How is physical activity defined?
How can I assess physical activity and sedentary behavior in the clinical setting?
What are developmentally appropriate physical activity recommendations?
What approaches can I use to counsel patients and families about physical activity?
How can I make physical activity counseling part of my clinical routine?
This chapter will address the following American College of Graduate Medical Education competencies: patient care, medical knowledge, and interpersonal and communication skills.
Patient Care: This chapter will help the pediatric health care provider understand the developmental dimensions of motor development and activity in order to help families implement appropriate and effective obesity prevention and treatment plans.
Medical Knowledge: This chapter will help pediatric health care providers understand and accurately assess fitness and physical activity: key elements in obesity prevention and treatment.
Interpersonal and Communication Skills: Effective promotion of physical activity strategies requires a family-centered approach and effective interpersonal and communication skills. This chapter will aid the pediatric health care provider in communication about physical fitness and physical activity assessment and recommendations.
Physical activity is considered an essential element of normal growth and maturation.1 The development of motor ability, locomotion, and physical activity was elegantly described by Strong et al2 as follows:
“Physical activity begins in infancy with pushing up, turning, crawling, and eventually walking, and it progresses to more complex activities as neuromuscular control develops. Basic movement patterns develop during preschool ages and are the foundation for a wide range of physical activities at later ages. With growth in body size and function and experience, basic movements are integrated and coordinated into more specialized and complex movement skills that characterize the free play, games, sports, and other activities of school-age youth. Guided instruction and supervised practice, specifically by qualified teachers, coaches, and others who work with children, are important in learning movement skills. Types and contexts of activities are variable and change with age during childhood and adolescence. Activities of 6-9 year old children are largely short-term and intermittent (as in non-sustained activities or games such as ‘tag’), and they help the child learn basic and more specialized motor skills. As youth move into the pubertal transition (about age 10-14 years, earlier in girls than in boys), these skills are incorporated into a variety of individual and group activities, often times being organized sports. However, it is also during this period that habitual daily physical activity begins to decline. Indeed, adolescence (12-18 years) is marked by the greatest annual decline in physical activity throughout the lifespan.3,4 Mature structure and function are approached or attained in late adolescence (age 15-18 years), so that physical activity programs can be more structured.”
Although all youth move through the age-related sequence described above, there is increasing concern about the physical activity levels of the US youth. Recent estimates from the National Health and Nutrition Examination Survey (NHANES) indicate that 58% of 6- to 11-year old children and 92% of 12 to 18 year olds fail to achieve 60 minutes per day of moderate-to-vigorous intensity physical activity.5
In this section, operational definitions and basic tenets for the components of overall physical activity and energy expenditure, both important for pediatric health care providers to understand in the context of pediatric obesity and weight management are provided.
Physical activity is defined as “any bodily movement produced by skeletal muscles that results in a substantial increase in energy expenditure over resting levels.”4 It is important to note that physical activity and physical fitness are not the same thing, even though these terms are often used interchangeably. Physical activity is a behavior (something that one does), while physical fitness is a biological or physical trait (something that one has—eg, cardiorespiratory fitness, muscle strength, etc) and reflects the ability to carry out tasks with energy to engage in leisure-time activities as well as respond to emergencies (Two Thousand Eight Physical Activity Guidelines for Americans, 2008). Components of physical activity include frequency (how often one engages in activity), intensity (level of effort one applies while being active), duration (amount of time spent doing activity), and type (identification of what activity is being performed). Physical activity takes many forms, or modes, that occur during leisure time, formal exercise or organized sports, occupational work, household chores, and transportation (eg, walking or biking to and from school) and can be performed at varying intensities ranging from light to vigorous.
The intensity of physical activity can be quantified by physiological measures such as heart rate, oxygen (O2) consumption, power output, or energy expenditure. The metabolic equivalent (MET) is widely used to classify activities into intensity categories (Table 7-1). One MET equals 3.5 mL O2/kg body weight/min and is based on a 70-kg adult consuming 250 mL O2/min (250 mL/min ÷ 70 kg = 3.5 mL O2/kg body weight/min) at rest. It is known that the resting metabolic rate of children is higher than adults, and thus the meaning of a MET in children has been questioned.6 Currently, there is a compendium of physical activities with associated intensity levels in youth7; however, over half of those activities represent adult values.
An additional classification of activity, or inactivity, is referred to as sedentary. Sedentary behavior refers to activities that do not result in a substantial expenditure of energy above one’s resting level (ie, 1 MET), and hence includes daily routine activities such as sitting, lying down, watching television, and sleeping.8 It is important to note that sedentary behavior was traditionally conceptualized as the opposite of being physically active, but is now being examined as its own entity. Additionally, much of the previous research and clinical assessments represented sedentary behavior solely by screen time. Screen time is the compilation of multiple activities that involve use of a screen—typically, watching TV, using a computer, and playing video games. It is important to emphasize that sedentary behavior is more than simply screen time; thus, capturing all aspects of sedentary behavior is encouraged.
Over the course of a day, the total daily energy expenditure (TDEE) can be quantified. The energy cost of physical activity, or physical activity energy expenditure (PAEE), is one component of the TDEE. Humans expend energy in a variety of ways, including basic cellular maintenance and repair, digestion, thermoregulation, locomotion, growth, and reproduction. More simply, the TDEE includes resting energy expenditure, thermic effect of feeding (TEF), and PAEE. The TDEE and its components are thought of as a pie chart with the resting energy expenditure, or basal metabolic rate, accounting for a substantial proportion of TDEE (60%), while the TEF is thought to contribute approximately 5% to 10% of TDEE. In general, PAEE contributes about 30% of TDEE. PAEE is definitely the most malleable component of TDEE (eg, from the extremes of complete sedentariness to highly active sportsmen). The energy cost of growth in children and adolescents should also be considered, but in general is less than 3% to 5% of TDEE.
PAEE can be further divided into 2 categories. The first is the energy expended in voluntary exercise which is defined as locomotor activity that is not required for survival or homeostasis and not directly motivated by any external factor. The second is habitual physical activities of daily living which generates nonexercise activity thermogenesis (NEAT). In industrialized societies, activity of daily living is often relatively low, and so it has been presumed that PAEE and TDEE are lower than in previous generations. Whether this is true and has contributed to a rise in obesity is controversial.9 However, Levine et al10 have emphasized the importance of NEAT in weight regulation because humans engage in a great deal of physical activity that does not qualify as voluntary exercise (eg, fidgeting, walking to the bathroom), termed spontaneous physical activity.11 In subsequent papers, the definition of NEAT in humans has been made more inclusive, for example, “NEAT is … akin to the energy expenditure of spontaneous physical activity. For a human it would be … performing all of our daily tasks such as walking, talking, yard work, and fidgeting.”12 Under this very broad definition, much of NEAT can be voluntary and even involve what is commonly viewed as “exercise.”11
Physical activity and/or TDEE can be measured by a variety of tools, including questionnaires/surveys/interviews, diaries, motion sensors, heart rate monitors, indirect calorimetry, and doubly labeled water. A thorough review of physical activity assessment methodology is beyond the scope of this chapter, and the reader is referred to other excellent resources.13, 14, 15, 16
In general, each method has its own advantages and disadvantages, and each method addresses a varying number of components of physical activity. Issues of reliability, validity, and feasibility are important considerations. In general, there is an inverse relationship between validity and feasibility (Figure 7-1). For instance, a feasible method in large population studies or the clinical setting such as a questionnaire may be a less valid measure of physical activity; on the other hand, the most valid measures of energy expenditure, doubly labeled water or indirect calorimetry, are not practical or feasible in large studies nor oftentimes in clinical settings, due to cost, time constraints, and technical expertise. The most accurate measures of PAEE and TDEE (indirect and direct calorimetry and doubly labeled water) are impractical for clinical purposes, and thus will not be reviewed here. Cost is another concern to researchers and practitioners. In general, the more features a device provides, the more it costs. In terms of objective assessment monitors, pedometers tend to be the cheapest models available, followed by heart rate monitors and accelerometers.
In a survey of 18 pediatric obesity centers providing Stage 3 or 4 treatment,17 100% reported that they assess physical activity. However, there was considerable variation in the assessment procedures reported. Greater than 80% used questionnaires or surveys or interviews. Questionnaires or surveys or interviews yield estimates of the amount of time spent in various activities, and these can be translated into energy expenditure, given estimates of the costs of various activities. They provide an assessment of all 4 components of physical activity (frequency, intensity, duration, and type). Questionnaires or surveys work well for large, population-based studies and for times that activity context may be desired. Interviews can be more cumbersome, but offer similar richness of information. However, these methods can be subject to recall bias. An additional concern in the child or adolescent population is cognitive ability to complete instruments or answer questions. It is generally recommended that children younger than 10 years do not participate in this methodology due to cognitive limitations.18 This has led researchers and practitioners to use proxy methods (ie, asking parents or teachers about children’s physical activity). Although proxy methods offer similar advantages as the questionnaire or survey or interview, one needs to note that parents and teachers do not necessarily observe the child across the entire day, limiting the information they can provide. Similarly, diaries provide assessment of all 4 components of physical activity but can be cumbersome for an individual to complete, and it is questionable how well children younger than 10 years could accomplish this task.
Because of issues with recall bias and cognition, researchers and practitioners often use objective measures of physical activity. These include heart rate monitors, pedometers, and various types of accelerometers or multisensor monitors. Most objective monitors can provide frequency, intensity, and duration of physical activity, but not type. Heart rate monitoring involves fitting a child with a belt capable of telemetry to a watch unit that is typically worn on the wrist. Although there are common cut-points to represent moderate and vigorous physical activity (eg, heart rate of 140 beats/min is moderate; heart rate of 160 beats/min is vigorous),19 the best way to implement heart rate monitoring requires measurement of an individual’s heart rate–oxygen consumption relation across a range of activities from sedentary to vigorous activity. In doing so, any given heart rate value can be converted to an oxygen consumption or energy expenditure value. This relationship tends to diminish at the extremes.
Pedometers are easy to use and provide objective assessment of total volume of activity, but they have some limitations. They use a mechanical lever system to detect steps taken, although some newer units incorporate accelerometer technology. Pedometers offer the total volume (number) of steps taken and can be reset daily. However, only some of the models record the steps in memory, requiring children to record daily step values and times the device was worn for models that do not have the memory capability, which introduces another challenge. Intensity levels cannot be assessed in many pedometers; only the models with accelerometers incorporated have the capability of providing intensity information.
Accelerometers are now commonly used in physical activity studies. These devices detect accelerations in movement in various planes (some are uniaxial, while others are bi- or triaxial) and translate them into movement “count” values. They provide information regarding frequency, intensity, and duration of physical activity. Several different monitors are available. These devices depend on calibration of output (counts) to meaningful units such as energy expenditure or minutes of time spent at various physical activity intensity levels based on independently determined count cut-points.20 Most previous work with accelerometry has related the movement counts to energy expenditure in a linear fashion. However, researchers have identified several problems with this approach. The primary concern is which cut-points to use to represent physical activity intensity levels for a particular population of interest (eg, high school adolescents vs preschool children)—also coined the “cut-point conundrum.”21 Recent advances in the field of accelerometry have involved sophisticated modeling techniques such as artificial neural networks to assess accelerometer output.22 These techniques also allow for identification of activity type, which was not possible using previous analytic methods. However, it can be difficult to apply these techniques and interpret the output. Beyond this, 1 accelerometer mounted at the hip frequently cannot detect activities that involve going uphill or various types of resistance exercise, and swimming is often an issue because many devices are not fully waterproof. New devices are being created to address some of these limitations, and some devices involve multiple accelerometers that are placed at different locations on the body and can communicate with each other. Until these devices are perfected, more traditional uses of accelerometry will remain in use.
It is also possible to triangulate data from multiple sources, such as a survey and an accelerometer. The best methods for combining information from multiple sources remain to be determined; however, it is a promising approach, given that more and more evidence suggests subjective and objective measures of physical activity may not quite be capturing the exact same things.
Assessment of sedentary behavior is slightly more challenging than that of physical activity because there has not been as much work to validate tools to capture sedentary behavior.23 Historically, time spent watching television and playing video games has been used as a marker of sedentary behavior. Many researchers are currently attempting to use existing physical activity measurement tools—namely, accelerometers—to assess sedentary behavior, but it is not clear that this is the correct approach. Many existing surveys do not ask varied enough questions to address all types of sedentary behavior that can occur, and it is not clear which accelerometer count cut-point should be used. Additionally, some evidence in adults exists that breaks (interruptions) in sedentary time are important,24 but this construct can be equally difficult to assess. Overall, it is conceptually agreed upon that sedentary behavior needs to be curbed to prevent and treat obesity, yet feasible and valid assessment of this construct has proven to be difficult.
Assessing physical activity or sedentary behavior in infants and toddlers brings about special challenges. Young children move differently than older children and adolescents—essentially, their physical activity typically occurs as short, sporadic bouts that can be of either high or low intensity.25 It is questionable if the usual classifications of intensity level (especially moderate and vigorous) apply to this population, and it is not clear if physical activity of any particular intensity level may be related to health outcomes. Young children are not capable of completing surveys or interviews, and proxy reports (as previously mentioned) may or may not reflect their true activity patterns. For this reason, objective measures such as accelerometry are often used in young children. In general, it is difficult to keep the devices in the proper position, and it is possible for children to tamper with the devices. However, children get used to wearing them. If the group of interest is infants, there are not widely accepted methods to assess intensity level, leaving total accelerometer counts as a preferred measure. Some individuals believe that direct observation is the best method for assessing young children. When observers are appropriately trained, direct observation can be a gold standard method. However, one must consider all the environments where data collection will take place and that, for at least a portion of the time, the Hawthorne effect is possible (ie, children may modify behavior or physical activity because they are being watched).
Twenty years ago (1993), a group of pediatric exercise scientists met and established the first physical activity recommendations for adolescents. They recommended daily lifestyle physical activity in addition to vigorous physical activity for at least 20 minutes per session 3 times per week.26 In 1996, the US Surgeon General’s Report on Physical Activity27 recommended 30 minutes of moderate physical activity on most, preferably all days of the week for all Americans.
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