Sample recruitment
Participants were recruited by BIB staff during routine home visits or baby clinics, conducted with families taking part in the BIB1000 focus sample from the BIB study, which aims to investigate growth trajectories and identify modifiable risk factors for childhood obesity [14]. BIB1000 includes over 1700 families who agreed to have additional measurements taken at six, 12 and 18 months, 2, 3 and 4 years [14]. The BIB1000 focus sample is composed by a sub-set of participant families who have consented to having additional data collected regarding growth, diet, feeding practices, PA, screen viewing and sleep [14]. Bi-lingual BIB staff (of both White British and South Asian ethnicities) showed the participant information sheet while briefly explaining the study to the parents in their preferred language, and filled in a contact sheet for those who showed an interest in taking part. The participant information sheet briefly described the study procedures, what was required from participants, the importance of understanding PA and SB levels and the factors influencing these, and contained contact details for the main investigators. Participant information sheets were available both in English and Urdu, to facilitate the recruitment of non-English speaking South Asian families. The contact sheets were collected from the BIB offices on a regular basis by the lead researcher for this physical activity measurement study, contact details were extracted into a database, and each family was assigned with a unique study ID. Parents were contacted by telephone at their preferred days and times, unless email was the preferred method of contact stated (in which case the first contact was via email). During the telephone call, the study procedures were explained with greater detail, and parents were provided with the opportunity to ask questions. For those who agreed to participate, current address was confirmed and a date and time was set for the first home visit. All recruitment telephone calls were made following a template guide, to ensure that no important information was missed. If there was more than 1 week between the telephone call and the first home visit, participants were also asked if they would like a reminder text message or telephone call to confirm if the scheduled date and time were still suitable. For participants who declared that they would like to think about the decision to take part in the study, the researcher asked for permission to call them again and the best time to do this.
Data collection procedures
After the recruitment telephone call, each family who agreed to take part in the study was assigned with a “family pack” consisting of: one checklist; one first home visit guide; parent and child informed consent forms; one anthropometric measurement sheet; one activity diary; one mother’s questionnaire form; one “end of data collection” interview; one magnet reminder to wear the accelerometers; one extra belt for the child’s accelerometer; and one accelerometer per participant family member. Each accelerometer was identified with a label which allowed participants to clearly identify each family member’s unique accelerometer, thus reducing the possibility of unintentional switches between children and their parents. The additional belt was included to allow parents to quickly replace the belt in case the original one needed washing (e.g. from issues resulting from potty training, as experienced by one family). The additional belt also enabled children to wear the ActiGraph GT3X+ during aquatic activities, providing a dry belt to place the accelerometer back on straight after the activity, thus avoiding loss of data because of waiting for the original belt to dry.
During the first visit, parents were reminded of study procedures, given the opportunity to ask questions, and requested to sign informed consent forms for themselves and their children. After formal consent was given, parents’ and children’s anthropometric measurements were taken by the first author, including height, weight, and waist circumference, hip circumference (parents only), and subscapular and triceps skinfolds (children only). Height was measured to the nearest one mm using a portable stadiometer (Holtain Ltd, Crosswell, UK), with a wooden standing platform placed on a firm horizontal surface. Weight was measured to the nearest 0.1 kg on a Tanita weighing scale (Tanita, model BC 418 MA, Tokyo, Japan) placed on a firm horizontal surface, with participants being barefoot and free of heavy clothing (e.g. outdoor jackets, sweat-shirts or work trousers). Waist circumference was measured with a Seca measuring tape to the nearest one mm at the level of the umbilicus directly over the skin, when this was allowed by the participants. In case participants considered this to be embarrassing or unsuitable (particularly due to cultural sensitivity in South Asian families), they were asked to either change into lighter clothing or remain with the thinnest possible layer of clothing for the measurement to be taken, with care to remove any bold creases that would introduce further error into the measurement. Whenever it was not possible to perform the measurements on the first home visit, these were taken during the last home visit or on a third appointment specifically arranged for this purpose up to 8 days after the last day of data collection. At the end of this first visit, the time and date for the last home visit was scheduled and participants were asked if they would also like to receive a text message during the first three days of data collection to remind them to wear the accelerometer. If participants requested this, mobile phone contact and usual wake-up times for both weekdays and weekend days were recorded, and a standardised text message reminder was sent roughly five minutes after reported wake-up times. All participants were advised to contact the author via email or telephone if there were any concerns or issues regarding any study materials or protocols during data collection.
During the second home visit, all monitors and study documents were collected, and any missing anthropometric measurements were taken. After completion of the questionnaire, a semi-structured “end of data collection” interview was conducted with the mother to assess if there were difficulties in wearing the accelerometer for both parents and the child, filling in the activity diary or the questionnaire, whether the week of data collection had been a “typical week” or more/less active (and why), and also if the child had been ill during that week. This was an informal one-to-one interview with several open-ended questions to allow parents to provide as much detail as possible on any issues and successful strategies used (see Additional file 1 for copy of interview guide). Upon completion of the study, all families were sent written reports informing them of the average PA and SB levels of the children and parents. These reports also contained current PA and SB guidelines [23] and tips on how families could achieve these recommendations.
Participants’ SES was assessed using the 2010 English Index of Multiple Deprivation (IMD) [24] with participants’ home postcodes. There are 32,482 postcode areas in England; the English 2010 IMD combines a range of economic, social, and housing indicators into a single deprivation score for each postcode area, thus allowing the ranking of areas from 1 to 32,482, according to their level of deprivation [24]. To facilitate interpretation, the ranking was converted to percentages where 0 % represents the most deprived area and 100 % represents the least deprived area.
Physical activity and sedentary behaviour measurement
Children’s and parents’ PA and SB were measured with the ActiGraph GT3X+ (The ActiGraph, Pensacola, FL, USA), during waking hours over eight consecutive days. The ActiGraph GT3X+ was set to record data at 60Hz, which was the maximum frequency possible for nine consecutive days of data collection. This nine-day period was programmed to give families the opportunity to still comply with the eight-day protocol if they forgot to wear the monitors on the first day. Each device was programmed to start recording at 5:00 am on the first day (the day after first home visit), and stop recording at 5:00 am on the 10th day (i.e. nine full 24-h days). At the first home visit, parents (most commonly mothers) were instructed and shown how to place the ActiGraph GT3X+ on themselves and the children at the level of the anterior superior iliac spine, underneath or on top of clothes, and to place it consistently on the same side of the body (see Fig. 1 depicting the accelerometer on a 3-year old child). We did not give strict guidance for placement on the left or right side of the body, to enable participants to choose the most comfortable side for them, and avoid switching sides during data collection (e.g. if participants mistakenly started wearing the device on the “wrong side” and corrected the placement half way through the collection period). Parents were advised to place the devices underneath children’s clothes, to avoid any issues resulting from children’s curiosity (e.g. removal of the device, or erroneous acceleration data resulting from the child playing with the device). Parents were instructed to place the accelerometers on themselves and their children after waking up in the morning, remove them at the time of going to bed, and to note down in the activity diary any other periods when the accelerometer was removed with the corresponding reason. Additionally, they were asked to record in the activity diary any day-time sleep (i.e. naps) that children may have during the data collection period.
After collecting the monitors at the second home visit, the raw accelerometry data (*.gt3x file) was downloaded and analysed in the proprietary software ActiLife (v.6). The raw accelerometry files were then transformed into an *.AGD file, displaying the acceleration counts by 5- and 60-s epochs (for children and parents respectively), using the low frequency extension filter. Due to the scarce research defining the correct criteria specifically for toddlers [25], wear-time validation and day inclusion criteria was undertaken in agreement with procedures used previously in preschoolers [26], toddlers [22], and infants [27]. Non-wear time was determined as ≥10 consecutive minutes of zero counts, which has been considered adequate for the typical activity patterns of this age group [28]. Following wear time validation, children’s *.AGD files were scanned to assess what time the accelerometer was placed on and taken off, thus defining the start and end of each day. The start of a day was set at the first consecutive minute of acceleration data higher than zero, and the end of a day was defined as the last epoch with counts higher than zero before a consecutive ≥180 min of zero counts. A valid day was defined according to the 70/80 rule [22, 27, 29]: 80 % of the period during which at least 70 % of the study population has recorded accelerometer data. Only days with ≥3 h and ≤18 h of valid wear time were included in this calculation to ensure that implausible wear times would not influence the definition of a valid day or the subsequent assessment of SB and PA levels [25, 30]. Since at least 70 % of the sample had ≥587.82 min of valid acceleration data, a valid day was defined as one containing ≥470.26 min of valid acceleration data (similar to previous studies with children aged ≤3 years) [22, 27]. The number of days needed for a reliable estimate of habitual SB and PA in children younger than 3 years has not yet been agreed on [24]. After investigation published elsewhere [31] regarding the minimum number of days needed to get reliable measurements of habitual PA/SB, which followed procedures similar to those used by Hinkley et al. [30] in preschoolers: children with ≥3 days of valid data were considered to provide enough data to assess habitual time spent in SB and PA. These three days did not necessarily include both week and weekend days, due to previous investigation showing no difference in PA/SB between week and weekend days in this sample [31].
For parents, accelerometry data cleaning procedures followed those previously employed in the literature [32, 33]. Non-wear time was defined as ≥60 min of consecutive zero counts, and a day was considered valid if wear-time was ≥600 min. Parents with ≥3 valid days were considered to provide enough data for inclusion in studies assessing habitual PA/SB. All procedures were reviewed and approved by the University’s Ethical Advisory Committee prior to the start of the study.
Statistical analysis
All scale variables were checked for normality of distribution by visual inspection of histograms and formally tested. Analyses were run in Stata (v.12) and SPSS (v.19) statistical software packages. Descriptive statistics (means, medians, standard deviation (SD), inter-quartile ranges (IQR), and percentages) were computed to describe the sample of provided contacts, recruitment uptake, compliance with the study protocol of eight days of wearing the accelerometer, and the sample providing ≥3 valid days of accelerometry data. Differences in these four variables between ethnicities, mother and father, and child’s sex were assessed with the Chi-square (χ2) or Fisher’s Exact test (when >25 % of cells had <5 expected counts). Mann–Whitney U or unpaired t-tests (when data were normally distributed) were used to compare the SES between those who accepted or refused to take part; those complying versus non-complying with 8-day protocol; and those providing ≥3 days of valid accelerometry data or not.