Awareness of wearing an accelerometer does not affect physical activity in youth

Eighty healthy participants (44 boys, 36 girls) from several primary care pediatrician offices in northern France volunteered for the study. The inclusion criteria were: (i) subjects were aged between 10 and 18 years old; (ii) informed consent was signed by the participant as well as his/her parents; (iii) absence of medical contraindication for daily PA (e.g. cardiovascular diseases, musculoskeletal pain); and (iv) no simultaneous inclusion in other biomedical studies. All participants underwent a medical examination to exclude potential contraindications for the study.

Before the study began, the aims and objectives were explained carefully to each adolescent and their parents. Written informed consent was obtained from the adolescent and the parents. Written informed consent was obtained from the adolescent and the parents. The study was approved by the Research Ethics Committee of the University of Lille (Comité Protection des Personnes, Nord Ouest IV, Lille, France). All procedures were performed according to the ethical standards of the Helsinki Declaration of 1975, as revised in 2008, and European Good Clinical Practice.

The study started in September 2013 and ended in June 2015. Participants were referred to the Clinical Research Centre of the Lille University Hospital. Body mass index was measured to the nearest 0.1 kg using an electronic scale (Seca, Hamburg, Germany) after removal of shoes or heavy outer garments. Height was measured to the nearest 0.1 cm using a stadiometer (Seca). The included participants were randomized using sealed envelopes between the blinded group (n = 40) and the nonblinded group (n = 40). The randomization was centralized in the study center using a 1:1 ratio of blinded to nonblinded participants, which was divided into height blocks to obtain 4 blinded participants and 4 nonblinded participants per block. Participants assigned to the blinded group were informed that the study targeted the reliability of a new device assessing body posture. However, participants assigned to the nonblinded group were aware of the device being an accelerometer for PA assessment. The participants in the nonblinded group were given full information about the device, i.e. how the device detects body movements, measures sedentary behaviors (e.g. sleeping, lying before the TV, playing video games) as well as vigorous activities (e.g. playing soccer, cycling, running). The explanations were given by the same researcher, who was not blinded about the randomization of the groups. Participants wore the accelerometer on their lower back under their clothing using an elastic belt and adjustable buckle. All participants were instructed to remove the device during contact sports, water-based activities (swimming, showering, and bathing), and overnight. To fairly assess PA patterns in both groups, the participants were asked to keep a log diary of when and why the device was removed (Fig. 1) [14]. The sport activities performed after removal of the device were classified in agreement with the Compendium of Energy Expenditures for Youth and time spent in moderate or vigorous PA was added. Activities between 4 and 6 metabolic equivalents of task (MET) were defined as moderate PA and activities with 6 or more MET were defined as vigorous PA [15]. According to consensus recommendations for assessing PA in youth, a minimum of 2 or 3 days measured is needed to estimate weekly usual PA behavior in children and adolescents [16‐18]. We a priori decided to record 4 days of PA behavior to ensure that our data reflected weekly PA, including leisure time. The accelerometers recorded activity for 4 consecutive days (2 school days and 2 school-free days) in free-living conditions. The devices were collected after the 4-day monitoring and the data were transferred from the device to a computer. To ensure the compliance of the subjects of the blinded group, the following 3 questions were asked: (i) Did you search online for any information regarding the device? (ii) Did you receive any information about this device by your peers? If so, which? (iii) Are you aware of the use or utility of this device?

The triaxial accelerometer used was the ActiGraph® Monitor (Model GT3X; ActiGraph, Pensacola, CA, USA) (46 × 33 × 15 mm; weight 19 g, additional technical features) [19]. The accelerometer measures acceleration and deceleration in 3 spatial dimensions according to a vertical vector (x), an anteroposterior vector (y), and a mediolateral vector (z). The vector magnitude (VM) was calculated as follows: VM = √(×2+y2+z2). The epoch interval for the accelerometer was set at 1 s. A computer was used to initialize and synchronize the accelerometer. Participants who recorded less than 10 h of activity per day were excluded from the analyses [18]. PA levels were categorized as follows: sedentary activity, 0–180 counts.15 s⁻¹; light activity, 181–757 counts.15 s⁻¹; moderate activity, 758–1112 counts.15 s⁻¹; and vigorous activity, >1112 counts.15 s⁻¹ [20]. The interinstrument reliability of this device is reported to be better for moderate and vigorous activities than for sedentary activity [19, 21]. Data were averaged and expressed in counts.min⁻¹.

The primary outcome was the overall total counts per minute over the 4 day-period. The secondary outcome was the daily time spent at each PA level (sedentary, light, moderate, vigorous, and moderate to vigorous) calculated over the 4 day-period. All outcomes were also analyzed over the 4-day period, and school days were separated from school-free days.

Based on the data of Martinez-Gomez and colleagues, we hypothesized that the mean of total counts.min⁻¹ would be reduced by 20% in the blinded group compared with the nonblinded group (corresponding to an absolute difference of 100 counts counts.min⁻¹) [22]. With a standard deviation of 150 counts.min⁻¹, a power of 80% and an alpha risk of 5%, 37 adolescents per group were required to detect this effect size (effect size of −0.67 considered as medium to large according to Cohen) [23]. To compensate for the potential missing data, inclusion of 40 subjects per group was required. Data were analyzed using SAS software (version 9.3; SAS Institute, Cary, NC). Statistical testing was conducted at the 2-tailed α-level of 0.05. Data were expressed as mean ± standard deviation (SD) or percentages as appropriate. Normality of distributions was assessed using histograms and the Shapiro–Wilk test. Data between groups were compared using Student’s t test. Cohen’s d effect sizes (standardized mean differences between nonblinded vs. blinded groups) were calculated and interpreted as small for absolute value = 0.2, medium for value = 0.5, large for value = 0.8 and very large effect for value = 1.3 [23].