Introduction

Assessment of maximal aerobic capacity (VO_2max) has been studied extensively using treadmills and cycle ergometers (Noonan and Dean, 2000). Research has shown that the specificity of testing (i.e., testing on the mode used for training) has positive effects on the testing outcome (Stromme et al., 1977). Stair climbing using a climb mill or stair stepper has gained popularity as a training mode so it may be a useful mode with which to perform exercise testing, particularly for firefighters, hikers, and others who engage in sports or activities that include stair climbing motions.

There are conflicting results when VO_2max derived from a maximal StairMaster or a stepmill test is compared to VO_2max values obtained from other modes. Some studies found peak VO₂/VO_2max to be higher when measured on a treadmill compared to a stair stepping device (Ben-Ezra and Verstraete, 1988; Luketic et al., 1993; Schuler et al., 1998). Other studies found no significant differences in peak VO₂/VO_2max values between stair stepping devices and treadmills (Holland et al., 1990, 1988).

Submaximal testing may be more applicable than maximal testing in commercial fitness settings, however. Research examining the validity of submaximal stair climbing exercise on a StairMaster 4000PT to predict maximal oxygen uptake found conflicting results for trained and untrained women. Participants who had previously trained on a step ergometer and had previous experience with cardiovascular training had no significant differences in VO_2max values measured from a maximal Bruce treadmill test and those estimated from the StairMaster submaximal test. The non-step trained group had significantly lower VO_2max values when estimated from the StairMaster test than when measured during the maximal Bruce test (Roy et al., 2004).

Using a maximal StairMaster (model not specified) test, Carroll et al. (1990) developed an equation (VO_2max = 66.852 - .178(HR3) – 3.413(sex) [HR3: HR at 60 Steps/Minute, sex: men = 1, women = 2] to predict VO_2max from submaximal stepping exercise. When cross-validated with 27 men and women, the equation’s predicted VO_2max values were found to be strongly correlated (r = 0.71) with measured VO_2max assessed from the maximal StairMaster test. The results from these studies suggest that submaximal StairMaster exercise may have potential as a method for estimating peak/maximal oxygen uptake.

One concern about using submaximal stair climbing to predict maximal oxygen uptake is that a number of studies have found elevated physiological responses at standard submaximal levels in stair climbing when compared to other modes of exercise. For instance, two studies found lactate levels to be higher during exercise on stair climbing devices when compared to exercise on a treadmill (Schuler et al., 1998; Zeni et al., 1996). Higher VO₂ and/or higher heart rates at similar workloads between a stair climbing device and a treadmill were found as well in other studies (Holland et al., 1990; Howley et al., 1992). Since submaximal testing typically relies on the heart rate to predict maximal oxygen uptake, an elevated heart rate at a given submaximal level will lead to a lower VO_2max prediction. Additionally, higher lactate levels and VO₂ may lead to greater discomfort during stair climbing tests when compared to treadmill tests. These responses may limit the ability to accurately predict VO_2max from a submaximal step climbing test.

Additionally, there is evidence that step climbing machines may not estimate MET levels accurately. Some studies found METs to be overestimated by a step climbing device when compared to measured METs (Holland et al., 1988; Howley et al., 1992; Luketic et al., 1993). Conversely, Butts et al. (1993) found that at all stepping rates but the slowest, MET values were underestimated by a StairMaster 4000PT. If the stair climbing device uses intensity to calculate predicted VO_2max but cannot predict METs accurately, it cannot predict VO_2max accurately.

While the StairMaster 4000 PT commonly examined in previous research is not in production any more, StairMaster and other commercial gym equipment manufacturers are producing stepmills or climb mills (StairMaster, 2019). One of these companies, Matrix, provides a pre-programmed submaximal testing protocol on their C5x climb mills (Matrix Fitness, 2012). The purpose of this study was to examine how the submaximal prediction of VO_2max from the Matrix C5x compares to VO_2max derived from a maximal test on a treadmill. A secondary purpose was to compare measured MET values to those estimated by the Matrix C5x.

Methods

Results

The mean height of participants was 171.2 cm (α 7.2). The mean mass was 69.1 kg (α 9.7), and the mean resting heart rate was 81 beats per minute (bpm) (α 14.2). Participants’ descriptive data are shown in Table 1. Means and standard deviations for maximal oxygen uptake are provided in Table 2.

For 44% of the participants (n = 7), the Matrix C5x stepmill reported a successful prediction of VO_2max. The mean EstVO_2max for these seven participants was 28.6 α 3.5 ml.kg^-1.min-¹, while the mean MeasVO_2max was 35.1 α 4.1 ml.kg-^1.min^-1. For the other nine participants, the Matrix C5x stepmill reported that the submaximal test had failed. Using the value provided by the C5x, the mean EstVO_2max for the nine participants for whom the test failed was 27.7 α 3.2 ml.kg^-1.min^{- 1}and the mean MeasVO_2max was 42.3 α 6.7 ml.kg^-1.min^-1. These nine participants were provided with additional stages of submaximal exercise following the one-stage C5x test failure, in order to collect data for CalcVO_2max. Four participants required three stages and one required four stages to achieve heart rates within the target range of 115-150 bpm. The remaining participants achieved target heart rates in two stages.

A two-way mixed ANOVA revealed no significant interaction (p = 0.54) between the method of determining VO_2max (Est vs. Meas) and group (Pass vs. Fail on stepmill test). There was a significant main effect of VO_2max method (p < 0.01, partial eta squared = 0.69). There was also a significant main effect of group (p = 0.04, partial eta squared = 0.28). Follow-up t-tests found significant differences in MeasVO_2max and EstVO_2max for both the group that had a failed test (p = 0.001) and the group that had a successful test (p = 0.047). There was also a significant difference (p = 0.03) between the passed and failed groups with regard to MeasVO_2max (35.1 vs. 42.3 ml.kg^-1.min^-1), but no significant difference in EstVO_2max (28.6 vs. 27.7 ml.kg^-1.min^-1).

Given no difference in EstVO_2max between participants whose stepmill tests had passed or failed, all 16 participants’ data were pooled for further analyses. A repeated-measures ANOVA revealed a significant main effect for the method of determining VO_2max at the p < 0.05 level (p < 0.001), with an overall large effect size calculated using eta squared of 0.54. Follow up paired t-tests revealed that MeasVO_2max (39.2 α 6.6 ml.kg^-1.min^-1) was significantly higher (p < 0.01, Cohen’s d = 1.69) than EstVO_2max (28.06 α 3.2 ml.kg^-1.min^-1). The mean CalcVO_2max (35.6 α 8.0 ml.kg^-1.min^-1) was significantly lower (p = 0.027, Cohen’s d = 0.55) than the mean MeasVO_2max and significantly higher (p = 0.003, Cohen’s d = 0.94) than the mean EstVO_2max.

Means and standard deviations for MET values are shown in Table 3. Submaximal MET values were obtained using three methods: reported by the C5x stepmill (METS_est), measured using indirect calorimetry (METS_meas), and calculated from step height and a stepping rate using the Holland et al.’s (1990) equation (METS_calc). METs derived in these three ways were compared for the first three stages of the submaximal step mill test using separate one-way ANOVAs. A significant main effect was found for each stage, and follow-up paired t-tests revealed a significant difference between METs_est and METs_meas in all three stages – Stage 1 (p < 0.01, Cohen’s d = 1.7), Stage 2 (p = 0.03, Cohen’s d = 0.78), and Stage 3 (p = 0.04, Cohen’s d = 1.42). In all three stages, MET values were significantly underestimated by the C5x. There was also a significant difference between METs_est and METs_calc in all three stages – Stage 1 (p < 0.01, Cohen’s d = 0.83), Stage 2 (p < 0.01, Cohen’s d = 1.79), and Stage 3 (p < 0.01, Cohen’s d = 2.02) - with METs_est being lower in each stage. There was no significant difference between METs_meas and METs_calc in stages 2 and 3, but METs_calc were significantly (p = 0.04, Cohen’s d = 0.58) lower in stage 1.

The correlations between MeasVO_2max and the two VO_2max prediction methods revealed a significant correlation between MeasVO_2max and CalcVO_2max (r = 0.70, p = 0.003) (Gibson et al., 2018; Roy et al., 2004). The correlations between MeasVO_2max and EstVO_2max (r = -0.40, p = 0.121) and EstVO_2max and CalcVO_2max (r = 0.07, p = 0.808) were not significant.

Correlations among METs_meas, METs_est, and METs_calc were also determined. In Stage 1, the C5x step mill reported all participants’ METs_est as 4.0. As such, a correlation could not be determined for this stage. In stages 2 and 3, METs_est and METs_calc were strongly and positively correlated. In stage 2, the correlation was r = 0.96 (p < 0.001) and in stage 3 the correlation was r = 0.98, (p = 0.004). METs_meas and METs_calc were moderately correlated (r = 0.53, p = 0.04) in stage 2. However, in stage 3, the correlation between METs_meas and METs_calc was not significant (r = 0.11, p = 0.86). The correlations for METs_est and METs_meas were not significant in stages 2 or 3 (p = 0.09 and p = 0.82, respectively). Correlations for stage 4 were not calculated as only one participant completed four stages of exercise during the submaximal stepmill test.

Discussion

The main purpose of this study was to compare directly measured VO_2max values to those predicted by a preprogrammed submaximal test performed on the Matrix C5x stepmill. Our results indicate that the C5x significantly under predicted VO_2max values. The mean value reported by the Matrix C5x was 28.1 ml.kg^-1.min^-1, which is considered “very poor to poor” for participants in this age range (American College of Sports Medicine et al., 2018), compared to the measured VO_2max value of 39.2 ml.kg^-1.min^-1, which is “poor to fair”. The C5x underestimated VO_2max for 13 of the 16 participants, with a mean difference of 13.1 α 6.8 ml.kg^-1.min^-1. Therefore, an individual evaluating their fitness using a Matrix C5x is likely to receive a discouraging and misleading result. Moreover, for over half of the participants, the submaximal test on the C5x “failed”, and so many users could feel compelled to perform the test again. Interestingly, VO_2max provided in spite of a “failed” test was not significantly different than that for a “successful” test, and VO_2max values were significantly underpredicted by the C5x across the board. One interesting finding was that participants for whom the test failed had a higher measured mean VO_2max value than those that had successful submaximal tests on the C5x. Of these nine participants, five needed more than two exercise stages (investigators manually provided additional stages when a test failed) to achieve target heart rates between 115-150 bpm. This, along with higher VO_2max values, is suggestive of higher cardiovascular fitness. The “failed test” error message from the C5x indicated “no increase in the heart rate”, which might suggest that the participant had not achieved the minimum baseline heart rate of 115 bpm and that this particular submaximal test is not suitable for more fit participants. However, four of the nine “failed” participants had heart rates above 115 bpm in the first stage of the C5x submaximal test. These inconsistencies make it difficult to draw conclusions about the Matrix C5x preprogrammed submaximal test.

With the exception of the equipment used (StairMaster vs. stepmill), our findings are almost identical to those of Roy et al. (2004) who found that a submaximal test performed using a StairMaster 4000PT resulted in a significant underestimation of VO_2max in non-step trained participants. The average measured VO_2max value for the non-step trained participants in their study was 37.6 ml.kg^-1.min^-1. StairMaster predicted VO_2max was 30.9 ml.kg^-1.min^-1, which was significantly lower than VO_2max measured during a maximal Bruce treadmill protocol. However, VO_2max values from the submaximal StairMaster test and the maximal treadmill test did not differ for step-trained participants (Roy et al., 2004).

Most of the previous research has been performed using a StairMaster (primarily the 4000 PT) rather than a stepmill device with revolving stairs, as was used in the present study. The research that has been done using a stepmill similar to the C5x has only used it for maximal exercise testing. For instance, Holland et al. (1990) found no significant differences in peak oxygen uptake (VO₂), heart rate (HR), ventilation (Ve) or respiratory quotient (RQ) obtained from a maximal test on a treadmill compared to a maximal test on a “steptreadmill” (StairMaster 6000). Ben-Ezra and Verstraete (1988) completed a study with firefighters comparing a maximal treadmill test to a maximal test on a StairMaster 5000 (revolving stairs of eight inches) and found that VO_2max, HR_max, and VE_max were all lower from the StairMaster 5000 test. No previous studies have examined submaximal estimation of VO_2max using a step mill.

Another purpose of this study was to compare measured METs to those reported by the C5x during each submaximal workload. Results indicated that the C5x underestimated METs in all stages. This finding is similar to that of Butts et al. (1993) who found that the StairMaster 4000PT significantly underestimated METs. A majority of the previous studies found the opposite result, however. Using a stepmill device, Holland et al. (1988) found significant differences between measured MET values and device-reported MET values in a study with coronary heart disease patients. During the last two stages of a maximal test (measured intensity = 5.8 and 7.2 METs), they found an overestimation of METs by a StairMaster stepmill, although in the first two stages (measured intensity = 3.4 and 4.7 METs), the values between stepmill-reported and measured METs were similar (Holland et al., 1988). Howley et al. (1992) found MET values reported by a StairMaster 4000PT to be 20% higher than measured values. Luketic et al. (1993) also found that the StairMaster 4000PT reported METs were higher across multiple stages of an exercise test when compared to measured METs. Because of the differences in equipment used and the conflicting results from previous studies, it is difficult to draw concrete conclusions or make comparisons between the present results and previous research. In general, however, it seems that the stair stepping devices that have been studied do not provide accurate estimates of submaximal oxygen uptake, which reduces their ability to provide accurate predictions of maximal oxygen uptake.

Previous studies have developed equations to predict VO_2max based on maximal testing performed on a StairMaster 4000PT (Butts et al., 1993; Holland et al., 1990). That approach might be warranted with stepmills in general and the Matrix C5x specifically. Unfortunately, the present study only completed submaximal exercise on the stepmill, and we were not able to obtain any clarification about the VO_2max prediction equation provided by Matrix. Attempts to calculate VO_2max using the provided equation and Wattage data from the C5x submaximal tests resulted in VO_2max values that were not only not the same as those displayed on the screen, but also not reasonable VO_2max values. It is possible that the results from the present study may have turned out differently if we had been able to use the Matrix prediction equation to calculate VO_2max values by hand. In an effort to find another way that VO_2max might be calculated using submaximal data from the stepmill, submaximal VO₂ was calculated using the Holland et al.’s (1990) equation, which uses stepmill height and a stepmill stepping rate to determine oxygen uptake (and therefore METs) for a given stage. The ACSM stepping equation includes a stepping rate pattern designed for bench step exercise (i.e. “up up down down”) and so was determined not to be an appropriate way to calculate VO₂ values from a stepmill where stepping only occurs in one direction (Holland et al., 1988; Luketic et al., 1993). The calculated submaximal VO₂ and MET values from the Holland et al.’s (1990) equation showed promise, as the values calculated did not differ significantly from measured MET values in two of the three stages examined. However, when those values were used to calculate VO_2max using the multi-stage prediction equation (Gibson et al., 2018; Roy et al., 2004), the mean CalcVO_2max value was significantly lower than the measured VO_2max value. Nevertheless, the correlation between the two was strong and significant. A review of the literature yielded no other options for predicting VO_2max values from submaximal stepmill exercise, although a recent study found that during treadmill exercise, the accuracy of the multi-stage equation to estimate VO_2max was improved when submaximal VO₂ used in the equation was measured rather than predicted (Peterman et al., 2021). Using measured submaximal VO₂ might improve the accuracy of stepmill-based VO_2max predictions as well, particularly as the ACSM stepping MET equation may not be applicable to stepmill exercise, and the Holland et al.’s (1990) equation has not been validated. However, measuring VO₂ may not be feasible in health-fitness settings, so there is a need to develop an equation accessible to the general population that can accurately calculate VO_2max values from stepmill exercise.

This study was limited by a small and convenient sample. Due to this, comparisons were not made between males versus females or trained versus untrained participants. The Matrix C5x also did not function as intended, with the preprogrammed submaximal test failing a majority of the time. The Holland et al.’s (1990) equation that we used in our calculations has not been validated, and thus may be considered a limitation. A major strength of this study is that it is a unique assessment of stepmill exercise and provides information on which to build future studies.

In summary, the present study’s findings align with previous research in that stair stepping devices do not provide accurate submaximal MET estimation. Most of the time, METs were overestimated. This can be misleading when estimating intensity or submaximal and maximal VO₂. If METs are overestimated, intensity and, thus, energy expenditure will be overestimated as well. If this equipment is used in fitness testing, the tester should be aware that the display may not reflect the user’s actual level of exertion. An inability to accurately estimate submaximal intensity also impacts the device’s ability to accurately estimate VO_2max. The present study did also find that the Matrix C5x stepmill did not provide an accurate estimate of maximal oxygen uptake and that the equation provided by the manufacturer was not useful for calculations by hand. With the growing popularity of stepmills like the C5x, and given this specific device provides a programmed submaximal test, the equations and methods behind submaximal testing on these devices need to be further researched.