This brief introductory chapter proposes a general categorization of physical activity that is appropriate to each of the topics discussed in this book. The impact of physical activity upon athletic performance, gastro-intestinal function and any related aspect of health depends upon the type of activity that is undertaken, and its pattern of intensity, frequency and duration.1 We will thus define each of these key terms.
Types of physical activity
Physical activity implies any type of body movement produced by the skeletal muscles that results in a substantial increase in an individual’s energy expenditure. The main potential components are the physical demands of occupational work, active physical leisure, exercise, sport and domestic chores, with occupational activity and leisure pursuits figuring largely in epidemiological studies.
Occupational work. The importance of occupational work to average daily energy expenditures has diminished greatly over the past 50 years, as workers have moved from agriculture to service and office jobs. Occupational activity is commonly assessed by job title, or by questioning employees on the time spent sitting, standing, lifting or engaging in heavy work. One problem with the use of job titles is that the physical demands of many occupations have diminished progressively through mechanization and automation.
The attraction of occupational activity for the epidemiologist is that an activity of relatively known intensity has been maintained for four to six hours per day, five days a week, for many years. However, there are often large socio-economic and cultural differences between “heavy” and “sedentary” workers, including the area of residence and features of lifestyle such as regular medical check-ups, smoking and alcohol consumption, and interest in and access to active leisure activities, and epidemiological analyses based on occupation must allow for these important covariates.
The standards of intensity also differ between occupational and leisure activity. Because heavy industrial work is sustained for 4–6 hours per day, and the pace is set by a supervisor, a machine or a conveyor belt, an energy expenditure of 31 kJ/min or more is judged as “very heavy” for a young or middle-aged worker. The situation may be further complicated by a task that involves only a small fraction of the body musculature, but requires maintenance of an awkward posture or involves exercise in a hot environment; such factors reduce the intensity of effort corresponding to a given intensity category.
Leisure activity. After work, a variable and growing period of commuting (usually passive, in a car, bus or train), completion of domestic chores and attention to personal hygiene, a typical person in developed societies now has three to four hours of “free” or discretionary time. This can be allocated between active and sedentary leisure pursuits and “do-it-yourself” tasks; automation of many domestic tasks is progressively increasing the discretionary component of daily life, but the growing cost of service industries is also increasing the need for many with smaller incomes to deal with household repairs. Although most epidemiologists have focussed on the active component of leisure time, some have also focussed on sedentary elements, such as the time spent watching television or working at a computer screen.
The volume of leisure activity is usually assessed by questionnaires that have a limited reliability and validity; in some instances, the absolute volume of activity may be exaggerated by a factor of three. There is growing interest in a more objective approach, using devices such as pedometers, accelerometers and GPS monitors. However, these instruments also have their problems, not recording such activities as swimming and cycling, and not taking account of energy spent in climbing hills. All data also suffer from the problem that leisure activity involves personal choice, and is highly seasonal; findings are thus likely to be invalid unless spread over the entire year.
Exercise. Exercise is a specific type of physical activity that is performed repeatedly with a specific purpose in mind, such as preparation for competition, or the improvement of health and fitness. In many with forms of chronic ill-health, the recommended mode of exercise, its intensity, frequency and duration, and speed of progression are regulated by a physician or a qualified health professional. Often, such exercise is undertaken in a fitness or rehabilitation class, and it is then possible to ascertain the energy expended with reasonable precision.
Sport. In North America, involvement in sport usually implies athletic competition or the use of a specially constructed facility such as a ski resort. However, in some European publications the term “sport” has been used to embrace many forms of leisure activity such as walking and jogging, as in UNESCO’s “Sport for all” programme. The categorization of subjects by sport participation is controversial, because selection for many sports is based upon body build (which in itself influences vulnerability to many conditions). Moreover, high-level sport participation is generally a feature of adolescence and young adulthood, and by middle age many former athletes have become fatter and less active than their non-athletic counterparts.
Household and other chores. Household chores are often neglected in considering total weekly energy expenditures. The automation of much domestic equipment has certainly reduced energy expenditures around the home, but substantial physical effort is still demanded for the care of dependents, both young children and the elderly. This task still falls mainly to women, and is not captured adequately by many of the popular physical activity questionnaires. A growing number of workers are adding active commuting (cycling or walking) to their daily activity; this is a form of activity that is not easily neglected, and can be quantitated rather precisely.
Patterns of physical activity
Intensity. Often, the intensity of aerobic effort is described in semantic terms, such as a “moderate” or “heavy” effort”, “brisk walking” or “vigorous getting about”, and sometimes there is a perceptual anchor such as “sufficiently vigorous activity to work up a sweat”, or “heavy enough exercise to make conversation difficult”. But in order to compare studies, it is helpful to quantitate such descriptions. The level of effort needed to undertake a given task depends on a person’s physical condition, and aerobic activity is thus conveniently categorized as a percentage of the individual’s maximal oxygen intake (O2max). Sweating and breathlessness typically imply a sustained bout of effort at 60–70% of an individual’s maximal oxygen intake.
For a typical 30-minute bout of activity involving the large muscle groups of the body, intensities range from resting (<10% of O2max) through light exercise (<35% of O2max) to fairly light (<50% of O2max), moderate (<70% of O2max) and heavy (>70% of O2max) to near maximal effort (100% O2max, an intensity that can only be sustained for a few minutes). The oxygen intake can be measured directly when exercising maximally and when performing the activity of interest, but in epidemiological studies inferences are more usually drawn from the heart rate, expressed as a percentage of maximal heart rate.
Some authors have chosen to present their data in absolute rather than relative units (for instance, a measured or estimated energy expenditure, measured in kJ/min) or as metabolic equivalents (METs, ratios to resting metabolism), but then the categorization of intensity must take account of the fact that the maximal oxygen intake of a 65-year-old is only about a half of that found in a 20-year-old. Possible intensities for a young adult range from 1 to 13 METs, or 5.2 to 67.6 kJ/min, and in a 65-year-old the range is from 1 to 7 METs, or from 5.2 to 36.4 kJ/min. Furthermore, at any age the percentages corresponding to each descriptor diminish if the duration of exercise is extended for more than a few minutes, as for example when industrial activity is considered.
Frequency. The frequency of activity is commonly reported as the number of times a given form of activity is practised per week. However, this type of recording ignores the seasonal nature of many leisure pursuits. Preferably, the investigator should record the number of times the activity was performed in the past week or the past month, and overcome the problem of seasonal variations by questioning various members of the test sample at differing points during the year.
It is commonly assumed that to have an effect upon health, a bout of physical activity must have a minimum duration. At one time, note was only taken of periods longer than 15 or 30 minutes, but there is now growing recognition that some benefits can accrue from the accumulation of several short bouts as from a single 30-minute session at the same intensity of effort (for example, 3 activity periods of 10-minute durations) over the course of a day. This is particularly true of responses that are linked to a reduction of body fat. Inevitably, fat losses are linked to the cumulative increase of energy expenditure over the course of a day. Moreover, there is a continuing increase of energy expenditure for some minutes following a period of vigorous energy expenditure, and it is conceivable that this effect can be augmented if the day’s physical activities are split into several segments.
Duration. The duration of exercise is usually recorded in minutes, but it is important to ensure that when this is reported a person includes only active time, omitting such factors as travelling to and from a gymnasium, changing and showering, discussions of exercise technique with a coach, and socializing with other exercisers.
Muscular versus aerobic activity. Muscular exercise is less often categorized than aerobic activity, although regular exercise for each of the main muscle groups of the body makes an important contribution to health. The intensity of a given isometric effort can be categorized as a percentage of the intensity of effort a person can make on a single occasion (the one-repetition maximum), and the cumulative stimulus can be expressed on this basis (for instance, 3 sets of 10 repetitions at 60% of 1 RM effort). Such information is sometimes available for participants in specific muscle-building programmes, but is rarely documented in population studies of physical activity.
Conclusions
Clarification of the effects of regular physical activity upon the normal functioning and health of the gastro-intestinal tract is dependent in part on a uniform and preferably objective categorization of activities between the studies performed in different laboratories. This chapter introduces methods of classifying the type of activity (occupational, leisure, exercise, sport, domestic and household chores) and its pattern (intensity, frequency and duration), noting also that muscular effort is often ignored in epidemiological assessments.
