Response to “Developing a Clinical Protocol for Habitual Physical Activity Monitoring in Youth With Cerebral Palsy”

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As a clinician and researcher, I appreciate timeliness of the Nicholson and colleagues article1 relative to the development of clinical protocols with the StepWatch (SW) accelerometer in children with cerebral palsy (CP). It is with both these perspectives, I would like to comment.
First, throughout the article the authors use the terminology of “physical activity” for the output from the SW device. “Physical activity” (PA) is defined as any body movement that expends energy2 and is typically quantified by total body movement “counts” via single to multiple axis accelerometers that are worn at the waist/wrist. Accelerometers have been employed to document optimal levels of PA (total body movement) to support optimal health outcomes. In contrast, “walking activity” (WA) is defined as when a stride or a step is taken, and is a category or subset of PA. The SW was specifically designed and validated to only capture a “step or stride” taken during walking (eg, worn on ankle, 2-dimensional accelerometer to movement of heel off the ground). In the vast PA monitoring literature, “walking” versus “physical” activity is clearly and uniquely defined.3–6 Unfortunately for consumers of our physical therapy literature, this mislabeling of the SW output as “physical activity” has been repeated elsewhere,7,8 thus adding more confusion.
From a public health perspective, “walking or steps taken” is a subset of overall PA. Employing the metric of “steps taken or walking” via pedometers (ie, Digiwalker and SW), there are population-based daily walking recommendations for elementary school-aged children. A total of 13 000 to 15 000 strides per day (26 000-30 000 steps) for boys and 11 000 to 12 000 strides/day for girls (22 000-24 000 steps) have been proposed to approximate the 60 min/day of moderate to vigorous physical activity (MVPA) for optimal growth and health.6 The mislabeling of the SW information as “physical activity” presented in this article is confusing to the clinician and novice researcher. It is essential in outcome measurement to employ tools that address the behaviors you expect to be influenced. For example, with this definition of “walking activity,” the SW or pedometer data (steps) before and after addition of an assistive device, orthopedic surgery or orthotic intervention appears at face value to be appropriate. In contrast, outcomes after a community-based PA program would be sampled by devices validated to capture PA (ie, actigraph).
Second, a full literature review/summary of the published protocols to date with the SW in children would have been helpful to inform this work. Such a review would specifically clarify the work completed in children with and without disability (vs adults/geriatric populations) as well as work relative the parameters of those protocols (ie, sample epochs, number of days monitored by functional level in CP, validation of SW to capture actual steps taken, and comparison of SW to other devices).
For example, the cut point of more than 40 strides/min as “high” intensity by Nicholson et al in this article is not based on what we know about pediatric walking activity across devices as well as with the SW. The default settings within the SW proprietary software (high-intensity walking ≥40 strides/min) are based on adult/geriatric walking data.9 Intense walking in adult US population samples has been documented at a peak stride per minute rate of 101.10 In contrast, intense walking bursts for children (<18 years of age) are documented at 108 to 146 peak strides/min rates.11–13 From a public health perspective, the literature has documented that MVPA to be 120 steps/min14 in 10- to 14-year-old boys and girls with another pedometer device (>60 steps/min with SW as it counts only one leg).
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