The minimum number of days needed to represent the daily PA of youth and children with CP varies by GMFCS level. Three days of PA data reliably captures PA for all children and youth with CP at GMFCS level I, II, III, or IV. When proper donning of the SW was confirmed with families, 85 of the 87 youth (99%) had at least 3 days of usable data over a 7-day span. Therefore, when given the SW for 7 days, there is a 99% chance of obtaining at least 3 days of usable PA data. These findings, and previous reports in the literature, indicate that a collection period of 1 week is an appropriate timeframe for a clinical protocol. Although participants' variability in total stride counts is high (average of 1080), the within-subject variability is smaller than the between-subject variability (average of 1993) allowing the single-day reliability to be high (0.73). Therefore, a minimum of 3 days per week gives an adequate representation of overall PA.
We found differences in mean daily stride totals between weekdays and weekends, confirming findings from previous reports. The difference in PA on weekdays versus weekends is most likely due to the school setting and the demands of a typical school day. This assumption is supported by our school year data compared with summer vacation data. Participants who wore the SW during the summer months (while not attending school) did not take significantly more steps on weekdays. When SW monitoring is being used to describe habitual activity, weekdays and weekend days should be noted. It may not be necessary to require a weekend day to be included in the monitoring. This is most likely a result of weekend days only accounting for 28% of overall time, and requiring 80% accuracy from the Spearman-Brown prophecy. Although including weekend days is likely to produce a different representation of habitual PA in comparison to weekdays only, any 3 or more days of PA should provide at least an 80% reliable representation of habitual PA. Because children and youth with CP take significantly more strides during the week, it is important to note the type of days included in a PA analysis, particularly when evaluating the outcome of an intervention. The number of weekdays and weekend days should be kept consistent between monitoring periods. A monitoring period with 2 weekend days and 1 weekday is likely to have different PA in comparison to a monitoring period with 3 weekdays. Clinical protocols should always note weekdays or weekend days, and include both as possible.
The low total on nonschool days suggests a need for youth with CP to be more physically active when not in school. PA on the weekends suggests motivation for a child to be physically active, and can affect the treatment plan. Families should be provided with education about and accessibility to active recreation outside of school. Using objective PA measurements to evaluate habitual PA can suggest specific PA patterns, allowing clinicians to direct the timing and environment of PA interventions.
Decreasing the period for data collection both increases the number of participants who can be monitored in the clinic and decreases the measurement burden for patients and families. The shorter data collection period allows the SW to be set to a shorter collection interval, which has the potential to detect PA intensity levels and identify types of PA. The 60-second interval was expected to capture less high-intensity activity compared with the 10-second interval. Our results did not support this assumption. Although we did not find a significant difference in the ability to detect high-intensity strides between collection methods with the default SW setting of more than 40 strides/min, this may be partially explained by the overall lack of high PA in this sample. If goals are to increase high-intensity PA, then the 10-second collection interval is more likely to detect individual change. For these reasons, a clinical protocol should consist of collecting SW data at 10-second intervals.
The wide variation in PA of the sample supports a high correlation between methods despite wide discrepancies in stride totals for some days. Participants had mean stride differences between the 2 SWs from 1% to 13% over the recording cycles. There were no clear trends in patient characteristics associated with discrepancies between SW methods. Acceptable data quality requires careful oversight in patient SW education, corresponding activity logs, and averaging over 3 or more days.
There are limitations to this study. The participants during the school year were not the same participants during the summer. Direct comparisons between PA during the school year and PA during the summer were not possible. Information on summer activities was not available. For example, there was no way to distinguish between children who went to camp during the week over the summer versus children who remained home. The weekday versus weekend differences, environment, and daily activity can influence PA, and care should be taken to receive and consider detailed activity logs. The PA collected was from participants at various stages of surgical recovery. Data collection occurred as part of a clinical preoperative or postoperative gait laboratory visit, with the postoperative visits ranging from 3 to 36 months postsurgery. Including data from various stages of functional recovery contributed to the large between-subject variance.
Habitual PA is one determinant of health and health-related fitness for youth with and without mobility limitations. Appropriate measurement tools and protocols to measure habitual PA are needed for children and youth with CP who are at risk for health complications associated with inactivity. The SW has been assessed and determined to be an appropriate measurement tool, but the implementation of a standard clinical protocol was still lacking. We now suggest and begin implementation of the protocol from this study. We recommend setting the SW to collect for 7 days at 10-second collection intervals. When the SW is returned, data should be analyzed if at least 3 days of adequate data is present. Weekdays and weekend days should be noted, and both should be included as possible.
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The Spearman-Brown prophecy requires the single-day intraclass correlations (ICCs) and the desired reliability (ICCt) as inputs. The ICCs were calculated for a single day of monitoring using equation (1).
where σ2 B is the between-subject variance and σ2 W is the within-subject variance.
To calculate the number of days needed to obtain 0.80 reliability, the Spearman-Brown prophecy formula (2) was used.
where N is the number of measures or days needed, ICCt is 0.80, and ICCs is the single-day reliability.