1. General considerations in deciding what to measure

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)

References:

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.