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1. General considerations in deciding what to measure
2. Methods of acquiring information on the physical activity of infants and children
2.1. Questionnaire or diary record
2.2. Direct, objective measurements of activity
2.3. Heart-rate recording
2.4. Methods of acquiring information on energy expenditure
J.V.G.A. DURNIN *
* Institute of Physiology, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K.
Measuring physical activity in infants and children poses some problems, and these are compounded if it is necessary or desirable to have an assessment of actual energy expenditure as well. This paper therefore begins with some general considerations on what measurements might be useful under various circumstances. Methods of acquiring information on the physical activity of infants and children (questionnaire or diary record, pedometers, accelerometers, actometers, video-recording, heart-rate) and the energy expended for it (indirect calorimetry, stable isotopes) are then discussed, pointing out their advantages and limitations.
Physical activity plays an important part in the normal development of infants and children. Its functional importance has many dimensions. We shall deal here primarily with the physiological and mechanical aspects; the psychological and social ones are discussed in other contributions to this workshop.
In many circumstances, the measurement of physical activity is important in infants and children to allow, for example, some assessment of whether its level may be so low, either for voluntary (i.e., cultural) or involuntary (i.e., nutritional) reasons, to cause some concern.
Measuring physical activity in infants and children poses some problems, and these are compounded if it is necessary or desirable to have an assessment of actual energy expenditure as well. The problems are compounded because (1) there are technical difficulties in directly measuring energy expenditure in young children, and (2) in some circumstances, a comprehensive assessment of the data requires an assessment of fat-free mass and of fat mass, and we do not have enough basic biological information on infants to allow an actual assessment of fatness or leanness. We can, for instance, measure the standard skinfold thicknesses of a 3-year-old boy and say their sum amounts to 32 mm. We can compare this with the average of some group of 3-year-old children of 25 mm. We cannot, however, derive from this the percentage fat of his body weight or his fat mass. It may therefore be justified to discuss briefly, if and when energy expenditure actually needs to be measured and when assessing some other parameter would be almost as useful.
Our answer will depend on the objective for the study, and on our attitude to the benefits of physical activity and whether we believe these benefits will depend upon the type of activity, its degree of strenuousness, and its duration. If our attitude is that light-to-moderate exertion of limited duration is what is desirable, then we probably do not require the measurement of actual energy expenditure if we can obtain a reasonably descriptive assessment of the type of activity. 'Running around playing with other children' (preferably with some indication of its duration) might be sufficient to convey the information that a specific activity of at least moderate degree has occurred, and the actual level or the energy expended in the 'running around' would be of minor importance.
On the other hand, if we believe that to obtain some of the important benefits of physical activity, even in childhood, necessitates that the activity is moderately severe, the assessment of this might often be too inaccurate without the corroboration of some objective measurement such as heart rate or energy expenditure.
However, in either case, we gain much useful information from a reasonably detailed report of the daily pattern and duration of activity. The precision of this information can have differing levels of importance, and it could well be that moderately precise data would be sufficient to give us all we need in order to put the individual child we are studying into some order of ranking. Very vague basic information could, of course, make our decisions open to so great a possible error that the situation would not be acceptable. But it is quite conceivable that data obtained by some of the techniques we shall be considering could be gathered in such a way that the technical problems and the labour involved do not become excessive and yet data is provided of adequate detail and validity. Thus, children could be ranked, on the basis of objective or semi-objective data, into perhaps one of 4 ranks: 'very active', 'moderately active', 'relatively inactive' and 'very inactive (HEIKKINEN, WATERS and BRZEZINSKI, 1983). These rankings could have an energy equivalent attached to them, if values of actual energy expenditure were needed.
For most purposes for which we might require knowledge of an infant's or child's physical activity, this sort of ranking would probably be adequate. This will be discussed in more detail later but, apart from special situations, actual measurements of energy expenditure might almost never be necessary. This would be practically advantageous for the investigation, since such direct measurements are technically demanding and costly.
The energy expended in physical activity can be important for two reasons: (1) the amount of physical activity is closely related to the total mean daily energy expenditure - a very inactive and a very active child will have different requirements for dietary energy - and (2) if dietary energy is limited by inadequate food availability, then physical activity may also be restricted, with consequent effects on the child's development.
Although it can be assumed that most healthy, free-living children would be naturally active, in some 'developed' societies there are at least two important influences which tend to reduce physical activity in children, each of them indirectly resulting from an increased material wealth in the community: (1) many children no longer need to walk to school since transport is provided, and (2) television has very much extended the time spent sitting passively, even by quite young infants. The Nielsen "Report on Television" (1990) indicates that in the U.S. 2- to 5-year-old children watch television on average 4 hours per day and 6- to 11-year-olds 3.5 hours daily.
An interesting example of the progressive reduction in physical activity of children in the U.K. over the past 50 years is provided by Table 1.
Table 1. Energy intakes of 14- to 15-year-old boys and girls (kcal/d)
1930's (ref. 1)
1960's (ref. 2)
1970's (refs. 2, 3 and 4)
1980's (ref. 5)
1. WIDDOWSON (1947)
2. DURNIN et al. (1974)
3. COOK et al. (1973)
4. DHSS (unpublished)
5. DH (1989)
The children studied by Widdowson in the 1930's were middle-class, and so were taller and heavier than comparable general population groups at that time. However, the secular increase in heights and weights of children, which has been occurring during the past 50 years, meant that the other boys and girls, who came from a wide socioeconomic range, had 'caught up' and all groups represented in the table had very similar heights and weights. The differences in intakes of these four groups of boys and girls cannot be explained, therefore, by differences in body mass. It would be more satisfactory if actual measurements of either energy expenditure or physical activity had been carried out on them, but the only likely explanation is a continued fall in physical activity surely not representative of a desirable state.
2.1. Questionnaire or diary record
2.2. Direct, objective measurements of activity
2.3. Heart-rate recording
2.4. Methods of acquiring information on energy expenditure
In adults, common ways of acquiring data on physical activity are (1) by questionnaire or record which may require an interviewer or may be self-administered; obviously, for young children a parent would be needed as an intermediary, (2) by some form of measurement of movement, including by direct observation, (3) by heart-rate recording, and (4) by direct measurement of energy expenditure.
Some questionnaires may be conscientiously filled in by a parent in 15-20 minutes, others may require the services of a skilled, experienced interviewer and may take 1-2 hours to complete. Which variant is chosen depends on the objective of the study, on the target population, and on the background and beliefs of the investigator.
Physical activity, or the lack of it, will affect mental and physical development almost from the age at which the infant begins to crawl around. Therefore, activity may need to be measured from the age of between 1 and 2 years. Since we are concerned here with problems of measuring energy and activity in infants and children, it is pertinent to ask the question: "Up to what age will these measurements be significantly different from a similar assessment in adults?". It is difficult to give a precise, generally valid answer to this question, since it will be affected by various factors such as the type of population (a poor village in a developing country or a middle-class group in an industrialized country, for example), the type of activity (children's games), weight and body composition, and probably others, in most situations it is likely that from the age of young adolescence there are no major differences from adults, so the age span discussed in this paper is roughly from 1-2 years up to 10 years.
In the older part of this age range, children can be questionned directly about their physical activity but filling out such a questionnaire would require the services of a skilled interviewer; it is improbable that any self-administered system will, in general, be suitable for children up to 10 years of age. Otherwise, the techniques of acquiring data on physical activity in children will often be very similar to the equivalent situation in adults.
Basically, the information needed about physical activity is (1) a detailed description of the activity, (2) the intensity (preferably, by objective assessment rather than subjectively), (3) the duration, and perhaps (4) the total energy expended in the activity (which may be estimated indirectly from existing tables and may not require actual direct measurements of oxygen consumption).
Data on these matters will permit us to speculate with reasonable precision about whether or not the child is active, how active he or she is and for how long, and roughly the likely energy expended in the activity.
To obtain the requisite information by questionnaire might be done via a 24-hour recall to the parent, plus an extrapolation to the previous days to arrive at an impression about the habitual activity levels of the child. However, it is difficult to imagine that what we learn in this way can be anything other than qualitative and, at best, may allow ranking of the child into one of 3 or 4 groupings. SARIS (1985), in studies on Dutch children aged from 4 to 12 years, used questionnaires of this sort, and examples of the type of questions asked are given in his paper.
A diary method of recording what an individual child actually does, used prospectively either by a parent or other observer, could, at least theoretically, produce useful data. However, it requires considerable dedication by the observer - perhaps noting down once every 2 or 5 minutes exactly what the child is doing at that moment. The recording has to be relatively simple, e.g., perhaps only the four categories of sitting, standing, walking, and running (with each of the last two being described in two levels - 'high' end 'low') would be adequate. The actual observation does not, of course, have to extend over the whole waking day and might, for preschool children, have a duration of only a few hours, from say, mid-morning to late afternoon. In order to obtain observations which would probably be more representative of normal behaviour, these observation periods should take place on at least 3 different days to avoid an unusual day giving bias to the results. For school children, much of the school period entails enforced inactivity, and the intervals between classes and the immediate after-school period might, for various reasons, be difficult to record and might not represent average behaviour; the week-end might be more representative of normal activity patterns.
Direct, or objective, measurements of activity have both advantages and disadvantages over a questionnaire or observational study. Firstly, because they involve an impersonal measurement and not a subjective impression, the data are more reliable. Secondly, because quantitative data are provided, more subtle assessments may be made than just gross subdivisions of activity. The disadvantages are both technical and financial. Studies that entail objective assessment of activity, require relatively expensive instruments. This may force the investigator to reduce the number of subjects from hundreds or even thousands of individuals who could be studied by a combination of questionnaire, or prospective diary records and observation, to no more than tens of individuals where instrumented, direct recording is performed. Lastly, many parents might be prepared to assist in the former method, and not wish the trouble and inconvenience of ensuring that a recording instrument continues to function properly.
Nevertheless, both approaches may be combined on occasion, with a large population being studied by questionnaire, and a subsample having objective measurements of activity to validate the larger data base.
Several techniques for measuring activity are available. Some simply record movement - such as pedometers, accelerometers, actometers, or electrical devices for counting steps. Others, such as the video-camera, may record activity in detail.
All of these techniques have been used in studies of physical activity in adults, and have been tried with varying degrees of success in children.
Pedometers have been in use for many years. While their design has improved recently, they can still occasionally give quite erroneous recordings and they do not, of course, necessarily detect activities performed in a more or less static position - arm movements, for instance. Even for a very rough estimation of activity they will have limited usefulness, and, for example, SARIS and BINKHORST (1977) found them very inaccurate.
An instrument, designed to provide an output similar to that of a pedometer but using a different principle, records an electrical impulse each time the shoe touches the ground. Even though, like pedometers, these instruments do not necessarily record all types of activity, they may have a role in a rough categorization of individuals into various groupings.
Accelerometers measure horizontal, lateral, or vertical movement, but counts may not differ in lying, sitting and standing, and work involving only the arms will not greatly increase the count rate of an accelerometer mounted on the trunk. These instruments are of most use in laboratory-based studies and are not particularly useful in large-scale field studies.
Actometers can be many different types of instrument; often they have the form of a modified wrist watch capable of recording acceleration and intensity of movement. They are inexpensive instruments and have been used successfully in adults and children for discriminating between groups of individuals with differing activity patterns. Studies have been described by SARIS and BINKHURST (1977), LAPORTE et al. (1979), TRYON (1987), and AVONS et al. (1988). However, they are not capable, as was originally hoped, of giving an indirect estimate of energy expenditure, and the fact that they need frequent calibration - for individual subjects, for the types of activity, and for the instruments makes it unlikely that they will be employed in large-scale studies in the near future.
Video-recording has a definite role in the assessment of children's activity patterns. However, obviously this is a technique which is unlikely to be practicable for large groups of individual children. The size of group which may be analysed by video-recording will vary depending on the objective of the study and, perhaps more importantly, on the financial resources available, since cameras and the actual analyses of the nature, frequency, and intensity of the movements of the child are all expensive.
There are obvious theoretical advantages of video-recording over direct observation and the recording of activity on a specially designed diary form. Direct recording requires immediate judgement of the actual activity and the noting down in some predetermined manner an abbreviated description of the activity. Often one observer may be required for one or, at the most, two children. If observations are needed over several hours, it may be difficult for the observers to maintain a satisfactory level of attention.
Video-recording can be used on several children simultaneously, and it can be played-back as often as required to make the appropriate decisions. It is also possible to use it in a more unobtrusive way, and it is more likely to be socially acceptable in both the developing and developed world. The practical problem, however, lies in the difficulty of analysing and quantifying the video-recordings to allow some ranking of individual children. It may provide the best way of assessing the activity of an infant or child in combination with some form of questionnaire or a diary record.
It is technically relatively easy to record heart-rate over prolonged periods of time. If only a record of heart-rate is needed, this is not an expensive technique and instruments are readily available. If a more elaborate break-down of the data is desired - such as the duration of several different ranges of heart-rate, to indicate differing levels of activity - say 80-100, 100-120, 120-140, over 140 beats/min - then this will become expensive and somewhat complex.
Heart-rate recorders can be purchased to provide almost any information on heart-rate that is desired. However, there is some confusion about what exactly can be derived from heart-rates, and some basic physiological facts need to be repeated.
Firstly, the heart-rate is related to cardiac output. This relation is closer at higher levels of exercise than at moderate and low levels, because of the many other influences on heart-rate, such as posture, effect of meals, psychological factors, the muscle masses involved, and so on. At levels which are most common for infants and children in ordinary daily life, heart-rate has a highly variable relationship to the level of physical activity, and this relationship can also vary within and between individuals.
Thus, heart-rate can be a good general indicator of activity, but can not be a very precise indicator of energy expenditure. Moreover, analysing heart-rate records, even using computerized techniques, often requires some expert decision-making and is a tedious procedure. An additional problem with young infants is that they tend to tamper with electrodes and connecting electrical wiring. We had to discontinue a study on 2-3-year-olds for this reason. Heart-rate recording has a definite place in the methodology of assessing physical activity in children, but has to be employed with discretion.
2.4.1. Indirect calorimetry
The classical technique of measuring energy expenditure by indirect calorimetry requires either the use of a Douglas bag to obtain samples of expired air for analysis or some form of respirometer. Neither method is ideal, especially in the case of small children. Moreover, since there is really no satisfactory respirometer on the market, the method has very considerable restrictions.
Neverthess, the Douglas bag technique can be utilized to provide the actual energy cost of a particular activity, and if measurements are carried out on the variety of activities of any individual child, together with information on the duration of these separate activities, then the total energy expended during any appropriate period of time can be calculated. If it is decided to use this technique, useful descriptions of the methodology and of problems incurred in a field study are given by SARIS (1982) in his account of his studies on children in Holland.
It has already been stated earlier in this article that there may be occasions when actual energy expenditure values are necessary in the assessment of the importance and implications of physical activity in infants and children. Nevertheless, these are rare situations, and the resources needed to carry out the measurements on anything other than a very small and statistically unsatisfactory scale, make it almost a prohibitive procedure. Fortunately, adequate information can generally be obtained without it on any relevant aspect of activity. It is usually possible to extract from published tables of energy expenditure, a value for a particular activity which might be reasonably appropriate for a given individual child.
2.4.2. Stable isotopes
The mean energy expenditure during a period of several days can be measured by an indirect estimate of CO2 production after giving a dose of the stable isotopes deuterium and 18O to the infant or child. The required number of days during which samples of body fluid have to be collected for the isotope analyses is still a matter of some uncertainty, but may be at least 7-10 days.
On the assumption that these measurements provide a valid estimate of total energy expenditure over these 7-10 days or more, is this technique of value in the present context? If there were only minor problems entailed in this procedure, we might well feel that such measurements provided some useful information. However, at least for the present, there are considerable technical difficulties with the mass spectrometers used to measure the isotopes, and there are also large financial costs for both the isotopes and the instrument. More importantly, the information obtained may be no more than would be available from an accurate measurement of energy intake, since no details about levels of physical activity can be provided. Estimates of total energy expenditure in a group of infants or children may well conceal important differences in the type, intensity, and duration of physical activity and therefore, in the present context, may have rather limited value.
The assessment of physical activity in infants or children is difficult but nonetheless feasible. For most purposes one, or a combination, of various techniques can be employed - questionnaire to a parent, observation (direct or by video-camera), pedometers, accelerometers, actometers, or heart-rate recorders. Careful and appropriate use of these methods will allow reasonably detailed data on the type, intensity, and duration of activity to be monitored. For this purpose, costly stable isotope techniques have no advantage.
The exact form of the detailed data will vary with the objectives of the study, and will obviously differ in the case of the relative importance of activity for physical development, and for socio-emotional or cognitive development. If nutritional factors are thought to be influencing the ability of the child to be active (e.g., malnutrition resulting in decreased activity) then the assessment of this may require some knowledge of the food intake. There are probably few circumstances when a measurement of the energy expenditure in physical activity will be really necessary, other than perhaps in providing a more exact estimate of the energy expended in specific activities.
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DURNIN, J.V.G.A., LONERGAN, M.E., GOOD, J., EWAN, A.: A cross-sectional nutritional and anthropometric study, with an interval of 7 years, on 611 young adolescent schoolchildren. Br. J. Nutr., 32, 169-179 (1974).
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