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The Health Consequences of Physical Inactivity and Sedentary Behavior

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INTRODUCTION

Over the past century, technological advances have reduced the need for human movement throughout the day. Currently, fewer people in the United States participate in walking or biking for the purpose of active transport. Additionally, technological innovations have led to a lower level of required movement to perform activities of daily living and household management.

In the 1960s, more than half of all American workers had more labor-intensive jobs that require a significant amount of time spent in moderate- to vigorous-intensity activities (1). Over the past decades, those jobs have been largely replaced by jobs that require little movement. Currently, less than 20% of all jobs require moderate-intensity physical activity (20). Less active commuting and longer school days have also led to an increase in sitting among youth (79,119). In effect, we have moved from societies where a substantial amount of human movement was required for daily survival to societies in which the necessity for human movement is quite low, with the average American adult spending close to 8 hours of their waking time per day on average in sitting activities and less than 30 minutes per day on average in moderate- to vigorous-intensity activities (51,115).

High levels of inactivity and sitting behavior, as seen in the U.S. population, have been linked to numerous poor health outcomes, including, but not limited to, obesity, cardiovascular disease, diabetes, and certain cancers (27,66). Therefore, individuals for whom activity is not a compulsory part of the day may need to plan opportunities for physical activity that are not otherwise necessitated. Likewise, individuals who spend much of their time in sitting behaviors may benefit from activity plans that include strategies for reducing the amount of time they spend seated. This chapter continues the theme of the previous chapter and touches on both physical inactivity and sedentary behavior.

Physical Inactivity

Definition of Inactivity and Current U.S. Guidelines for Physical Activity

Physical inactivity describes individuals or groups of individuals who are not meeting a certain threshold of moderate to vigorous activity, often based on the established guidelines (41). The American College of Sports Medicine (ACSM) recommends moderate- to vigorous-intensity cardiorespiratory exercise training for ≥30 minutes per day on ≥5 days per week for a total of ≥150 minutes per week, vigorous-intensity cardiorespiratory exercise training for ≥20 minutes per day on ≥3 days per week (≥75 min ∙ wk⁻¹), or a combination of the two to achieve a total energy expenditure (TEE) of ≥500–1,000 metabolic equivalent (MET) minutes per week. Additionally, resistance training, neuromotor, and flexibility exercises are also recommended (38). The Centers for Disease Control and Prevention (CDC) also recommends 60 minutes per day or more of moderate to vigorous activity for children and adolescents (ages 6–17 yr) (92). As seen in the previous chapter, there are many positive health effects of physical activity. As a result, physical inactivity is considered a risk factor for numerous poor health outcomes (66).

Deconditioning

Significant reduction or cessation in exercise and increases in inactivity results in partial or complete reversal of the physiological adaptions to exercise, known as deconditioning. Common causes for deconditioning are reductions in typical levels of physical activity, aging, casting, paralysis, and bed rest.

The process by which adaptations to exercise are gradually lost or reduced is commonly known as detraining. Because detraining relates specifically to exercise, it is the form of deconditioning that is most relatable to inactivity. The effects of detraining are most notable in muscle tissue, including the heart (50,86,110). Reduced metabolic function can also happen quickly in response to reductions in exercise (86,121). Additionally, decreases in exercise that are maintained over a longer period of time, in the form of an inactive lifestyle, can result in decreased bone mineral density, metabolic dysfunction, and neuromuscular complications (66,71). As a result, inactivity is linked to numerous poor health outcomes including certain cancers, diabetes, dyslipidemia, hypertension, immune deficiencies, metabolic syndrome, neurological disorders, depressive disorders, osteoporosis, overweight/obesity, oxidative stress, and sarcopenia, among others (66). Additionally, the World Health Organization (WHO) had estimated that inactivity is responsible for 3.3 million deaths a year, making it one of the most important underlying causes of death worldwide (126).

Prevalence of Inactivity

Rate estimates for physical inactivity vary across populations and may even differ within the same population depending on the measurement instrument used to capture physical activity. In the U.S. population, self-reported estimates of physical activity from questionnaires suggest that 45%–75% of adults and youth from large population-based surveys are inactive based on the CDC guidelines (32,62,74,116). Estimates of inactivity based on objectively collected data tend to be higher than those based on self-report. Accelerometer data collected from the National Health and Nutrition Examination Survey (NHANES) between 2003 and 2006 suggested that 58% of youth 6–11 years, >90% of adolescents aged 12–19 years, and >95% of adults aged 20 years or older would be considered inactive based on the CDC guidelines (115). Other studies using objective measures have reported similar results (60,116).

It is worth noting that differences in estimates of inactivity between subjective measures (like self-report recall questionnaires) and objective measures (like pedometer and accelerometers) may be due in part to inherent biases that differ across instruments (49,111). Although there are many validated self-report measures, these instruments are more prone to overestimate higher intensity activity and underestimate lower intensity activity due to various types of reporting bias (49). Although accelerometers are generally considered the most valid instrument for assessing time spent in different intensities of activity, they can underestimate nonambulatory activities such as bicycling, swimming, and upper body resistance training (depending on monitor placement) (111). Despite quantitative differences, the general agreement between measurement instruments provides stronger evidence to support the finding that the majority of Americans could be classified as inactive. With similar results being reported in other countries (44,67), reducing the prevalence of inactivity in the population has become a major area of focus in the chronic disease prevention efforts of the CDC and the WHO (15,125).

Exercise is Medicine

In response to the high prevalence of inactivity worldwide, the ACSM spearheaded the Exercise is Medicine® (EIM) (3) program as a global initiative to decrease population levels of inactivity. EIM calls for clinical inactivity screening and encourages physicians and other health care providers to include physical activity when designing treatment plans for patients by referring patients to evidence-based activity programs. The main premise behind the EIM initiative is based on the existing evidence for the importance of reducing physical inactivity in the prevention and treatment of certain health conditions and diseases. The primary stakeholders in the EIM campaign are health care providers, exercise professionals, and community resources (Table 2.1). Large initiatives, like EIM, hope to reduce rates of inactivity by better identifying individuals who may benefit most from increasing activity and by making activity-related resources more accessible.

Sedentary Behavior

Definition and Measurement

The remainder of this chapter focuses on another component of movement that is separate from exercise and physical activity: sedentary behavior. First, it is necessary to establish the meaning of the term sedentary. Currently, two different definitions of sedentary continue to be used in the literature. In one definition, individuals or groups of individuals who are not meeting physical activity recommendations have been referred to as sedentary (10,89,91). Operationally, this definition would be the same as physical inactivity, and the two terms have been used interchangeably. A more recently established definition that better distinguishes sedentary from inactive would be those individuals or groups who perform high amounts of low energy expenditure activities, which have been referred to as sedentary behaviors (113). In this way, a person could be both sedentary and inactive but would not necessarily have to be both. Distinguishing the two terms is important because it allows for a greater understanding of the health consequences of being inactive versus being sedentary (vs. being both inactive and sedentary).

Sedentary behaviors have been more formally defined as waking behaviors that take place while in a sitting or reclined position and result in an energy expenditure of ≤1.5 METs; where 1 MET is the energy cost of resting quietly (often defined in terms of oxygen uptake as 3.5 mL ∙ kg⁻¹ ∙ min⁻¹) (89,91,113). This definition focuses on both the postural and energy expenditure aspects of sedentary behavior while differentiating it from sleep. Based on this definition for sedentary behavior, light-intensity activity would then be defined as seated and nonseated activities with an energy expenditure between 1.5 and 3.0 METs.

Historically, feasibility concerns with direct observation have caused health-related studies of sedentary behavior to rely heavily on subjective self-report questionnaires which have been shown to do reasonably well at capturing certain domain-specific sedentary behaviors, such as watching television, sitting at a computer, or sitting while commuting. With the wider availability of objective measurement instruments, including accelerometers and inclinometers, it has become possible to gain more accurate and precise estimates of the total time a person spends in sedentary pursuits. Tremblay et al. (113) have suggested that sedentary behavior should be described in accordance with the SITT formula (similar to frequency, intensity, time, and type [FITT] for exercise) which includes Sedentary behavior frequency (number of bouts), Interruptions (or breaks), Time (or duration), and Type (or mode).

The subdiscipline of biology dedicated to the study of the body’s response to short-term and long-term sedentary behavior with a particular focus on identifying unique mechanisms that are distinct from the biological basis of exercising is known as sedentary physiology (113), also sometimes referred to as inactivity physiology (46,47). Both sedentary behavior and exercise can be viewed as regions along a continuous scale of MET values, referred to as the movement continuum (Fig. 2.1). Because of this relationship on the continuum, some of the principles that apply to exercise physiology could also be seen as pertaining to sedentary physiology (e.g., progression, individuality, specificity) (113). Therefore, deconditioning studies involving a downshift in the intensity of movement to the level of sedentary behavior, such as complete mobility loss (as with casting or paralysis) (43,94), bed rest (110,131), or loss of gravity (microgravity/space flight) (55,108) can be seen as providing evidence of the physiological effects of both inactivity and sedentary behavior.

FIGURE 2.1. The movement continuum for waking movement with reference to sedentary and exercise physiology.

In light of the fact that sedentary behavior and exercise are often viewed as two parts of one continuum, it is important to note the physiological responses and adaptions to sedentary behavior are not necessarily the opposite of exercise adaptions or responses. This finding is likely due in part to the fact that the relationship (dose-response) between activity intensity and various disease outcomes is often not constant (linear) across the continuum (70). Additionally, there is evidence that sedentary behavior may negatively modify the effects of physical activity on health outcomes, essentially causing a shift in the dose-response curve between physical activity and certain disease outcomes so that more activity is necessary to reduce the disease risk when a person is also sedentary (28,63). There is also evidence that sedentary behavior and moderate- to vigorous-intensity physical activity may cause different physiological effects that contribute separately to certain disease pathways (47,48,113). Support for this observation can be seen in the fact that sedentary behavior has been shown to modify the effects of physical activity on health outcomes. Specific evidence from physiological research is presented in the next section of this chapter as it pertains to specific health outcomes.

Health Effects of Sedentary Behavior

Sedentary Behavior and Overall Mortality

Several studies have examined the connections between sedentary behavior and mortality, from either all causes or related to specific disease pathways (8,31,40,64,118,123). A meta-analysis of 18 studies that included over 700,000 individuals concluded that sedentary behavior was associated with a 49% increase in the risk of all-cause mortality (hazard ratio [HR] = 1.49; 95% confidence interval [CI] = [1.14, 2.03]) and a 90% increase in the risk of cardiovascular death (HR = 1.90; 95% CI = [1.36, 2.66]) (123). Another, more recent meta-analysis looking at 14 studies also found significant associations with all-cause mortality and cardiovascular mortality. That study also found that sedentary behavior was associated with a 13% increase in cancer mortality (HR = 1.13; 95% CI = [1.05, 1.21]) (8). The later study also examined the effect of sedentary behavior on all-cause mortality stratified by physical activity level. The results indicated that sedentary time was associated with a 30% lower risk for all-cause mortality in those with high physical activity levels when compared to those with low physical activity levels, suggesting that activity may attenuate some but not all of the association between sedentary behavior and all-cause mortality.

Another study that examined all-cause mortality in a large cohort of 40- to 79-year-old men and women with diabetes found that those individuals reporting more than 12 hours of sedentary behavior per day had a 21% increase in mortality risk (HR = 1.21; 95% CI = [1.08, 1.37]) compared with those individuals reporting less than 6 hours of sedentary behavior per day after controlling for physical activity levels (40). The study also found those individuals who were highly sedentary (≥11 h ∙ d⁻¹) and least active (<9.2 MET-h ∙ d⁻¹ of activity) had a 75% increased risk of all-cause mortality (HR = 1.75; 95% CI = [1.45, 2.11]) when compared with the least sedentary (<7 h ∙ d⁻¹) and most active (≥20.5 MET-h ∙ d⁻¹) individuals.

The vast majority of studies related to sedentary behavior and mortality examined all-cause mortality only. A smaller number of studies also looked at cardiovascular and cancer mortality. However, one study utilizing data from the National Institutes of Health and American Association of Retired Persons (NIH-AARP) Diet and Health Study (N = 221,426 individuals aged 50–71 yr; mean follow-up 14.1 yr) was able to examine associations between reported television watching and over a dozen of the leading causes of mortality in the United States (64). This study concluded that there were significant risk increases for death from cancer, heart disease, chronic obstructive pulmonary disease, diabetes, influenza/pneumonia, Parkinson disease, liver disease, and suicide with increased television watching.

Sedentary Behavior and Cardiovascular Disease

One of the earliest well-known studies in the field of physical activity epidemiology can been seen as providing early scientific evidence for the health effects of both physical inactivity and sedentary behavior on cardiovascular outcomes. This seminal study by Morris et al. (84) was conducted on a large cohort of London Transport Executive employees from 1949 to 1950. The results suggested the more sedentary bus drivers had close to twice the risk of fatal coronary heart disease as the bus conductors, whose job required both standing and walking to collect passenger tickets. Additional occupational studies followed that supported these findings (1,83,85,90,101).

Recently, there have been several review articles and meta-analyses that have summarized the evidence for the associations between sedentary behavior and the development of cardiovascular disease (8,26,35,123). The results of one meta-analysis suggested that for each 2 hours per day increase in screen time, there was a 17% increased risk for cardiovascular disease (HR: 1.17; 95% CI = [1.13, 1.20]) (35). When examined by comparing those individuals in the lowest category of screen time with those in the highest category of screen time, the researchers found that there was a risk increase for cardiovascular disease of 125% for those in the highest category after adjusting for important covariates including physical activity. Although not all studies included in these recent meta-analyses showed evidence for an association between cardiovascular disease and sedentary behavior, the pooled results from all four meta-analysis studies, respectively, suggested that overall, there were important, significant associations. A meta-analysis summarizing the results of studies examining the connections between sedentary behavior and cardiovascular mortality is discussed later in this chapter with other mortality outcomes.

Bed rest studies have been used to understand the biological relationship between sedentary behavior and cardiovascular disease outcomes. The results of bed rest studies indicate that prolonged periods of sedentary behavior are associated with deleterious changes in vascular function including reduced peripheral vascular function, increased blood pressure, and significant decreases in brachial artery diameter (12,25,45,97). Space simulation has also provided evidence for decreased endothelium-dependent vasodilation and increased endothelial cell damage in women after more than a month of head-down bed rest (25). That study also showed that the effects were mediated by physical activity, suggesting that the physiological connection between endothelial function and sedentary behavior could be the same as the physiological connection between exercise and endothelial function. It has also been proposed that sedentary behavior may be associated with cardiovascular disease through the effects on adiposity and obesity (47).

Sedentary Behavior and Obesity

Long-term weight gain, leading to obesity, results from a sustained positive energy balance that is achieved when caloric food intake exceeds TEE (24). The main components of TEE are basal metabolic rate, the thermic effect of food, and activity thermogenesis. Activity thermogenesis which may account for as much as 30% of TEE can be divided into exercise activity thermogenesis (EAT) and nonexercise activity thermogenesis (NEAT) (73).

NEAT is defined as all physical activities other than exercise for the purpose of physical fitness and typically accounts for more TEE than EAT (even in most individuals performing regular exercise) (73). Energy expenditure related to NEAT has been shown to vary by as much as 2,000 kcal per day between individuals (9). Differences in NEAT may be the result of differences in active transport, work-related activity, nonexercise leisure activity, and/or activities of daily living.

By definition, sedentary behaviors are those waking behaviors with the lowest energy expenditure. As a result, benefits to energy expenditure can be achieved by converting sedentary time to increased intensity activity (both exercise and nonexercise activities). In addition to the importance of sedentary behavior to energy balance, it has also been suggested that the deleterious effects of sedentary behavior on metabolic function and the association between certain sedentary behaviors, such as watching television, and excess caloric intake may also provide important connections between sedentary behaviors and obesity (11).

Currently, there are over a dozen review articles summarizing the association between sedentary behavior and weight gain, adiposity, and obesity (16,19,23,56,69,78,95,96,99,102,112,114,120,129). The majority of these studies have been conducted in children and adolescents. Two review studies in youth conducted meta-analyses examining the relationship of television watching and the development of obesity (114,129). Both of those studies reported positive associations between television watching and obesity. Odds ratios (ORs) reported in Zhang et al. (129) indicated that after controlling for other important variables, there was a 47% increased pooled odds of developing obesity in youth with the highest reported television watching versus those with the lowest (OR = 1.47; 95% CI = [1.33, 1.62]). Furthermore, they reported a linear dose-response relationship of a 13% increased risk of childhood obesity with every hour per day spent watching television (p < .0001).

Studies in adults examining the relationship between sedentary behavior and obesity have reported both positive and negative findings (96,112,120). An earlier study in Pima adults suggested that increased television watching (≥3 h ∙ d⁻¹) was significantly associated with higher body mass index (BMI) in men but not in women (34). More recently, it has been suggested that there may be a bidirectional relationship between sedentary behavior and obesity-related variables, such that highly sedentary adults are prone to weight gain, whereas adults who are overweight and obese are prone to become more sedentary over time (37,42). A longitudinal study by Golubic et al. (42) suggested that after controlling for important covariates, objectively measured sedentary time was related to changes in fat mass. The authors also found that a 1.5-hour reduction in sedentary time was associated with a 1.4-kg reduction in body weight.

Sedentary Behavior and Diabetes/Metabolic Disease

In relation to impaired glucose tolerance and Type 2 diabetes mellitus, the results of several studies suggest that sedentary behavior has a strong influence on lipoprotein lipase (LPL) activity in skeletal muscles (7,46,47). LPL is an enzyme that is necessary for hydrolysis of the triglyceride contained in lipoproteins. LPL binds to circulating lipoproteins when present on the vascular endothelium. Higher circulating levels of LPL in skeletal muscle have been linked with higher plasma glucose levels (33,61). More specifically, higher levels of circulating LPL have been shown to cause preferential use of lipids as an energy source, which can lead to insulin resistance (33,106). One study in mice and rats showed that both acute and chronic sedentary behavior lead to a decrease in LPL activity in weight-bearing skeletal muscle. It has also been suggested that sedentary behavior may affect diabetes development through its influence on gene activation and deactivation (6) and on the regulation of β cell function (30).

Currently, there are numerous public health studies that have linked the amount of time spent watching television to diabetes-related outcomes and increased risk for diabetes development (29,31,36,57,58,100,122,128). An analysis of television watching time and the development of diabetes in the Diabetes Prevention Program suggested that in individuals at high risk for diabetes, there was a 3.4% increased risk for diabetes with each 1 hour per day of reported television watching (over ~3 yr of follow-up). However, this risk was attenuated when adjusting for body weight (100). Additionally, total sedentary behavior, obtained from accelerometer output, has also been linked to diabetes-related outcomes and the development of diabetes in several studies (22,30,52–54). Specifically, one study that examined the cross-sectional relationship between average total sedentary minutes per day from the accelerometer and cardiometabolic biomarkers in a representative sample of U.S. adults found that total sedentary minutes per day was positively associated with insulin, homeostasis model assessment of steady state beta cell function (HOMA-%B), and insulin sensitivity (HOMA-%S) (p < .05) (53).

Sedentary Behavior and Cancer

A systematic review and meta-analyses that included all forms of reported cancer suggested that individuals participating in higher amounts of sedentary behavior were at increased risk of developing cancer (HR = 1.13; 95% CI = [1.05, 1.21]) (44). There have also been a number of studies examining the connections between sedentary behavior and specific forms of cancer. The most examined associations have been with colorectal, breast, endometrial, ovarian, and prostate cancers (8,21,75–77,93,130).

Although the findings for an association with sedentary behavior for any one form of cancer have been mixed, recent large meta-analyses for women’s breast cancer and for colon cancer suggest that overall, sedentary behavior may confer an increased risk for the development of these cancers (130). For women’s breast cancer specifically, the pooled OR for developing breast cancer was 1.08 (95% CI = [1.04, 1.13]) across all 21 sites examined (130). The results were similar when only studies controlling for BMI or only studies controlling for physical activity were included. In the case of colon and rectal cancer, the meta-analyses included 23 studies, representing over 4 million individuals (21). The pooled relative risk (RR) of developing colon cancer was 1.30 (95% CI = [1.22, 1.39]) for sedentary individuals. Including only studies controlling for physical activity yielded similar results, whereas controlling for BMI gave attenuate but significant estimates of risk increase (RR = 1.24; 95% CI = [1.14, 1.35]) with higher sedentary behavior. Overall, the findings for an association between sedentary behavior and rectal cancer were not significant in that study.

The exact mechanisms by which sedentary behavior may be linked to the development of cancer are not completely understood. Some of the mechanisms that have been proposed include increased adiposity (98) and decreased insulin sensitivity/insulin resistance (39,80). It is plausible that different biological mechanisms may be involved with the formation of different cancers. For example, in the case of breast cancer, one study showed a positive association between sedentary behavior and increased breast density, a known risk factor for breast cancer, specifically (124).

Sedentary Behavior and Bone Mineral Density

Sedentary behavior has been shown to be related to reductions in bone mineral density (13,65,82,127,131). Results from studies of sedentary behavior and bone formation suggest that sedentary behavior does not lead to increased bone formation but causes rapid increases in bone absorption that leads to lower bone mineral density and increased risk of fracture or osteoporosis (59). Evidence for the effects of prolonged periods of sedentary behavior on bone mass has been gained from studies involving space flight and bed rest. One study, by Kim et al. (65), showed that in healthy males, markers of bone resorption including urinary calcium, deoxypyridinoline, and Type 1 collagen cross-linked N-telopeptides were increased during a 14-day period of bed rest. Other studies reporting similar results have shown that bouts of daily aerobic exercise did not offset the negative effects of bone metabolism that resulted from bed rest (108,131). In relation to this finding, the results of a literature review on the health effects of cycling, a form of exercise performed while sitting, suggests that road cycling does not confer significant osteogenic benefits (88). This conclusion was based on the findings of lower bone mineral density in key regions including the lumbar spine, pelvis, hip, and femoral neck.

A number of population-based studies have provided evidence for the link between sedentary behavior and reduced bone mass density and related poor health outcomes (17,18,81,87,107,109). One study in older adults (aged 55–83 yr) at high risk for physical frailty reported a 36% increased risk (HR = 1.36, CI = [1.02, 1.79], p < .05) of developing physical frailty over a 2-year follow-up for each hour per day of objectively recorded sedentary behavior at baseline, after controlling for important covariates including time spent in moderate to vigorous physical activity (109). Studies in adolescents have also provided evidence for the negative effects of sedentary behaviors on bone mineral density in younger populations. A study reporting on over 1,200 youth (aged 8–22 yr) from the NHANES suggested that the associations between sedentary behaviors and lower bone mineral density were not attenuated by time spent in moderate to vigorous activity but may be attenuated by reported frequency of strength training and vigorous play (17).

Sedentary Behavior and Physical Function in Older Adults

Physical independence has been associated with longer survival (4) and higher quality of life (68) in older adults. Several studies examining the associations between total time spent in sedentary behavior (measured objectively) and physical function in older adults concluded that there were important associations between higher levels of sedentary behavior and reduced physical function (72,103–105). One study found that there were significant negative associations between total sedentary time and a composite z score for function that was based on six function variables: chair stand, arm curl, 8-ft up and go, 6-minute walk test, chair sit and reach, and back scratch (103). These findings were significant after controlling for BMI, gender, age, register time, and minutes per day of moderate to vigorous activity. Another study (104) that specifically examined the effects of breaking up sedentary time on function in older adults found that breaking up sedentary time was associated with increased physical function, as determined through a composite z score based on results from a battery of tests (chair stand, arm curl, 8-ft up and go, 6-min walk test, chair sit and reach, and back scratch). This association was still significant after adjusting for time spent in moderate to vigorous activity and overall sedentary time.

Sedentary Behavior Goals and Guidelines

Just as sedentary physiology can be seen as a complementary field of research to exercise physiology, so can sedentary behavior reduction be seen as a complement to, without becoming a replacement for, setting exercise goals. Current evidence suggests that individuals who are highly sedentary even if they engage in prescribed levels of physical activity could benefit from reductions in sedentary behavior. Furthermore, for individuals who are finding it difficult to achieve physical activity goals due to health/physiological, psychological, or other reason, sedentary behavior reduction may provide a more accomplishable proximal goal toward increasing physical activity.

Currently, there are no standardized protocols for reducing sedentary behavior and the approaches used have typically been different than those taken to increase levels of moderate to vigorous physical activity. Whereas exercise is viewed as a planned moderate or vigorous activity done in bouts of 10 minutes or more, on most days, sedentary reduction is encouraged to take place throughout each day and can occur in short “microbursts” or longer bouts. Although health research provides evidence for both reducing total time spent sitting and breaking up long bouts of sitting, there are no specific guidelines for sedentary behavior as there are for physical activity (48). However, some organizations and national governments have adopted general guidelines related to sedentary behavior (Box 2.1). Originally, these guidelines all focused around screen-based activities in children and adolescents. More recently, evidence gained from new research has allowed for some of the guidelines to be extended to adults and to other modes of sitting including transportation sitting (5,14). Currently, more evidence of the specific nature of the dose-response relationship between sedentary behavior and specific health outcomes is needed in order to create more explicit guidelines including those specific to different health outcomes.

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