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Active Commuting and Physical Fitness: A Systematic Review.

Physical fitness (PF) is considered an excellent biomarker of health. One possible strategy to improve PF levels is active commuting. This review, performed accordingly to the Preferred Reporting Items for Systematic Reviews guidelines includes scientific articles published in peer-reviewed journals up to December 2019 that aim at examining the relationship between active travel/commuting and PF. The search was performed in three databases (PubMed, Scopus, and Web of Science). Sixteen studies were included in this review. Findings from the 16 studies were unclear. From the eleven studies on children and adolescents screened, eight were cross-sectional, one prospective cohort, one quasi-experimental, and one experimental. From the five studies on adults, four were experimental and one cross-sectional. Body mass, waist circumference, skinfolds, fat mass, cardiorespiratory fitness, upper and lower strength tests were performed in children, adolescents, and adults. Agility and speed tests were performed only in the young age groups. Majority of the investigations on young ages and adults have shown positive effects or relationships between active commuting and several attributes of PF. However, to avoid misconceptions, there is a need for future robust investigation to identify potential mediators or confounders in this relationship. More robust investigations are essential to understand how and whether decision-makers and public health authorities can use active travel/commuting as a strategy to improve PF in all ages.


Introduction
Physical inactivity is one of the main risk factors for mortality worldwide [1,2]. Therefore, there is a global need to promote strategies to increase physical activity (PA) levels. PA can be performed in several contexts such as work, organized sports, recreational activities, home activities, and active travel/commuting [2][3][4][5]. Active travel/commuting is an ecological and non-motorized transport mode for all ages, which can be characterized by a form of displacement through PA from/to home and workplace/school. Active commuting increases individual energy expenditure and is easy to incorporate in normal daily routines [6,7]. Active travel/commuting, such as cycling or walking, seems to be an effective strategy to improve daily PA levels; however, it might also improve physical fitness instruments for assessing active travel or active commuting, main results, and investigation quality.

Study Quality and Risk of Bias
The Quality Assessment Tool for Quantitative Studies checklist was used to assess the articles' quality [26]. This checklist includes 19 items, which assessed the following criteria: selection bias, study design, confounders, blinding, data collection methods, withdrawals and dropouts, intervention integrity, and analyses. The 19 items were divided in the 8 sections listed above and for each section a score of strong, moderate, or weak methodological quality was given. From the interpretation of the scores of each section, an overall score was given to each article. The study quality and risk-of-bias procedure was performed by two authors (S.G.; A.M.).

Synthesis of Results
This systematic review examined the association between PF and active travel/commuting in young and adult populations. A synthesis of the results and characteristics (such as, design, participant characteristics and sample size, measures, main results, and investigation quality) of each included article are presented. A narrative review of the included studies was performed.

Search Results
A total of 1313 articles were identified during the search. Of those, 683 were identified as duplicates, resulting in 603 articles for the title and abstract screening. In this phase, 27 articles were extracted for full text read, from which 11 were excluded for not meeting the inclusion criteria, namely: three were not focused in active travel/commuting, two were systematic reviews, four were cited in previous systematic reviews, and two were written in Korean or Japanese. Thus, 16 articles were identified as relevant. The flow chart of study selection is presented in Figure 1.

Investigation Characteristics
Tables 1 and 2 present the characteristics of the studies for children/adolescents and adults, respectively. Seventeen articles were included for final qualitative analysis, with 11 studies targeting the young population, i.e., children (up to 13 years old) and adolescents (up to 18 years old), and five

Investigation Characteristics
Tables 1 and 2 present the characteristics of the studies for children/adolescents and adults, respectively. Seventeen articles were included for final qualitative analysis, with 11 studies targeting the young population, i.e., children (up to 13 years old) and adolescents (up to 18 years old), and five studies that focused on the adult population. Every study was either focused on children/adolescents or on adults. None of the included studies were focused on children/adolescents and adults at the same time. All studies were mainly completed in Europe, for both populations.

Children/Adolescent
From the 11 studies focused on children/adolescents, four were performed in Spain, two in England, two in Norway, one in Sweden, one in Brazil, and one in Colombia. Furthermore, eight were cross-sectional [3,12,[27][28][29][30][31][32], one prospective cohort [33], one quasi-experimental [34], and one experimental [35]. The CRF was the PF attribute assessed the most (nine studies), while MF was assessed in three studies [3,27], and agility in two studies [3,31]. Only one study assessed the speed [31]. Active commuting was reported by the participants in most studies (ten studies), except for one study in which the parents reported how their child usually went to school [29]. Distance from house to school, used in one study, was calculated by Google Maps. Seven studies showed a positive association between PF levels and active commuting, mainly in participants who cycled. Two studies observed a positive effect of active commuting on PF attributes assessed in girls but not in boys, and the other five studies found no relationship between active commuting with body composition, CRF, upper strength, and lower strength. Only two studies reported results related to body composition variables. From 11 investigations subjected to methodological quality analysis, one strong, nine moderate, and one weak investigation were identified (Table 1).
In six studies activity, commuting was positively associated with PF [12,27,28,30,33,35]. Active commuting by cycling improves CRF [12,30,35], body composition [12], and muscular strength [27]. One experimental investigation [35] concluded that active commuting improves the CRF, while another quasi-experimental investigation [34] did not find associations between active commuting and PF. The only prospective investigation screened showed that active commuting by cycling in children over a span of six years, increased the PF in 14% [33]. On the other hand, in four studies an association between active commuting and PF was not observed [3,29,32,34]. In one study, mixed results were observed. Girls who actively commuted to school showed better levels of upper limb strength and velocity. However, non-significant associations were observed in boys [31].

Adults
Results of the studies in adult are presented in Table 2. For adults, five studies were identified: two in Denmark, one in Belgium, one in Finland, and one in Switzerland. From these five studies, four were experimental and two cross-sectional. The CRF was assessed in all studies, while MF was assessed in only one study. Variables of active commuting were assessed mainly by self-report, GPS (global position system), and Google Maps. From the 5 studies with adults, two were classified as having strong methodological quality, one moderate, and two weak methodological qualities. In general, a positive effect of active commuting on PF attributes was observed. Cycling to work has the potential to increase physical performance in an untrained people [36], and bicycle commuting improve CRF and reduced body fat [13,24,37]. Four weeks of active commuting can lead to improvements in CFR [38].

Discussion
The aim of this systematic review was to examine the association between PF and active travel/commuting in young and adult populations. Studies published until December 2019 were identified according to the inclusion criteria. A total of 16 studies were systematically reviewed. Some studies, in young and adult samples, demonstrated that active commuting is related to PF levels. However, in young populations, four studies did not find positive effects of active commuting on PF levels. Overall, the results between active commuting and PF levels in adults seem to be more consistent than those at young ages.

Children/Adolescents
Firstly, CRF is an attribute with a higher genetic component and increases only 8%-9% with three weekly bouts of 20 min of PA, at 80%-90% of maximum heart rate, for 10-12 weeks [39]. Secondly, some tests are flawed, with respect to the estimation of peak VO 2 in mL/kg/min at young ages [40]. Other factors that can explain the non-association between active commuting and CRF in young ages are as follows: The age of participants, mode of active commuting (e.g., walking or cycling), lower active commuting distance, high-deprivation neighborhoods, frequency of commuting, and the overall amount of time spent being an active commuter [12,28,30,33]. When examining the association between active commuting and MF, the authors observed that cyclists had higher handgrip strength and walkers had higher vertical jump peak power when compared with non-active commuters [27]. Furthermore, positive association was observed between active commuting and upper limb strength in girls but not in boys [31]. This result is crucial to retain, in order to understand the various impacts of active commuting on girls and boys. Different results in PF between girls and boys can be explained because the girls usually have lower levels of PA and MF than boys. The specific type of physical exercise (walking or cycling) promotes specific physiologic adaptations, which act in several intensities on several attributes of PF [41,42].
The positive association between active commuting and CRF in prospective study [35] is in accordance with previous investigations, which concluded that promoting active commuting to school among children and adolescents may be a useful strategy to improve CRF and other health outcomes [11,43,44]. Our discoveries highlighted the variability of the investigations' results, which can be explained by different methodologies applied and/or non-control of other confounders. The social environment and the neighborhood characteristics are crucial factors to be considered when promoting active commuting in youth [30]. Overall, active commuting is associated with healthier levels of PF among youth. The findings from this systematic review concur with those from previous systematic reviews [18,19].

Adults
From the five studies screened, a positive association between active commuting and several PF attributes was observed. A cross-sectional investigation showed a positive association between active commuting and metabolic health, along with the beneficial impact on the promotion of the PA levels in adults. However, no association between active commuting and CRF and MF was found [13].
Four intervention investigations used cycling to analyze the potential positive effects of active commuting on PF levels. After one year of experimental investigation, the authors concluded that cycling to work had a positive effect on CRF levels on the intervention group [36]. The experimental investigation, performed over six months, showed active bike commuters presented better CRF than the non-active commuting group, but not when compared with the group that performed vigorous PA in leisure time [24]. Both active groups had similar improvements [24]. However, it seems that for the same period of intervention, the intensity of PA plays an essential role in the improvement of CRF [45]. The majority of studies performed in adults indicated that active commuters had greater cardiovascular fitness, especially those who cycled to and from work. Cycling to and from work seems to be an essential tool to reduce the time required to expend a given quantity of energy. Additionally, high-intensity physical exercises seem to be a fundamental exercise component to increase CRF, and they serve as a protective factor against several metabolic diseases [24,45,46]. Active commuting improved the cardiometabolic health and CRF in both groups, but with slow effects in the active commuting group when compared with the leisure-time vigorous-intensity group [24]. Previous investigations have shown the impact of lifestyle exercise on adiposity, while the changes in CRF seem to be more dependent on the exercise intensity [45,47].  Several limitations have been identified in these investigations, which can explain the results. However, it was possible to identify potential mediators or confounders, which can influence the relationship between active commuting and the results of several attributes of PF [13,23]. Although most studies examined the effects of active commuting and non-active commuting, an experimental investigation analyzed the effect of E-Bike versus bike commuting on CRF in overweight adults, which concluded that both bike systems increased CRF levels, even if the bikes were electrically assisted [38].
Ambiguous or inconsistent results observed mainly among children and adolescents in this systematic review can be the result of the several methodologies used to access PF attributes, the type of active commuting, the type of population, the geographic area, and the environmental context. Additionally, the measures of active commuting extensively varied (e.g., frequency, duration, and distance) among the investigations screened in this systematic review. Due to all these factors, rigorous comparison among investigations is highly limited.
This systematic review is not without some limitations. Firstly, active travel/commuting was self-reported in all the included studies. This may be subject to bias, especially at young ages. Secondly, most investigations do not control the intensity of active commuting. Thirdly, studies were mainly focused on a specific world area in high-or mid-income countries. Finally, the terms selected to identify investigations that examined active commuting and PF could have excluded several articles (e.g., ones in which the predefined terms were found neither in the title nor the abstract).
Overall, the results seem to indicate that active travel/commuting and PF are positively associated in young and adult population. Walking and cycling are common modes used by young active commuters. Young cyclists had higher CRF level, while walkers had better MF levels. In adults, cycling is the principal mode of active commuting and is associated with greater PF levels, especially CRF. Active commuting by cycling increases the intensity level of physical activity and seems to be an excellent strategy to improve the PF levels. These findings highlight that active commuting promotes health status. However, for all entities who promote active commuting, it is essential considering factors such as age, sex, and environment at the moment to select the adequate active commuting mode.

Conclusions
Findings from this review suggest that among younger ages, active travel/commuting is inconsistently related to PF and that several factors should be considered to compare the effectiveness of active commuting in improving PF outcomes in children and adolescents. Findings of studies in adults demonstrated the positive effect of active commuting on CRF and body composition. Overall, most studies have shown a positive relationship between active commuting and several attributes of PF. Additionally, active commuting, by cycling, seems to have a more positive impact on several PF attributes. However, there is a need to identify potential mediators or confounders in this relationship, in order to avoid misconceptions. More investigations on this topic are essential to understand how and whether decision makers and public health authorities can use active travel/commuting as a strategy to improve PF in all ages. In order to achieve that, it is important that future investigation pursue a more detailed approach when examining active travel/commuting, especially taking into account its context and specificity.

Conflicts of Interest:
The authors declare no conflict of interest.