Measuring physical activity
Measuring physical activity levels can reveal lots of useful information about a patient, and can help inform exercise prescription. For example, it might be possible to find out if someone accumulates most of their physical activity during the week, or at the weekend, through occupational activities or active transport. Measurements of physical activity can also reveal what level of intensity of physical activity a patient is engaging in, and whether they are meeting physical activity guidelines.
In other words, tools used to measure physical activity have the capacity to measure the frequency, intensity, time and type of physical activities being performed. However, not all tools used to measure physical activity can measure all of these variables.
For the purpose of this step we will concentrate on methods used to measure physical activity which are used in clinical situations. Whole room calorimetry, indirect calorimetry, observation and doubly labelled water are expensive methods of measuring physical activity and require training and skill to administer and analyse. These methods are generally reserved for research and will not be covered in this step.
Common clinical methods of measuring physical activity include heart rate monitoring, questionnaires, accelerometers, and pedometers.
Since not all methods of measuring physical activity give you the same detail, before choosing a method to measure physical activity, think about what information you would like to find out.
Heart rate monitors
Heart rate monitors are an excellent method of indirectly measuring the intensity of physical activity as well as the duration and frequency. On their own they do not give you information on the type of activity being performed but can easily be used in combination with an activity diary to gain this information if needed.
- A disadvantage is that heart rate is affected by more than just activity levels. When a person gets nervous, stressed or hot, their heart rate will increase regardless of their physical activity levels.
- There is a linear relationship between heart rate and energy expenditure during steady state work loads. In order to increase the accuracy of heart rate monitoring it is advisable to plot the relationship between work rate and heart rate for an individual before measurement begins. This way you will know what work rate was being achieved for a given heart rate.
- Difficulties can arise with thin children and very thin adults if a heart rate strap is used as this can become loose. With the very obese patient placement of the heart rate strap is important. The telemetry signal transmitting the heart rate to a watch or similar device can be weaker or not function at all if soft tissue results in the monitor being further away from the heart than expected.
Estimation equations have been developed to convert heart rate to energy expenditure. This way you can determine how many calories are being expended. Remember however that this is just an estimation of calorific expenditure and subject to assumptions and error. With such equations the person’s age, fitness, gender and body mass will all effect the accuracy of results.
Accelerometers are small light devices and can be attached almost anywhere on the body, and give you information on the duration, frequency, time and intensity of physical activities. They do not however give information on the type of activity being performed.
- The acceleration of the body can be measured in one (vertical), two (vertical and medio-lateral) or three (vertical, medio-lateral and anterior-posterior) planes of movement.
- Accelerometers quantify acceleration of the body in an output known as counts per minute. When a person moves, the body is accelerated in relation to the muscular forces responsible for the acceleration of the body, and in theory, to energy expended. Counts per minute can then be converted into minutes of activity in various intensities or calories expended.
- Accelerometers are evolving quickly. These devices are now able to measure sleeping patterns and sedentary activity patterns as well as physical activity.
- However they are not able to measure the intensity associated with resistance exercises or walking uphill. When used in combination with an activity diary you can determine what type of activity a patient was engaging in.
The most common sites for attachment are at the level of the hip, on the ankle and on the wrist. Since physical activity patterns vary so much it is advisable to wear the monitor for a minimum of four days. Preferably one of these four days should be a weekend day.
Pedometers count steps taken. They are simple and affordable, and are frequently used in public health initiatives and in large-scale research.
- Some pedometers give output in terms of steps per minute which gives an indication of the intensity of certain types of exercise.
- Others simply report steps taken. Those that report steps taken do not measure intensity. For example, if someone walks or sprints 100 steps, only 100 steps will be registered.
- It is generally recommended that adults achieve 10,000 steps a day in total (Tudor-Locke et al, 2011).
As pedometers are now readily available on smart phones, they are a good first step with which patients can learn to monitor their own physical activity levels.
New generation activity monitors
Some devices have combined several methods of measurement together in order to provide a more accurate representation of physical activity levels.
- For example, wearing a heart rate monitor which has an inbuilt accelerometer, or a watch which monitors heart rate but also has GPS capabilities.
- These devices are expensive but useful for populations where the energy cost of activities is less dependent on movement alone, for example, children with cerebral palsy.
Questionnaires can be very subjective, and unfortunately the accuracy of questionnaires and surveys often falls victim to recall bias. They are however affordable, easy to use and administer, and often don’t take long to complete. Some are completed in private by the patient, while others are administered in an interview format by the clinician.
- People are known to overestimate their physical activity levels when asked to report on them subjectively. This is especially true for those who are overweight or obese.
Despite this, questionnaires can help you gather essential information quickly and should be encouraged over no measurement of physical activity at all and over simple informal probing. If you work a lot with a particular patient population, have a look at the literature, and see if you can find a questionnaire which is commonly used and validated in that population. Some examples of questionnaires include:
The International Physical Activity Questionnaire is an example of a commonly used tool to determine how physically active someone is (as we have seen in Week 1. There is a long and short version of this questionnaire, and it is available in several languages. Although it has been validated in several populations, it is not considered as valid as other more objective measures such as accelerometers since it relies on patient recall and honesty.
The World Health Organisation (WHO) developed the Health Behaviour in School Children (HBSC ) questionnaire which gives an overview of physical activity levels in children. This tool aims to give an overview of physical activity levels in children rather than a detailed account. In order to inform individualised exercise prescription in paediatric populations, more detail may be needed than that acquired by the HBSC.
Measuring physical activity in paediatric populations
Finally a note on the physical activity levels of children. The behaviour of children differs from that of adults, as we will see in Steps 2.22 and 2.23. Physical activity can be vigorous, spontaneous, short-lived and is often associated with play. Objective tools such as accelerometers are recommended to measure the physical activity levels of children rather than questionnaires, especially if used to inform an individual exercise prescription.
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