Physical Activity Monitoring: Use of Accelerometers in Individuals with Spinal Cord Injury

Amy M Hayes, PT MS; L Yeats; Jon N. Myers, PhD; B. Jenny Kiratli, PhD

Objectives: The objective of this study is to investigate the validity of using accelerometers for measurement of habitual activity in individuals with Spinal Cord Injury (SCI). Accurate and portable measurement of habitual activity levels will be useful for determination of profiles for cardiovascular disease (CVD) risk, as elevation of CVD in this population is thought to result primarily from a reduction in the ability to do physical activity and/or the sedentary lifestyle secondary to paralysis. This study is part of a larger project investigating habitual activity levels and determinants of cardiovascular health after SCI. However, there are no objective measures of activity in common use. While the accelerometer, a small electronic motion counter, has been shown to be effective for approximation of physical activity in ambulatory individuals, its validity remains to be demonstrated in those with SCI.

Methods: Eleven healthy male subjects with SCI were recruited: 7 with paraplegia and 4 with tetraplegia; mean age 45.8 ± 8.6 years (range: 31-56 years). Each subject performed an individualized maximum exercise test to obtain heart rate (HR) and VO2 (amount of oxygen consumed) values. HR and activity were monitored for a continuous 24-hour period by use of a portable ECG Monitor (Mortara), accelerometers, and activity diaries and questionnaires. Physical activity monitoring was accomplished by use of single channel accelerometers (Computer Science and Applications, Inc) placed on the dominant wrist, dominant upper arm, and non-dominant wrist (in 6 subjects only). Regression analysis was performed to determine the linear relationship between HR and VO2 obtained during the exercise test, in order to generate individualized equations for estimation of energy expenditure (EE) for each participant. Total activity counts (a sum of accelerometer counts) were compared with EE estimates from both the questionnaire and the 24-hour HR values (using the individualized equation for EE from the exercise test).

Results: Total activity counts from different accelerometer placements were highly correlated with each other. For the purpose of these analyses, accelerometer counts from the dominant wrist were used; this location is both acceptable/comfortable for the subject and appears to provide a representative estimate of activity. There were no significant differences between subjects with paraplegia and tetraplegia with respect to total activity counts, EE estimated from HR data, or EE determined from questionnaire for the 24 hour periods. Total activity counts significantly predicted EE from the 24 hour HR data (p=0.0036, R2=0.68) but not EE from the questionnaire (p=0.8749, R2=0.06). Further, EE from the questionnaire did not predict EE from the 24 hour HR data (p=0.4525, R2=0.08).

Conclusions: Total activity counts from accelerometry were able to predict a significant proportion of EE as estimated from HR data using individualized regressions from maximal exercise tests. An inadequate prediction of daily EE was demonstrated by use of questionnaire data alone. Although 24 hour HR-derived EE provides a reasonable estimate of daily energy output, this methodology is unwieldy for broad application. Our preliminary findings indicate that accelerometer use in individuals with SCI may have a role for population monitoring of daily physical activity and thus assessment of CVD risk.

Funding Acknowledgment: This study was funded by the VA Rehabilitation Research and Development Service, project # B2549R.