The present study demonstrates a good reliability of PA obtained by the triaxial accelerometer and the uniaxial accelerometer in adolescents in free-living conditions. Indeed, even if significant differences between the triaxial and uniaxial devices were found for sedentary, light and moderate activities, these differences were small (less than 2.1%) and decreased with PA intensity level. Such low difference appears to be negligible from a clinical research point of view. Moreover, the Intra Class Correlation that is a measure to express consistency and agreement between 2 methods was superior to 95%, demonstrating a very good reliability between uniaxial and triaxial accelerometers. Therefore, these data indicate that these two devices assess PA level in a similar way and that data obtained with these two devices in different studies on PA patterns could be considered comparable.
Results from different studies showed that the triaxial accelerometer was more precise than uniaxial accelerometer to assess PA and energy expenditure in children and adults [18, 19]. We also hypothesized that the triaxial accelerometer could assess PA levels better because it measures movements in the three dimensions of space whereas the uniaxial accelerometer measures only in one dimension, and so it may lack some movements . However, our results demonstrate that the uniaxial is as efficient as the triaxial accelerometer in the assessment of PA levels in FLC in adolescents. One hypothesis explaining this discrepancy could be the low impact of anteroposterior and mediolateral axis in the calculation of the VM using the triaxial accelerometer. This is supported by a recent study showing that the vertical axis (x) predicts similarly the activity energy expenditure than the VM (which is the square root of the sum squared of activity counts in each vector) . Therefore, the vertical axis seems the most important one in the assessment of PA using the triaxial accelerometer, and compares the uniaxial accelerometer.
Our results agree with those of Macfarlane et al., who compared the validity of six methods to assess PA in daily life using questionnaires, heart rate monitoring, and accelerometers . In the study by Macfarlane et al., the same accelerometers were worn for seven consecutive days by 49 Chinese subjects aged 15-55 years. As in the present study, they found that the triaxial accelerometer RT3 and the uniaxial accelerometer ActiGraph gave similar results for time spent in light, moderate, and vigorous activity. One important additional point in our study is that the differences between the two devices are also low (2.1%) for sedentary PA, which was the predominant level of PA for most of the adolescents; even a minimal difference in this category could yield a large difference in terms of total PA. Paul et al. showed also, although these two types of accelerometer (ActiGraph and Actical) predict PA on the same scale (counts ⋅min-1), the results differed . This agrees with our findings because total PA was 204.136 counts ⋅ 24 h-1 for the RT3 and 353.252 counts ⋅ 24 h-1 for the ActiGraph. However, our results show clearly that these two devices distinguish different intensities of PA in the same manner.
Two studies have been previously performed in adults to compare different prediction equations of energy expenditure using both RT3 and Actigraph [24, 25]. The first study was performed in adults on the capabilities of eight previously published regression equations for three commercially available accelerometers (ActiGraph, Actical and RT3) to predict daily energy expenditure . Eighty-five subjects completed one overnight stay in a room calorimeter where they engaged in typical activities in free living conditions (walking, jogging, deskwork...). The authors concluded that most energy expenditure prediction equations showed differences of < 2% in the moderate and vigorous intensity categories, while, several regression equations under or overstimated the energy expenditure against the direct calorimetry . The second study was realized in 13 subjects during 7 days where the total daily energy expenditure was measured with ActiGraph and Tritrac (predecessor of the RT3 accelerometer) regression equations against the doubly labeled water . Of the 14 different regression equations examined, only two developed for the ActiGraph accelerometer were not significantly different from the doubly labeled water method. All equations for the RT3 accelerometer showed significant difference with the doubly labeled water method. Authors concluded that the results from these two studies imply that researchers may want to avoid using accelerometers to predict energy expenditure in free-living conditions, instead using these instruments only to measure patterns of PA. Our results reinforce this conclusion showing good concordance and agreement of the two accelerometers in the assessment of PA patterns.
A 2 or 5 s epoch is preferred for assessing PA in children since most of their spontaneous activities are very short and do not exceed one minute . A 15 s epoch can be used with adolescents . However due to technical constraints (RT3 does not have the capacity to record data for 1 week with a short second epoch) a 1 min epoch was chosen for the study. Whether or not using a 1 s epoch would change the comparison between these 2 devices is unknown but we could speculate it would not change significantly the comparison of the devices but rather the physical activity patterns.
In the present study, we used predictive equations obtained previously in our group for the two accelerometers. However, other groups have published different predictive equations, we cannot exclude that the results of the comparison between these two accelerometers should have been different using other equations [27–29].