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Time allocation to different activities

The habitual physical activity of children and adolescents differs among societies with different cultural characteristics and among groups of different socioeconomic conditions in the same society. For example, while many children in rural areas of developing countries partake in domestic chores or are part of their community's labor force from an early age (Rodgers and Standing, 1981), most children in industrialized countries attend school for several hours, and those in a better socioeconomic situation do not have any work obligations.

Many studies have addressed various aspects of the time allocated by children to their daily activities. These have been performed with diverse objectives by researchers whose main interests are in nutrition, physiology, anthropology, human behavior or economics. Methods have included continuous or spot observations, recall interviews with children or caretakers, subject or observer diaries, and analysis of heart rate patterns. Results have been analyzed and presented as specific activities or classified according to their purpose or physical effort.

Table 10 Mean differences between measured MBR in children of different races and BMR calculated from Schofield's equations (1985)

^a Stunted: >1.5 s.d. below the NCHS median of height-for-age. Underweight: <95% of weight-for-age and weight-for-height in comparison to Colombian children of upper socioeconomic groups (Rueda-Williamson et al, 1969). All others: adequate height and weight for age.
^b + indicates that Schofield's formulas give higher values, and - indicates lower values.
^c Mixed: various degrees of mixture between caucasian and indigenous.

Quantification of total daily time distribution

The variety of methods and the lack of a standard for presenting the data make it difficult to compare across societies and to combine the results of different studies. This is further impaired by the selective nature of some studies that focus on one type of activity, and by incomplete information, such as indicating children's involvement as a percentage of activities performed without information on the time period. We, nevertheless, made an effort to compare and combine information after a critical revision of studies with time allocation data.

From a review of more than 70 studies that had some information, we identified 39 with data of sufficient quality and completeness to quantify children's total daily time allocation (Table 11).

Table 11 Studies used to evaluate and quantify children's time allocation (see 'References' for full bibliographic information)

We classified activities according to two types of characteristics:

(1) Intensity of effort and energy expenditure: (a) sleep, (b) sedentary, (c) light, (d) moderate, (e) heavy. When those categories were used by the investigators, their criteria for classification were respected. When not, we allocated the time to the corresponding category according to the description of the activity or to the children's heart rate, following the criteria shown in Table 12.

(2) Nature or purpose of the activity: (a) sleep, (b) school, (c) domestic chores, (d) production (with or without wages), (e) non-work activities. Table 13 gives descriptive examples. 'Recreational activities' are mentioned in some studies. These are non sedentary leisure activities that involve more effort than the general 'non-work activities'.

Classification of activities according to their physical effort permits making estimates of total daily energy expenditure of children with different lifestyles. Most studies that describe the nature of activities, such as in Table 13, do not indicate the degree of physical effort involved. They must be assigned an energy cost, or at least an intensity of effort, to allow comparing with studies that allocate time according to the level of energy expenditure.

Table 12 Criteria to classify the physical effort of activities according to the children's heart rate

Table 13 Selected examples of activities classified according to their nature or purpose

Although the energy cost of some activities listed in Table 13 has been measured by indirect calorimetry, that of many others has not (see review by Torun, 1990a). Furthermore, many tasks involve a variety of specific activities with different energy demands (for example, house cleaning can involve light dusting or heavy sweeping), and pauses of different length may be interspersed with the actual physical endeavor. Consequently, we made an empirical estimation of the physical effort involved in the activity categories of Table 13, based on the energy costs that have been measured, the descriptions available in some studies, our own experience, and the assumption that domestic and productive activities in developing societies involve more physical effort than their equivalents in developed countries or urban centers. This is shown in Table 14. As with all empirical estimations, this can later be modified but it is a starting point to compare studies.

The age groups were classified as 2-5, 5-9, 10-14 and 15-19 years, as this was the age breakdown allowed by most of the reviewed studies. In addition to the overlap between the 2-5 and 5-9 groups, there was some overlap between the other categories, as some studies presented data on children aged 9-11 or 13-15.

Tables 15 and 16 show the factorial distribution of the time allocated by boys and girls, respectively, to activities with different energy demands. They are presented separately for children from industrialized countries, cities in developing countries, and rural areas in the latter, as the activities performed and the energy expenditure involved vary in each of those settings.

Table 14 Effort empirically assumed to be required by the activities listed in Table 13

^a Described as such in some studies. They Are non-work activities. that Are not sedentary

Table 15 Weighted averages of time allocated by boys to activities that require different levels of physical effort^a

^a Sources are listed in Table 11. Averages were weighted on the number of children in each study; refer to the text for explanation of procedure when the exact number of children was not known or it was too large in relation to other studies.
^b Some numbers of children are approximations, as some studies do not give exact figures.
^c Expressed as multiples of BMR or Physical Activity Level. Not calculated when time allocation was reported in only one study.
^d Energy cost of activities, in multiples of BMR, as suggested by Torun (1990^a)

Table 16 Weighted averages of time allocated by girls to activities that require different levels of physical effort^a

^a Sources are listed in Table 11. Averages were weighted on the number of children in each study; refer to the text for explanation of procedure when the exact number of children was not known or it was too large in relation to other studies.
^b Some numbers of children are approximations, as some studies do not give exact figures.
^c Expressed as multiples of BMR or Physical Activity Level. Not calculated when time allocation was reported in only one study.
^d Energy cost of activities, in multiples of BMR, as suggested by Torun (1990a)
^e In one of the two studies 24 girls were studied longitudinally four times at 3-month-intervals.

Time distributions were calculated as weighted means from several studies, weighting them for the number of children involved, and rounding the time to the nearest half-hour. In studies that only presented the number of households, the number of children was assumed to be either 50 or 33% of those households, depending on other information related to the study. When the number of boys and girls was not given, equal numbers were assumed for each sex. When a study greatly outnumbered the sample size of all others for that sex and age category, only 50% of its sample size was used to calculate the weighted mean in order to reduce the bias of the results towards a single study. For example, 8 of 9 studies on boys 10-14 aged years old in industrialized countries involved between 11 and 171 children, whereas the ninth study involved 360; a weight of 180 was given to that study.

Tables 15 and 16 show that, compared with children in industrialized societies, children in developing rural areas sleep less at night, participate longer in moderate and/or heavy physical activities, and have a greater energy expenditure in relation to their basal metabolic rate. There are very few studies on children in cities from developing countries, but their physical activity falls between the other two groups, resembling more that of children in industrialized countries than that of their rural counterparts. Within the same type of society, there were no striking differences between boys (Table 15) and girls (Table 16).

In terms of the nature or purpose of the activities, children of school age in industrialized countries spend between 4.5 and 7.5 h at school during school-days. In developing countries, children in urban areas spend similar amounts of time at school, although many from the lower socioeconomic groups do not attend school at all, especially after 12 years of age. School attendance is less among their rural counterparts, who average between 0.5 and 2 h per day (Table 17).

Table 17 also shows that children in rural traditional societies of developing countries begin domestic and productive work at preschool age, and from 10 years onwards they have an important daily workload. Girls are involved in domestic work longer than boys and, after 9 years of age, boys spend more time than girls in production and wage-earning chores.

Estimations of total daily energy expenditure

Total daily energy expenditure was estimated from the time allocations in Tables 1:5 and 16, and the energy costs of sedentary, light, moderate and heavy activities suggested by Torun (1990a) as shown in those tables; the energy cost of sleep was assumed to equal basal metabolic rate. The results, expressed as PAL or multiples of BMR, are shown in the last column of those tables.

Table 17 Time allocated to school attendance, domestic work, productive work and non-work activities by children of native, traditional, rural populations from several countries^a

^a Bangladesh (Cain, 1977), Borneo (Colfer, 1981), Botswana (Mueller, 1984), Guatemala (Loucky, 1988), Indonesia (Nag et al, 1978; Hart, 1988), Ivory Coast (Berio, 1984), Kenya (Munroe et al, 1983; Munroe & Munroe, 1989), Papua/New Guinea (Grossman, 1984), Panama (Franklin & Harrell, 1985), Peru (Munroe et al, 1983; Johnson & Johnson, 1987), Philippines (Carbañero, 1980), Nepal (Nag et al, 1978; Acharya & Bennett, 1981), Venezuela (Paolisso & Sackett, 1987), Western Caroline Islands (Turke, 1988).

PALs were converted into kcal/kg/day applying Schofield's equations (Schofield, 1985) to the body weight at the mid-point of the age intervals shown in Table 18 (i.e. 7.5, 12.5 and 17.5y). The NCHS/WHO median weight for age was used for children in industrialized countries, and it was assumed that the average weights for children in urban and rural developing areas corresponded to the 30th and 20th centiles of the NCHS values, respectively. The remarkable agreement with the estimates of total daily energy expenditure by the doubly-labeled water and heart rate methods (Figure 6) suggests that the criteria for classification of activities shown in Tables 13 and 14 and the factors used to assign them an energy cost (Tables 15 and 16) were good estimates.

Tables 15, 16 and 18 suggest that total energy expenditure expressed as PAL is similar for boys and girls within each age group and geographic/developmental category. In industrialized countries, it is constant between 5 and 14 years (and similar to cities in developing countries), and it increases by about 5% after that age. In rural developing societies, daily energy expenditure increases with age, as a reflection of children's increasing involvement in energy-demanding chores.

An analysis of the estimates of total daily energy expenditure shown in Table 18 indicates that, based on multiples of BMR, children of 5-9, 10-14 and 15-19 years spend about 10,15 and 25% more energy in rural developing societies than in industrialized countries. When expressed as kcal/kg, the corresponding increments in energy expenditure are about 15, 25 and 30% for the three age groups, respectively.

Table 18 Estimates of total daily energy expenditure of children based on the data shown in Tables 15 and 16, and the median weights assumed for the age span

^a Children in industrialized countries: NCHS median for mid-point of age range (i.e., 7.5, 12.5 and 17.5y); children in developing urban centers: 30th centile; children in rural societies: 20th centile.
^b Basal metabolic rate was converted to kcal/kg/day using the formulas suggested by Schofield (1985).

Conclusions

We believe that more insightful information on children's time allocation and its energy cost is lying unanalyzed in existing databases of nutritional, physiological and anthropological studies. Efforts must be made to retrieve, analyze and present them in a standard manner to allow making better estimates of children's energy expenditure and requirements, as well as of the behavioural and social implications of their time distribution.

The data that we were able to analyze indicates that, beginning at least at 5 years of age, children in rural areas of developing countries spend more time in activities that require more physical effort than children in cities or industrialized countries.

It seems that time allocation of physical activity is similar in urban areas of industrialized and developing countries, but more information is needed from the latter to confirm this notion. Information is also needed on the time allocated to activities by children and adolescents of different socioeconomic groups.

Table 19 Mean 24-hour physical activity levels of children and adolescents in industrialized countries and in cities of developing countries (based on data in Tables 2, 4, 7, 15 and 16)^a

^a Excluding studies with mean PAL < 1.40 for children over 5 years, and > 1.90 for all ages.
^b DLW: doubly labeled water; HR: heart rate monitoring; TA: time allocation; TM: time-motion/diary. (St): stunted or mildly underweight; otherwise, normal.
^c Weighted mean. Number of children in parenthesis.

The conversion of time allocation data to energy expenditure gives reasonable results when activities such as those listed in Table 13 are assigned the intensity of effort shown in Table 14, and the energy equivalents shown in Tables 15 and 16 are applied to sleep, sedentary, light, moderate and heavy activities.

When time allocation is converted into energy expenditure expressed as PAL, there is practically no difference between boys and girls within the same type of society.

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