Recent Trends in Sedentary Time: A Systematic Literature Review

This paper systematically reviews and synthesizes the relevant literature on sedentary time research. A bibliometric analysis was conducted to evaluate the publications from 2010 to 2020 in the Web of Science (WoS) core collection database. Derwent Data Analyzer software was used for the cleaning, mining, and visualization of the data. Historical trends of the topics, main contributors, leading countries, leading institutions, leading research areas, and journals were explored. A total of 3020 publications were studied. The United States, the United Kingdom, and Australia are the three most productive countries. The Australian institution Baker Heart and Diabetes Institute led the list of productive institutions, and Ekelund U published the most papers. Sedentary time raised the concerns of scholars from 106 research areas, and public health was the dominant field. Physical activity, accelerometer, children, and obesity were the most frequently used keywords. The findings suggest that sedentary time is rapidly emerging as a global issue that has detrimental effects on public health. The hotspots shifted in the past 10 years, and COVID-19 was the most popular topic of sedentary time research.


Introduction
Sedentary time is rapidly emerging as a global issue that has detrimental effects on public health [1]. Sedentary behavior is defined as any waking activity with very low (≤1.5 MET) energy expenditure [2]. It is typically characterized by time spent sitting or screening in various domains of life, including leisure [3,4], occupation [5,6], and transportation [7,8].
The first serious discussions and analyses of sedentary time emerged in the 1960s. A study published in 1965 sought to determine the relationship between the sedentary time and blood pressure of railroad employees aged 40 to 49 [9]. The effect of exercise on systolic time intervals in sedentary individuals and rehabilitated patients with heart disease was studied in 1971 [10]. The results of a study of the epidemiology of brain infarction associated with occlusive arterial disease showed that sedentary living may be a cause of atherosclerosis [11]. Over the past decades, scholars worldwide have employed epidemiological research methodologies or combinations of experimental designs [12,13], such as cross-sectional studies [14][15][16], longitudinal studies [17][18][19], cohort studies [20][21][22], intervention studies [23][24][25], and quasi-experimental studies [26][27][28], to develop population-based research. Accelerometer-based measurements or self-reported methods are widely used to assess the attribution of sedentary time and light or moderateto-vigorous physical activity, and samples are taken across all ages (children, adolescents, adults, and old adults), genders, countries, and socioeconomic subgroups [29][30][31][32][33]. Previous research has identified that sedentary behavior and physical activity are determined by or correlated with individual socioeconomic status, the environment, and related health policies [34][35][36]. The relationship between a sedentary lifestyle; the amount, intensity, and frequency of physical activity and diabetes, cardiovascular disease, metabolic syndrome, obesity, cancer, and all-cause mortality has been deeply researched and analyzed [37][38][39][40]. Excessive uninterrupted sedentary time is strongly associated with an increased risk of many chronic diseases [41][42][43][44][45]. It is well recognized that approaches to prevent such diseases and promote health benefits should seek to both increase regular physical activity and decrease sedentary behaviors [46].
Quite a few meta-analyses and systematic reviews of observational studies have previously examined the relationship between sedentary time and unfavorable health outcomes [47]. Researchers have reviewed studies on the pattern of sedentary behavior among older adults in care facilities [48], the effectiveness of physical activity and sedentary behavior interventions in reducing sedentary time in adults [49,50], the necessity of reducing occupational sedentary time among general practitioners and other occupations [51,52], and the effects of sedentary behavior interventions on biomarkers of cardiometabolic risk and asthma [53,54]. However, there is little information available in the literature that presents a distinct perspective by conducting bibliometric analysis to summarize the overall view of the sedentary time research field. Bibliometric analysis is widely used as a valid tool to quantitatively evaluate the distribution of active countries, institutions, authors, and collaborations, and find the hotspots and the research trends in various research areas such as chemistry [33], mechanical engineering [55,56], energy [57], optics [58], software engineering [59], medicine [60][61][62], health [63], economics [64,65], art [66], management [67], urban planning [68], social work [69], etc.
This paper aims to provide a general overview of the sedentary time research area organized as follows: (1) leading countries, institutions, authors, journals and research areas; (2) collaboration patterns between countries and institutions; and (3) research trends and hotspots. This will be accomplished by analyzing author keywords and highly cited papers.

Methodology and Data Source
Bibliometric analysis is a statistical evaluation of published papers and academic research. Different from systematic review papers, bibliometric methods can analyze massive papers, and show the overall picture of sedentary research from the perspective of the literature. The analysis was based on publications related to "sedentary time" published during the 2000-2020 period. The data were obtained through the Web of Science (WoS) core collection database in May 2021 using a retrieval query of "sedentary time", searching the "topic" field, and defining the document type as "article and review". There are some other related keywords in sedentary time research, but in the pre-investigation, we found that the core literature is within the scope of "sedentary time" retrieval, whereas there are quite a few irrelevant documents in the retrieval results of "sedentary behavior", "sitting time", "screen time", and "sedentary lifestyle". Finally, 3060 publications were collected from the Science Citation Index-Expanded (SCI-E) and Social Science Citation Index (SSCI). After analysis of the yearly outputs, we found that the number of publications in the first 10 years was extremely small and unstable. Therefore, this study limits the scope of the research to 3020 studies published in 2010-2020. Other related publications may have been excluded as a result of the search restrictions mentioned above. We excluded some publications related to ecology, veterinary sciences, energy fuels, marine freshwater biology in the WOS research field. Papers originating from England, Scotland, Northern Ireland and Wales were categorized under the United Kingdom. The impact factor (IF) for each journal was acquired from the 2019 Journal Citation Reports (JCR). The Derwent Data Analyzer (DDA10.0 build 27,330, Search Technology Inc., Norcross, GA, USA), a statistical analysis tool applied for data cleaning, data mining, and data visualization, was used to process and analyze the data extracted from the 3020 publications and form figures. All publications referring to sedentary time during 2010-2020 were assessed for the following bibliometric indicators: publication outputs, countries, international collaborations, institutions, research areas, journals, authors, most cited papers per year, and author keywords.
It is worth mentioning that some related papers may not be involved in the results. If there were no required words in the title, abstract, or keywords, some related papers could be ruled out. This issue may produce some deviations and certain limitations.

Results
From 2010 to 2020, 3020 publications were contributed by 120 countries/regions to the "sedentary time" research field. Of these 3020 publications, 88 were Essential Science Indicators (ESI) highly cited papers, and 3 were ESI hot papers. The growth trend of sedentary time research over time is presented in Figure 1. The yearly number of publications increased from 32 (2010) to 537 (2020). The total number of studies published on this topic has increased more than 15-fold. The results show that about three-quarters of the literature was published in the last five years (2016-2020), which indicates that sedentary time is an important risk factor impacting public health, resulting in widespread concern among scholars worldwide.

Number of Publications and Citation
The 3020 publications were from 87 countries/regions. The United States, the United Kingdom, and Australia were the top three most productive countries/regions. The USA was the leader of this field and published 914 sedentary time research papers since 2010. This number was 30% of the total number of publications. The United Kingdom and Australia were also productive in sedentary time research. As shown in Figure 1, the yearly outputs of the three countries/regions grew rapidly between 2011 and 2020.
The number of publications and citations of the top 20 most productive countries/regions in terms of the number of publications related to sedentary time research can be found in Table 1. Of the 20 most productive countries/regions, 13 were from Europe, 3 were from the Americas, 2 were from Asia, and 2 were from Oceania.  (111), and Germany (109). In terms of publishing influence, the United States led the list of total citations (TCs) with 21,980. Despite the high number of publications and citations from the United States, the average citations per publication (ACPP) was relatively low, at only 24.05. Canada led the ACPP rankings at 31.39, and China was in next-to-last place (13.81).

Cooperation of Countries/Regions
Publications were defined as internationally cooperative if the paper was coauthored by researchers from more than one country [70]. As shown in Table 1, of the top 20 countries' publications, a large proportion were internationally cooperative, especially for Portugal (86.07%) and Belgium (84.71%). This demonstrated that sedentary time raised the concern of scholars worldwide who exchanged ideas with each other. In the 20 countries/regions, 13 European countries comprised a more significant share (over 60%) of papers with international co-authorship relationships. As for SP%, we also observed that the most productive country, the United States, was in next-to-last place (40.37%). The United States was the most active country that had partnerships with 63 countries, followed by the United Kingdom (60) and Spain (47).
The academic collaboration network of the 20 most productive countries/regions is shown in Figure 2. DDA software was used to draw the network diagram based on the co-occurrence matrix. The size of the circles is proportional to the degree of contribution each country. The lines among these circles represent the cooperation between countries/regions, and the thickness of the lines implies the total number of collaborative publications [71][72][73]. The 20 most productive countries/regions had intensive cooperation with the other countries/regions, especially the United States, the United Kingdom, Australia, and Canada, which were listed in the top four countries/regions.

Contribution of Leading Institutions
The top 20 most productive institutions in sedentary time research, along with their total numbers of publications, citations, and h-indexes, are listed in Table 2. Most of these institutions were from the top three productive countries/regions. Among the top 20 institutions, ten were from Australia; four were from the United Kingdom; and the United States, Belgium, Norway, Spain, the Netherlands, and Sweden each had one. The top three in the list were all Australian research institutions, with the top three h-indexes 47, 45, and 39, respectively. As for ACPP, Monash University located in Australia led the list with 61.39, Loughborough University ranked second with 59.44, and the University of Queensland from Australia was third with 58.58. It was clear that there were no institutions from developing countries. Comparing the research influences of the top 20 institutions that had a prominent role in developing and promoting the field, China, Brazil, and other developing countries still have a long way to go.  As shown in Figure 3, institutions from Australia, especially the Baker Heart and Diabetes Institute, the University of Queensland, Deakin University, the University of Melbourne, Australian Catholic University, Curtin University, the University of Western Australia, and Monash University, had a much closer collaborative network among the top 20 most productive institutions. In addition, cooperation among the University of Cambridge, the Norwegian School of Sport Sciences, Ghent University, and Vrije University Amsterdam located in Europe was particularly frequent. This was possibly attributed to the convenience of their geographical locations, promoting the development of collaboration.

Contribution of Leading Research Areas
The research field shown in Table 3 is derived from the core collection of Web of Science, a discipline classification system. Every journal and book covered by Web of Science core collection is assigned to at least one of the subject categories. Every record in Web of Science core collection contains the subject category of its source publication in the Web of Science Categories field. (The WOS research field is detailed in http://images.webofknowledge. com/WOKRS535R102/help/WOS/hp_subject_category_terms_tasca.html, accessed date: 22 July 2021). Sedentary time was a multidisciplinary field covering 106 research areas, and the top 20 research areas ranked by the number of publications are illustrated in Table 3. "Public, Environmental & Occupational Health" dominated the research area listed with 842 papers, followed by "Sport Sciences" (526) and "Nutrition & Dietetics" (359). The results show that the top three research areas comprised nearly 60% of the total publications. The topic was closely related to public health issues; therefore, it was conceivable that most of the top 20 WoS research areas involved various subdisciplines of medicine, and some subjects corresponded to the research of diseases caused by sedentary behaviors, such as "Endocrinology & Metabolism", contributing a share of 6.92%; "Pediatrics" (5.96%), "Geriatrics & Gerontology" (4.30%), "Cardiac & Cardiovascular Systems" (3.11%), "Oncology" (2.68%), etc. As for ACPP, "Medicine, General & Internal", "Physiology", and "Endocrinology & Metabolism" led the list with 43.93, 36.13, and 34.55.  Researchers from "Endocrinology & Metabolism", "Multidisciplinary Sciences", "Environmental Sciences", "Geriatrics & Gerontology", and other areas became associated with this field later, but the number of published papers grew faster. "Environmental Sciences" entered the table of the top three most productive research areas in 2020. Several research areas, including "Clinical Neurology", "Respiratory System", "Psychology", and others, had sustained output, but their growth was not significant.

Contribution of Leading Journals
Papers related to sedentary time were published in 612 journals. As listed in Table 4

Contribution of Leading Authors
Among the 11,249 authors who contributed to sedentary time research, 8195 authors published only one paper, and 9 authors published more than 50 papers. The top 20 most prolific authors based on the number of publications are presented in Table 5.   (36). Given that the h-index and ACPP, as a reflection of a publication's quality, can reveal the influence of an author in sedentary time research, it can be seen that Owen N., Dunstan D.W., and Healy G.N., with higher h-indexes and ACPPs, had more significant influences in the field [74].

An Analysis of Author Keywords
The author keywords provide critical information on the current research status and hotspots, and such keywords have been proven to play a key role in analyzing future development trends [75][76][77]. Overall, 3056 author keywords from 3020 papers were analyzed. It is crucial to mention that some publications that did not have author keywords may be excluded from statistical analysis. Among the author keywords, 2027 (66.33%) of the author keywords appeared only once, 429 (14.04%) were used twice and 165 (5.40%) were used three times.

An Analysis of Keyword Categories
The top 20 author keywords were chosen to show the relationship between topics ( Figure 6). Each node represented a keyword. The data next to the keywords represented the total number of publications from this topic. The lines between the keywords show the co-occurrence of keywords. As observed from the chart, we divided the top 20 author keywords into the following categories: 1.
The measurements used to monitor sedentary time and the effective way to change the pattern of sedentary behavior: physical activity/exercise, intervention, and accelerometry. Accelerometers are commonly used as a device to assess sedentary time; engaging in regular physical activity is widely regarded as a valid measurement to prevent a range of health risk factors across all age, gender, ethnic and socioeconomic subgroups [41,[78][79][80][81]. Some intervention studies aiming to increase physical activity or reduce sedentary time have also been conducted [82][83][84]; 2.
Related diseases: the majority of epidemiological evidence has adversely associated high levels of sedentary time and unhealthy sedentary lifestyle with an increased risk of chronic diseases, as listed in the keywords, including obesity/adiposity [92,101], type 2 diabetes [102,103], and cardiovascular disease [104]; 4.
Biomedical health indicators: body mass index (BMI) is used extensively to select sample ranges, limit sample conditions, and detect changes in sample populations before and after experimental studies [105,106].

An Analysis of the Most Cited Papers
Although the citation impacts of publications will be affected by means of variations [107], it is still a widely used measurement to evaluate scientific publications. The most cited publications in the sedentary time field by year during 2010-2020 are presented in Table 6. The most highly cited publication was "Too Much Sitting: The Population Health Science of Sedentary Behavior" [108] published in Exercise and Sport Sciences Reviews in 2010. It was authored by Owen N. et al., and it led the list of total citations with 1214. In terms of TCY, "Sedentary Behavior Research Network (SBRN)-Terminology Consensus Project process and outcome" authored by Tremblay M.S. et al. [109] from Canada ranked first with 203. Of the most cited publications, five papers had coauthors from institutions in more than one country. Four papers were from institutions in Canada and the United States. Researchers from the United Kingdom contributed to two papers. Institutions from Australia, Norway, New Zealand, Belgium, Brazil, Sweden, and the Netherlands also published highly cited papers. It is worth mentioning that the most cited papers for each year were published in top journals in the field of medicine and sports science, such as the International Journal of Behavioral Nutrition and Physical Activity [46], Diabetologia and Medicine and Science in Sports and Exercise [110].
In 2010, Owen N. et al. published a paper showing that sitting time, TV time, and time sitting in automobiles increase premature mortality risk [108]. Following this direction, the relationship between sedentary behavior and health indicators was the hotspot of sedentary time studies during the following years; researchers studied sedentary behaviors and subsequent health outcomes in children, adolescents, and elderly individuals. Researchers have studied the relationship between sedentary behavior and health indicators in children and youth and found that decreasing any type of sedentary time is associated with a lower health risk [111]. Researchers have also examined the association of sedentary time with diabetes, cardiovascular disease and cardiovascular and all-cause mortality; investigated the influence of sedentary behavior on cardiometabolic disease and discovered that regular activity breaks are more effective at decreasing postprandial glycemia and insulinemia [112]; compared physical activity and sedentary time; and studied the effects of sex, age, education, and body mass index [110].
In 2015, Biswas A. et al. published their paper which quantified the association between sedentary time and hospitalizations, all-cause mortality, cardiovascular disease, diabetes, and cancer [47]. Since then, researchers have begun to pay attention to the quantitative analysis of sedentary time and have promoted research in this field. Warburton D.E.R. et al. mentioned that sedentary time is associated with independent health risks and that physical activity should be part of an integrated approach to enhance healthy lifestyle behaviors [43]

Discussion
Eighty-seven countries/regions contributed 3020 papers to sedentary time research from 2010 to 2020, indicating that sedentary time is a global public health issue and attracting worldwide attention. In the past 10 years, the number of sedentary time research papers has increased by 15-fold. A group of researchers and journals focus on this field. The rapid increase in the number publications also revealed the necessity and urgency of sedentary time research.
Currently, increasingly more countries/regions have put efforts into the study of sedentary time. Western Europe, North America, and Oceania were the most active regions in terms of the number of publications. The top 20 countries in the list were all developed countries, except for China and Brazil. The possible reason is that developed countries with high levels of income pay more attention to public and individual health. This was further confirmed by the most active institutions and authors. The majority of the top 20 most productive institutions were from the United States, the United Kingdom, and Australia. Among the 11,249 authors who contributed to sedentary time research, none of the top 20 most productive authors were from China, Brazil, India, or other Asian, South American, and African countries.
The obvious change in the number on the bubble of the author keywords showed the trend of sedentary time research. Physical activity, accelerometers, children, and adiposity were core directions of sedentary time research which have concerned researchers for a long time. In recent years, the number of publications has maintained steady growth. The study of adolescents and old adults started later than that of children, and such papers began to appear after 2013. Currently, sedentary time research has formed the trend of synchronous development of research on children, adolescents, adults, and older adults. Additionally, related diseases such as type 2 diabetes and cardiovascular disease have also been the focus of sedentary time research in recent years.
An analysis of the highly cited papers by year can also conclude that the hotspots of sedentary time research have shifted many times in the past 10 years. Along with the shift of hotspots, significant progress has been made with the work of researchers. Quantitative analysis of sedentary time, including the standardization of sedentary behavior, has become the focus of research since 2015. Since 2020, billions of individuals have been asked to stay at home due to COVID-19, and COVID-19 has become a hotspot of sedentary time research. Stay-at-home orders bring sedentary lifestyles; the relationship among the physical activity, diet, and sedentary time of students, workers, and other groups was studied [116,117].
It is worth noting that sedentary time research involves a wide range of topics including TV time, sitting time, screen time, etc., which cross over with those of other research fields [118,119]. However, there are many differences between them. Sedentary time research is mainly from the public health perspective. Other research fields, such as exploring TV time, are based on theories and methods in market management [120], sociology [121], communication [122], and other disciplines.

Conclusions
In this study, we analyzed the sedentary time research literature published from 2010 to 2020 based on bibliometrics and the DDA software. The United States, the United Kingdom, and Australia were the three most productive countries/regions. Three Australian institutions, Baker Heart and Diabetes Institute, the University of Queensland, and Deakin University, lead in the table of the most productive institutions. Regarding the subject field, sedentary time research has distinct multidisciplinary characteristics, especially for public health. The top 10 journals in terms of the number of publications, such as BMC Public Health, the International Journal of Behavioral Nutrition and Physical Activity, and PLoS ONE, published 16.19% of the total sedentary time research papers. Ten researchers, represented by Ekelund U., Owen N., and David W.D., published more than 50 papers during the past 10 years.
Physical activity, accelerometer, children, and adiposity were the most frequently used words. In the past 10 years, sedentary time research has been conducted at various levels, with a series of discussions on the sedentary time of children, adolescents, adults, and elderly individuals. In recent years, researchers have sought to quantify the association between sedentary time and health indicators. Currently, COVID-19 is the most popular topic of sedentary time research.
This study can help potential scholars to better understand sedentary time research on a global scale, providing useful information for relevant scholars to further develop research in this field. Institutional Review Board Statement: Not applicable; the study did not involve humans or animals.
Informed Consent Statement: Not applicable; the study did not involve humans.
Data Availability Statement: All data generated or analyzed during this study are included in this published article.

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