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Modern analyses of football games require precise recordings of positions and movements. The ZXY arena tracking system reports the position of players wearing a dedicated chip (transponder) at high time resolution. The main issue addressed here is the quality of the system's output data. Filtering the data to reduce noise may affect the outcome adversely. Therefore, we have examined the precision of the data given, possible influence by sources of noise, the effect of the filtering, and the accuracy of the built-in calculations. The system's reported positions of the transponders at rest and during different types of movements, including accelerations, were recorded and compared with the true positions, speeds, and accelerations. The reported position has a random error of ≈0.2 m, defining the system's upper spatial resolution. The error in signals interrupted by a human body was of that magnitude or less. There was no significant influence of nearby transponders. Filtering the data delayed the time resolution. Consequently, accelerations were dampened and delayed, causing an error of 1 m for sudden changes in position. Moreover, fluctuations of the foot speed of a running person were not accurately reproduced, but rather, averaged over time periods >1 s. Results calculated from measured values appeared accurate and were readily reproduced in a spreadsheet output. In conclusion, the ZXY system reports the position with little random error. Its main limitation is caused by averaging of the signals.

Background: Limited evidence exists regarding prospective associations for physical activity (PA) and sedentary time (SED) with body mass index (BMI) and muscle strength in young children. We aimed to determine prospective associations for PA and SED with change in BMI and standing long jump over 2 and 4 years in children aged 3-5 years at baseline., Methods: A sample of 262 Norwegian children (50% girls) was followed from 2015 to 2017 and/or 2019. PA and SED (hip-worn ActiGraph GT3X+) were measured at baseline and BMI and standing long jump at baseline and at follow-ups. Multivariate pattern analysis was used to determine prospective associations between the triaxial PA intensity spectrum (0-99 to ≥15,000 counts per minute) and the change in outcomes., Results: We found significant prospective associations between the PA intensity spectrum and standing long jump at 2- (explained variance = 5.8%-7.7%) and 4-year (explained variance = 4.8%-5.6%) follow-ups. Associations were negative for SED and positive for all PA intensities. We found no associations between PA/SED and BMI., Conclusions: Our findings suggest that PA and SED can predict future lower body muscle strength but not BMI in early childhood.

The aim of this study was to determine associations between weather and playground surface conditions and time spent outdoors and in moderate-to-vigorous PA (MVPA) in preschool children aged 3-6 years. We included 1201 children (mean age 4.8 years, 51% boys) from 68 preschools in Norway who provided 12,030 days of observation during 2015-2016. Preschool MVPA was measured by accelerometry (ActiGraph GT3X+) for 10 consecutive weekdays. During this period, outdoor time and playground surface conditions were logged by staff and weather conditions were obtained from the Norwegian Meteorological Institute. Associations were determined using linear mixed models. Associations for MVPA depended on whether the analyses were adjusted for outdoor time or not. In unadjusted analyses, rainfall, wind, and wet, icy, and mixed playground conditions were negatively associated with MVPA, while temperature, snowfall, and dry playground conditions were positively associated with MVPA. In adjusted analyses, temperature and wet and mixed playground conditions were negatively associated with MVPA, while snowfall and dry and snowy playground conditions were positively associated with MVPA. Outdoor time and MVPA should be addressed simultaneously to inform interpretation of associations with weather characteristics and intervention development in early childhood education settings.

Reliability of accelerometer-determined physical activity (PA), and thus the required length of a monitoring period, appears to depend on the analytic approach used for its calculation. We compared reliability of objectively measured PA using different resolution of data in a sample of 221 Norwegian 2-6-year-old children providing 2-3 valid 14-day periods of accelerometer monitoring (ActiGraph GT3X+) during September-October, January-February, and May-June 2015-2016. Reliability (intra-class correlation [ICC]) was measured for 1-14 days of monitoring across the measurement periods using linear mixed effect modelling. These results were compared to reliability estimated using different resolution of data using the Spearman-Brown formula. The measured reliability improved only marginally with increased monitoring length and levelled off after 5-6 days. Estimated reliability differed substantially when derived from different resolution of data: 3.9-5.4, 6.7-9.2, 13.4-26.7 and 26.3-87.7 days of monitoring was required to achieve an ICC = 0.80 using an hour-by-hour, a day-by-day, a week-by-week and a period-by-period approach, respectively. Reliability could not be correctly estimated from any single resolution of data. We conclude that reconsideration is needed with regard to how reproducibility of objectively measured PA is analysed and interpreted.

Background: Reliability of the Actigraph GT3X+ accelerometer has not been determined under normal wear time criteria in a large sample of subjects and accelerometer units. The aim of this study was to assess contralateral hip difference and inter-instrument reliability of the Actigraph GT3X+ monitor in adults under long-term free-living conditions., Methods: Eighty-seven adult subjects (28 men; mean (standard deviation) age 31.3 (12.2) years; body mass index 23.7 (3.1) kg/m2) concurrently wore two GT3X+ accelerometers (174 units in total) attached to contralateral hips for 21 days. Reliability was assessed using Bland-Altman plots, mixed model regression analyses and absolute measures of agreement for different lengths of data accumulation (single-day-, 7-day- and 21-day periods)., Results: There were no significant differences between contralateral hips (effect size ≤0.042; p ≥.213). Inter-instrument reliability increased with increased length of data-accumulation. For a 7-day measurement period (n = 232 weeks), limits of agreement were ±68 cpm (vertical axis) and ±81.3 cpm (vector magnitude) for overall physical activity (PA) level, ±51 min for sedentary time, ±18.2 min for light PA, ±6.3 min for moderate PA, ±3.5 min for vigorous PA, and ±6.7 min for moderate-to-vigorous PA., Conclusions: The Actigraph GT3X+ accelerometer is a reliable tool for measuring PA in adults under free-living conditions using normal data-reduction criteria. Contralateral hip differences are very small. We suggest accelerometers be attached to the right hip and data to be accumulated over several days of measurement.

Background: In adults, a minimum of 3-5 days of accelerometer monitoring is usually considered appropriate to obtain reliable estimates of physical activity (PA). However, a longer period of measurement might be needed to obtain reliable estimates of sedentary behavior (SED). The aim of this study was to determine the reliability of objectively assessed SED and PA in adults., Methods: Eighty-seven adult subjects (28 men; mean (standard deviation) age 31.3 (12.2) years; body mass index 23.7 (3.1) kg/m2) wore the GT3X+ accelerometer for 21 subsequent days, for which the reliability of different wear time criteria (8 to 12 h/day and 3 to 5 d/week) was explored. Variance partitioning along with the Spearman-Brown prophecy formula was used as the basis for determining intraclass-correlation coefficients (ICC) and the number of monitoring days needed (N) to achieve an ICC = 0.80. Week-by-week reliability was reported using ICC, Bland-Altman plots and absolute measures of agreement., Results: Seven-10 days of monitoring was needed to reliably assess overall- (axis 1 and vector magnitude (VM) counts per minute (CPM)) and moderate-to-vigorous PA (MVPA), 3-4 days was needed for light PA (LPA), whereas the number of days needed for SED depended on whether adjustments were made for wear time (6-8 days) or not (13-15 days). The week-by-week ICC was ≥0.70 for all variables, with limits of agreement being ±267.8 cpm for CPM, ±352.3 cpm for VM CPM, ±76.8 min/day for SED, ±57.8 min/day for LPA and ±43.8 min/day for MVPA, equal to 1.0-1.6 standard deviations, when adjustment was made for wear time., Conclusions: For most variables, more than one week of measurement was needed to achieve an ICC = 0.80. Correcting for wear time was crucial to reliably determine SED. Considerable week-by-week variability was found for all variables. Researchers need to be aware of substantial intra-individual variability in accelerometer-measurements.