Academic journal article Research Quarterly for Exercise and Sport

Estimating Energy Expenditure with the RT3 Triaxial Accelerometer

Academic journal article Research Quarterly for Exercise and Sport

Estimating Energy Expenditure with the RT3 Triaxial Accelerometer

Article excerpt

The RT3 is a relatively new triaxial accelerometer that has replaced the TriTrac. The aim of this study was to validate the RT3 against doubly labeled water (DLW) in a free-living, mixed weight sample of adults. Total energy expenditure (TEE) was measured over a 15-day period using DL W. Activity-related energy expenditure (AFt) was estimated by subtracting resting energy expenditure and thermic effect of feeding from TEE. The RT3 triaxial accelerometer was worn over 14 consecutive days. TEE and AEE were estimated using the RT3 proprietary equation. Thirty-six adults ages 18-56 years (56 % women) with an average weight of 75.9 kg (SD = 14.8) completed all measurements. Compared to DLW, the RT3 underestimated TEE by 539 kJ (4 %) and AEE by 485 kJ (15 % ) on average. The RT3 provided a relatively accurate assessment off tee-living activity-related energy expenditure at the group level and generally underestimated total and activity-related energy expenditure compared to DLW.

Key words: physical activity, activity counts, motion sensor, body composition


Accurate measurement of physical activity (PA) is a prerequisite to monitor population health and design effective interventions. Accelerometers are one of the most commonly used methods for objectively measuring PA under field conditions (Welk, 2002). They are small, noninvasive devices that measure the rate of accelerations (i.e., intensity) produced by body movement in one (vertical; uniaxial) or three planes (anterior-posterior, lateral, and vertical; triaxial).

Of the triaxial accelerometers, the Tritrac-R3D (Leenders, Sherman, & Nagaraja, 2006) and Tracmor (Bouten, Verboeket-vam de Venne, Westerterp, Verduin, & Janssen, 1996) have been validated in adults against doubly labeled water (DLW). More recently, StayHealthy, Inc. (Monrovia, CA) bought the technology and rights to the Tritrac and developed a smaller (71 x 56 x 28 mm, 65.2 g) device called the RT3 triaxial accelerometer. According to the manufacturer, the activity counts recorded by the RT3 and Tritrac-R3D are comparable (Stayheahhy Inc., 2005).

There are few published data concerning the RT3 accelerometer. Some researchers have found good intramonitor variability, but expressed concerns regarding intermonitor variability (Esliger & Tremblay, 2006; Powell & Rowlands, 2004). The RT3 has been shown to correlate well with oxygen consumption in a laboratory setting during moderate and vigorous activities in both men and boys (Rowlands, Thomas, Eston, & Topping, 2004), but it had significantly higher activity counts compared to the Tritrac in some activities (Rowlands et al.). To date only one study has validated the RT3 against DLW in free-living conditions (Jacobi et al., 2007). At the group level, Jacobi et al. reported good agreement between RT3-derived PA-related energy expenditure and DLW measurement. However, the study was conducted using an overweight and obese female sample. To the best of our knowledge, there has been no published validation of the RT3 against DLW in a more representative population. Hence, the aim of this study was to validate energy expenditure estimates of the RT3 against DLW in free-living, mixed weight men and women.



The participants were 36 adults (20 women and 16 men), ages 18-64 years (M = 39 years; see Table 1). They were predominantly of European ethnicity (82%), while the remaining were New Zealand Maori (9%), Asian (6%), and Tongan (3%). Most were white collar professionals (69%), with 17% students, 8% blue collar workers, and 6% unemployed. According to international (World Health Organization, 1998) body mass index (BMI) cut-off points, participants were on average slightly overweight (MBMI = 25.7 kg/[m.sup.2]), which is typical of the New Zealand population (M BMI = 26.1 kg/[m.sup.2]; Russell et al., 1999). Half the participants were at normal weight, and the other half were overweight. …

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