Physical activity measurement

To the author’s knowledge two PA scales have been assessed for validity and reliability in a population of people with psychosis; the short version of International Physical Activity Questionnaire (IPAQ) and the 7 day PA recall questionnaire. The latter scale was found to be reliable but with questionable validity (Soundy et al, 2007b), whereas the IPAQ was found to have a correlation coefficient of 0.68 for reliability and 0.37 for criterion validity (compared against accelerometer), based on total reported minutes of physical activity (Faulkner et al, 2006). This was similar to reliability and validity information for the IPAQ used in the general population with scores of reliability of 0.8 and criterion validity (compared against accelerometer) of 0.30 (Craig et al, 2003). It was therefore considered appropriate for use in this study.

International Physical Activity Questionnaire (IPAQ) (Craig et al, 2003) (see appendix four)

Data from the IPAQ were categorised into low, medium and high activity levels, and PA was expressed as a continuous measure of the amount of energy expended (METs) over a 7 day period. The intensity of PA was also derived, producing a score for the amount of walking, moderate and vigorous intensity

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activity. Scores for the amount of time spent and frequency of each intensity of PA were also calculated.

In addition to the psychometric properties of the IPAQ, there are a further three reasons for choosing the IPAQ. Firstly, it is deemed as being easy to administer and relatively undemanding for both respondents and investigators (Faulkner et al, 2006), especially as the short form was only 4 questions long. Secondly, it assessed walking behaviour which is reported as being the most common type of PA for people with psychosis (Richardson et al, 2005; McLeod et al, 2009).

Thirdly, the IPAQ has been frequently used in a variety of countries and in population surveys (Craig et al, 2003), producing comparable data and promotes consistency in measurement (Faulkner et al, 2007). The IPAQ is easily available at http://ipaq.ki.se/ and in a number of different languages.

IPAQ scoring

The scoring of the IPAQ followed the protocol outlined on http://www.ipaq.ki.se/scoring.pdf.

Following data collection and input, the IPAQ data was reviewed. Craig et al (2003) state that PA data is usually skewed; they therefore recommend measures to be taken in an attempt to normalise the data. In line with these recommendations for scoring IPAQ, values less than 10 minutes of PA was transformed to 0 and daily activity bouts more than 180 minutes were capped at 180 minutes.

The scores for IPAQ were presented as a categorical outcome into low, medium and high PA and a number of continuous outcomes. The continuous outcomes consisted of: total volume of PA, volume of vigorous, moderate and walking, total amount of PA, amount of vigorous, moderate and walking and frequency of vigorous, moderate and walking.

The volume of low, medium, high and total PA was calculated and converted to METs. METs are multiples of resting metabolic rate. One MET is equal to a person’s rate of energy expenditure when at rest. As a guide moderate PA is classed as between 3.0 – 6.0 METs and vigorous PA is anything greater than 6.0

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METs (Pate et al, 1995). The MET values recommended for use when scoring IPAQ data are based upon a compendium of physical activities (Ainsworth et al, 2000). An average MET score for all types of walking, moderate-intensity and vigorous-intensity activities was derived and the following MET values were recommended when calculating volumes for PA on the IPAQ:

Walking = 3.3 METs*

Moderate-intensity = 4.0 METs Vigorous-intensity = 8.0 METs

* 3.3 METs is classed as moderate intensity PA.

To calculate weekly METs of each of the categories, the following equation was used: Frequency (days) x Duration (minutes) x Intensity (MET value). The scores of each of the categories were summed to calculate the total physical activity METs for the week.

The frequency of PA was reported, which is the number of occasions each of the different intensities of PA were undertaken in a week. The total amount of PA and amount of PA for each intensity was also reported which is frequency x duration and therefore does not include METs.

Categorising the data

The categories outlined below were cut-off points based upon PA guidelines (Craig et al, 2003).

High volume of PA

The two criteria for classification as ‘high’ were:

a) vigorous-intensity activity on at least 3 days achieving a minimum Total PA of at least 1500 MET-minutes/week

Or

b) 7 or more days of any combination of walking, moderate-intensity or

vigorous-intensity activities achieving a minimum Total PA of at least 3000 MET-minutes/week.

108 Medium volume of PA

The pattern of activity to be classified as medium was either of the following criteria:

a) 3 or more days of vigorous-intensity activity of at least 20 minutes per day Or

b) 5 or more days of moderate-intensity activity and/or walking of at least 30 minutes per day

Or

c) 5 or more days of any combination of walking, moderate-intensity or vigorous intensity activities achieving a minimum total physical activity of at least 600 MET-minutes/week.

Low volume of PA

Participants not achieving medium or high levels of PA were categorised as having low PA levels. If they did not achieve medium levels of PA, they were not achieving public health recommendations of 150 minutes a week of moderate-vigorous PA a week.