Correlation analysis of oxygen consumptions between stepper and treadmill movements and implication for rehabilitation

Background: Stepper movement is popular as a home-based exercise. However, it has not been fully investigated how much energy is consumed during movement. This study aimed to investigate oxygen consumption for stepper movements at different cadence levels and to look into whether there are correlations between stepper movement and treadmill walking. Methods: Twenty two healthy volunteers (10 males, 12 females, aged between 18 and 40 years) participated in the study. The participants randomly performed stepper movements and treadmill walking at three different cadences, a self-determined comfortable cadence, a 20% higher and a 20% lower ones than the comfortable one. Their oxygen consumption was measured using Oxycon mobile ® system. A set of parameters, e.g. VO 2 , Metabolic unit, Energy expenditure, etc were obtained. Statistical analysis was carried out to investigate correlations between stepper and treadmill movements in terms of oxygen consumptions. Results: Strong correlations were found between two types of movements, most of metabolic parameters had correlation coe�cients ranged proximately between 0.5 to 0.8, p<0.001, e.g. 0.789 for VO 2 , 0.790 for energy consumption, 0.826 for ventilation, all p<0.001. In the conformable cadence level, VO 2 for stepper movements was 826.49±56.02 (ml/min) while VO 2 for treadmill walking was 787.16±56.02. Usually, stepper movements have oxygen consumption similar to or slightly higher than that in treadmill walking. Conclusions: This study indicates that stepper movement has similar oxygen consumption level to treadmill at a comfortable cadence level and at a 20% higher than the comfortable one, but stepper movement consumes more energy than treadmill walking by approximately 10% at lower cadence (i.e. 20% lower than the comfortable one). The study shows that the stepper movements is highly correlated with treadmill walking in terms of oxygen consumption, and thus stepper movement data provided can be used as a reference for the healthy to estimate their energy consumption in rehabilitation at home.


Background
In rehabilitation, through different people select different exercise [1,3,8], both stepper and treadmill machines are popular [5,6].Stepper machine is easy to use, much less expensive, occupies less area, and no guidance is needed for its use as compared to the treadmill.As the home exercising machine, a stepper seems to be better than treadmill.
Stepper machine has various functions.It helps rehabilitation by stimulating the movement of ascending steps [21].Steppers have a very strong impact on gluteal muscles, legs and lower back.It is an apparatus found to be very useful for rehabilitation by many researchers [2,4,11,15,16].There have been a number of studies done on stepper to assess the feasibility for rehabilitation, however, most of them were limited to using electromyography (EMG) which has been considered as the most useful method to measure the muscle activity [15].However, a few researchers stated that the EMG assessment is limited to the location of the muscles selected.In the situation with deeper muscles containing high threshold motor units the assessment through EMG gets limited in comparison to the ne low threshold motor units.This suggests that there is a need for other ways to nd the muscle activity which is the starting point for rehabilitation [15].
The measurement of oxygen consumption is very useful for the assessment of physiological considerations, especially during rehabilitation of patients [9,10,14,19,20,22].There are numerous rehabilitation strategies that can be used, however, the better approach being an individualised approach [18], where each and every individual is assessed separately and the rehabilitation programme is speci cally developed according to personal circumstance.
Test using the stepper machines has been used traditionally for submaximal testing due to their lower oxygen values in comparison to treadmill [13].Balance issues, fatigue and higher rate of stepping are also some of the disadvantages which could be related to the use of stepper [13].However, despite the fact that stepper machines are an important tool in rehabilitation, there has not been any signi cant research done to analyse their usefulness in terms of energy consumption [19].
Treadmill, on the other hand, is a common used equipment in rehabilitation [5,7,12].Its research has been done for many years and has a reasonably completed database.It provides more collective form of physiological stress, as walking required by the participant results in higher uptake of oxygen and increased heart rate [1].Today, it is popular for users to use treadmills in gymnasiums [23].However, for those who cannot go to gymnasiums for some reasons, stepper exercise is an option [24,25,26].Therefore, the research question is if the stepper movements could replace treadmill movements and whether correlations exist between stepper movement and treadmill walking.It is hypothesized that there are some kinds of correlations between two movements.
The aim of this study was to investigate oxygen consumption during stepper movement and to analyse whether stepper movement and treadmill walking are correlated or not.Hopefully, this research would provide clinicians with a useful database and allow them to guide patients in rehabilitation.

Data inclusion and exclusion
This was a single centre, observational study of a cohort of the healthy.The inclusion criteria was that the healthy, age 20-50, male or female and the exclusion was without any physical and medical disabilities or injuries.They were required to sign a consent form before they took part in the study.This project was approved by the university research ethics committee.

Equipment
Stepper Reebok® mini side stepper (REM-7580) was used in this experiment.It consists of foot plates (Figure 1a), a knob to adjust the resistance and console (Figure 1b) that displays count, total count, time, calories and scan side to side action.Dimensions: H22, W51, D45 cm.

Treadmill
Vision tness TM T 8500 treadmill was used for the experiment.Three different speeds with no inclination were used for the entire data collection.The treadmill picture is ignored.

Mobile Oxycon ® System
The mobile Oxycon system consists of the following components (Figure 2 Transmitter (Data exchange unit -DEx with transmitter antenna attached).

Laptop with the USB interface cable.
A vest for carrying the DEx and the sensor box SBx/ CPx Protocol Proper calibration was done before every data collection to get the accurate data.

Taking Cadence
To identify a reasonable cadence for each participant, a method was designed as below.The participants were asked to walk at their self-determined conformable walking speed for 1 min and their steps were counted for 15 seconds during walking.The procedure was repeated three times and the average cadence was taken as baseline, then 20% higher and 20% lower cadences were calculated from the baseline cadence.Three cadence values were then input into the Tempo Perfect Metronome Software (free from the internet) 17 that produces the beat sound accordingly for taking the corresponding data collection.This protocol was designed according to walking which is a commonly used movement daily and everybody has their own comfortable cadence.We asked each participant to do so that the cadence was calculated reliably.

Data Collection
The rst data collection was done for the resting data by asking the subject to sit on a chair.
After ve minutes the oxygen mask was removed and the subjects were allowed to relax for ve minutes.The stepper or the treadmill machines were then chosen randomly to avoid any advantage of the machine being used rst.The data for three different cadences were collected on each machine.A total of seven sets of data (one resting, three different cadences each on stepper and treadmill machines respectively) were collected.All the subjects were instructed to use the stepper or treadmill machine in tandem with the beat produced by the Tempo Perfect Metronome Software, and made sure they did so accordingly (Figures 3).
The data collection on stepper was done by setting the machine to the least resistance.On treadmill no inclination was used.The data collection for each cadence was done for ve minutes on stepper or treadmill with ve minutes rest in between data collections, and thus each data collection was done as a fresh one.The participants were reminded that they had the freedom to discontinue from the study any time during the experiment, if they developed any distress or di culties.All the data collected were stored in the database of Oxycon Mobile ® Software.The data were converted to .txtformat and were stored on a ash drive for the analysis.
The Oxycon Mobile ® can give different variables as below.
VO 2 -Oxygen uptake in millilitres per minute (ml/min) VCO 2 -Carbon dioxide output in millilitres per minute (ml/min) VE-Ventilation (the movement of air between the atmosphere and the lungs via inhalation and Variables given by Oxycon Mobile ® system The results on the major parameters, e.g.VO 2 are shown in Table 1, and each parameter is analysed and plotted, one by one, as below.
VO 2 mean ml/min The level of oxygen consumption (VO 2 mean, ml/min) in the S1, S2, S3 showed strong correlation with T1, T2, T3, where S1 and T1 are baseline cadence in stepper and treadmill movements, S2 and T2 represents 20% higher cadence than the baseline one, and S3 and T3 20% lower cadence than the baseline one.Mean values showed that S3 consumed more energy than T3 by approximately 14% (Table 1).The correlation coe cient between stepper and treadmill movements in VO 2 is very strong (0.789, p< 0.001) and a linear regression was constructed as seen in Figure 4. EqO 2 mean in S1 (28.992) was very close to T1 mean value (28.33).It was found that the difference between S3 and T3 was signi cant, and T3 was higher than S3 by approximately 6% (Table 1).The correlation coe cient was also strong (0.489, p <0.001) and linear regression as seen in Figure 5.The mean at Rest was observed at 33.560, which was signi cantly higher than the other situations, because EqO 2 is reversely proportional to VO 2 .

Volume of CO 2 expired
VCO 2 , which is the production of carbon dioxide per minute, was recorded.It is found that S1 and S3 were higher than T1 and T3 by approximately 16% and 11% respectively (Table 1).The correlation coe cient between two types of movements is 0.778 (p<0.001) as seen in Figure 6.

Respiratory exchange ratio (RER)
RER mean values showed a correlation between stepper and treadmill as in Figure 8, and S1 and S2 higher than T1 and T2 by approximately 8% and 6% respectively (Table 2).The correlation coe cient is 0.507 with p< 0.001, again con rming the correlation between the stepper and treadmill movements.was signi cantly higher than T3 by approximately 14% as shown in Table 2.The correlation coe cient was 0.790 with p<0.001 as in Figure 9, showing strong correlation between two types of movements.

Ventilation (VE)
VE mean value showed that S1 was higher than T1 by approximately 9% in Table 1.The correlation coe cient between stepper and treadmill is 0.826 with p<0.001 as Figure 10.

MET using stepper and treadmill
Mean value for MET showed that S3 is higher than T3 by approximately 15% in Table 2.The correlation coe cient for MET was 0.598 with p<0.001 as Figure 11, so two type of movements were correlated.

Qtc
Qtc results showed correlation between stepper and treadmill as well, with the coe cient as 0.806 with p<0.001 as in Figure 12.It is also found that S3 is higher than T3 by approximately 11% as in Table 2.

Analysis of objective data
As whole, stepper movement has a higher oxygen consumption than that in treadmill walking.This could be partly attributed to that most users are not familiar with stepper movements while they are familiar to treadmill walking as they horizontally walk every day.This point indicates that stepper movement is not an exercise as simple as treadmill, and in fact stepper movement requires the body to consume more energy than treadmill walking in order to maintain balance, especially in lower cadence (i.e.20% lower than a comfortable one).From the results, it is estimated that in lower cadence, stepper movements consumes roughly 10% more energy than treadmill walking.
The reasons for differences or similarities could be that treadmill walking is similar to our daily movements and thus saves some energy while stepper movement is related to balance control and thus spends slightly more energy.The detailed mechanisms are to be explored in the future.
The results of the present study showed signi cant correlation of oxygen consumptions between using the stepper and treadmill.Also, almost all parameters analysed showed that there are linear correlations between two types of movements, and nearly half of the correlation coe cients reached or close to 0.8.Therefore, some of the regression functions provided are useful and would be used to describe the relationships between stepper movements and treadmill walking.In other words, users can use stepper movement data at home to estimate how much energy they could have consumed in treadmill walking in gymnasiums.The database produced from this study allows clinicians and physiotherapists to prescribe an exercise plan for the patients who 1) physically cannot go to gymnasium and have to stay at home, and 2) cannot horizontally walk in rehabilitation.To our best knowledge, there is little previous research done on this topic [4,7,11].

Sample size and power
As this study is brand new and there was no previous data available, we did a power analysis after data collection.Given the standard deviation as 280 in VO 2 , clinical difference as 170 and power as 80%, the estimated sample size should be 22.Therefore, the power with the sample size in this study is reasonable.

Limitation of the study
Lack of ability of a few participants to work accurately on the stepper initially as they were not trained how to work with beep produced by the Tempo Perfect Metronome software.Sometimes it was getting di cult for them to keep balance on the stepper also.Regarding to BMI, there were two participants with extremity values 30 and 40 respectively, and hopefully their data may have not largely affected the general outcomes.General subject population (10 men and 12 women with age ranged between 18-40 years and with unknown habitual physical activity level were measured in this study, and, therefore, it is di cult to associate the results to gender and functional level.In this study, the gender has not been considered as a factor as we did consider general population and compared the data within-subject.
future study In the future, the studies may focus on muscle force, muscle activities and kinematics and kinetics in the lower limbs in stepper movements.Further research should be done for patients, who may give practical feedbacks to different machine and movement types.If patients were not like stepper due to its stability, a handle or support could be designed for them.Also, a future study would consider for male and female separately and for different physical active levels.

Conclusion
This study indicates that stepper movement has similar oxygen consumption to treadmill at a comfortable cadence level and higher one (i.e.20% higher than the comfortable one) but stepper movement consumes more energy than treadmill walking by approximately 10% at a lower cadence (i.e.20% lower than the comfortable one).The study shows that stepper movement is highly correlated with treadmill walking in terms of oxygen consumption, and thus the stepper data provided can be used as reference for the healthy at home.In the future, the experiment could be done for the patients and similar analysis could be applied in rehabilitation.
VE-Ventilation (the movement of air between the atmosphere and the lungs via inhalation and        Linear correlation and regression on EE using stepper and treadmill.
Page 24/26   Linear correlation and regression on Qtc using stepper and treadmill to PC (Power calibration unit -PCa).

Figure 1 a
Figure 1

Figure 12
Figure 12 It is derived from the "Indirect Fick" Equation (Fick principle for CO 2).According to the equation the cardiac output (pumping of blood from heart/min) equals CO 2 production (VCO 2 ) divided by the difference of content of CO 2 in venous blood and the arterial blood (CvC0 2 -CaCO 2).So Qtc= VCO 2 /CvCO 2 -CaCO 2 and it is calculated in Litres per minute (L/min).the data collected were checked for any errors and any participant with inappropriate data was requested to repeat the data collection.All the data were analysed using SPSS ® (version 21).The general linear model for repeated measurement was used to compare differences between various groups of data.When some variables were not normal distributed, non-parametric methods were used to double check p values.Pearson's correlation coe cient and linear regression were used to analyse whether two variables are linearly correlated or not, and if correlated a regression function will be constructed.The signi cant level was set at p<0.05.
EE-Energy expenditure, which is the total energy consumed and is calculated as kilo calories per day.MET -Metabolic unit, in Oxycon Mobile ® system, 1 MET=3.5 ml/min/kg Qtc -All Results Participants Twenty two participants took part in the study, and they have ages mean 29.27 standard deviation (S.D.) 6.97 ranged 21-49 years old, heights mean 168.45 SD 10.14 ranged 155-191 cm, weights mean 71.69 SD 10.12 ranged 51-120 kg, body mass index (BMI) mean 25.01 SD 4.92 ranged 20.4-40.7,and 10 male and 12 female.

Table 1 .
VO 2 oxygen consumption and other parameters using stepper and treadmill

Table 2
Respiratory exchange ratio and parameters in stepper and treadmill measurements At Rest, the lowest mean of energy consumption (EE) was recorded as 1791.723.It was found that S3 exhalation) RER -Respiratory Exchange Ratio, which is Carbon dioxide output and the Oxygen uptake (RER=VCO 2 /VO 2 ) EqO 2 -Breathing equivalent for Oxygen, which is the minute volume and the Oxygen uptake (EqO 2 = VE/VO 2 ) EqCO 2-Breathing equivalent for Carbondioxide, which is the minute volume and carbon dioxide output (EqCO 2 = VE/VCO 2) EE-Energy expenditure, which is the total energy consumed and is calculated as kilo calories per day.MET -Metabolic unit, in Oxycon Mobile ® system, 1 MET=3.5 ml/min/kg Qtc -It is derived from the "Indirect Fick" Equation (Fick principle for CO 2).According to the equation the cardiac output (pumping of blood from heart/min) equals CO 2 production (VCO 2 ) divided by the difference of content of CO 2 in venous blood and the arterial blood (CvC0 2 -CaCO 2).So Qtc= VCO 2 /CvCO 2 -CaCO 2 and it is calculated in Litres per minute (L/min).