Validity of Accelerometry for Measurement of Activity in People with Brain Injury

Sean M. Tweedy; Stewart G. Trost


Med Sci Sports Exerc. 2005;37(9):1474-1480. 

In This Article

Abstract and Introduction

Purpose: To evaluate the validity of a uniaxial accelerometer (MTI Actigraph) for measuring physical activity in people with acquired brain injury (ABI) using portable indirect calorimetry (Cosmed K4b2) as a criterion measure.
Methods: Fourteen people with ABI and related gait pattern impairment (age 32 ± 8 yr) wore an MTI Actigraph that measured activity (counts·min-1) and a Cosmed K4b2 that measured oxygen consumption (mL·kg-1·min-1) during four activities: quiet sitting (QS) and comfortable paced (CP), brisk paced (BP), and fast paced (FP) walking. MET levels were predicted from Actigraph counts using a published equation and compared with Cosmed measures. Predicted METs for each of the 56 activity bouts (14 participants × 4 bouts) were classified (light, moderate, vigorous, or very vigorous intensity) and compared with Cosmed-based classifications.
Results: Repeated-measures ANOVA indicated that walking condition intensities were significantly different (P < 0.05) and the Actigraph detected the differences. Overall correlation between measured and predicted METs was positive, moderate, and significant (r = 0.74). Mean predicted METs were not significantly different from measured for CP and BP, but for FP walking, predicted METs were significantly less than measured (P < 0.05). The Actigraph correctly classified intensity for 76.8% of all activity bouts and 91.5% of light- and moderate-intensity bouts.
Conclusions: Actigraph counts provide a valid index of activity across the intensities investigated in this study. For light to moderate activity, Actigraph-based estimates of METs are acceptable for group-level analysis and are a valid means of classifying activity intensity. The Actigraph significantly underestimated higher intensity activity, although, in practice, this limitation will have minimal impact on activity measurement of most community-dwelling people with ABI.

The health risks associated with habitually low levels of physical activity are both serious and well documented.[2,11,20] National and international public health authorities agree that, in many countries, the general population are insufficiently active and that this represents a major public health concern.[5,6,19,23] Unfortunately, evidence indicates that people with disabilities are likely to be among the most physically inactive sections of the population.[19]

Included among people with disabilities are people with acquired brain injury (ABI)-injury to the brain regardless of cause (including mechanical trauma, hypoxia, infection, and stroke) that results in diminution of physical, cognitive, social, and/or behavioral function that is likely to be permanent.[7] Only one study has specifically investigated physical activity participation among people with ABI. In a sample of 717 people (430 with ABI and 287 nondisabled), 14.9% of people with ABI self-reported exercising 3× wk-1 for the previous 6 months, compared with 23.0% of the nondisabled cohort.[10] The available evidence and the nature of the impairments that accompany ABI suggest that physical activity among this population is likely to be very low. To increase activity among people with ABI requires targeted interventions that take account of their unique characteristics.

To develop an evidence base for population-specific physical activity interventions, Sallis and Owen[17] identified five areas of research need, one of which is to establish valid and reliable methods for measuring physical activity in the target population. Impairments of memory and cognition frequently affect people with ABI and will limit the validity of self-report measurement methods such as diaries, activity logs, and questionnaires. Objective measures of physical activity, such as those provided by hip-worn accelerometers, are likely to be more appropriate in people with ABI, although, to date, the validity of these instruments in people with ABI has not been evaluated.

The Manufacturing Technologies Inc. (MTI) Actigraph (formerly known as the CSA Actigraph) is an accelerometer that has been shown to be a reliable and valid tool for measuring physical activity in nondisabled populations.[8,15,18,22] It measures physical activity in counts, and Freedson and colleagues[8] have validated a regression equation for converting Actigraph counts into METs, the conventional unit for expressing physical activity intensity (1 MET = 3.5 mL·kg-1·min-1). To date, research has not examined the validity of the Actigraph for use in people with ABI. Such studies are important because many people with ABI have gait pattern impairments (e.g., hemiplegic gait and ataxic gait), and there is some evidence that perturbations in gait may influence accelerometer-based estimates of physical activity.[16] Moreover, because the Freedson equation was developed using nondisabled adults walking on a treadmill, it is not clear whether this equation would be valid for converting counts obtained from people with ABI walking outdoors.

The purpose of this study was to evaluate the validity of using the MTI Actigraph for the following purposes in people with ABI: to differentiate between different intensities of walking activity, to estimate MET levels from Actigraph counts using Freedson's prediction equation,[8] and to estimate MET levels for the purpose of classifying activity intensity according to accepted convention (i.e., light, moderate, vigorous, and very vigorous).