Measuring moderate-intensity walking in older adults using the ActiGraph accelerometer

Background: Accelerometry is the method of choice for objectively assessing physical activity in older adults. Many studies have used an accelerometer count cut point corresponding to 3 metabolic equivalents (METs) derived in young adults during treadmill walking and running with a resting metabolic rate (RMR) assumed at 3.5 mL.kg(-1).min(-1) (corresponding to 1 MET). RMR is lower in older adults; therefore, their 3 MET level occurs at a lower absolute energy expenditure making the cut point derived from young adults inappropriate for this population. The few studies determining older adult specific moderate-to-vigorous intensity physical activity (MVPA) cut points had methodological limitations, such as not measuring RMR and using treadmill walking. Methods: This study determined a MVPA hip-worn accelerometer cut point for older adults using measured RMR and overground walking. Following determination of RMR, 45 older adults (mean age 70.2 +/- 7 years, range 60-87. 6 years) undertook an outdoor, overground walking protocol with accelerometer count and energy expenditure determined at five walking speeds. Results: Mean RMR was 2.8 +/- 0.6 mL.kg(-1).min(-1). The MVPA cut points (95% CI) determined using linear mixed models were: vertical axis 1013 (734, 1292) counts.min(-1); vector magnitude 1924 (1657, 2192) counts.min(-1); and walking speed 2.5 (2.2, 2.8) km.hr(-1). High levels of inter-individual variability in cut points were found. Conclusions: These MVPA accelerometer and speed cut points for walking, the most popular physical activity in older adults, were lower than those for younger adults. Using cut points determined in younger adults for older adult population studies is likely to underestimate time spent engaged in MVPA. In addition, prescription of walking speed based on the adult cut point is likely to result in older adults working at a higher intensity than intended.