Influence of Anatomical Placement of Accelerometers on Prediction of Physical Activity Energy Expenditure in Wheelchair users

Introduction Physical activity energy expenditure (PAEE) is inherently difficult to measure in free-living conditions, particularly in populations where movements and movement patterns are atypical, such as manual wheelchair users. The aim of this study was to (i) assess the mechanical reliability of the Actigraph GT3x+ (Actigraph, USA) accelerometer and (ii) assess the influence of its anatomical placement on PAEE estimation in manual wheelchair users. Methods Mechanical reliability: Ten GT3x+ units were attached to a multi-axis shaker table. A testing schedule which comprised various acceleration conditions to replicate a range of physiological movements was conducted along each of the three measurement axes. Human validity: Eleven manual wheelchair users (mean ± SD: 34 ± 11 years, time since injury: 13 ± 15 years, body fat: 25 ± 13 %) completed five activities; deskwork and wheelchair propulsion (2, 4, 6, 8 km.hr-1). A GT3x+ accelerometer was worn on the right wrist, upper arm and waist. The relationships between physical activity counts (PAC) from each unit and metabolic rate (Cosmed K4b2, Italy) were subsequently assessed and bias ± 95 % limits of agreement (LoA) calculated. Results Mechanical reliability: CV ranged from 0.2 to 4.7 % (intra-unit) and 0.9 to 5.2 % (inter-unit) in all axes. ICCs were 1.0 for all stages in each axis. The absolute bias ± 95 % LoA values within units were 0.4 ± 4.1 counts.5 s-1, -0.1 ± 4.6 counts.5 s-1 and 0.3 ± 4.2 counts.5 s-1 for x, y and z axes respectively. Human validity: PAC at each anatomical location were significantly (p < .01) associated with metabolic rate (wrist; r = .96 , upper arm; r = .91 , waist; r = 0.73). The SEE for each correlation was 2.88, 4.15, and 6.89 KJ.min-1 for wrist, upper arm and waist respectively. Using the generated regression equations the absolute bias ± 95 % LoA values were 0.46 ± 5.71 kJ.min-1, 0.33 ± 8.18 kJ.min-1 and -0.03 ± 13.56 kJ.min-1 for wrist, upper arm and waist respectively. Discussion The findings of the mechanical reliability testing demonstrate that the Actigraph GT3x+ is a reliable tool for assessing accelerations within the physiological range of interest. Of the three anatomical locations considered, a wrist-mounted accelerometer provides the most valid prediction of PAEE in manual wheelchair users. Future studies should assess the validity of such devices and anatomical positions for predicting PAEE during more complex representative daily activities performed by manual wheelchair users.