Author's response to reviews Title: Agreement between pedometer and accelerometer in measuring physical activity in overweight and obese pregnant women Authors: Tarja I Kinnunen (tarja.kinnunen@ncl.ac.uk) Peter WG Tennant (peter.tennant@ncl.ac.uk) Catherine McParlin (catherine.mcparlin@ncl.ac.uk) Lucilla Poston (lucilla.poston@kcl.ac.uk) Stephen C Robson (s.c.robson@ncl.ac.uk) Ruth Bell (ruth.bell@ncl.ac.uk) Version: 2 Date: 6 May 2011 Author's response to reviews: see over
1 Author Response Letter Manuscript Reference: 2802255904576243 Authors: Tarja I Kinnunen, Peter WG Tennant, Catherine McParlin, Lucilla Poston, Stephen C Robson and Ruth Bell Title: Agreement between pedometer and accelerometer in measuring physical activity in overweight and obese pregnant women Dear Professor Gabriella Anderson, Thank you for considering our paper and for providing us with the opportunity for resubmission. The following letter details our response to each of the referees comments. The original comments are included in italics. Reviewer 1, Stephanie Prince: Overall impressions: This was a very well, clear and concise paper on the measurement discrepancies between pedometers and accelerometers in overweight pregnant women. The methods employed were appropriate and the conclusions justified. I would recommend this for publication. Background Discretionary revisions: 1. This section was thought to be well written. The description of discrepancy between self-report and direct measures is well founded (perhaps the authors may wish to highlight this by providing an example of the level of agreement between a self-report and direct measurement of PA in a pregnant population as per their references # 5-7). We have included a brief description of two recent studies comparing accelerometers and pedometers in pregnant women (Connolly et al. 2010, Harrison et al. 2011). As these are the most pertinent studies to the aims of our paper we have not extended the Background further. 2. Third paragraph: I would also bring to the authors attention that some recent work has been done in the field looking at validation of pedometers against accelerometers in pregnant women. Please see the following reference: Connolly CP, Coe DP, Kendrick JM, Bassett DRJ, Thompson DL. Accuracy of Physical Activity Monitors in Pregnant Women. Medicine & Science in Sports & Exercise 2010;Publish Ahead of Print. The devices were validated using treadmill walking, rather than free-living conditions used in the present study, but it would be good to reference the similarities/differences between the two studies. This study was not available at the time of submission, but we have updated the manuscript to include this paper. Methods The study participant, data collection, and categorization sections were clearly written. Minor Essential Revisions: These revisions refer to the statistical analyses section. 1. Can the authors comment to why they did not use means, standard deviations (SDs) and range rather than median and inter-quartile ranges? The median represents the middle point of the data,
2 but it is unclear as to how this provides better information regarding the spread and distribution of the step counts across the women. The authors would be urged to include means and SDs. In our data, most of the physical activity variables were not normally distributed and therefore it was not appropriate to report means and SDs. Consequently, we used non-parametric tests and reported medians and IQRs for all variables. It might be confusing for the readers if we reported both medians and means as the statistical analyses were made for the medians only. Results Minor Essential Revisions: The authors make use of the Bland-Altman method to examine % agreement between the two measures for overall step counts. Could they provide the mean differences between the two measurements rather than just the means and limits of agreement and include this in Figure 3? This information can be found in the text on page 11 in Results: "There was no significant difference between the overall step counts recorded by the pedometer compared to the accelerometer (medians 5961 vs. 5687 steps/day, p=0.37)." Discussion This section was very well written. There are no suggestions for improvement other than the discussion of the reference mentioned above for the introduction. Reviewer 2, Tiago Barreira: Major Compulsory Revisions 1. The main problem with this manuscript is the lack of clarity in the conclusion. The readers don t come out with a precise answer about the devices; the article raises more questions than gives answers. It is not clear if one device is better than the other or even if the step counts from the devices can be used interchangeably or not. We have now clarified our conclusions in the abstract. This study does not answer the question which of the monitors is more accurate, but it shows that the monitors do not agree well with each other and the step counts cannot be used interchangeably. 2. There is a need to update the references, which will lead to re-writing a large portion of the manuscript. There is a study examining the validity of pedometers and accelerometer in pregnant women. See provided reference. We have updated the references and modified the introduction and discussion to include studies that were published after our article was submitted to BMC Public Health. This most notably allows us to include two very recent studies comparing accelerometers and pedometers in pregnant women (Harrison et al. 2011, Connolly et al. 2010). 3. It is very important to make a clear distinction between some of the accelerometers references. Most of the references refer to the Actigraph 7164 and in this study the accelerometer used was the GT1M and they are not similar in their measurement. When describing the Actigraph accelerometers used in different studies (references 18, 19, 20, 26, 38), we now specify in Methods and Discussion whether they used the 7164 model or some other
model. We have also added a reference to a recent study by Kozey et al. (2010), which suggested that the 7164 model detected more steps but less counts than the GT1M model (see the second paragraph in Discussion). However this difference did not affect activity intensity classifications and the authors concluded that the monitors should be comparable when estimating habitual activity levels. 4. There are a large number of studies that show the superiority of piezoelectric pedometers over the Yamax (a spring lever pedometer). Even the references used to make the point that the Yamax is a superior device are questionable to support this statement. We have now included the following sentence and reference in the Discussion when describing the accuracy of Yamax Digi-Walker pedometer: On the other hand, another study showed that tilt angle reduces the accuracy of spring-levered pedometers (such as Yamax Digi-Walker) in overweight and obese adults (Crouter et al. 2005). 5. It is also known that the spring lever pedometer have problems with tilt angle caused by large waist circumference or high BMI, which could be a problem in this population. The authors state that high BMI is not a problem, which is not true. We have discussed the possible effect of tilt angle on the results in the Discussion as follows: The accuracy of the latest Actigraph accelerometer model (GT3X) and Yamax Digiwalker SW-200 was recently investigated in 30 pregnant women (Connolly et al. 2010). Both monitors underestimated the number of steps especially at slow walking speeds, but positioning the monitors at a tilt angle did not correlate with the percentage of actual steps detected by either monitor. In contrast, Crouter et al. (2005) suggested that the tilt angle reduced the accuracy of spring-levered pedometers in overweight or obese adults. The tilt angle may also reduce the accuracy of accelerometers in assessing vertical movement, which may happen more often among overweight and obese than normal weight people [17]. The tilt angle was not directly measured in the present study. However, BMI and gestational age did not significantly modify the results of the Bland-Altman plot suggesting that the potential effect of the tilt angle on the results may have been the same regardless of BMI or gestational age. 6. The recommendation of +- 500 steps/day based on 10 min of MVPA is not accurate; 10 min of MVPA would require at least 1000 steps or more. There are a large number of references making the relationship between MVPA and 100 steps/min. Revise references and recommendation. We agree with the reviewer that 10 min of MVPA would give at least 1000 steps, not 500 steps. Our purpose was to say that ±500 steps/day corresponds to a range of 1000 steps/day, which obviously was not clear for the readers. We have clarified the sentence in the manuscript as follows: We propose that they should be no larger than ±500 steps/day (i.e. a range of 1000 steps/day), which is likely to correspond to a maximum of 10 min difference in the duration of MVPA, such as brisk walking [32], and may therefore be of clinical and public health importance. 7. The reported problem with lack of accuracy of pedometers at slow walking speeds is also a problem with accelerometers. See reference provided. We have revised the text in Discussion as follows: This may also be the case with some accelerometers, although the GT1M model used in our study has been shown to have lower intermonitor variability and lower sensitivity for light intensity activity than the preceding 7164 model (Rothney et al. 2008, Kozey et al. 2010). On the other 3
4 hand, the previous CSA model has also been reported to erroneously detect slightly more nonsteps e.g. when travelling by a motor vehicle [38]. 8. The information about epoch length does not make sense. The cut points used in this study were developed for 60 sec epochs. Physical activity cannot be classified using those cut points and shorter epochs unless the authors provide references to justify this approach. We have now used 60s epoch length in all analyses and reported those results. The number of women with at least 3 valid days and pedometer step counts is now 58 instead of 62. The results were essentially the same as before, when comparing pedometer and accelerometer data. The absolute durations of activity at each intensity level have changed slightly. The results are now also comparable with the results of the study by Harrison et al., as they also used 60s epoch length. We have removed the discussion on epoch length as it is no longer relevant. 9. There are a few participants that had very low numbers for step counts measured by the pedometer and high difference compared to the accelerometer. The authors try to explain this by attributing to different wear time of the devices, but this is probably not the case. It doesn t make sense that the person would wear one device and not the other. It appears that the pedometers were in a position that did not work well or it was just equipment malfunction. Those are definitely outliers and the deletion of those points should be explored. We can only speculate on the reasons for very low pedometer step counts. As the reviewer suggested, it is possible that some of the pedometers failed to detect steps accurately (possibly because of tilt angle). However, we did not find any evidence of equipment malfunction, e.g. the same monitors worked well when used in subsequent women. Furthermore, it is not possible to exclude the possibility that the devices were not worn for the same time. We have carefully considered the possibility of excluding some outliers from the analyses and came to the conclusion that it would be better not to do so. Firstly, this is because any outlier definition would be very arbitrary. For example, for 3 of the 4 women with the lowest pedometer step counts (from 224 to 402 steps/day) the difference between pedometer and accelerometer step counts varied from 1158 to 2215 steps/day only while for the fourth the difference was 5612 step/day. On the other hand, there were several women with higher pedometer step counts for whom the difference between the monitors was several thousand steps/day. Should these women also be excluded? More importantly, this study is about measuring agreement in the real world. Excluding potentially valid individuals because of low (but not impossible counts) therefore risks biasing the results and altering the conclusions. Finally, there was no evidence that the parametric assumptions of the regression-based Bland- Altman method were violated, suggesting there were no 'outliers' for the most important variable on test i.e. the level of agreement. 10. The authors make it clear that the difference between step counts measured by the devices were in opposite direction in the low activity participants and high activity participants but they do not do a good job explaining why those differences happened. Why did the pedometer count more steps at the high end? Why the pedometer counted fewer steps in the low end? This is a good question. Unfortunately we are not able to provide any more exhaustive answer to it than already presented in the Discussion. 11. The conclusion in the abstract is not supported by the results. It is not clear why accelerometers would be a better choice than pedometers.
5 We have revised the abstract conclusion: Pedometer and accelerometer steps cannot be used interchangeably in overweight and obese pregnant women. 12. The Actigraph is not the only accelerometer that has outputs that are highly correlated to DLW. See reference bellow. As previously mentioned the Actigraph used in reference 26 is not the same used in this study. We have revised two sentences in the Discussion and incorporated the reviewer s comment as follows: Although the previous version of the Actigraph accelerometer was the only commercially available accelerometer [26] that correlated reasonably with doubly labelled water, most validation studies have been conducted in controlled environments. Validity is lower when applied to freeliving settings [17]. Two armband accelerometers have recently been shown to be highly correlated with doubly labeled water in free-living conditions (Johanssen et al. 2010). 13. Why were the 60 min interval and the 500 min total time chosen as standards? Any references for it? To our knowledge, there is no consensus for defining a valid day, but approximately 10 hours is often used (Mâsse 2005, Corder et al. 2007). The same applies to the length of the period with no counts that should be excluded. Minor Essential Revisions Introduction 1. The information about quality trials is irrelevant to the article, should delete. We feel the sentence However, the available evidence is limited and larger and better quality trials are needed (at the beginning of the Background) is indeed relevant for this article: We believe a number of researchers will be interested in whether simpler and cheaper pedometers provide comparable results to accelerometers and can therefore be used in future physical activity trials. We would therefore prefer to keep this sentence in the text. Methods 14. Make clear that the cut points were created for a different version of the Actigraph. We have added the following sentence to the Methods when describing accelerometer data collection. These cut points were originally developed for CSA Model 7164 accelerometer. Results 1. Provide the whole information about the Mann-Whitney U test and not just the p-value. We now report medians (for age) or percentages (for smoking and education) for all background variables, where there were statistically significant differences between the included and the excluded women. Discussion 1. The authors use mean across the discussion. Shouldn t it be median. If that is not true make clear why the change happened.
6 We agree with the reviewer that we should use medians when referring to our own results. We found only one instance where we had quoted mean instead of median and we have revised that (the last paragraph where reporting our conclusions). When discussing the results of Bland-Altman plots, the situation is different as we used the mean of accelerometer and pedometer steps on x- axis (Figure 2). Use of the mean is hence appropriate when discussing these results. When referring to results of other studies, we refer to whatever measure of central tendency was reported in the original paper. 2. The information about the Actigraph counting steps refers to a previous version of the accelerometer. Any information on the version used in this study? We have added the following sentence in the Discussion: This may also be the case with some accelerometers, although the GT1M model used in our study has been shown to have lower intermonitor variability and lower sensitivity for light intensity activity than the preceding 7164 model (Rothney et al. 2008, Kozey et al. 2010). 3. There is no need to mention the problem with cycling. We have removed this sentence from the Discussion. Yours Sincerely, Tarja Kinnunen, Peter Tennant, Catherine McParlin, Lucilla Poston, Stephen Robson and Ruth Bell References: Connolly CP, Coe DP, Kendrick JM, Bassett DRJ, Thompson DL. Accuracy of Physical Activity Monitors in Pregnant Women. Medicine & Science in Sports & Exercise 2010; published ahead of print Corder K, Brage S, Ekelund U. Accelerometers and pedometers: methodology and clinical application. Curr Opin Clin Nutr Metab Care 2007:10:597 603. Crouter SE, Schneider PL, Bassett DR, Jr. Spring-Levered versus Piezo-Electric Pedometer Accuracy in Overweight and Obese Adults. Med Sci Sports Exerc 2005;37:1673 1679. Harrison CL, Thompson RG, Teede HJ, Lombard CB. Measuring physical activity during pregnancy. Int J Behav Nutr Phys Act 2011,8:19. Johannsen DL, Calabro MA, Stewart J, Franke W, Rood JC, Welk GJ. Accuracy of armband monitors for measuring daily energy expenditure in healthy adults. Med Sci Sports Exerc 2010;42:2134-2140. Kozey SL, Staudenmayer JW, Troiano RP, Freedson PS. Comparison of the ActiGraph 7164 and the ActiGraph GT1M during Self-Paced Locomotion. Med Sci Sports Exerc 2010;42:971 976. Mâsse LC, Fuemmeler BF, Anderson B, Matthews CE, Trost SG. Catellier DJ, Treuth M. Accelerometer Data Reduction: A Comparison of Four Reduction Algorithms on Select Outcome Variables. Med Sc. Sports Exerc 2005;37:S544-S554. Rothney MP, Apker GA, Song Y, Chen KY. Comparing the performance of three generations of ActiGraph accelerometers. J Appl Physiol 2008;105:1091 1097.