Estimation of 24-Hour and Physical Activity Energy Expenditure
It is very costly, and in most cases impractical, to measure 24-hour energy expenditure or to monitor physical activity energy expenditure through either direct or indirect calorimetry. These two methods of measurement are used only for research and clinical applications. Therefore, most professionals use energy expenditure prediction equations, movement analysis devices, or both, to estimate the energy cost of physical activity and the 24-hour energy expenditure for their clients. Tools used to monitor physical activity and estimate energy expenditure range from expensive and sophisticated machines, which are found only in health centers, to inexpensive gadgets and activity diaries, which can be found in almost any setting.
The most plausible tools for measuring either 24-hour energy expenditure or physical activity energy expenditure in the field are pedometers, accelerometers, and heart rate monitors. Pedometers are more suited for monitoring physical activity than 24-hour energy expenditure, whereas accelerometers and heart rate monitors are well suited for both.
Pedometers have been around for several decades. The pedometer itself measures the number of steps taken during the day. The summation of these steps is converted to a distance, and energy expenditure is estimated based on the distance traveled. Pedometer estimates for physical activity energy expenditure correlate moderately well with indirect calorimetry measures (Brown, Miller, and Eason 2006). However, only the total distance traveled is recorded on the pedometer, and there is no indication of the intensity of the physical activity. Therefore, pedometers are useful for gaining insight into 24-hour energy expenditure, but do not offer any reference to exercise intensity or activity patterns throughout the day. An advantage of pedometers is that they are relatively inexpensive; even children can learn how to use them.
Accelerometers work on a principle that is different from that of pedometers. Accelerometers contain tiny force transducers that continuously measure the intensity, frequency, and duration of movement for extended periods of time. The forces measured by the accelerometer are summed and recorded as counts per time frame. There is no consensus about the accelerometer count thresholds for defining mild, moderate, and high exercise intensities. Nonetheless, accelerometers are valid and reliable for monitoring physical activity counts in both children and adults. Correlation coefficients between accelerometer counts and indirect calorimetry measures range from about 0.60 to 0.85, which represent fairly high correlations (Brown, Miller, and Eason 2006).
The advantage of accelerometers over pedometers is that accelerometers can measure the intensity of energy expenditure throughout the day, and this information can be downloaded to a computer. The computer then generates the data and pinpoints the fluctuations in energy expenditure at any time of day. The computer also uses regression equations to calculate the actual energy expenditure from recorded activity counts.
Heart Rate Monitors
Heart rate is strongly related to respiratory rate and energy expenditure across a wide range of values. Heart rate monitors are similar to accelerometers in that they can accumulate data from short or long bouts of activity throughout the day. Heart rate data can also be downloaded to a computer, and the magnitude of fluctuations in heart rate during the day can be pinpointed. Regression equations are used to convert heart rate measures to energy expenditure.
Activity Surveys and Diaries
Activity diaries necessitate that the participant (or an adult observer in the case of young children) make a record of every activity undertaken throughout the day. The person describes the nature of the activity and the time spent participating. This record includes activities that are sedentary as well as those that require physical exertion. Predetermined values for the energy expenditure of each activity noted in the diary are applied, and the energy expenditure is summed across time and throughout the day.
Activity surveys are similar to activity diaries, but rather than record the actual events at the time they occur (or shortly thereafter), recorders estimate the activity of an average day or an average week or month. In other words, people describe their usual routines over a period of many days, rather than recording actual events over a period of a few days. Calculations for energy expenditure are performed as with activity diaries to get the estimated energy expenditure.
The accuracy of physical activity surveys and diaries is variable; they range from being rather poor indicators of actual physical activity to being relatively good measures of physical activity. Activity surveys and diaries for children tend to be less accurate than those intended for adults. Nonetheless, physical activity surveys and diaries are commonly used to determine physical activity levels in both children and adults, because they are inexpensive, unobtrusive, and easily administered.
Many physical activity surveys have been designed for adults. Some of these have been intended for specific populations, or constructed specifically for independent research studies. The reliability and validity of these surveys is variable. The most popular of these surveys were collected and published by the American College of Sports Medicine (1997) several years ago.
One of the most popular physical activity surveys is the International Physical Activity Questionnaire (Craig et al. 2002; IPAQ 2011). The IPAQ comes in a long and short version, and in several languages, and can be downloaded (IPAQ 2011). Both versions ask respondents to record their health-related physical activity for the past seven days. Both versions can be either administered by a professional (in person), or self-administered. The long version consists of 27 questions that focus on job-related physical activity, transportation-related physical activity, housework, recreation and sport activity, and sedentary or sitting time. The short version of the form asks only seven questions about time spent in vigorous physical activity, moderate intensity activity, walking, and sitting.
A popular physical activity diary for older children and adolescents is the Previous Day Physical Activity Recall (PDPAR; Children’s Physical Activity Research Group 2011). The PDPAR was designed to provide accurate data on the type, frequency, intensity, and duration of physical activities; these are then used to estimate physical activity energy expenditure (Weston, Petosa, and Pate 1997). The PDPAR is an activity diary that is segmented into seventeen 30-minute intervals. Participants are given a list of 35 numbered activities in which youth normally engage. They record the number of the activity in which they participated for any given 30-minute interval of the previous day. For the selected activity, they also record the intensity as being very light (slow breathing and little or no movement), light (normal breathing and movement), medium (increased breathing and moderate movement), and hard (hard breathing and quick movement). An estimated energy expenditure value is then calculated for each activity within the given time frame.