Interest in the health implications of sedentary behavior has exploded in the past decade. Despite the proliferation of discussion and research during this period, our understanding of the physiological mechanisms, health outcomes, and measurement of sedentary behavior in both adults and children is limited. Most of the research has focused on adults, with an emphasis on evaluating the association of sedentary behavior with health outcomes such as longevity, metabolic syndrome, diabetes, cardiovascular disease, and obesity. The child research has primarily evaluated the effect of sedentary behavior on body composition in children who are developing typically. The effect of sedentary behavior on children with a diagnosis of cerebral palsy (CP) is unknown.
Sedentary behavior is usually defined by position (sitting or reclining) and by energy expenditure (<1.5 metabolic equivalents of task [METs]).1 Although it is often associated with decreased moderate to vigorous physical activity (MVPA), the 2 constructs are not synonymous. They have different physiological mechanisms, definitions, methods of measurement, and outcomes. Owen et al2 coined the term “active couch potato” to describe adults who meet the recommended guideline of 150 minutes per week of MVPA but who are still at risk for metabolic consequences from too much prolonged sitting. Sedentary behavior and decreased physical activity are separate constructs with independent outcomes, and individuals can receive the benefit of performing MVPA but still be negatively affected by sedentary behavior.
The aims of this article are to introduce the definitions, health outcomes, physiological mechanisms, measurement methods, and interventions associated with sedentary behavior in the typical population (adults and children, 6-12 years) and to review the literature related to sedentary behavior in children with CP. The challenges of developing an assessment and intervention framework for school-aged children with CP are discussed.
Sedentary behavior has not been consistently defined in the literature, with many studies evaluating reduced physical activity rather than true sedentary behavior.3 , 4 Sedentary behavior is most often defined functionally as “sitting without being otherwise active”5 (p190); it is unknown whether movements such as fidgeting or arm/leg movement while sitting are enough to negate the deleterious effects of being sedentary. Researchers frequently use screen time (eg, television viewing, gaming, and computer use) as a proxy activity to represent sedentary time. However, screen time may not be an accurate estimate of total sedentary time because it does not account for a person's total daily sedentary time.6 Some definitions have also included transportation time,2 , 3 lying down, and desk work,4 and sedentary hobbies and sitting and socializing,7 as proxy activities for sedentary time. Yates et al8 (p293) defined sedentary behavior in terms of muscle activity, stating that sedentary behavior is a “non-exercise activity that involves sitting or lying,” and that in these postures most of the body's large muscle groups are relaxed. They contrast these positions with static standing where, even though a person may not be moving, a large proportion of the body's muscles are working to maintain an upright posture. Sedentary behavior is defined physiologically as 1.0 to 1.5 METs.2–4 , 8 Light-intensity activity (LIA) is defined as greater than 1.5 to 3.0 METs, and MVPA is defined as 3.0 to 8.0 METs.2 , 3 The definition of sedentary behavior used in the literature is the same for adults and for children who are developing typically.
The Sedentary Behaviour Research Network recently published a standard definition for sedentary behavior including both physiological and functional components as “any waking behavior characterized by an energy expenditure less than or equal to 1.5 METs while in a sitting or reclining posture.”1 (p540) They also suggest that the term “inactive” be used to describe people who are not achieving the recommended amount of daily MVPA, thus distinguishing between sedentary behavior and decreased physical activity.
HEALTH OUTCOMES ASSOCIATED WITH SEDENTARY BEHAVIOR
The health outcomes of sedentary behavior in adults have been examined2–4 predominantly by evaluating associations and outcomes rather than causal relationships. A review by Proper et al4 included 19 prospective studies, of which 14 were considered to be of high methodological quality, and a recent review by Wilmot et al6 included 18 studies, of which 16 were prospective. Fifteen of these studies were considered to be of moderate to high quality. The reviews concluded that sedentary time is associated with diabetes,6 cardiovascular disease, all-cause mortality,4 , 6 and mortality from cardiovascular disease. The majority of the studies reviewed measured sedentary behavior by self-report.
Other negative health outcomes have been associated with adult sedentary behavior, including metabolic dysfunction (eg, decreased lipoprotein lipase [LPL] activity, decreased insulin sensitivity, decreased glucose uptake, decreased levels of high-density lipoprotein cholesterol, and increased plasma triglyceride levels),2 , 3 bone health concerns,3 vascular health, cardiovascular health,2–4 body weight issues (including increased waist circumference, obesity, and body mass index [BMI] gain),2–4 and cancer (colon in men3 and endometrial in women3 , 4).
The majority of child sedentary behavior outcome research has focused on health associations between sedentary behavior and measures of body fat mass.9 , 10 Health outcomes for child sedentary behavior have been reported for BMI and other indicators of fat mass,9 , 10 bone mass,11 aerobic fitness, and metabolic syndrome.9 Other outcomes such as self-esteem, academic achievement, and prosocial behavior have also been evaluated for children.9 Less is known about the health outcomes of sedentary behavior for children than for adults. Chinapaw et al10 caution that there is not yet enough evidence for negative health effects in children and adolescents. However, because sedentary time increases with age,7 and childhood sedentary behavior patterns tend to continue into adulthood,12 it may be important to intervene early.
PATTERN OF ACCRUAL OF SEDENTARY TIME
The pattern of sedentary time accrual, not just the total sedentary time, may influence some of the health outcomes of sedentary behavior. Healy et al13 report that an increased number of breaks in sedentary time, independent of the length and intensity of the breaks, and of MVPA time, was associated with more positive health outcomes, specifically decreased body fatness, decreased triglycerides, and improved 2-hour plasma glucose. The authors hypothesize that the absence of skeletal muscle contraction in prolonged sitting contributes to the decreased clearance of plasma triglycerides and oral glucose from plasma. They acknowledge that it was not possible to determine the required frequency, length, or duration of breaks needed for the improved metabolic outcomes. A recent study14 found that breaks in sedentary time, independent of total sedentary time, were associated with a decrease in waist circumference, and improvements in fasting plasma glucose and C-reactive protein levels. The inflammatory marker C-reactive protein is associated with an increased risk of coronary heart disease and vascular mortality. The authors hypothesize that inflammation may be an additional way that prolonged sitting can affect cardiovascular disease risk. Most recently, it has been reported that 2-minute breaks every 20 minutes of sedentary time, compared with continuous sitting, result in an improvement in glucose and insulin levels in adults who are overweight and obese.15
Kwon et al16 investigated the longitudinal changes in the frequency of sedentary breaks over a 10-year period in children between the ages of 5 and 15 years. Over 5 assessment periods, they reported that the frequency of sedentary breaks decreased over childhood and adolescence. Children and youth also had a lower frequency of breaks during weekday school hours than after school or on weekends. No research has yet been conducted to determine whether the frequency of breaks in sedentary time influences health outcomes for children.
The mechanisms causing the deleterious effects of sedentary behavior have not been studied extensively. Many researchers who discuss sedentary behavior have actually evaluated the effect of a lack of MVPA,3 but the physiological mechanisms of MVPA are different from those of sedentary behavior. The deleterious effects of sedentary behavior are not due simply to a lack of MVPA but are more likely related to a lack of frequent intermittent LIA throughout the day.2 , 14 Light-intensity activity refers to all waking activity that is not sedentary or MVPA and includes standing, stepping, and many activities of daily living.2 , 17
Two physiological mechanisms have been proposed to explain the negative effect of sedentary behavior: a lack of contractile activity of the muscles, which in part results in decreased LPL regulation,18 , 19 and blood vessel remodeling.20 The mechanisms that explain the detrimental health effects of sedentary behavior are not the same (or even simply the opposite) of the mechanisms explaining the positive health effects of MVPA.
Sedentary behavior is characterized by a reduction of muscle activity and may place a person at a greater risk of developing metabolic diseases. Hamilton et al19 postulate that the lack of skeletal muscular contraction during sedentary time may suppress LPL. Lipoprotein lipase is an enzyme that is necessary for the uptake of triglycerides as well as for producing high-density lipoprotein. Animal studies have shown a quick decrease in LPL activity in the skeletal muscles with inactivity, resulting in a significant decrease in the clearance of plasma triglycerides. Interestingly, only the inactive muscles are affected by decreased LPL activity, suggesting that a lack of local contractile activity may be the problem.18 Light-intensity activity appears to activate LPL in the skeletal muscles,19 and there is evidence that inactivity causes LPL suppression. Hamilton et al19 report a much greater decrease in LPL activity (10 times), specifically in the red oxidative muscles, during prolonged sedentary activity compared with LIA. Conversely, there was only a modest increase (2.5 times) in LPL activity when comparing MVPA with LIA, and the increase was in the white glycolytic muscle fibers, showing that the LPL mechanism during sedentary activity is different from that during MVPA.
In a recent review, Thijssen et al20 examined the effects of sedentary behavior on the vascular system. They report independent effects of sedentary behavior and MVPA on the vascular system. Being sedentary may lead to a decrease in the diameter of the arterial lumen, which appears to be related to an enhancement of the vasoconstrictor pathway that contributes to the regulation of vascular tone. In contrast, physical activity may enhance vasodilation and facilitate healthy vasculature. In addition to the decrease in the diameter of the arterial lumen, being sedentary may also cause an increase in the thickness of the arterial walls. Both of these vascular changes are predictive of cardiovascular problems.
The physiological mechanisms underlying the negative effects of sedentary behavior are not yet well understood. Further investigation is required to test and refine these theories and to be better able to relate these mechanisms to the proposed health outcomes.
Some of the researchers evaluating activity measurement have investigated the measurement of MVPA rather than the amount of sedentary activity. It is important to ensure that true sedentary behavior is measured rather than the absence of MVPA. Subjectively, it has most frequently been measured by self-report of proxy activity—most commonly television viewing. Accelerometers have become the standard objective method of measurement to collect information regarding the quantity, intensity, and patterns of movement.3 Many accelerometers also include an inclinometer, which indicates whether someone is sitting or standing, an important distinction when measuring sedentary activity. Accelerometers cannot distinguish what type of activity is occurring and will not register movements when the body's center of gravity is relatively stable, such as riding a bike.21 Accelerometers also do not provide direct information about energy expenditure; they measure the acceleration of movements, and prediction models are used to estimate energy expenditure on the basis of the counts.22 Generally as the counts per minute increase, so does the intensity of the movement.7 Cut-points based on counts per minute are used to estimate the category of activity intensity (eg, sedentary, LIA, and MVPA).23 The cutoff of fewer than 100 counts per minute has been commonly used in the literature to describe sedentary behavior in adults.2 More recently, Kozey-Keadle et al24 have advocated for the use of a cutoff of 150 counts per minute for the ActiGraph (Actigraph, LLC, Pensacola, Florida) monitor as they found that this has improved accuracy in identifying sedentary behavior when compared with direct observation. Cut-points are specific to the accelerometer type.25
Sedentary behavior in children has been measured in a number of ways (see Table 1), both objectively and subjectively. Measurement in children is more challenging than that in adults because children tend to have more sporadic movement patterns than adults,23 which may be more difficult to remember and tally. Sedentary behavior in children has been measured most often by self-report or proxy report of television viewing.10 Television viewing may not be a valid indicator of sedentary activity in children, because it may comprise only a portion of a child's daily sedentary time, and because children may engage in other activities while also watching television. Television viewing may also be associated with dietary habits such as snacking on high-calorie food,26 , 27 which could confound the relationship between television viewing and health outcomes.27 Although self-report and proxy-report measures have been extensively used to measure child sedentary behavior, their validity and reliability have not been extensively evaluated.23
Accelerometers are used to objectively measure child sedentary behavior,7 but they have the same limitations as with adults. Various accelerometers have been validated for use with children who are developing typically, compared with observational measures such as direct observation, and physiological measures such as indirect calorimetry.23 Cut-points used to define sedentary behavior vary widely in the literature, as do the measurement intervals or epoch lengths. The cut-point of fewer than 100 counts per minute is commonly used for children, although counts as high as 1100 counts per minute have been used.7 Shorter epochs (eg, 15 seconds) may be more appropriate for use with children because they are more accurate in capturing the irregularity in their patterns of play and movement.23
Another method that has been used to measure child sedentary behavior is the measurement of calorie expenditure. One such device is the BodyBugg (BodyMedia, Pittsburgh, Pennsylvania) armband.28 Unlike traditional accelerometers, it does not need movement to differentiate between different activity intensities. The BodyBugg records a variety of physiological measurements such as heat flux, skin temperature, galvanic skin response, and activity counts via a 3-axis accelerometer. An algorithm is used to estimate calorie expenditure or METs. This algorithm has been used with children, with a low average measurement error (1.7%).
INTERVENTIONS FOR CHILDREN
In 2002, American children aged 9 to 13 years exceeded the recommended guidelines of 1 to 2 hours a day29 of daily leisure screen time, with an average of 4.5 hours.30 With the mounting evidence for the detrimental effects of sedentary behavior, researchers have begun to investigate effective interventions for children with typical development. Kamath et al31 evaluated 34 randomized controlled trials in a systematic review. The aim of the review was to determine the effectiveness of behavioral interventions with the goal of changing behaviors (eg, increasing physical activity, decreasing sedentary behavior, improving eating habits, and decreasing poor eating habits) to prevent obesity in children aged 2 to 18 years. None of the interventions had a significant effect on BMI when compared with the control group. They did find that interventions to decrease sedentary behavior were more effective in children (6 to 11 years) than in adolescents, with longer treatment periods (>6 months), and for interventions that comprised multiple cognitive components compared with only 1 or no cognitive component. The researchers reported that interventions to decrease unhealthy behaviors might be more effective than interventions to encourage healthy behaviors. If this assumption is true, focusing on reducing sedentary behavior may be more effective than encouraging an increase in MVPA. Research suggests that increased sedentary behavior is associated with reduced LIA,2 but it does not appear to affect participation in MVPA,32 supporting the theory that sedentary behavior and physical activity are separate constructs. Interventions to decrease sedentary behavior and increase physical activity are both important.9
In a novel study, sitting desks were replaced with sit/stand workstations in 2 of 4 first-grade homeroom classrooms.28 The treatment group children were instructed to sit and stand as they wished, and the students in the 2 nonadapted classrooms served as controls. The BodyBugg armband was used to measure calorie expenditure in the children. After 12 weeks, 70% of the students in the treatment group stood through the whole 2-hour analysis period. The remaining 30% of students stood an average of 75% of the time. Calorie expenditure for students in the treatment group was 17% greater than that in the control group. There was also anecdotal improvement in behavior and achievement.28 This study increased standing (LIA) and thereby decreased sedentary behavior just by altering available equipment. It would be interesting to see if this trend would continue as children age and classroom demands change. Children and youth take fewer breaks from sitting as they age and fewer breaks during the school day than during their leisure time,16 and therefore effective school day interventions may contribute significantly to decreasing prolonged sedentary time. The Transform-Us! study33 is a randomized controlled trial to evaluate the individual and combined effects of behavioral and environmental interventions to decrease sedentary behavior and increase physical activity in 8- and 9-year-old children.33 This study is strong methodologically and compares different intervention strategies. These types of intervention studies are needed to understand the true effects of decreasing sedentary behavior of children.
CHILDREN WITH CP
Very little is known about the effects of sedentary behavior in children with CP. The functional and physiological parameters of the definition of sedentary behavior developed for the typical population may not be appropriate for children with CP. If so, then the sparse body of research reporting health outcomes using these parameters may also be inappropriate for this population. Children with CP demonstrate atypical muscle tonus (ie, spasticity, hypotonia, athetosis, ataxia, or a combination) and challenges with muscle co-contraction, balance, and coordination. These motor impairments often make sedentary behaviors such as sitting more challenging and as a result may require more energy expenditure than is reported for children without motor challenges. Thus, sitting activities may exceed the physiological parameters of sedentary behavior (1.0–1.5 METs) when measured in children with CP. In contrast, activities considered nonsedentary for the typical population, such as quiet standing, may in fact be sedentary for some children with CP, especially those who use equipment such as a standing frame to maintain the position. Some children using a standing frame may use the trunk and lower extremity muscles while positioned in a standing frame, whereas others may be inactive and supported by the standing frame straps. These sitting and standing examples illustrate that it may not be appropriate to extrapolate sedentary functional behavior definitions developed for children who are developing typically to children with motor disabilities. In addition, the heterogeneity of motor abilities in children with CP may make it impossible to determine a standard functional definition of sedentary behavior for them. Research to determine the muscle activity and energy expenditure of children with CP in different positions and with and without supportive equipment is necessary to unravel the true nature of their sedentary behavior.
Electromyographic recordings could be used to determine the amount of trunk and lower extremity muscle activity of children positioned in various adaptive equipment, such as standing frames and seating devices, to differentiate between active and inactive postures and the influence of supportive devices. No study of sedentary behavior to date has evaluated muscle activity directly, despite the theoretical assumption that a lack of muscle activity contributes to the negative health outcomes associated with sedentary behavior. Caloric expenditure measurements could also assist in understanding the activity level of children with CP in “typical” sedentary behaviors.
Investigations of energy expenditure levels and the amount of activity or movement in children with CP have focused primarily on ambulatory activities not considered to be sedentary. Measurement tools used have been predominantly a measure of walking energy expenditure34 , 35 or a step36 or activity37 , 38 count using various types of accelerometers. The majority of research supports the assumptions that children with CP use more energy to walk34 , 39 and that the energy requirements of walking increase with the severity of involvement.34 Little is known about the energy expenditure of children with CP when sedentary. Johnson et al40 have reported that adults with athetoid CP have significantly higher resting energy expenditure than healthy adults. It is unknown if this increased resting energy expenditure is higher than 1.5 METs, the upper limit for energy expenditure for the physiological definition of sedentary behavior.
A number of accelerometry devices have been validated to measure the physical activity levels of children with CP who are ambulatory. The StepWatch (Orthocare Innovations, Oklahoma City, OK) monitor has been validated for children with CP who are ambulatory,36 and a recent study validated the ActiGraph accelerometer for 8- to 16-year-old children with CP who are ambulatory (Gross Motor Function Classification System [GMFCS] levels I to III), using oxygen uptake (
O2) as the reference standard.38 To measure sedentary behavior, not physical activity, the ActiGraph was able to be used to reliably identify a 10-minute rest period of quiet sitting. Sedentary behavior is one of the variables used in an Australian study,41 and the researchers plan to evaluate the criterion validity of the ActiGraph accelerometer compared with direct observation to establish counts per minute cut-points to distinguish sedentary behavior in young children with CP. Accelerometry has not been validated for the measurement of other perceived sedentary behaviors with children with CP, such as desk work.
The measurement of sedentary behavior in children who are nonambulatory presents unique challenges. Accelerometry has not been validated for use with children who use wheelchairs. The standard placement on the hip may not accurately measure activity levels because the trunk movements may not be identified.42 , 43 From an energy expenditure perspective, it is unknown whether limb movements alone increase the body's energy expenditure above the defined sedentary level. Once validated, the BodyBugg armband or another similar device that provides an indirect measurement of energy expenditure may be a more suitable measurement tool for this population.
Effect of Sedentary Behavior
The long-term effects of sedentary behavior are also virtually unknown for children and adults with CP. Considering that reduced muscle activity has been identified as a factor contributing to the negative health outcomes of sedentary behavior in persons in the typical population, and considering that persons with CP may have increased rather than decreased muscle activity in postures currently defined as “sedentary,” it may be that the deleterious health effects of these postures do not apply to persons with CP. In a study involving adults with spinal cord injury, spasticity was found to be protective against metabolic syndrome.44 The same could be true in children with CP.
With such limited understanding of an appropriate definition of sedentary behavior for children with CP, how to measure it, and what the long-term health implications are, a discussion of potential intervention strategies is clearly premature but very intriguing. Fitness programs to increase the physical activity levels of children with CP are a popular rehabilitation intervention strategy.45–47 The short-term improvements reported from exercise programs do not seem to be long-lasting,45 and improvements in aerobic capacity may not result in functional improvements.48 , 49 Improvements in muscle strength are also equivocal.50 In addition, commitments to a regular exercise schedule may be challenging for parents, and community accessibility is an issue.51 Considering these factors and the idea that intervention is more effective when focused on decreasing unhealthy behaviors rather than increasing healthy behaviors,31 interventions focused on decreasing sedentary behavior may be more effective than the current intervention emphasis on increasing physical activity. This change in intervention emphasis may be particularly effective with nonambulatory children with more severe motor impairments. What could interventions focused on decreasing sedentary behaviors look like for children with CP in wheelchairs? Having a classroom assistant encourage “wiggling” or “sit to stand” once an hour instead of taking children out of their wheelchairs at lunchtime for a prolonged period? The use of supported standing desks for some classroom time instead of prolonged sitting? Wheelchair exercises often during the day? Walking classroom breaks? Interventions will be limited only by the imagination of therapists, but not until the “landscape” of sedentary behaviors in children with CP is clearly mapped out. This involves a definition of what comprises sedentary behavior, validated measures, and longitudinal studies to understand the long-term implications of sedentary behavior in adulthood for persons with CP. Achievement of this knowledge base can only occur with the formation of collaborative research teams, instituting an international sedentary behavior research agenda for persons with CP.
Despite myriad reports discussing the implications of sedentary behavior published in the last decade, the mechanisms of sedentary behavior and the resulting outcomes are still not clear. The identification of valid measurement options and effective intervention strategies is just emerging with adults and children in the general population. Almost no information is available for children with motor impairments such as CP. The study of sedentary behavior within this population may lead to innovative rehabilitation intervention strategies. Sedentary behavior represents an important construct that requires more evaluation.
Dr P. Manns read and critiqued the physiological mechanisms section of the manuscript.
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