accelerometers provide an objective measure of habitual activity which is not dependent on self - report , and are superior to pedometers because they measure the intensity of physical activity as well as frequency .
they are small portable devices , particularly suitable for measuring physical activity in free - living living conditions .
accelerometers operate on the principle that they measure change in velocity over time ( acceleration ) ( m.s ) , enabling intensity of physical activity to be quantified .  
accelerometers can be uniaxial , usually sensitive to movement in the vertical plane , or biaxial or triaxial , with movement also detected in the anteroposterior and/or lateral planes .
a known limitation with uniaxial ( vertical ) accelerometers is that they underestimate nonambulatory activities that do not involve vertical movement of the trunk ( when waist mounted ) such as cycling .
triaxial accelerometers have a theoretical advantage over uniaxial monitors to capture non - ambulatory activities , although in reality they provide similar information due to dominance of detecting movement in the vertical plane .  
studies of the validity and reliability of accelerometers in children and protocols to standardise their use are available [ 46 ] .
 accelerometer counts  have been calibrated against energy expenditure , and researchers have published thresholds of activity counts equating to different intensities of physical activity [ 79 ] . there has been a focus on measuring moderate to vigorous physical activity ( mvpa ) , as the level of physical activity deemed to improve health .
however , there remains a lack of consensus in defining the intensity of physical activity with accelerometers , which is making comparison between studies difficult .  
for example , in uk school children there is a wide variation in the proportion reaching the department of health 's   recommendation of 60 minutes of moderate intensity activity per day , from 2.5% in 11 year olds   to 92% in 13 - 14 year olds .
this large difference is thought to be due to different thresholds of accelerometer counts used to define moderate intensity activity .   
the utility of an instrument includes factors relating to first , the monitor , such as technical limitations and data loss due to malfunction , and second , the participants , including   adherence to data collection protocols and use of the monitor within their social context .
a study of the feasibility of using accelerometers in a population- based cross - sectional study of young adolescents shows data loss due to malfunction in 8.5% of participants , and 50% of the remaining students had the full 7 days of recording . using accelerometers in intervention studies to measure change in physical activity requires multiple testing and poses potentially greater practical challenges for researchers .  
, a new group - based intervention for the treatment of childhood obesity for 711 year old children and their parents .
this paper gives further details on the practicalities of using accelerometers to measure change in habitual physical activity , providing new insight into their utility in children who are obese .
families for health  , a treatment intervention for children who were obese or overweight and their parents from coventry ( england ) .
the first group of families commenced the programme in september , with followup in december and june ; and the second group of families started the programme in january , with followup in april and october .  
children 's physical activity levels were measured using a 7-day recording with a uniaxial ( vertical ) accelerometer with step - count function ( gt1 m actigraph , fort walton , florida ) at three timepoints : baseline , end - of - the programme ( 3-months ) , and 9-months .
as far as was practically possible , children wore the same monitor ( serial number ) at each time point , to remove  between unit  variation .
children were asked to wear the accelerometers during waking hours , removing them at bedtime and also during bathing and swimming , since they are not waterproof .  
the accelerometer was worn on an elastic belt around the waist , positioned on the right hip .
a recording over 7 consecutive days provides a reliable estimate of usual physical activity in children allowing for differences between weekday and weekend .
the data collection interval was 60  sec , chosen to allow the storage of a week of data .
we acknowledge that this epoch length ( rather than a shorter epoch length ) may underestimate mvpa .  
children wore the monitor for a day before the data collection started in order to allow for habituation .
a 3% increase in normal levels of activity has been found on the first day of recording in children .
children , sometimes with help from parents , completed an activity diary alongside the accelerometer for 7 days ( see the appendix ) .
this was a pictorial  tick box  diary recording activities each hour , with a column for free text additional comments .
there was also a space to record the time the accelerometer was put on and taken off .
we included a child 's record in the analysis if there were at least 4 complete days of data available , taken as the minimum needed to obtain a reliable measurement of habitual physical activity in children ( reliability of 0.80 ) .  
we defined a complete day as one where there was 7  hrs   of data , after excluding periods in the day when the accelerometer appeared not to have been worn .   
although 10 hours of worn time is often used , the reliability between 7 and 10 hours is not substantially different [ 18 , 19 ] .   in practice , wear time was usually greater than 7 hours .  
nonwear time was identified from the data by periods of 20 minutes of consecutive zero counts , making it unlikely that the monitor was worn [ 20 , 21 ] .  
there is , however , no consensus for the minutes of continuous zeros for identifying non - wear time , ranging from 10 minutes   to 180 minutes   in studies in children . in some cases , the number of minutes has not been specified .   at each time point for each child , the mean accelerometer counts per minute and the mean daily step count were calculated . the mean daily time spent in moderate and vigorous physical activity ( mvpa ) was calculated using two different thresholds for mvpa , which are in use for the actigraph :  
 ( i ) freedson equation ( see [ 3 , 7])activity counts were translated into mets using the freedson equation with 4  mets used as the cutoff for mvpa .
mets = 2.757 + ( 0.0015  countsmin  )  ( 0.08957  age [ yr ] )  ( 0.000038  countsmin   age [ yr ] ) . 
 
activity counts were translated into mets using the freedson equation with 4  mets used as the cutoff for mvpa .
mets = 2.757 + ( 0.0015  countsmin  )  ( 0.08957  age [ yr ] )  ( 0.000038  countsmin   age
( ii ) puyau ( see ) an activity count of 3200  countsmin   was used as the cut - off for mvpa .
minutes spent in light physical activity was also calculated using puyau 's cut - off of 800 to 3199 accelerometer counts . 
 
an activity count of 3200  countsmin   was used as the cut - off for mvpa .
minutes spent in light physical activity was also calculated using puyau 's cut - off of 800 to 3199 accelerometer counts . to account for the hierarchical nature of the data induced by family clustering we fitted linear mixed models with random family effects to examine the differences between baseline , the end of the programme and 9-month followup , for counts per minute , minutes of mvpa , minutes of light physical activity , and stepcounts .
analyses were conducted using sas version 9 . semistructured interviews with 12 parents were carried out at their home by one of the authors ( wr ) just after the 
interviews obtained parents ' perceptions of the research measurements , which were required to optimise data collection and minimise respondent burden in subsequent research .
the stem question was  what did you think of the research aspects of the programme , such as the measurements of height , weight and waist ; questionnaires and interviews ; and activity monitor ?    
interviews were recorded , transcribed and analysed using the framework approach , using nvivo ( version 7 ) . only qualitative data with bearing on the use of accelerometers are presented .
group interviews were also carried out with 16 children at the end of the programme .
no child mentioned the accelerometers and so these did not provide any information and will not be discussed further .
28 children ( 9 males , 19 females ) aged 713 years who were overweight or obese   were recruited to the  families for health 
22 children also completed followup measurements both at the end of the programme and at 9-month followup . at each time point , around half of the children had accelerometer data for all 7 days ( 7  hrs worn time per day ) : 46% ( 13/28 ) at baseline , 41% ( 9/22 ) at the end of the programme , and 50% ( 11/22 ) at the 9-month followup .
over 90% of children had at least 4 days of data :   baseline 93% ( 26/28 ) , end - of - programme 96% ( 21/22 ) , and 9-month follow - up 91% ( 20/22 ) .
most records included at least one valid weekend day : baseline 86% ( 24/28 ) , end - of - programme 86% ( 19/22 ) , and 9-month follow - up 86% ( 19/22 ) .
there were five records ( from five different children ) with less than 4-days data . in four cases this was because the child had not worn the accelerometer for sufficient days and in one case it was because the battery in the accelerometer failed .
adequately completed activity diaries ( all days completed ) were available from 68% ( 19/28 ) of children at baseline , 64% ( 14/22 ) at the end - of - programme , and 55% ( 12/22 ) at the 9-month followup .
the other diaries were either not returned at all ( n = 6 ) or completed poorly ( n = 21 ) with days missing and/or lacking any detail in the final column about what children were doing . 

table 1 gives the changes in children 's physical activity after participation in the childhood obesity treatment programme .
no significant change from baseline was demonstrated in the average minutes spent per day in mvpa at the end of the  families for health  intervention or at the 9-month followup using either method of calculation .
the average daily step count was unchanged at the end of the programme , but increased significantly at the 9-month followup .
this increase may indicate that children were becoming more active in daily living , although this activity was not of sufficient intensity to be picked up by change in the summary measures from accelerometer activity counts .  
the children from the two pilot groups of  families for health  differed significantly in their response from baseline to the end of the programme , but not from baseline to the 9-month followup .
for example , children from group 1 reduced their mean daily mvpa using the freedson equation from 71 minutes in september ( baseline ) to 64 minutes in december ( end of programme ) ( 7  mins , 95% ci 22 to 6 ) , whereas group 2 showed a significant increase from 40 minutes in january ( baseline ) to 55 minutes in april ( end of programme ) ( 15  mins , 95% ci 1 to 30 ) ( p = .028 ) .
likewise , there were similar differences for the accelerometer counts , with group 1 showing a reduction of 87  countsmin   ( 95% ci 180 to 7 ) from september to december , whereas group 2 had an increase of 157countsmin   ( 95% ci 44 to 358 ) from january to april ( p = .015 ) .
this may reflect a seasonal effect , with less activity in the winter months , rather than any differential impact of the programme .  
the difference in the mean mvpa calculated by the two methods was striking : 60.2 versus 15.9 minutes at baseline ( table 1 ) . using the freedson equation the cut - off point of 4  mets equated to a mean activity count of 1834  countsmin   at baseline , although this ranged from 1510 countsmin   for the youngest child ( 7  yrs ) to 2515  countsmin   for the oldest child ( 13.7  yrs ) .
thus the activity count which defines mvpa is much lower using the freedson equation [ 3 , 7 ] in comparison with the 3200 countsmin   recommended by puyau . as a consequence ,
at baseline 10 of the 26 ( 38% ) children met the daily recommendation of 60 minutes of moderate intensity physical activity   when the freedson equation was used , whereas no children met this recommendation when a cut - off of 3200  countsmin   was used .
a number of children had very high peak activity counts ( countsmin  ) for some days .
greater than 20,000  countsmin   is considered the threshold for biological plausibility for the actigraph , above which data would be invalid .
recordings showing these levels of accelerometer counts were explored using visual inspection of the data alongside the diary .  
figure 1 shows an example in which this child reached 28,000  countsmin   between 3 - 4  pm . the diary showed that this child was participating in trampolining at school ( see the appendix ) .
four children specifically mentioned trampolining in their diary , suggesting that this is a common activity .
at least two other children also had trampolines in their gardens , and in one diary  playing in garden  was recorded at the time when there were very high peak counts , indicating that they may have been on their trampoline .
one other child with very high activity counts did not return her diary but her mother at interview commented about the trampoline in the garden :  she 's like 
mum i 'm going to go out and do my exercise  cos it 's light ' and every night she 's in the garden on the trampoline , she loves it .     
the four records where trampolining was verified by the diary were analysed both with and without values above 20,000  countsmin  , resulting in a mean reduction of 181  countsmin    for the daily record and 50  countsmin   when averaged over the weekly record .
for the example in figure 1 , removal of six minutes of data between 3 - 4  pm where the activity counts were above 20,000  countsmin   led to a reduction from 500 to 342  countsmin   for that day , and from 413 to 388  countsmin   for the summary record .
these high counts are not  invalid  data per se , but it is of note that trampolining can lead to very high activity counts which can influence the summary record
.    the physical activity the children were doing may also be undermonitored , due to the measurement abilities of the accelerometer used .
a known limitation with uniaxial ( vertical ) accelerometers is that they underestimate activities that do not involve vertical movement of the trunk .
the diaries highlighted the wide range of activities in which children commonly partake which primarily involves horizontal movement ( e.g. , cycling , nonmotorised scooter riding , roller blading , ice skating ) and may therefore be only partially monitored by the accelerometer ( table 2 ) .
some children did not wear the accelerometers because they found them unacceptable when carrying out their usual activities of daily living including going to school , and playing sport .
interview data confirmed that one boy was forbidden by the coach to wear the accelerometer when playing rugby , because being a contact sport it posed a risk of injury to others .
additionally , this child was a regular swimmer  
 ( i )  the only problem was that with some of the activities that child-4 does , he ca n't really wear it so you 're not really getting the data for him when he 's doing the activities .
because he ca n't wear it when he 's swimming and he was going swimming most mornings , he ca n't wear it when he is playing rugby so you haven't really got a true reading of when he has done his activities .
( i )  the only problem was that with some of the activities that child-4 does , he ca n't really wear it so you 're not really getting the data for him when he 's doing the activities . because he ca n't wear it when he 's swimming and he was going swimming most mornings , he ca n't wear it when he is playing rugby so you haven't really got a true reading of when he has done his activities .
two parents complained that the elastic belt holding up the accelerometer was too obtrusive , and one parent suggested a clip as an alternative :   
 ( i ) 
i think when you tightened it a bit the thing [ remaining elastic belt ] came down , we tried to sort of shove it in the one together , she was alright after .  
i think when you tightened it a bit the thing [ remaining elastic belt ] came down , we tried to sort of shove it in the one together , she was alright after .  
( ii )  i think because of the belt . you know like the pedometers maybe if it was something like that .
[ clip ]   when you 've got the belt its more  ugh  , you 've got this big thing that goes right the way around you , and sometimes it flaps down , trying to tuck it in .
( ii )  i think because of the belt . you know like the pedometers maybe if it was something like that .
[ clip ]   when you 've got the belt its more  ugh  , you 've got this big thing that goes right the way around you , and sometimes it flaps down , trying to tuck it in .
when one of us ( wr ) collected the monitors from their homes , several parents said that their child had not wanted to wear the monitor at school .
one   boy ( aged-11 ) said that the accelerometer had caused him to be bullied by another pupil :  you are wearing it because you are fat  .  
girls in particular were unwilling to wear the accelerometer at school , due to stigma and bullying : 
 ( i )  she did n't want to take it to school with her , she did n't want people knowing that she was wearing it .
so she only really wore it when she was at home and you were only talking about a couple of hours between coming home and going to bed again and a lot of it ,  cos of the weather and different bits and pieces , she has been sitting . and
again , you know he [ friend on programme ] did take it to school though , but maybe it 's a girl thing .
to be honest her dad turned around and said she could n't take it to school because it could get damaged or broken .
but i suppose he said that but maybe he was thinking he did n't want people saying stuff about her .
( i )  she did n't want to take it to school with her , she did n't want people knowing that she was wearing it .
so she only really wore it when she was at home and you were only talking about a couple of hours between coming home and going to bed again and a lot of it ,  cos of the weather and different bits and pieces , she has been sitting . and
again , you know he [ friend on programme ] did take it to school though , but maybe it 's a girl thing .
to be honest her dad turned around and said she could n't take it to school because it could get damaged or broken .
but i suppose he said that but maybe he was thinking he did n't want people saying stuff about her .
a fitness test in a fun way may be more productive than a monitor purely because you are relying on the children wearing the monitors , not getting teased at school , filling the diary out .
.     a fitness test in a fun way may be more productive than a monitor purely because you are relying on the children wearing the monitors , not getting teased at school , filling the diary out .
school could have a lot to do with it , she is conscious because of her weight and because her t - shirt is quite tight that it is going to be showing .
( iii )  it is just getting them to put it on . school could have a lot to do with it , she is conscious because of her weight and because her t - shirt is quite tight that it is going to be showing .
( mother-3 , girl aged-10 ) .   she also added that wearing the accelerometer was a particular problem during sport at school , due to embarrassment :  
 ( i )  she is bothered about wearing it during the activities like netball and things because she is frightened her t - shirt is going to come up and because it is other classes , it 's not just her own class .
i did ask her today if she could wear it for netball tonight , i said it 's only for practice , everybody knows you now , nearly everybody knows that you have got it on .
( i )  she is bothered about wearing it during the activities like netball and things because she is frightened her t - shirt is going to come up and because it is other classes , it 's not just her own class .
i did ask her today if she could wear it for netball tonight , i said it 's only for practice , everybody knows you now , nearly everybody knows that you have got it on .
accelerometers may be missing the physical activity of some of the children , either because of the technical limitations of the uniaxial accelerometer or due to children 's unwillingness to wear them at school or during sport .
however , around half of the children wore the monitor very conscientiously and provided 7 days of recording .
as one parent reported :  
 ( i )  i think wearing the monitors to start with made child-19 aware that it was serious , you know that there was a reason for doing it , he never once did n't wear it , he never once said he could n't wear it or threw a tantrum , even when he went to his dad 's . and his dad was n't particularly supportive of the whole thing  even on the weekends when he was playing football with his dad ,
when we went to watch him [ play in a football match ] i said  have you got your monitor on ?  , and he was like 
yeah  and showed it , so he took it seriously , he knew it was serious .
( i )  i think wearing the monitors to start with made child-19 aware that it was serious , you know that there was a reason for doing it , he never once did n't wear it , he never once said he could n't wear it or threw a tantrum , even when he went to his dad 's . and his dad was n't particularly supportive of the whole thing  even on the weekends when he was playing football with his dad ,
when we went to watch him [ play in a football match ] i said  have you got your monitor on ?  , and he was like 
yeah  and showed it , so he took it seriously , he knew it was serious .
this paper provides insight into the utility of accelerometers for measuring activity in children ( in this case , children who were overweight or obese ) , including the acceptability of their use ; an analysis of the physical activity which may be undermonitored ; the results from different data reduction methods to derive minutes of mvpa .   around 50% of the children provided 7 days of data and 90% had 4 days of data ( the minimum required to be included in the analysis ) .
this compliance is similar to a study in young adolescents in which 50% had 7 days and 86% had at least 4 days of valid accelerometer data , although a higher proportion of children who were overweight ( 66% ) versus nonoverweight ( 46% ) had 7 days of data .  
our experience with the actigraph monitor was good with only one recording lost due to a fault with the monitor .
participant factors included forgetting to put the accelerometer on but some nonuse was related to children being unwilling to wear the monitor , particularly in school , because they were too conspicuous .
in some cases the accelerometer had an unintended consequence of stigmatising the children and putting them at risk of bullying .
because of missing data , only 18 of the 22 children who completed all the other research measurements had accelerometry data at each timepoint .
this is reasonably good adherence to the protocol , and suggests that accelerometers are acceptable to most children but there are some children , in particular girls , who do not find it acceptable to wear accelerometers during their usual activities of daily living and , in some cases wearing the accelerometer was   potentially harmful , because it encouraged bullying .
accelerometers may have missed some of the physical activity that the children were engaged in for two reasons .
first , children engage in a wide variety of physical activities which may not be captured by the uniaxial ( vertical ) accelerometer due to technical limitations .
triaxial accelerometers , in principle , have a greater potential to capture the diverse activities in which children partake .
for some sports children were requested to remove the monitors such as swimming ( requested by the researchers ) and rugby ( requested by the coach ) , but some children were embarrassed and chose not to wear the monitor during sport at school .  
thus the accelerometers are likely to have underestimated physical activity in the children in the current study , but the degree of underestimation is likely to vary between children .  
uk guidelines recommend  at least 60 minutes of at least moderate intensity physical activity each day  for children .
the proportion of children estimated to meet this standard varies widely [ 12 , 13 ] .  
different conclusions may be due to different thresholds of accelerometer counts used to define mvpa .   in this study
we had initially used the freedson equation with a 4  mets threshold [ 3 , 7 ] to derive minutes of mvpa .
however , we received feedback that the values for mvpa were too high , and reanalysis was conducted using a threshold of 3200  countsmin .
these two methods yielded very different results for both the minutes spent in mvpa , consistent with other studies [ 5 , 27 ] , and for the proportion of children reaching 60 minutes of physical activity per day .
it is now accepted that the freedson equation overestimates children 's mvpa , and that the correct cut point is between 3000 to 3700  countsmin [ 5 , 27 ] .
thus our data using puyau 's cut - off of 3200  countsmin for mvpa is likely to be the most accurate . using this cutoff , none of the 26 children with 4 days of data at baseline achieved 60 minutes of daily moderate intensity physical activity , which is consistent with the low levels of physical activity in overweight and obese 11 year - old children in the alspac cohort .    
a common activity in children  trampolining  results in very high activity counts , and our data suggest that it may result in physical activity being  overmonitored  .
the physics of trampolining shows a peak acceleration of 4 g , although the effect on the results from accelerometers in children is not widely discussed .
activity counts above 20,000 were recorded during trampolining , thought to be beyond biological plausibility for the actigraph .
very high activity counts with trampolining may negate the use of summary measures using raw activity counts ( i.e. , total daily activity counts or average countsmin ) used in some studies with children , unless the trampolining data is removed prior to analysis .
trampolining could also affect the number of minutes spent in mvpa , because even gentle trampolining may take a child to an activity count above the threshold for mvpa .
these findings suggest that trampolining should be given a specific column on our daily diary .
trampolining is physical activity , but further assessment of its impact on summary measures is warranted , and further consideration should be given to how best to account for trampolining in analysis .
caution must be exercised in the quality control and reduction of accelerometer data in children where trampolining is a relatively common activity .  
this study has contributed an insight into the perceptions of parents on the use of the accelerometer with children who are obese .
the findings are strengthened by using a diary alongside the accelerometer records to aid interpretation .
limitations of the study include that standardisation across the followup timepoints , although attempted , was not always possible due to logistics .
followup was made at 3 and 9 months from baseline , whereas a 12-month follow - up is desirable in intervention studies so that results are not distorted by known seasonal changes .
a further limitation is that , whilst children were offered the opportunity of commenting on any aspect of the study , they were not explicitly asked about their perception of wearing the monitors in their daily living .
researchers should be aware that not all physical activity is likely to be monitored by uniaxial accelerometers .
further validation studies in children performing activities such as scootering ( non - motorised ) , cycling , roller blading , and ice skating should compare the output from triaxial and uniaxial monitors .  
researchers must also be aware of the potential for harm , such as stigma and bullying of the obese child when they are singled out to wear the monitor to evaluate an obesity treatment intervention , and make efforts to minimise these risks .
acceptability to children , in particular girls , could be improved by using attachments other than the elastic belt , such as a clip .
improved communication with the child 's school about the childhood obesity treatment intervention may also be of value to increase the acceptability of wearing the accelerometer at school .
further studies with children are indicated to gain their perception of wearing the accelerometers , similar to the study in adults by perry . in conclusion , although accelerometers are recognised as an objective measure of physical activity , the analysis of the diary records and interview data suggest some issues with their use in children .
 
 objectives . to investigate the use of accelerometers to monitor change in physical activity in a childhood obesity treatment intervention . methods . 
 28 children aged 713 taking part in  families for health  were asked to wear an accelerometer ( actigraph ) for 7-days , and complete an accompanying activity diary , at baseline , 3-months and 9-months . 
 interviews with 12 parents asked about research measurements . 
 results . over 90% of children provided 4 days of accelerometer data , and around half of children provided 7 days . 
 adequately completed diaries were collected from 60% of children . 
 children partake in a wide range of physical activity which uniaxial monitors may undermonitor ( cycling , nonmotorised scootering ) or overmonitor ( trampolining ) . 
 two different cutoffs ( 4  mets or 3200  countsmin1 ) for minutes spent in moderate and vigorous physical activity ( mvpa ) yielded very different results , although reached the same conclusion regarding a lack of change in mvpa after the intervention . 
 some children were unwilling to wear accelerometers at school and during sport because they felt they put them at risk of stigma and bullying . 
 conclusion . 
 accelerometers are acceptable to a majority of children , although their use at school is problematic for some , but they may underestimate children 's physical activity . 
1. Introduction
2. Methods
3. Results
4. Discussion
 Activity Diary to Use alongside the Accelerometer  (Completed Here for Child 16B when Trampolining, 3-4pm) 