7][8][9] and monitoring [10], as there are a number of field tests available that allow whole school classes to be assessed in one session [11,12]. Therefore, PE teachers have several quality tools to assess the students' CRF. Notwithstanding, evidence regarding the contribution of PE classes for the development of CRF in children and adolescents is inconsistent [13,14] and most studies examine school-based physical activity intervention programs [15] instead of curricular PE.
Although the school setting and PE classes offer a platform that might help for improving [7,8] and monitoring [10] of CRF, recent studies suggest that in the last decades CRF appears to have declined in children and adolescents worldwide [16,17]. Due to its importance, this evidence is of great concern. In order to begin reversing the decline in CRF, understanding how PE classes contribute to the improvement and maintenance of CRF in children and adolescents is vital. To the best of our knowledge, there is no study available that summarizes findings regarding the effect of PE classes on the CRF of students. Thus, the aims of this review were: (1) to summarize literature findings on the contribution of PE classes for improving CRF in children and adolescents; and (2) to define, based on this review, potentially relevant factors for promoting CRF in PE classes.

Study identification
Four relevant electronic databases (PubMed, ERIC, SPORTDiscus, and Web of Science) were comprehensively searched to identify peer-reviewed articles published up to July 2019. Definition of search terms was discussed among the authors. The identified search terms were: "physical education" AND cardiorespiratory OR cardiopulmonary OR cardiovascular OR endurance OR aerobic OR fitness OR PACER OR FitnessGram OR VO 2 OR "physical condition" OR "physical aptitude". Search terms were used in each database to identify potential articles with abstracts for review.

Study selection and selection criteria
Primary source articles, relating PE classes and CRF, published up to July 2019 in peerreviewed journals were eligible for inclusion. Specific inclusion criteria were: (a) having crosssectional or longitudinal and observational or interventional study designs; (b) targeting school-aged children or adolescents; (c) measuring CRF, heart rate or CRF test results as an outcome; (d) having statistical analyses of the CRF, heart rate or CRF test results outcomes reported; (e) focusing on PE classes or PE interventions that did not extended time or frequency of the classes; and (f) published in English, French, Portuguese, or Spanish. Articles that did not meet all the inclusion criteria were excluded. Titles and abstracts of the retrieved articles were independently assessed for eligibility for inclusion by two authors (AM, MP). Duplicates from the electronic database search were deleted. Full texts of all eligible articles were retrieved, and other possible relevant studies were searched in the references of those articles. Two authors (AM, MP) reviewed the text of potential studies, and decisions to include or exclude studies in the review were made by consensus.

Data extraction and harmonization
Based on the PRISMA statement [18] a data extraction form was applied. Relevant data was extracted from manuscripts by one author (MP); coding was verified by two other authors (AM, ERG). Divergences were discussed among authors and solved. Data extracted included study design, sample size, age, country, content of PE / intervention, outcome measure, method and main findings. Outcome measures were either a direct (e.g. VO 2 max) or indirect measure (e.g. number of laps) of CRF, or heart rate during exercise. Main findings are presented as a description of the contribution of PE classes to the CRF.

Study quality and risk of bias
The Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies [19] was used to appraise risk of bias (study quality). This tool comprises a 14 item checklist for longitudinal studies, while for cross-sectional studies only 11 items could be applied. According to the criteria, each longitudinal and cross-sectional study was rated either good (when meeting 10-14 and 8-11 criteria, respectively), fair (when meeting 5-9 and 4-7 criteria, respectively), or poor (when meeting 1-4 and 1-3 criteria, respectively). Study quality was assessed by two researchers (AM, MP) independently and discrepancies were discussed and solved by agreement.

Literature search
Fig 1 presents the flow diagram of studies through the systematic review process. The systematic literature search identified a total of 582 studies. Additionally, one study was identified through a manual search and added to the review process. Out of these 583 studies, 225 were duplicated and thus removed, resulting in a total of 358 studies for title and abstract screening. After excluding studies at the title and abstract screening (n = 268), 90 studies were eligible for full-text reading and 66 were excluded with reasons. Therefore, a total of 24 studies were identified as relevant and included in this review.

Risk of bias of included studies
Risk of bias of included studies was assessed by the Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies [19] and is presented in Table 1. Most studies (19 out of 24) were classified as 'fair', one study received a 'poor' classification, and the other four studies were considered 'good'.

Study characteristics
Study characteristics are summarized in Table 2. Included studies presented several designs (intervention, observational cross-sectional and observational longitudinal), outcome measures (VO 2 max, heart rate, test result), and methods to asses CRF. Studies from 14 countries were included, most of them with a mixed sex sample. Table 3 shows the main findings and characteristics of the studies included in this review. Included studies ranged from 1969 to 2017, demonstrating that scientific interest in the contribution of PE to promote CRF is close to 50 years old. Overall, 10 studies have found a neutral effect of PE classes in students' CRF, eight studies found that PE indeed contributed to the improvement of CRF and six studies revealed mixed findings, when PE classes where controlled for others variables (e.g. body mass index, intensity). Although 24 studies were included in this review some presented more than one relevant finding, therefore, 33 findings regarding the contribution of PE to the promotion of students' CRF are presented. This resulted in 16 findings indicating that PE did contribute to the improvement of students' CRF, whereas 14 findings point to a neutral effect and 3 findings suggesting that students' CRF decreased during   a given time period in a PE program. However, there was some heterogeneity in the study populations, as well as PE class characteristics included in this review that must be considered. Findings from younger students (n = 7) [13,[20][21][22][23][24], ranging from 6 to 12 years, showed mainly that participation in PE classes improved the students' CRF [13,21,22], and in two of these studies the improvements were due to high intensity (whether fitness oriented or game oriented) PE classes [23,24]. Notwithstanding, two other studies from this set concluded that PE had a neutral students' CRF [20,23]. On the other hand, findings from older students (n = 26), with a wider age range (approximately 11 to 19 years), were mixed. A total of 15 findings from 14 studies showed that PE had a neutral effect on students' CRF (n = 15) [14,22,[25][26][27][28][29][30][31][32][33][34][35][36]. While, 11 findings from nine studies reported that PE had a positive effect in children's and adolescents' CRF [26][27][28][36][37][38][39][40][41].   Studies concluding that PE classes had a neutral effect on students' CRF are supported by two main findings: (1) children and adolescents participating in PE classes did not improve or decreased their CRF during a given time period (n = 7) [14,22,[26][27][28][29]35]; and (2) PE classes did not provide sufficient intensity for achieving an aerobic benefit (n = 5) [28,30,31,33,34]. Besides these findings, two other studies found that students participating only in PE classes had lower CRF levels than their peers participating in school-sponsored sports programs [32] and in extracurricular organized physical activities [25].

Main findings
Almost all studies (n = 10) with findings indicating that PE contributed to improving CRF in students are related with the intensity level of the classes. Six studies indicated that high intensity PE classes, involving fitness activities or aerobic training, improved the students'

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CRF in a given time period [26][27][28][37][38][39]. Furthermore, three other studies showed that high intensity and fitness oriented PE classes had more than 50% of time spent in moderate-to-vigorous physical activity (MVPA) [28,40,41]. Finally, one study identified fitness activities as the greatest provider of time in MVPA, compared with individuals games and team games [40].
One study [36], in which the analysis was divided into three groups according to body mass index classification, showed that: while normal-weight and overweight girls enrolled in PE showed improvements in fitness, as well as maintenance of these improvements; obese girls, enrolled in the same PE program, did not.
From the review of these results, three potential relevant factors for promoting CRF in PE classes were identified: students' age, PE classes' intensity, and students' weight status.

Discussion
This review summarizes literature findings from studies published up to July 2019 on the contribution of PE classes for promoting CRF in children and adolescents. Twenty-four studies were included and systematically reviewed. Overall, this review revealed that findings regarding the contribution of PE classes to the promotion of CRF are mixed. Several findings suggested that PE has a neutral effect on students' CRF, while others reinforce its importance. However, higher intensity PE classes consistently demonstrated having a positive contribution in promoting students' CRF. Additionally, some other potentially relevant factors for promoting CRF in PE classes were identified, such as age and weight status. Review findings are discussed accordingly to these factors.
All studies were focused on school-aged children, however, due to the wide age range of the studies' populations, findings were organized in two age groups. This separation enabled some differences in findings between younger and older students to be found. While for older ages PE seems to be less effective in promoting students' CRF [14,22,[25][26][27][28][29][30][31][32][33][34][35][36], for younger ages almost all studies suggested that PE classes improved the students' CRF [13,[21][22][23][24]. From a physiologic standpoint, CRF naturally increases as children grow-up. This increase is fairly linear in boys until later adolescence, whereas in girls it plateaus around age 13 [42,43]. Furthermore, during the early stages of adolescence, participation in physical activity and the corresponding physical fitness begins to show some decline [44]. Thus, increasing CRF, related to body growth, occurring at younger ages and decreasing participation in physical activity in older students may explain why improvements in CRF for a given period of time are more frequently found in younger children and adolescents. Another possible reason for the apparently less effective contribution of PE to the improvement of CRF in older students is motivation. Motivation to participate in PE seems to decline in the late elementary and high school years [45,46], possibly resulting in decreasing physical activity both during PE and in leisure time. Considering these findings, PE may have a bigger role to play in promoting older students' CRF than it does in younger students, through providing MVPA opportunities.
Aerobic exercise has been shown to increase CRF by about 5-15% in youth [42,47]. Additionally, improvements in CRF, involving structural and functional adaptations, as well as in the oxidative capacity of skeletal muscle occur with regular MVPA participation [42]. In this review, five studies reported that PE classes did not provide sufficient intensity for achieving an aerobic benefit [28,30,31,33,34] and thus, did not contribute in a consistent manner to promote students' CRF. It is clear that CRF in youth increases with activity of sufficient intensity, leading to improvements in maximal stroke volume, blood volume, and oxidative enzymes after exercise [48]. Consequently, time spent in MVPA during PE classes should be monitored and adequate to promote health. Also, findings suggesting that PE has a positive contribution in improving CRF were mainly related to the intensity level of the classes. The majority of studies examined in this review involved intervention programs built to increase PE class intensity without increasing the number of classes or curricular time dedicated to PE [23, 26-28, 37-39, 41]. In fact, four of these studies reported that students participating in PE classes from the intervention programs increased their CRF levels, while students participating in regular PE classes, i.e. classes that were not part of the program, decreased or maintained their CRF levels [23,[26][27][28]. Considering the importance of CRF for health, the lack of intensity in PE classes is worrying. From a public health perspective, PE has the potential to provide the tools to face the current youth obesity and sedentary epidemic [49]. However, in order to effectively contribute to CRF and health, it is urgent to find strategies to increase the intensity of PE classes.
One study [36], examining whether an eight months PE program improved students' CRF when considering body mass index categories, suggested that although normal-weight and overweight girls showed improvement in fitness and maintenance of these effects over the next two years, their obese peers did not. Physical activity and body mass index are inversely correlated in children and adolescents [50]. Also, studies of usual physical activity in children suggest that the overweight and obese are less active [51,52] and have poorer fundamental movement skills than their normal-weight counterparts [53]. Mastering of fundamental motor skills is strongly related to physical activity in children and adolescents and is critical to fostering physical activity since these skills are the foundation for advanced and sport-specific movement [54]. Furthermore, a recent meta-analysis found that overweight and obese students were, respectively, 27% and 54% more likely to have school absenteeism than their normal weight peers [55]. All these factors may contribute to a greater ineffectiveness of PE programs among obese students. Therefore, PE should not only provide sufficient intensity to promote health, but also be based on developmentally appropriate motor activities to nurture self-efficacy and enjoyment and encourage ongoing participation in physical activity.
High quality PE together with appropriate approaches to fitness as part of health education programmes has been shown to promote fitness and healthy lifestyles [6]. On the other hand, when applied inappropriately and without context fitness monitoring can have the opposite result [56]. Therefore, mixed findings found in this systematic review may be due to the variations in the quality of PE. It is important that high quality PE is provided and promoted in schools, as it benefits not only students' CRF, but also promotes future healthy lifestyles.
The present review has some limitations that should be acknowledged. Even though study bias was assessed according to their methodological quality [19], they were not weighted or ranked, thus, findings from studies with poorer quality and smaller sample sizes were given no less importance than other findings. Nonetheless, only one study was classified as 'poor' quality. Additionally, included studies had a wide publication date range, from 1969 to 2017, and were from different cultural and socioeconomic contexts, which could have implications on what PE classes represent as well as PE curricula. Also, 16 studies were not accessible to the authors. Grey literature research was not included. Despite the developed efforts for reducing publication bias by the authors, this should be taken into account.
Notwithstanding, to the best of our knowledge, this is the first systematic review focused on the contribution of PE classes or PE interventions that did not extend time or frequency of the classes to the improvement of students' CRF. Furthermore, an extensive research strategy comprising four different databases and several keywords was used. Future research should continue to investigate the contribution of PE to the improvement of CRF and other fitness attributes, and examine which curricula offers the best opportunity to improve fitness, health and promote life-long physical activity behaviours.

Conclusions
Review findings from the 24 included studies suggest that PE classes can contribute to the promotion of students' CRF. Some potentially relevant factors for promoting CRF in PE classes were identified, such as intensity, age and weight status. Exercise intensity is essential to promote CRF and other health outcomes in youth [42,57,58], thus it is not surprising that higher intensity PE classes demonstrated improvements in CRF. Findings from studies of younger students more consistently reported improvements in CRF than findings from studies of older students. Therefore, as older students may be more vulnerable to decreasing physical activity levels [44], PE should be keen in providing tools and opportunities to improve and maintain CRF levels in these ages. Regarding weight status, overweight and obese students should be a priority concern, as they may have more difficulty in improving CRF than normal-weight peers. Given that CRF is an independent health predictor and that decrease of CRF is a global trend, more efforts should be done to promote CRF in PE classes. High quality PE is needed as it can be a successful strategy in improving CRF levels.