The ongoing obesity epidemic has heightened interest in physical activity as a behavior that influences energy balance and body composition (^13,34). Because an increased prevalence of obesity has been observed in young children as well as in older segments of the population (^9,19,20), both researchers and professionals have shown a growing interest in the promotion of physical activity in children of preschool age (^1,5,8,33). Interest in this issue has been boosted by sociodemographic trends that have placed an increasing number of young children in structured child care settings. Expert panels have emphasized the importance of preschool programs promoting physical activity and limiting physical inactivity to decrease young children's risk of becoming overweight (³¹).

Research on promotion of physical activity in young children requires application of measures of physical activity that are valid and reliable when applied to this unique group. Many measures of physical activity that have been developed for use with older groups are clearly inappropriate for preschool children. For example, self-report measures of physical activity cannot be used with children younger than approximately 10 yr (²⁴). Some measures of physical activity may be useful with young children only if special modifications are made to accommodate the unique characteristics of this age group. The purpose of this article was to provide an overview of the methods that have been developed for measuring physical activity in children of preschool age. Emphasis will be given to direct observation and accelerometry, but some less expensive and less burdensome methods will be reviewed as well.

DIRECT OBSERVATION SYSTEMS

Principles of direct observation.

Direct observation is the method by which a trained observer records physical activity behavior for a predetermined period. Specific codes that correspond to characteristics of the physical activity behavior are recorded. Direct observation systems can provide much information, including physical activity intensity (e.g., sedentary, light, moderate, vigorous), type (e.g., running, skipping, sitting, standing), environmental context (e.g., use of portable or fixed equipment), social context (e.g., group composition), location (e.g., inside, outside), and prompts (e.g., encouragement to increase or decrease physical activity). Typically, the observer watches one child at a time. Direct observation systems vary in the length of the observation period, with some prescribing observation for an entire day and others requiring 30- to 120-min sessions. The study setting plays a role in determining the number of observations. For example, if physical activity is to be measured in the preschool setting, multiple sessions during a shorter time frame are more practical. However, for the home setting, measurements across an entire day or portion of a day, at multiple times of the year, may be more appropriate.

Two direct observation systems that have been used extensively with young children-the Children's Activity Rating Scale (CARS) (²⁸) and the Children's Physical Activity Form (CPAF) (²¹)-focus solely on physical activity intensity. The CARS system categorizes activity intensity across five intensity levels. The intensity levels in CARS are defined as follows: 1) stationary-no movement, 2) stationary-with movement, 3) translocation-slow/easy, 4) translocation-moderate, and 5) translocation-fast (²⁸). The CPAF has four intensities: 1) stationary-no movement, 2) stationary-limb movement, 3) slow trunk movement, and 4) rapid trunk movement (²¹).

Two observation systems, in addition to measuring activity intensity, also measure other physical activity domains. The Children's Activity Timesampling Survey measures four domains: 1) intensity level, 2) environment, 3) participants or others in the presence of the child, and 4) type of interaction (e.g., encouraging an increase or decrease in physical activity) (¹¹). A child is observed for 10 s, and codes are recorded during the next 10 s. Behaviors of Eating and Activity for Child Health: Evaluation System (BEACHES) measures 10 categories: 1) environment, 2) physical location, 3) activity level, 4) eating behavior, 5) interactor, 6) antecedents, 7) prompted event, 8) child response, 9) consequences, and 10) consequent event (¹⁷). A child is observed for 25 s, and the observer has 35 s to record the appropriate codes (¹⁷). The Observational System for Recording Activity in Children - Preschool Version (OSRAC-P) was developed by Brown et al. (²) to measure the type, intensity, and contexts of preschoolers' physical activity. Specifically, it collects data on 1) physical activity intensity, 2) physical activity type, 3) location, 4) indoor activity context, 5) outdoor activity context, 6) activity initiator, 7) group composition, and 8) prompts. Individual children are observed during 30-min sessions, with 5 s of observation followed by 25 s of recording, so that 60 cycles per 30-min session are completed per child.

Advantages and disadvantages.

Direct observation systems have several advantages over other measurement tools. They can provide information on both the type of physical activity and the intensity at which it is performed. Some systems also assess social and environmental factors that can influence physical activity behavior. In addition, direct observation systems are feasible for use in both preschool and home settings (^2,17). There is the possibility of participant reactivity; however, in a study of 5- to 6-yr-old children, only 16.6% of participants reacted to observers (²⁸). The main disadvantage of direct observation systems is that training of the observers is burdensome and time-consuming. Refresher training and regular interobserver reliability tests are necessary. In addition, extensive observer time in the field is required.

Validity and reliability of direct observation.

Direct observation systems have been validated against other measures of physical activity. CARS has been validated extensively against indirect calorimetry (²⁸), the Caltrac accelerometer (¹⁸), and the Actiwatch accelerometer (⁷). The CPAF system has been validated against HR monitors during physical education classes; the mean correlation between the CPAF and HR was 0.64 (²¹). A BEACHES assessment found that activity levels recorded by the system increased with increases in HR and energy expenditure, although no correlation coefficients were provided (¹⁷). Reliability of BEACHES was 94%-99%, and median κ values ranged from 0.71 to 1.0 (¹⁷). In terms of reliability, the OSRAC-P has demonstrated κ and interobserver agreement above 0.80 (²). The validity and reliability details for these systems are presented in Table 1.

ACCELEROMETRY

Principles of accelerometry.

Accelerometry is widely used as an objective measure of physical activity in preschool children. Accelerometers are small and unobtrusive devices that can be worn easily by preschool children; they typically are worn on an elastic belt and placed at the right side of the hip (^10,26,32). A piezoelectric sensor detects accelerations, which are converted from an analog signal to a digital signal, typically within a range (0.1-3.6 Hz) that specifically allows for human movement to be measured. Uniaxial accelerometers measure movement in the vertical plane only; the ActiGraph (ActiGraph, Fort Walton Beach, FL) is an example of a uniaxial accelerometer (^10,26). Omnidirectional accelerometers have the capacity to measure movement in multiple planes; however, they can only measure one axis at a time. The Actical (Mini Mitter) is an omnidirectional accelerometer that functions as a single-axis accelerometer (²⁷). The user orients the Actical in the axis deemed most important, which is typically the vertical axis. In addition, the Actical is waterproof and can be worn while swimming and bathing (²⁷).

Advantages and disadvantages of accelerometry.

Accelerometry provides an objective measure of physical activity, avoiding the biases that can be introduced by self-report or proxy report of activity. It allows researchers to measure physical activity intensities and patterns during all waking hours, for several days, for a large number of subjects. The risk to participants and the burden of taking part in a study is minimal. The potential for reactivity exists, although this is expected to be minimal. To our knowledge, there are no studies of reactivity to accelerometers in preschool children. The staff burden is moderate and is significantly lower than with direct observation.

A disadvantage of accelerometers is that they do not provide information on activity type or context. In addition, accelerometers are limited in their ability to measure non-weight-bearing activities, such as cycling and upper limb movements. In studies of preschool children, the cooperation of preschool staff and parents is required to ensure that accelerometers are worn correctly. Finally, accelerometers can fail, leading to loss of data (³³).

Physical activity intensity cut points.

For accelerometry data to be reduced to metrics that express physical activity in terms of intensity or energy expenditure, accelerometers must be calibrated against criterion measures. The resulting calibration equations can then be used to develop cut points for a wide range of activity intensities. The relationship between accelerometer counts and energy expenditure is population- and instrument-specific, and cut points vary widely, depending on the population and instrument under study. The cut points developed in a study of adults, for example, should not be applied to children, and cut points developed for the ActiGraph cannot be applied to the Actical. Variations in the time sampling interval and their corresponding cut points need to be considered. For example, the cut points developed on the basis of a 1-min sampling interval cannot be applied to data collected using 15-s intervals. Because young children tend to be active in sporadic bursts of energy, short (e.g., 15-s) time sampling intervals are recommended for studies of preschool children.

Researchers have conducted several calibration studies to determine appropriate cut points for preschool children (^25,27,29,30). Pate and colleagues used indirect calorimetry as the criterion measure to determine cut points for the ActiGraph 7164 and Actical accelerometers (^25,27). The differences in the resulting cut points reflect the differences in the two accelerometers. Sirard et al. (³⁰) conducted a study to assess the need for age-specific cut points for preschool children. They found that different cut points may be needed for different ages of children, even within the narrow preschool age range (Table 2). Two studies have developed sedentary activity cut points for young children, using direct observation as the criterion measure (^29,30). The cut points obtained in these studies are quite different, reflecting the differences in sampling intervals (60 vs 15 s), observation systems, and populations of children. A summary of these studies is presented in Table 2. The findings of these and other calibration studies indicate that accelerometry can estimate activity intensity or energy expenditure in young children if the cut points selected are valid for the population and accelerometer under study.

Validity and reliability of accelerometry.

In the preschool population, accelerometer calibration studies have used both indirect calorimetry and direct observation as criterion methods. The validity of the ActiGraph 7164 is high for moderate-to-vigorous physical activity (²⁵), vigorous physical activity (²⁵), and sedentary behavior (²⁹). The Actical also demonstrated high validity for preschool children (²⁷). For reliability, the established cut points are moderate-to-good for the ActiGraph (^25,29,30) and for the Actical (²⁷) accelerometers. Both the ActiGraph and the Actical accelerometers are valid measures of physical activity in preschool-aged children. Validity and reliability details for accelerometry are presented in Table 2.

PEDOMETERS

Pedometers are similar to accelerometers, in that they measure movement in the vertical plane and are typically mounted on the right hip. However, pedometers measure the frequency of movement not the intensity of movement. This type of data is easier to interpret than accelerometry data; no initialization or downloading is required, and step counts can be read directly from the device. In addition, pedometers are a cost-effective alternative to accelerometry because of their significantly lower cost. A disadvantage of pedometers is that they do not provide information on the type, intensity, or context of physical activity.

There is evidence that pedometer step counts provide an adequate assessment of physical activity in preschool-aged children. Pedometer step counts are strongly correlated (r = 0.73) with moderate-to-vigorous physical activity captured using accelerometry (⁴). Researchers have also found that pedometers are strongly correlated with the CARS direct observational system, with significant correlation coefficients of 0.64 (¹⁵), 0.59 (²²), and 0.86 (¹⁶).

Participant burden associated with wearing a pedometer is minimal. Preschool children may want to open or reset the counter, and in some cases, researchers have used wire to prevent the device from being opened (⁴). There are no studies of reactivity to pedometers in preschool children; however, in elementary school-aged children, researchers concluded there was no reactivity when the pedometer was sealed (^23,35) or unsealed (²³). The level of data collector burden depends on the design of the study, primarily whether data collectors or the parents/preschool staff records participants' step counts at the end of each day (^4,22).

Additional Measures

HR monitors.

HR monitoring also can be used to measure physical activity in preschool-aged children. This method assumes a linear relationship between increasing physical activity and HR. However, HR also is influenced by age and emotional stimuli, and at higher intensities of physical activity, the relationship becomes nonlinear. The resting HR also has to be adjusted for when determining the increased HR associated with physical activity. However, there is no consensus regarding the definition of resting HR in preschool children, and different definitions lead to different estimates of physical activity (¹⁴). If HR monitors are used to measure physical activity in preschoolers, Durant et al. (⁶) determined that just more than 4 d of measurement are needed to attain reliability of 0.80.

Proxy reports.

Some studies of physical activity in preschool children do not have the resources needed to successfully implement direct observation or an objective measure of physical activity. For these studies, proxy report by parents may be a suitable option. Preschool children cannot recall and report their activity, but parents and, in some cases, teachers can do so with a modest level of reliability.

Burdette et al. (³) developed a brief checklist for parents to record the time their preschoolers spent outdoors over two weekdays and one weekend day. When compared with accelerometry, time spent outdoors was positively correlated with total physical activity. A second parental report design used parental opinion about their child's physical activity in relation to other children their age and gender (¹²). These proxy approaches for the measure of physical activity should be interpreted with care.

CONCLUSIONS

Before 1990, there was a very limited scientific literature on the measurement of physical activity in young children. However, in the past decade, interest in this issue has grown substantially. Systems for direct observation of physical activity have become quite sophisticated, and the psychometric properties of these instruments have been well described. Likewise, accelerometry has become a well-established measure of physical activity in young children as well as older groups. Direct observation, which can provide information on type and context of physical activity, is an excellent complement to accelerometry, which provides detailed information on the intensity and duration of physical activity but no contextual information. Pedometry and HR monitoring have been shown to be applicable as measures of physical activity in young children, but these methods have been studied less extensively than direct observation and accelerometry. Proxy reports of physical activity are attractive because of their low burden, but they have limited validity.

No funding was received for this work.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.