BARREIRA, TIAGO V.; KATZMARZYK, PETER T.; JOHNSON, WILLIAM D.; TUDOR-LOCKE, CATRINE
Children and adolescents can benefit from a physically active lifestyle (30). The 2008 Physical Activity Guidelines for Americans (30) recommend that children and adolescents engage in at least 60 min·d−1 focused primarily on moderate- to vigorous-intensity physical activity (MVPA). With the escalating use of body-worn objective activity monitors (including pedometers and accelerometers), there has been an increasing interest in translating this time- and intensity-based recommendation into a reasonably approximate amount of ambulatory activity expressed as steps per day (1,3,6,18,20). Although steps per day is known to be associated with time spent in higher intensity activities (1,3,22), it is more clearly an indicator of total volume of physical activity. A recent review has introduced the concept of using cadence (steps per minute) to describe step accumulation patterns as a way to study free-living ambulatory behavior (29). Improving cadence is known to be the primary strategy for increasing free-living walking speed in adults (23) and children (12), and although stride lengthening supersedes this strategy as running speed incrementally increases, cadence still increases at least to some degree (17). Recently, cadence has also been used to describe intensity of ambulation in laboratory studies of young people (10,11,15,16). For example, Graser et al. (11) asked 10- to 12-yr-old boys and girls to wear a pedometer and walk on a treadmill at 3, 3.5, and 4 mph and concluded that, in general, cadences above 120 steps per minute were associated with MVPA. In addition, we know from short-distance walking tests (8) that expected cadence at self-selected preferred walking speed is 126.14 ± 10.29 steps per minute for 10-yr-olds.
We recently published 2005–2006 National Health and Nutrition Examination Survey (NHANES) accelerometer data to describe cadence patterns of free-living US adults ≥20 yr (25). We reported that US adults accumulate ≅4.8 h·d−1 at zero cadence during accelerometer wearing time, ≅8.7 h·d−1 between 1 and 59 steps per minute, ≅16 min·d−1 at cadences of 60–79 steps per minute, ≅8 min·d−1 at 80–99 steps per minute, ≅5 min·d−1 at 100–119 steps per minute, and ≅2 min·d−1 at cadences ≥120 steps per minute (25). In a separate analysis of these data (24), we reported the descriptive epidemiology of peak 30-min cadence (defined as the average steps per minute recorded for the 30 highest but not necessarily consecutive minutes in a day) and peak 1-min cadence (defined as the steps per minute recorded for the highest single minute in a day). We found that US adults average a peak 30-min cadence of approximately 71 steps per minute and a peak 1-min cadence of approximately 101 steps per minute. Further, both peak cadence indicators displayed significant and consistent decrements with increasing age and levels of obesity as defined by categories of body mass index (BMI).
We know of only one study of free-living cadence in children or adolescents at this time. Bjornson et al. (5) used the StepWatch Activity Monitor (Orthocare Innovations, Oklahoma City, OK) to examine the stride rate patterns of 428 normally developing children and adolescents 2–15 yr. Their analysis was from a comparison cohort and was not based on a representative sample, and they did not report peak cadence to the extent that we presented in our analysis of NHANES adults (25). Therefore, there is an opportunity to analyze the NHANES children and adolescent data to explore peak cadence rates and cadence patterns in a sample representative of the US population. Because physical activity guidelines for young people recommend ≥60 min·d−1 of healthful physical activity, it makes sense that peak cadence indicators be extended to reflect this focus. Further, changes with growth and development require that cadence data be examined by sex and age groupings (6–11, 12–15, and 16–19 yr). Therefore, these analyses of the 2005–2006 NHANES accelerometer data served the dual purposes of describing in children and adolescents 1) free-living step accumulation patterns using cadence and 2) peak cadence indicators (i.e., peak 60 min, peak 30 min, and peak 1 min) by sex and age groups and by BMI category.
The National Center for Health Statistics ethics review board approved all NHANES protocols, and all participants were asked to provide informed consent. Parental/guardian consent was obtained for minors. As in previous research that described steps per day in children and adolescents using these 2005–2006 NHANES data (28), 2610 participants who were 19 yr or younger, had at least one valid day of accelerometer data (defined later), and had complete height and weight data were included in these analyses. BMI was transformed into the following categories: normal weight, overweight, and obese on the basis of sex- and age-specific cutpoints for children and adolescents (7).
The NHANES is designed to assess the health and nutrition status of adults and children in the United States and, in the 2005–2006 survey cycle, used the ActiGraph model 7164 accelerometer (formerly the CSA/MTI AM-7164, manufactured by ActiGraph of Fort Walton Beach, FL) to assess step-defined physical activity (as well as other outputs not focused on here). The ActiGraph 7164 (5.08 × 3.81 × 1.27 cm), which is no longer distributed by the manufacturer, has been well described previously (9). Briefly, it used a single-axis accelerometer designed to measure vertical accelerations while worn at the hip. The output frequency was 0.25 to 2.5 Hz, digitized by an analog-to-digital converter, and stored in 1-min intervals. Step data are not available from the 2003–2004 NHANES cycle, which also collected accelerometer data.
NHANES participants who were at least 6 yr and not limited in their ability to walk or otherwise wear an accelerometer were recruited to wear it for seven consecutive days during all waking hours (removing it at night and for any water activities, including showering). Height and weight were measured using standard procedures. Additional information about all measurements, including more details about the accelerometer protocol, can be found at http://www.cdc.gov/nchs/data/nhanes/nhanes_05_06/BM.pdf.
Data treatment and analysis.
A SAS macro (http://riskfactor.cancer.gov/tools/nhanes_pam/) supplied by the National Cancer Institute was used to determine nonwear time (defined as ≥60 consecutive zeros, allowing minimal interruptions). To keep consistent with previous analyses of these data, a valid day was defined as having at least 10 h of accelerometer wear time and at least one valid day of data, which was shown to eliminate bias due to more active participants wearing the accelerometer for more days (28).
Adhering closely to procedures published describing cadence patterns in adults (25), cadence was extracted from the data without any manipulation (e.g., censoring below any specific activity count cutpoint ) because our objective was to present the full range of cadence as detected by the instrument. Cadences were then organized into bands of approximately 20-step-per-minute increments: 1–19, 20–39, 40–59, 60–79, 80–99, 100–119, and 120+ steps per minute. The amount of time (min·d−1) and steps per day accumulated within each cadence band were computed, as well as the time spent at zero steps per minute (excluding nonwear time). In addition, the peak 60-min, peak 30-min, and peak 1-min cadences were computed by rank ordering the data for each day for each participant and then averaging steps per minute accumulated over the top 60, 30, and 1 min (as appropriate) across valid days.
All analyses were conducted using SAS 9.2 (SAS Institute, Cary, NC) sample survey procedures to account for the complex NHANES sampling design. The SURVEYREG procedure followed by pairwise contrast analysis was used to compare cadence bands and peak cadence values between age, sex, and BMI groups. We set the α level at 0.001 to conservatively interpret significance amidst multiple comparisons in this large sample. To ensure national representativeness of results, sample weights were used to account for oversampling and nonresponse.
The total minutes of wear time for the overall sample and by sex are presented in Table 1. Mean wear time displayed a narrow range from 804 to 843 min·d−1 (13.4 to 14.0 h·d−1) across the sex–age groups. There were no significant differences for wear time among the groups.
The number of minutes per day and steps per day accumulated within each cadence band for the overall sample and by sex are presented in Table 2. During wear time, US children and adolescents spent, on average, 242 min·d−1 (≅4 h·d−1) at zero cadence, ≅534 min·d−1 (8.9 h·d−1) between 1 and 59 steps per minute, ≅22 min·d−1 at cadences of 60–79 steps per minute, ≅13 min·d−1 at 80–99 steps per minute, ≅9 min·d−1 at 100–119 steps per minute, and ≅3 min·d−1 at cadences ≥120 steps per minute. On average, approximately 7500 steps per day were accumulated between 1 and 59 steps per minute, and fewer than 400 steps per day were accumulated at 120 or more steps per minute. Significant differences were noted between boys and girls for time at zero cadence and for the number of steps per day and minutes per day accumulated at cadences of 20–39, 40–59, 60–79, and 80–99, all P < 0.001.
Table 3 displays minutes per day and steps per day accumulated within each cadence band by age group. Time spent at zero cadence ranged between 189.5 and 298.6 min·d−1 (about 3 to 5 h·d−1), and fewer than 4 min·d−1 were spent at cadences of 120 or more steps per minute in all age groups. Overall, children and adolescents accumulated more than 60% of steps per day at cadences between 1 and 59 steps per minute. The minutes per day and steps per day accumulated within each cadence band for each sex–age group are presented in Figure 1A and Figure 1B, respectively. Time spent at zero cadence ranged from ≅3 h·d−1 for 6- to 11-yr-old boys and girls to ≅5 h·d−1 for 16- to 19-yr-old boys and girls. Time spent at 1–19 steps per minute ranged from ≅6.7 h·d−1 for 6- to 11-yr-old girls to ≅6 h·d−1 for 16- to 19-yr-old girls, and steps per day accumulated within this cadence band on average ranged from about 2731 to 2161 for the same sex–age groups. Time accumulated within each incremental cadence band was progressively lower until the lowest value of 1 min·d−1 was observed in the 120+ steps per minute cadence band for boys 16–19 yr. Significant differences were observed between sex–age groups for all except the 80–99 cadence band.
The peak 60-min, peak 30-min, and peak 1-min cadences are presented in Table 4. The peak 60-min cadence ranged from a low of 67 steps per minute for the 16- to 19-yr-old girls to a high of 75 steps per minute for 12- to 15-yr-old boys. The peak 60-min cadence mean was 72 steps per minute for the total sample. Significant differences were observed between boys and girls for the total sample and within the 12- to 15-yr-old age group. In addition, statistically significant differences in peak 60-min cadence were apparent between the 6- and 11-and 16- and 19-yr-old age groups. The peak 30-min cadence ranged from 82 steps per minute (for both boys and girls age 16–19 yr) to 88 steps per minute (for boys age 12–15 yr), and significant differences were noted between the 6- and 11- and 16- and 19-yr-old age groups. The peak 1-min cadence ranged from 108 steps per minute (for boys age 16–19 yr) to 124 steps per minute (for girls age 6–11 yr). Significant differences were identified for both sexes between the youngest and oldest age groups and between 12- and 15-yr-old boys and girls. Peak 60-min cadence was 73, 71, and 67 steps per minute for normal-weight, overweight, and obese BMI-defined weight categories, respectively. Significant differences were observed between the normal-weight and obese groups for all peak cadence indicators and also for peak 1-min cadence between the overweight and obese groups.
Free-living cadence has been previously derived from the NHANES adult accelerometer data and described as minute-by-minute step accumulation patterns organized into incremental cadence bands (25) and also as peak cadence, a simple indicator of best natural ambulatory effort (24). Because we know of no other study of free-living cadence in children or adolescents in representative samples at this time, these analyses extend the adult findings. For all age groups and for both sexes, the majority of time (mean = 10.5 h·d−1) is spent at zero cadence or accumulating 1–19 steps per minute during the monitored day (mean = 13.7 h·d−1). Thus, on average, 3.2 h·d−1 is spent in cadence bands accumulating 20 or more steps per minute and only a relatively small amount of time (<3 min) is spent at a cadence ≥120 steps per minute. However, US children and adolescents do average approximately 50 min·d−1 taking 60 or more steps per minute, a cadence range indicative of purposeful and incrementally higher intensity ambulatory activities (25,26).
To facilitate comparison, the previously collected adult data (25) are displayed alongside the data for children and adolescents by age group (6–11, 12–15, and 16–19 yr) in Figure 2. Also, as previously described, this sample of US children and adolescents averaged approximately 13,000 (boys) and 12,000 (girls) accelerometer-determined steps per day (28), considerably more than the US adults who averaged approximately 9600 steps per day (27). Despite apparent differences in volume of steps per day, when categorized into incrementally higher cadence bands, the children’s and adolescents’ data display step accumulation patterns similar to adults. The most notable difference is that US children and adolescents average approximately 40 min·d−1 less at zero cadence (i.e., nonmovement) and approximately 30 min·d−1 more at cadences ≥40 steps per minute than US adults.
In adolescents, short-distance gait tests of similarly age boys and girls demonstrate that girls self-select a higher cadence under controlled testing conditions (31,32). These objectively monitored NHANES data show that there were no statistically significant differences, however, between boys’ and girls’ free-living highest single minute or best 30 min of the day, suggesting that these indicators are overall more a reflection of natural behavior than sex-associated height/leg length differences. Table 2 reveals that, although there are no sex differences regarding time spent at cadences ≥100 steps per minute, boys spent significantly more time at cadences between 20 and 99 steps per minute, and this is reflected by the significantly higher peak 60-min cadence in boys compared with girls. Compared with the shorter peak cadence indicators, the peak 60-min cadence will be more affected by the relative persistence of a behavior during a day. The observed sex difference in peak 60-min cadence (i.e., boys > girls) is consistent with what is known based on other physical activity measurement approaches (4).
Extrapolating from these cross-sectional data, it seems that as children age, they tend to spend increasingly more time at zero cadence, shifting on average from 189.5 min·d−1 at ages 6–11 to 298.6 min·d−1 at ages 16–19. Children age 6–11 yr also tend to spend more time and take more steps per day at cadences <80 steps per minute than the 12- to 15- and 16- to 19-yr-old groups. Although significant differences were found, the overall time spent and steps per day taken at cadences of 100–119 and 120+ steps per minute were similarly low (on average) for all the age groups highlighting the need to enhance promotion of activities that can elicit such cadences across the board.
Similar to the study by Bjornson et al. (5), which reported only peak 1-min cadence, the older age groups analyzed herein had slightly lower values than the younger age groups. As well, the peak 30-min and peak 1-min cadence values for the 16- to 19-yr-old adolescents were higher than those previously reported for adults (peak 30 min = 63 steps per minute and peak 1 min = 91 steps per minute) (24). Although differences between age groups could be attributed to development that occurs with age, the mechanical work and the muscular efficiency during walking for children ≥10 yr are not different from adults when normalized to body mass (21). During late adolescence, both boys and girls are likely to have attained adult height (19). In addition, Beck et al. (2) tested children between 2 and 15 yr and demonstrated that for an average speed of 1.04 m·s−1, the stride length was 76% of the child’s height. With those results in mind, it is possible to conclude that as children get older and taller, they can walk faster at a similar cadence rate or decrease cadence to walk at a similar speed. However, the present study focuses on the description pattern of sex- and age-specific step accumulation and as an expression of naturally occurring ambulatory behavior. Certainly, the conclusions about time spent at zero cadence cannot be influenced by development or stride length, and, therefore, it more clearly represents a behavior.
Children and adolescents classified as obese had lower values for all peak cadence indicators than those classified as normal weight by BMI. Peak cadence is an indicator used to capture natural best-effort ambulatory activity expressed under free-living conditions. As an average of the best minutes of a day, it is shaped not only by its magnitude but also by the number of minutes considered. Even when considering only the best single minute (where magnitude would matter most), obese children and adolescents attain lower values than their normal-weight peers. This free-living finding is consistent with gait analyses studies that also show that obese children have lower cadences at all walking speeds (13,14). Interestingly, the NHANES data demonstrate that even overweight children/adolescents had a higher peak 1-min cadence than obese children/adolescents. There was no difference, however, in the peak 30-min or peak 60-min cadences. The longer duration peak cadence indicators increasingly reflect the persistence of the behavior. Overall, it seems that obese children/adolescents do not achieve cadences that are as high as those attained by either overweight or normal-weight children. However, neither overweight nor obese children sustain these cadences for accumulated durations when compared with normal-weight children. This is an interesting nuance that we do not believe has been presented previously and thus requires additional investigation to illuminate underlying factors contributing to these BMI-defined behavioral differences.
As previously stated, we know from short-distance walking tests (8) that the expected cadence at self-selected preferred walking speed is 150 steps per minute for 6-yr-old children and 126 steps per minute for 10-yr-olds. However, in this representative sample of free-living US children and adolescents, only a relatively small portion of time was spent at comparable cadences. In addition, just a few minutes were spent taking ≥120 steps per minute, which Graser et al. (11) suggested to be associated with MVPA in 10- to 12-yr-old boys and girls. Unfortunately, there has been little research beyond the study by Graser et al. that provides alternative age-specific values. We would expect the value to be higher in younger children and lower as adolescents develop into adults. The MVPA-associated cadence value for adults is ≥100 steps per minute (26). Interpreted against the results of gait speed tests and this crude single-cadence threshold offered by Graser et al., our findings are consistent with the hypothesis that many US children and adolescents have relatively few minutes in daily life when they engage in activities that elicit step accumulation patterns indicative of normal walking speeds, let alone those associated with MVPA. In all sex-by-age groups, the average peak cadences were well below the 120 steps per minute associated with MVPA for children (11). Only six children (of 2610) had a peak 60-min cadence above 120 steps per minute, and only 22 were above 100 steps per minute. The average time above 120+ steps per minute was only 2.5 min for the entire sample and was 3.0, 2.1, and 2.0 min for the 6–11, 12–15, and 16–19 age groups, respectively.
Many limitations in this study must be acknowledged. This report is focused on ambulatory physical activity, and it does not fully capture the more complex activities that include upper body movements (e.g., gymnastics, volleyball) or water activities like swimming that would also be considered of at least moderate intensity. This is a descriptive study of step accumulation patterns of US children and adolescents. Additional analyses are planned to examine the association of cadence indicators with various health parameters. Whereas at higher cadences, the values represent actual stepping rates for 1 min, at lower cadence bands, we are representing a pattern of how steps are accumulated within a minute. It is not realistic to think that children and adolescents are walking in “slow motion” at a step rate of <40 steps per minute. The lower cadence steps were likely accumulated at a much faster rate and in a time interval shorter than 1 min. As intensity of activity increases and sustained durations are shortened, the effect will be larger. Unfortunately, it is not possible to investigate bouts in a more flexible manner because the 1-min data collection interval was preset. We acknowledge that dividing detected steps by smaller time frame denominators suggests higher stepping rates; however, Tudor-Locke and Rowe (29) have recently argued that “the purpose of studying free-living minute-by-minute step accumulation patterns is to relay the execution and relative persistence of naturally occurring ambulatory behavior.” Finally, these analyses represent population averages that minimize individual realities. For example, if two children obtained peak 60-min cadence values of 120 steps per minute and a third child obtained 70 steps per minute, the “sample” average would be only 103 steps per minute. The few cases with very high peak cadence values are thus reduced by the very larger number of cases with very low values.
In summary, this study provides descriptive data for US children and adolescents for time and steps per day accumulated within incrementally higher cadence bands and peak cadence indicators for 60, 30, and 1 min. These reference data are important and can be used for surveillance, tracking, comparison, screening, intervention, and evaluation purposes. On average and for all age groups and for both sexes, the majority of time during the day is spent at the 1- to 19-steps-per-minute and zero-cadence (10–11 h·d−1) bands, and a very small amount of time (<3 min·d−1) is spent at cadences ≥120 steps per minute. Although there are no sex differences with regards to time spent at cadences ≥100 steps per minute, boys have a higher peak 60-min cadence than girls, reflecting their higher amounts of time spent at 20–99 steps per minute. A finding that bears repeating is that, overall, children and adolescents seem to have relatively few minutes in daily life in which they engage in activities that elicit step accumulation patterns indicative of normal walking speeds (derived from short-distance gait tests), let alone those associated with MVPA. The results also indicate that interventions to increase physical activity and, in particular, opportunities for persistent brisk ambulation or faster types of locomotion are necessary at an early age for both boys and girls if they are to establish and sustain habitual healthy ambulatory activity levels before entering adulthood.
The authors thank Meghan Brashear for help with data management and statistical analysis.
The authors did not receive funding for this project.
The authors have declared that no competing interests exist.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
1. Adams MA, Caparosa S, Thompson S, Norman GJ. Translating physical activity recommendations for overweight adolescents to steps per day. Am J Prev Med
. 2009; 37 (2): 137–40.
2. Beck RJ, Andriacchi TP, Kuo KN, Fermier RW, Galante JO Changes in the gait patterns of growing children. J Bone Joint Surg Am
. 1981; 63 (9): 1452–7.
3. Beighle A, Pangrazi RP. Measuring children’s activity levels: the association between step-counts and activity time. J Phys Act Health
. 2006; 3 (2): 221–9.
4. Belcher BR, Berrigan D, Dodd KW, Emken BA, Chou CP, Spruijt-Metz D Physical activity in US youth: effect of race/ethnicity, age, gender, and weight status. Med Sci Sports Exerc
. 2010; 42 (12): 2211–21.
5. Bjornson KF, Song K, Zhou C, Coleman K, Myaing M, Robinson SL. Walking stride rate patterns in children and youth. Pediatr Phys Ther
. 2011; 23 (4): 354–63.
6. Cardon G, De Bourdeaudhuij I. A pilot study comparing pedometer counts with reported physical activity in elementary schoolchildren. Pediatr Exerc Sci
. 2004; 16 (4): 355–67.
7. Cole TJ, Bellizzi MC, Flegal KM, Dietz WH Establishing a standard definition for child overweight and obesity worldwide: international survey. BMJ
. 2000; 320 (7244): 1240–3.
8. Dusing SC, Thorpe DE. A normative sample of temporal and spatial gait parameters in children using the GAITRite electronic walkway. Gait Posture
. 2007; 25 (1): 135–9.
9. Freedson PS, Melanson E, Sirard J. Calibration of the Computer Science and Applications Inc. accelerometer. Med Sci Sports Exerc
. 1998; 30 (5): 777–81.
10. Graser SV, Groves A, Prusak KA, Pennington TR Pedometer steps-per-minute, moderate intensity, and individual differences in 12- to 14-year-old youth. J Phys Act Health
. 2011; 8 (2): 272–78.
11. Graser SV, Pangrazi RP, Vincent WJ. Step it up: activity intensity using pedometers. J Phys Educ Recreat Dance
. 2009; 80 (1): 22–4.
12. Hillman SJ, Stansfield BW, Richardson AM, Robb JE Development of temporal and distance parameters of gait in normal children. Gait Posture
. 2009; 29 (1): 81–5.
13. Hills AP, Parker AW. Gait characteristics of obese children. Arch Phys Med Rehabil
. 1991; 72 (6): 403–7.
14. Hills AP, Parker AW. Locomotor characteristics of obese children. Child Care Health Dev
. 1992; 18 (1): 29–34.
15. Jago R, Watson K, Baranowski T, et al.. Pedometer reliability, validity and daily activity targets among 10- to 15-year-old boys. J Sports Sci
. 2006; 24 (3): 241–51.
16. Lubans DR, Morgan PJ, Collins CE, Boreham CA, Callister R The relationship between heart rate intensity and pedometer step counts in adolescents. J Sports Sci
. 2009; 27 (6): 591–7.
17. McArdle WD, Katch FI, Katch V. Exercise Physiology: Energy, Nutrition, and Human Performance
. 6th ed. Baltimore (MD): Lippincott Williams & Wilkins; 2007.
18. Olds T, Ridley K, Dollman J, Maher CA The validity of a computerized use of time recall, the Multimedia Activity Recall for Children and Adolescents. Pediatr Exerc Sci
. 2010; 22 (1): 34–43.
19. Roche AF, Davila GH. Late adolescent growth in stature. Pediatrics
. 1972; 50 (6): 874–80.
20. Rowlands AV, Eston RG. Comparison of accelerometer and pedometer measures of physical activity in boys and girls, ages 8–10 years. Res Q Exerc Sport
. 2005; 76 (3): 251–7.
21. Schepens B, Bastien GJ, Heglund NC, Willems PA Mechanical work and muscular efficiency in walking children. J Exp Biol
. 2004; 207 (Pt 4): 587–96.
22. Tanaka C, Tanaka S. Daily physical activity in Japanese preschool children evaluated by triaxial accelerometry: the relationship between period of engagement in moderate-to-vigorous physical activity and daily step counts. J Physiol Anthropol
. 2009; 28 (6): 283–8.
23. Terrier P, Schutz Y. Variability of gait patterns during unconstrained walking assessed by satellite positioning (GPS). Eur J Appl Physiol
. 2003; 90 (5–6): 554–61.
24. Tudor-Locke C, Brashear MM, Katzmarzyk PT, et al.. Peak stepping cadence in free-living adults: 2005–2006 NHANES. J Phys Act Health
. In press.
25. Tudor-Locke C, Camhi SM, Leonardi C, et al.. Patterns of adult stepping cadence in the 2005–2006 NHANES. Prev Med
. 2011; 53 (3): 178–81.
26. Tudor-Locke C, Craig C, Brown W, et al.. How many steps/day are enough? For adults. Int J Behav Nutr Phys Act
. 2011; 8 (1): 79.
27. Tudor-Locke C, Johnson WD, Katzmarzyk PT. Accelerometer-determined steps per day in US adults. Med Sci Sports Exerc
. 2009; 41 (7): 1384–91.
28. Tudor-Locke C, Johnson WD, Katzmarzyk PT. Accelerometer-determined steps per day in US children and youth. Med Sci Sports Exerc
. 2010; 42 (12): 2244–50.
29. Tudor-Locke C, Rowe DA. Using cadence to study free-living ambulatory behaviour. Sports Med
. 2012; 42 (5): 381–98.
30. US Department of Health and Human Services. 2008 Physical Activity Guidelines for Americans: Be Active, Healthy, and Happy!
Washington (DC): 2008.
31. Waters RL, Hislop HJ, Thomas L, Campbell J Energy cost of walking in normal children and teenagers. Dev Med Child Neurol
. 1983; 25 (2): 184–8.
32. Waters RL, Lunsford BR, Perry J, Byrd R Energy–speed relationship of walking: standard tables. J Orthop Res
. 1988; 6 (2): 215–22.