Title
Step accumulation per minute epoch is not the same as cadence for free-living adults
Authors Names
Philippa Margaret Dall1, Paul Robert Walker McCrorie2, Malcolm Howard Granat1, Benedict William Stansfield1.
Author Affiliations
1School of Health and Life Sciences, Glasgow Caledonian University, Glasgow, UK, G4 0BA.
2Social & Public Health Sciences Unit,Medical Research Council, Glasgow, UK, G12 4RZ.
Corresponding author
Dr Philippa Dall
Senior Research Fellow, School of Health and Life Sciences,
Glasgow Caledonian University,
Cowcaddens Road.
Glasgow, UK
G4 0BA,
(+44) 141 3318003 (phone)
(+44) 141 3318112 (fax)
Running Title
step accumulation versus cadence in adults
Disclosure of Funding
This study was funded by the School of Health and Life Sciences, Glasgow Caledonian University.
Conflict of interest statement
Malcolm Granat is a co-inventor of the activPAL activity monitor, and director of PAL technologies.The other authors report no conflict of interest.
Abstract
Purpose:The term cadence has been used interchangeably to describe both the rate of stepping and the number of steps in a minute epoch.This is only strictly true if walking is continuous within that epoch.This study directly compared these two outcomes in minute epochs of data from free-living adults to assess the scale of any difference between them.
Methods:A convenience sample of healthy adults wore an activPAL activity monitor for seven days.The event record output of the activPAL, providing the start time and duration of each stride to the nearest 0.1s, was used to calculate step accumulation (number of steps), duration of walking and cadence (number of steps/ duration of walking) for each minute of measurement.
Results:Data from 117 individuals (78 female, mean age 46±16 years, mean BMI 24.9±3.7 kg·m-2) were analysed.Twenty-one percent of minutes (n=310/day) contained walking.The distribution (most minutes less than 40 steps/min) and mean (34±9 steps/min)of step accumulation, was very different from that of cadence (most minutes between 60-100 steps/min, mean 76±6 steps/min).Only 12% of minutes with stepping were walked continuously, while 69% of minutes with stepping contained less than 30s of walking.This is key to the difference between step accumulation and cadence, and means that cadence cannot be reconstructed from step accumulation without also knowing the duration that was walked.
Conclusion:Step accumulation, the number of steps in a fixed period of time, and cadence, the rate of stepping whilst walking, are not interchangeable outcome measures.It is vitally important that unambiguous terminology is used to describe the rate of stepping so that the outcomes of studies can be correctly interpreted.
Key Words step rate; objective measurement; public health; accelerometer; outcome measures; ambulation
Introduction
Paragraph Number 1Cadence (rate of stepping) can be measured by body-worn sensors during habitual physical activity. Cadence, as an outcome measure, is of interest as it can be used to indicate the intensity of walking activity (18), and act as a proxy of usual walking speed, which is predictive of mortality in older adults (16).Public health guidelines for physical activity refer both to the duration and the intensity of activity, for example recommending that adults spend 30 minutes a day in physical activity that is at least moderate intensity (9).There is a growing consensus that a cadence of 100 steps/min represents a reasonable minimum level for moderate physical activity in adults and adolescents (1,2,8,11,13,19).These studies measured indirect energy expenditure using portable calorimetry while participants walked on a treadmill or over ground across a range of speeds.For each measurement point, participants walked for at least 6 minutes, and cadence was therefore assessed during continuous periods of walking.
Paragraph Number 2Threshold values for cadence have recently been applied to data collected from large population based surveys (17).TheNational Health and Nutrition ExaminationSurvey (NHANES) from 2005-2006 collected objective accelerometer data from a sub-sample of 3744 participants, using an Actigraph activity monitor.Data was provided as the number of steps taken within successive minute-long epochs, and was presented using the term cadence.Within this population, the vast majority of minutes that contained any stepping (95%)had less than60 steps in the minute-epoch.There was a relatively small number of minutes of those containing any stepping (1%) with more than 100 steps in the minute-epoch.This was perhaps unexpected, given that observational studies of apparently healthy adults walking, have demonstrated that many people do at times walk continuously with cadence greater than 100 steps/min in free-living conditions (18).
Paragraph Number 3In using the term cadence for the number of steps recorded within a minute-long epoch, there is an underlying assumption that walking was continuous throughout theminute.Non-walking breaks within that minute, however, would lead to a situation where the number of steps recorded was not identical to the actual cadence of walking.In that case, the data presented by the Actigraph representedthe accumulation of the number of steps taken within the minute (step accumulation), but not the actual rate at which those steps were taken (cadence).An analogy would be driving a car at a speed of 30 miles per hour.If driven at that speed for a wholehour, the distance travelled (analogous to steps) would be 30 miles, and the speed (analogous to cadence) would also be 30 miles/hour.However if the car was only driven for half an hour, the speed (i.e. cadence) would still be 30 miles/hour, but the distance travelled (i.e. steps) would only be 15 miles.
Paragraph Number 4The validity with which cadence and step accumulation can be used as interchangeable terms, depends on the relative proportion of minutes with continuous walking to those with non-continuous walking, and how close the duration of non-continuous walking is to the full minute.When using an event-based approach, where data is analysed as a set of events representing continuous periods of walking rather than artificially imposed epochs (5), studies have shown that walking is often comprised of short events(3,12).For example, it has been found that the mean length of a walking event was of the order of a minute (3),indicating that there are many walking events of less than 1 minute duration.It is therefore possible that these short walking events would have a large effect on the relationship between cadence and step accumulation.
Paragraph Number 5The purpose of this analysis was to quantify the scale of any discrepancy between step accumulation and cadence, by directly comparing the two outcome measures of using data from an activity monitor which records the duration of walking as well as step count.We hypothesise that step accumulation will be lower than cadence in some epochs, but are unclear as to how many epochs this will effect or how different the two outcome measures will be.
Method
Paragraph Number 6This study is an analysis of data collected from healthy adults during their usual activities.Throughout the article we define step accumulation as the number of steps taken within a minute-long epoch, regardless of the actual time spent walking within that minute.We define cadence as the number of steps within that minute divided by the duration of walking in that minute, representing the actual stepping rate within the minute.Both step accumulation and cadence have the same units(steps/min).
Paragraph Number 7A convenience sample of 121 healthy adults were recruited via staff working in a Scottish University, and data was collected between October 2007 and October 2009.Participants were included if they were aged over 18, lived in the community, and had no mobility problems.Ethical approval was granted by the School of Health & Social Care Research Ethics Committee, Glasgow Caledonian University, and all participants provided written informed consent.
Paragraph Number 8Physical activity data was collected using the activPAL activity monitor (Pal Technologies Ltd, Glasgow).This small, light, monitor is worn on the front of the thigh, and attached using double sided adhesive sticky pads.The device records acceleration at a 10Hz sampling rate, and proprietary software classifies the data into categories of sit/lie, stand and walk.The data is time stamped to the nearest 0.1s, and the start time and duration of each step can be provided in the output.The monitor is valid for posture allocation, step count and cadence in adults and older adults (4,6,7,14).
Paragraph Number 9On day one, participants met with a researcher and were provided with the monitor and instructed in its use.Basic demographic data (age, sex, BMI, employment status) were collected from each participant.Participants were asked to start wearing the monitor that day before midnight, and to then wear the monitor for seven consecutive days whilst going about their usual activities.Monitors were worn overnight but not during water-based activities.Participants were provided with additional sticky pads, and advised to change them each day.On day nine, participants removed the monitor and returned it to the researcher, data was analysed from midnight to midnight for days two to eight, inclusive.
Paragraph Number 10Data from the activPAL activity monitor was downloaded using proprietary software (PALProfessionalv5.9.1.1).Data was visually inspected to ensure that the monitor had been worn, and a participant was included if there was data for at least four days.Data output from the event record provided by the activPAL software was used for this analysis.This output provided a series of records representing a sitting event, a standing event, or a stride.For each record the start time and duration were reported to the nearest 0.1s.Custom software (written in Matlab, Mathworks) used the start time and duration of each stride to divide the data into minute-long epochs starting at midnight on day two.Each stride has a duration, and we assumed the stride count to be equally spread across that duration.For each stride occurring entirely within an epoch, stride count for the epoch was increased by 1, and the duration of that stride was added to the total duration of walking in that epoch.For strides that occurred across epochs, both the stride count and the duration of the stride were split between the two epochs in proportion to the duration in each.For example, if 30% of a 2 second long stride occurred in the first epoch, the stride count of that epoch would be increased by 0.3 (30% of the stride count of ‘1’), and the duration of walking in the epoch would be increased by 0.6s (30% of the duration of 2s).The number of strides was then doubled to provide the number of steps in each minute epoch.
Paragraph Number 11Three primary outcome measures were derived for each minute epoch, step accumulation (the total number of steps in the epoch), duration of walking, and cadence (total number of steps in the epoch/duration of walking).The difference between cadence and step accumulation (cadence – step accumulation) was also calculated for each minute epoch.Additionally, for minute epochs where the step accumulation and cadence agreed (i.e minutes where walking was continuous) the cadence was reported as “agreement” cadence.
Paragraph Number 12For the whole sample, the number of minutes analysed, number of minutes with walking, and proportion of those minutes where walking was continuous were calculated.For minute-epochs containing walking, the frequency distribution of minute-epochs was reported for the outcome measures, step accumulation, cadence, and duration of walking.Although not normally distributed, mean and standard deviation were used to illustrate the central tendency of the outcome measures.To assess whether there was a relationship between step accumulation and cadence, a scatter graph was created.As there were a large number of points to be graphed, a random selection of 1 in 64 data points (~4,000) were used.The frequency distribution of cadence was also reported for those minute-epochs with continuous walking (i.e. when step accumulation was equal to cadence).Finally, to assess the impact of using step accumulation or cadence on the public health message, the number of minutes for each participant which had a value of 100 steps/minute or more were calculated both for step accumulation and for cadence.The actual duration of walking in minutes with a cadence of over 100 steps/minute was also calculated for each participant.Data were not normally distributed, and were presented as median and inter-quartile range for the population, and were compared using related-samples Wilcoxon signed rank tests.
Results
Paragraph Number 13Four participants were excluded as they had fewer than 4 days of data.Therefore, the multiday physical activity records of 117 participants (78 female) were included within the analysis.The participants had a mean age of 46 ± 16years with a wide range (23-82 years old).Mean BMI (24.9 ± 3.7 kg·m-2) was within the normal category, ranging from underweight to obese (17.7-38.4 kg·m-2).Ninety-six participants were in employment, and 83 of them worked full time.
Paragraph Number 14A mean of 6.8 days of physical activity was recorded from each participant giving a total of 797 days of data.The mean daily number of minutes that included walking was 310, representing 21.6% of all minutes within each 24 hour period.In total, there were 247,453 minutes with stepping and an associated 8,424,866 steps.Participants walked an average of 10,627 ± 4,157 steps/day ranging from 3,389 to 32,244 steps/day, and just over half the participants (n=59) took an average of more than 10,000 steps/day.
Paragraph Number 15The distribution of step accumulation and cadence across the range of values were very different (figure 1).Step accumulation was distributed across the range 0-180 steps/min,with the largest contribution to the number of minutes walking at the lower end of the step accumulation range, below 40 steps/min (table 1).However, there was a small peak in the distribution of step accumulation at about 100-120 steps/min.The mean (± standard deviation) step accumulation across all participants was 34 ± 9 steps/min.Cadence was distributed very differently, across the range 20-180 steps/min, with stepping concentrated around 50-100 steps/min.The mean cadence across all participants was 76 ± 6 steps/min.
Paragraph Number 16The difference between cadence and step accumulation for each minute ranged between 0 and 170 steps/min, similar to the ranges of the individual variables.As illustrated by a random sample of points (figure 2), there was an upper limit on the value of step accumulation when walking occurred during the whole minute, and at higher cadences (> 140 steps/min) step accumulation tended to agree with cadence.However,within these limits, there was no apparent relationship between the step accumulation and cadence of individual minutes.
Paragraph Number 17Walking was continuous in only 12% (38±28 min/day) of minutes with walking (figure 3).A large proportion of the minutes with stepping contained only a short duration of walking within that minute, for example 32% of minutes with stepping contained less than 10s of stepping, whilst69% of minutes contained less than 30s of stepping.Where a minute contained continuous walking there was agreement between step accumulationand cadence.The distribution of the cadence for these minutes demonstrated a concentration of outcomes between 90-130 steps/min (Figure 4).The mean cadence across all participants for those minutes with continuous stepping was 109 ± 9steps/min, over three times that of the average step accumulation (34 steps/min) and approximately fifty percent higher than the mean value of cadence (76 steps/min).
Paragraph Number 18There were significantly fewer minutes a day with a step accumulation of over 100steps/min (median (interquartile range), 29 (26) min, p<0.01) compared with minutes with a cadence greater than 100 steps/min (65 (41) min).As indicated above, many of those minutes will not have contained a full minute of walking.However, the duration of walking within minutes with a cadence greater than 100 steps/min (39 (34) min) was also significantly higher than the number of minutes with a step accumulation greater than 100 steps/min (p<0.001).
Discussion
Paragraph Number 19The term cadence has widespread use in the literature when describing stepping activity.The rate of stepping per time interval is considered important as it is indicative ofhow intensively someone is engaging in physical activity.However, the term cadence has been used to describe both an output that is the total number of steps within a minute epoch (step accumulation), and an output that is the rate of steps taken within a period of walking (cadence).It is clear from this direct comparison of the two concepts on the same habitual physical activity data, that step accumulation and cadence are very different outcome measures.They are significantly different both in terms of their distribution, and the summative information that can be derived from the data. For example, mean cadence in this population (76 steps/min) was twice that of mean step accumulation (34 steps/min).