Study population
The study population was randomly selected from a larger cohort of patients which was prospectively formed to study the physical activity behavior of patients one year after THA. This cohort consisted of patients who had undergone primary THA at University Medical Center Groningen or Martini Hospital Groningen. Selected patients were contacted by mail or phone and asked to participate in this study. From the 86 contacted patients, 44 were enrolled in the reliability study and 39 patients also in the validation study. The remaining patients were not willing to participate for various reasons. These patients did not show any differences in main characteristics (age, gender) compared to the patients in the study population. The study took place from March 2007 to September 2007. In this period we did not observe large differences in weather conditions between measurements, which could have influenced physical activity behavior.
The study was executed in accordance with the regulations of the Medical Ethical Board of University Medical Center Groningen. Written informed consent was obtained from all patients.
Study design
As part of the prospective study, all patients were sent a questionnaire with an explanatory letter one year after THA. This self-administered questionnaire contained the SQUASH as well as some demographic questions. After completion and return of the questionnaire, those patients who were enrolled in the reliability study (reliability group) completed the SQUASH for a second time 2 to 6 weeks later. This period was considered to be long enough to prevent patients from copying the SQUASH from memory, and short enough to prevent large changes in physical activity levels. Patients who also consented to participate in the validation study (validation group) wore an accelerometer, the ActiGraph™ GT1M monitor (Actigraph™, LLC, Pensacola, Florida, USA), during the two weeks following completion of the second questionnaire. These patients kept a diary in which they noted periods of noncompliance with the Actigraph and/or exceptional activities.
Physical activity questionnaire
The SQUASH [see additional file 1] was used to assess the physical activity behavior of the study population. It is structured in a way that allows comparing the results to national and international physical activity recommendations. The SQUASH contains questions on commuting activities, leisure-time and sports activities, household activities, and activities at work and school. It consists of three main queries: days per week, average time per day and intensity (effort). In order to keep the questionnaire short and easy to fill in, intensity of household activities and activities at work and school are prestructured into two categories, light or intense, while time spent on activities at work and school is depicted in average time per week.
Calculation of the activity score per week from the SQUASH
Patients were asked to refer to an average week in the past few months. Using the Ainsworth compendium of physical activities [18, 19], activities were assigned a MET value. One MET is defined as the energy expenditure for sitting quietly. Based on the Dutch physical activity guideline [6], activities were subdivided for adults and older adults (up to age 55 and older) respectively into three intensity categories. For adults activities with a MET-value between 2 and < 4 were classified as light, between 4 and < 6.5 as moderate, and ≥ 6.5 as vigorous intensity. For older adults activities between 2 and < 3 MET were classified as light, between 3 and < 5 MET as moderate, and ≥ 5 MET as vigorous intensity. Activities with a MET value lower than 2 were not included because they are considered to contribute negligibly to physical activity level. Based on reported effort in the questionnaire, activities were assigned an intensity score and a total activity score; activity scores for separate questions were calculated by multiplying total minutes of activity by the intensity score.
Activity monitor
Physical activity was also assessed by means of the ActiGraph™ GT1M activity monitor. This is a compact (3.8 × 3.7 × 1.8 cm), light-weight (27 gr) uniaxial accelerometer, measuring and recording time-varying accelerations ranging in magnitude from approximately 0.05 to twice gravitational acceleration. It is band-limited to a frequency range of 0.25 to 2.5 Hertz, so that normal human motion is detected and motion from other sources rejected. The ActiGraph™ collects and reports physical activity in "counts". Counts are the summation of the accelerations measured during a user-specified time interval (epoch), and represent the intensity of activity in that epoch. In this study, data were collected for each minute during a two-week period. Patients were instructed to wear the monitor during the time they were not asleep, except when showering, bathing or swimming. The monitor was firmly attached to a belt on the waist (sagittal line).
Calculation of activity from the activity monitor
Activity counts per minute were converted to MET values using the equation published by Freedson et al. [20] (MET value = 1.439008 + (0.000795 * counts/minute)), with cutoff points for the intensity categories consistent with those of the SQUASH. After this conversion, time spent per week in the different intensity categories as well as total time of activity was calculated. Furthermore, mean counts per minute were calculated by dividing the total count over two weeks by the total number of minutes the ActiGraph™ was worn. For purposes of reproducibility of this reference method, the activity level was only calculated for days in which the monitor was worn for 12 hours or longer. Assuming one sleeps for 8 hours a day, this time period represents at least 75% of the available time (16 hr). For purposes of comparability to the reference period of the SQUASH, the monitor had to be worn for at least seven days.
Statistical analysis
The data were analyzed using the Statistical Package for the Social Sciences (SPSS, Chicago) software (version 14). Descriptive statistics were used to describe the main characteristics of both study populations.
Reliability of the SQUASH was determined by calculating Spearman's correlation coefficient between the activity scores of the separate questions as well as the total activity scores from both administrations. Additionally, a Bland & Altman analysis was performed for the total activity scores [21].
Spearman's correlation coefficients were used to determine relative (or concurrent) validity of the scores on the SQUASH using the Actigraph™ as criterion measure. To this end, the scores of the first SQUASH were used to exclude the possibility of biases resulting from an increased awareness of activity or a learning effect. Spearman's correlation coefficient was assessed between total activity score of the SQUASH and mean counts per minute of the ActiGraph™. Spearman's correlations were also assessed between total time spent in activity, as well as time spent in different intensity categories of physical activity, according to the SQUASH and the ActiGraph™. Additionally, Bland & Altman analyses were performed.
To examine the capability of the SQUASH for categorizing patients according to their physical activity level, the kappa statistic for the tertiles of both activity scores and activity counts as well as the percentage of exact agreement between the tertiles were calculated. This was also performed for the capability of the SQUASH to determine if patients complied with the guidelines of health-enhancing physical activity.