The main purpose of this study was to use indirect calorimeter to identify a step-rate cut-point associated with activity intensity in a field environment. To our knowledge, this was the first attempt to establish a walking target for Chinese people. We identified the optimal step-rate cut-point was 105 step/min for MPA and 130 step/min for VPA. Applying cut point for MPA to calculate the walking steps and distance taken to meet PA guidelines, 30 min of moderate-intensity activity corresponds to 3100 steps in young men and 3200 steps in young women, or roughly 3150 steps for both. If steps are converted to walking distance, it is about 2 km.
We tested four different walking speeds in this study. Three of them were significantly corresponded with PA Compendium.20
When compared with EE reference from PA Compendium, the EE measured from our study was higher for three walking speeds. Previous studies showed inconsistent results when comparing measured EE with compendium reference. Some reported higher value,21
others reported lower value.22
The inconsistency might be due to difference in sample characteristics, testing methods and test environment.21
Therefore, it is not proper to perform complete result comparisons for different test condition. For the current study test setting might be a contributor to the difference. We conducted the walking test in a field setting, not on a treadmill. Our previous study found that walking-related energy expenditure in the field was different from treadmill testing.19
However, the intensity of these three walking speeds in present study was between 4.0 and 5.5 MET, which was in the range of 3–6 MET as moderate intensity identified by PA Compendium.
Objective measurement method and larger sample size allowed this study to establish the step-rate cut-point related to intensity (METs) as a minimum threshold for MPA walking and VPA walking. To date, four other studies have used indirect calorimeter to validate a step-rate cut-point associated with moderate or vigorous intensity walking. Simon et al14
measured the step rate and intensity on a treadmill. Different cut points were obtained from different statistical method, and the author concluded walking at 100 step/min on ground level would meet the moderate-intensity walking recommendation. Tudor-Locke et al15
determined that 96 and 107 step/min as the minimum threshold for moderate-intensity walking, and 125 and 136 step/min for vigorous-intensity for young men and women. The two other studies supported the 100 step/min as moderate-intensity walking cadence while emphasised inter-individual variation of step rate were apparent due to anthropometric differences such as height and leg length.26
Our finding corresponded closely with these previous studies, although our cut point was slightly higher. The similar findings of these studies are encouraging given the differences between the sample characteristics and methodologies, which offer some evidence that will support the development of a consensus step-rate recommendation for the people in different countries.
In addition, we found that there was significant difference of step rate between young men and women at the same walking speed; therefore, it seems proper to have different cut-point recommendation for men and women separately. The gender differences in the mean step rate may be caused by differences in height and leg length. At the same walking speed, female's step frequency is higher than male's due to shorter height and shorter lower limb. However, since the difference of step rate between men and women was less than 10 steps/min, considering the needs to establish the walking recommendation in a relatively simple way, we think that single step-rate recommendation would be more effective in PA promotion and intervention application. Therefore, we suggest 105 and 130 step/min cut-points to be corresponded to MPA and VPA. However, if the recommendations of individualised step rate would be developed in the future, physical differences such as height, leg length and gender should also be considered.
In order to associate our step-rate cut-point with PA guideline,3
the minimum walking steps of 3150 steps daily for MPA were considered based on our study results. It should be emphasised that 3150 steps need to be taken above the basic number of daily steps.15
Recent study has reported the daily walking steps goal for American people is 8000 steps, derived from accelerometer data,10
but an earlier study reported 10 000 steps.8
Since people have different PA patterns, it is difficult to establish consistent total number of walking steps for everyone. Moreover, there is not a comprehensive walking recommendation if only walking steps is involved but not the intensity. Therefore, it is practical and useful to provide a general suggestion that how many extra steps individual needs to take above the daily activities and how fast to walk for health promotion. Specially, Chinese young adult should walk at least 30 min with a minimal 105 steps/min, or 3150 steps or 2 km with the same step rate daily to meet PA guidelines. They will perform vigorous-intensity activity if 130 step/min is reached and this will provide more health benefit.
China is experiencing rapid economic growth. With the increase of private car ownership and the reduction of the intensity of work, a rapid decrease of PA levels of Chinese people has caused widespread concern. Data from 2007 China PA and Exercise Survey showed that the proportion of young people aged 20–29 years to participate in regular exercise is the lowest among survey population aged 16–70 years old, with only 6.2%.28
Walking is the most common and easy exercise. Walking above the certain speed can improve the health of people.29
Therefore, a walking recommendation built on scientific evidence for Chinese young adults is a meaningful step to help them promote the PA levels.
One strength of the present study was large sample size and EE measurement in field settings, which provide sufficient power to identify the step-rate cut-points accurately. Another strength was to provide Chinese young adults a walking recommendation in form of relative flexible assistive tool. People can achieve their own exercise goal by using different calculations, such as step rate, walking during, total walking steps and/or walking distance. There were a number of limitations in this study. The first limitation was the small age range of young participants. It was known that gait and energy expenditure will be different between older and young individuals.30
At the same walking speed, older people will have the gait of shorter step length and faster step rate.31
Therefore, the cut-point established might not be suitable for older population. The second limitation was the use of a constant (3.5 ml/kg/min) as an estimated value of resting energy expenditure, instead of a direct measurement for calculating METs. Although the use of this constant is widely accepted in the scientiﬁc literature, it is likely to overestimate resting-energy expenditure at the individual level.23
Then, the step-rate cut-point developed by the estimated value of MET might be overestimated. The third of limitation was lack of cross-validation. Considering the data collected under the current controlled environment may be different from the real environment, future study should focus on establishing the validity of the current cut-points through independent validation studies in real-life field walking.