Validity of a ramp protocol based on whole-body climbing-specific exercise for peak oxygen uptake in physically active adults

Abstract

The present study aimed to verify the validity of peak oxygen uptake ($\:\dot{\text{V}}$O_2peak) between climbing-specific walking (CLIMIT) and cycling tests in physically active adults. A total of 101 physically active adults participated in this study. Participants were randomly assigned to either cycling or CLIMIT test. The cycling test included an initial period of 2 min at a constant load of 50 watts. The workload was increased by 25 watts every 30 s. During the CLIMIT test, an initial period of 2 min at a constant step length (SL) of 18 cm, a step frequency (SF) of 80·min^–1, and a viscous intensity (VI) of 100 F*v·min^–1. SL and VI were increased by 2 cm and 50 F*v·min^–1 every 30 s, respectively. SF was reduced only by 20·min^–1 until the end of the test. Pearson’s correlations of $\:\dot{\text{V}}$O_2peak, physiological variables, and energetic contributions were determined during both tests. Cycling and CLIMIT tests showed strong interclass correlations, acceptable biases, and narrow 95% limits of agreement of $\:\dot{\text{V}}$O_2peak. Strong relationships of physiological variables and energetic contributions between both tests were observed. The CLIMIT test and $\:\dot{\text{V}}$O_2peak showed a high validity. This may lead to another useful exercise for cardiovascular/-metabolic tests.

Introduction

The level of peak oxygen uptake ($\:\dot{\text{V}}$O_2peak) is a vital indicator of mortality risk^1,2,3. A relatively low $\:\dot{\text{V}}$O_2peak (i.e. women < 35.1 mL·kg^–1·min^–1; men < 44.2 mL·kg^–1·min^–1)⁴ is associated with an increased risk of cardiovascular diseases, insulin resistance, type 2 diabetes mellitus, obesity, metabolic disorders, endocrine diseases, and higher all-cause mortality^1,2,3,4. Therefore, the use of $\:\dot{\text{V}}$O_2peak values is clinically relevant, particularly in assessing disease severity and serving as a strong predictor of mortality^3,4. Furthermore, $\:\dot{\text{V}}$O_2peak has been found to be valid and reliable for determining cardiorespiratory and metabolic fitness, as well as for predicting endurance performance, such as success in marathons, alongside other physiological parameters in sports science and sports medicine^2,5.

In light of this, individual cardiovascular fitness and health conditions are determined as maximal oxygen uptake ($\:\dot{\text{V}}$O_2max). $\:\dot{\text{V}}$O_2max is defined as a plateau or leveling off of oxygen uptake despite an increase in exercise intensity during a standardized ramp or incremental test². Therefore, this variable indicates the upper exercise limit of the cardiorespiratory system^2,6. However, the plateau area in $\:\dot{\text{V}}$O₂ as $\:\dot{\text{V}}$O₂ increase < 150 mL·min^–1 or < 2 mL·kg^–1·min^–1 is not always achieved with most used criteria (failure of heart rate to increase with an increase in workload, blood lactate concentration (La^–) > 8.0 mmol·L^–1, rating of perceived exertion > 17 Borg scale, and a peak respiratory exchange ratio ≥ 1.10 or > 1.0) during the test^6,7,8,9. On the other hand, $\:\dot{\text{V}}$O_2peak is also used interchangeably during a single maximal test, considering the limitations of participant characteristics, exercise modality, and specific test protocols^2,10.

The $\:\dot{\text{V}}$O_2peak determined during the standardized ramp or incremental test can be affected by diverse factors, such as the modality and test duration². For instance, ~ 20% higher levels of $\:\dot{\text{V}}$O_2peak can be reached during a treadmill running protocol compared to a cycling protocol because of the familiarity of the participants or a larger involved skeletal muscle mass^2,11. Despite the report of higher $\:\dot{\text{V}}$O_2peak data on a treadmill, the assessment of $\:\dot{\text{V}}$O_2max or $\:\dot{\text{V}}$O_2peak using a cycling ergometer is a standardized test in many different sports disciplines. The measurement of $\:\dot{\text{V}}$O_2max or $\:\dot{\text{V}}$O_2peak using a cycling ergometer is well established due to standardization, safety, and reproducibility^12,13,14. In comparison to a treadmill, it requires little specific technique and permits for safe at maximized exercise intensity and stationary measurement using an automatic protocol¹². A recent study has demonstrated the high validity of $\:\dot{\text{V}}$O_2max values between cycling and swimming tests. This indicates an interchangeable (upper vs. lower extremities) use of the cycling ergometer protocol related to a swimming test¹².

Nowadays, there are challenges in administering and advocating physical activity¹⁵. As another form of climbing-based exercise, stair climbing is an easy and accessible method to increase physical activity^16,17. In this aspect, stair climbing exercise can be a low-cost, practical, and feasible way to improve cardiovascular health^15,16,17. However, a stair-based whole-body climbing-specific ramp protocol test for $\:\dot{\text{V}}$O_2peak has not been standardized or validated yet.

Furthermore, thorough analysis of energy system contributions including oxidative, glycolytic, and phosphagen systems can offer valuable insights into the physiological and metabolic responses that occur during exercise. By applying mathematical models (PCr-La^–-O₂ method) to $\:\dot{\text{V}}$O_2peak test data, metabolic contributions can be calculated using measured $\:\dot{\text{V}}$O₂ and La^{–6,18,19,20,21,22,23,24,25,26}. Understanding these contributions enhances the interpretation of metabolic efficiency and performance capabilities while supporting metabolic shifts toward physiological outcomes, such as the achieved $\:\dot{\text{V}}$O_2peak^{18,19,20,21,22}.

A test protocol for the stair climbing-based machine CLIMIT may more accurately reflect daily life movements than traditional test protocols for cycling, making it a more accessible option for specific populations. Therefore, the present study aimed to compare ramp test protocols using a similar stair climbing-based machine CLIMIT and a standardized cycling ergometry. The test of CLIMIT included concentric and eccentric movements on lower and upper extremities with continuous incremental step length, step frequency, and viscous intensity. This study also aimed to verify the validity of the attained $\:\dot{\text{V}}$O_2peak between climbing-specific and standardized cycling ramp tests in physically active adults. In addition, relationships of physiological parameters and energy system contributions between both ramp tests were verified. We hypothesized that $\:\dot{\text{V}}$O_2peak, physiological parameters, and energetic contributions would be highly correlated between a climbing-specific ramp test and a standardized cycling ramp test in physically active adults.

Materials and methods

Participants and study procedure

The required sample size for the present study was estimated using the G*Power software program version 3.1.9.4 (Heinrich Heine University, Düsseldorf, Germany), considering an effect size of 0.29, an alpha error probability of 0.05, and a statistical power of 0.80. The effect size was determined based on previous studies^6,12,23,24. Considering a 10% dropout rate, a total of 101 physically active individuals (74 males and 27 females) were randomly recruited to participate in this study. They performed different exercises such as running, road cycling, Pilates, yoga, and resistance training sessions for at least 6 to 12 h per week. They had no pre-existing cardiovascular, pulmonary, or metabolic diseases or musculoskeletal disorders¹. Anthropometric data of participants are as follows (mean ± standard deviation; SD): age of 30 ± 8 years, height of 173 ± 8 cm, body mass of 73 ± 14 kg, body fat of 19 ± 6%, and body mass index of 25 ± 4 kg·m^–2 (Table 1). They did not take any medication before or during tests. They abstained from nicotine and alcohol for 24 h before tests. This study was approved by the Institutional Review Board (IRB) of CHA University (IRB No. 1044308-202401-ER-152-02). Approved ethical protocols were according to the principles of the Declaration of Helsinki. All participants provided written informed consent.

Table 1 Anthropometric data and weekly training program of participants (n = 101).

All participants attended a laboratory visit for measuring cardiorespiratory fitness during cycling and CLIMIT ramp tests. Tests were performed at a temperature of 25^◦C and a relative humidity of 48%. Anthropometric data of all participants were measured using an 8-point tactile electrode segmental multi-frequency bioelectrical impedance analysis (1–1000 kHz range; i55, Mediana Co., Ltd., Wonju, Republic of Korea)²⁵. The study procedure was conducted using a crossover approach after anthropometric measurements to avoid synergistic effects between the two tests. All participants were randomly assigned to start the testing protocol with either the cycling ramp test (n = 50) or the CLIMIT ramp test (n = 51). They were instructed not to change their diet for at least 5 days before the test day. No food intake was allowed for any participants 2 h before main ramp tests⁶. After cycling or CLIMIT ramp test, participants were advised to continue low-intensity jogging until blood lactate concentrations (La^–) were below 2.0 mmol·L^–1 when La^– values were over 2.0 mmol·L^–1 before the next ramp test^6,26 (Fig. 1).

Fig. 1

Study design and procedure. This study was performed using a crossover design. Participants (n = 101) were randomly allocated to either a cycling ramp test (n = 50) or a similar stair climbing-based ramp test (n = 51). After an anthropometric measurement, both tests were separated until a reduction of blood lactate concentrations under 2 mmol·L^–1 between the first and second tests. La^–, blood lactate concentrations; SL, step length; SF, step frequency; VI, viscous intensity; $\:\dot{\text{V}}$O₂, oxygen uptake; W, watt.

Cycling and CLIMIT ramp test protocols for $\:\dot{\mathbf{V}}$O_2peak

Cycling and CLIMIT ramp tests were performed using continuous incremental ramp protocols on a SRM cycling ergometer (No. 2307, 2024, Schoberer Rad Messtechnik, GmbH, Jülich, Germany) and a similar stair climbing-based machine CLIMIT (75 × 100 × 200 cm; 150 kg) (CLIMIT; Ronfic Co., Ltd., Busan, Republic of Korea) with a breath-by-breath technique portable gas analyzer (MetaMax 3B; Cortex Biophysik, Leipzig, Germany) (Fig. 1). The spiroergometry was calibrated before each test and between tests using 15% O₂ and 5% CO₂ (Cortex Biophysik, Leipzig, Germany). The turbine volume transducer was calibrated using a 3-L syringe (Hans Rudolph, Kansas, United States). Values of heart rate (HR) were continuously recorded during both cycling and CLIMIT ramp tests for HR_peak and HR_mean (Polar H10; Polar Electro, Kempele, Finland). Capillary blood was collected from the earlobe (20 µL) before and at 1-min intervals (1st to 5th) after each ramp test to determine resting and highest La^– among five values (resting La^– and peak La^–) using an enzymatic-amperometric chip sensor system (Biosen C-line; EKF diagnostics sales, GmbH, Barleben, Germany). A general initial warm-up, such as familiarization, was performed for 10 min by cycling at 2 W·kg^–1 on the cycling ergometer²⁰ when the first test was a cycling ramp protocol. Otherwise, warm-up, such as familiarization was performed for 10 min using whole-body movement at a step length (SL) of 20 cm, a step frequency (SF) of 80 steps·min^–1, and a viscous intensity (VI) of 100 F (N)*v·min^–1 on the CLIMIT when the first test was a CLIMIT ramp protocol. F (N) was calculated according to Eq. (1):

$${\text{F }}\left( {\text{N}} \right){\text{ = c (N}\cdot\text{s/m)}\cdot\text{v }}\left( {{\text{m/s}}} \right)$$

F (N) = force, c = viscous damping coefficient, m = meter, N = newton, s = second, v = velocity.

Participants performed a cycling ramp protocol for $\:\dot{\text{V}}$O_2peak which included an initial phase of 2 min at a constant load of 50 watts (W). After this initial phase, the workload was increased by 25 W every 30 s until exhaustion^13,14.

A CLIMIT ramp protocol was implemented on a stair climbing-based whole-body exercise machine. Repeated stair whole-body movements with hand grips and fixed foot pedals on the CLIMIT were crossed, in which a concentric movement on the left side of upper and lower extremities was performed while another eccentric movement on the right side of upper and lower extremities was conducted (Fig. 1). During the ramp test, these movements included an initial period of 2 min at a constant SL of 18 cm, SF of 80 steps·min^–1, and VI of 100 F*v·min^–1. Afterward, SL and VI from 200 were increased by 2 cm and 50 F*v·min^–1, respectively, every 30 s until exhaustion while SF was decreased only by 20 steps·min^–1 (from 80 to 60 steps·min^{– 1} [fixed]), which remained until the test ended (Table 2). The ramp protocol of CLIMIT was established based on our internal pilot measures. Furthermore, the selected combination of SL, SF, and VI was the only one to elicit a linear $\:\dot{\text{V}}$O₂ response.

Table 2 A developed CLIMIT ramp test protocol.

The investigator verbally encouraged all participants during both ramp tests to maintain the effort to measure their highest performance for as long as possible. According to most used criteria of previous studies, the test process was tried to reach $\:\dot{\text{V}}$O₂ plateau (< 2 mL·kg^{– 1}·min^{– 1}) with La^– > 8.0 mmol·L^{– 1}, and respiratory exchange ratio (RER) (> 1.0) or until volitional exhaustion by the participant^6,7,8. However, the $\:\dot{\text{V}}$O₂ plateau of participants was not always reached during both ramp tests. Therefore, absolute and relative $\:\dot{\text{V}}$O_2peak levels were determined as an averaged value of the highest $\:\dot{\text{V}}$O₂ during a 15-s duration at the end of each test^6,8. Moreover, peak cycling and CLIMIT workloads (_W$\:\dot{\text{V}}$O_2peak, _SL$\:\dot{\text{V}}$O_2peak, and _VI$\:\dot{\text{V}}$O_2peak), peak heart rate (HR_peak), and RER were defined as values through the same section as $\:\dot{\text{V}}$O_2peak determination^6,9. Further averages of $\:\dot{\text{V}}$O₂ and HR during both ramp tests were analyzed as $\:\dot{\text{V}}$O_2mean and HR_mean.

Energetic contributions (PCr-La^–-O₂) during cycling and CLIMIT ramp tests

During cycling and CLIMIT ramp tests, contributions of three energy systems, oxidative (W_Oxi), glycolytic (W_Gly), and phosphagen (W_PCr) systems, were analyzed. $\:\dot{\text{V}}$O₂ data of all participants were measured at 5-min rest in the standing position, during ramp tests (10 min approximately), and after ramp tests (6 min in the sitting position) such as Off $\:\dot{\text{V}}$O₂ kinetics^{6,19,20,21,26,27}. The timing of $\:\dot{\text{V}}$O₂ measurements was marked by investigators at the end of the ramp tests, specifically when participants stopped the tests before reaching 10 min. Subsequently, further $\:\dot{\text{V}}$O₂ measurements were taken immediately to assess Off $\:\dot{\text{V}}$O₂ kinetics.

The W_Oxi value was analyzed by subtracting the resting $\:\dot{\text{V}}$O₂ from $\:\dot{\text{V}}$O₂ during both ramp tests using the trapezoidal method. The measured area of the $\:\dot{\text{V}}$O₂ curve was divided into sections. Summarized data of the trapezoid were utilized to calculate the integral value. The value of $\:\dot{\text{V}}$O_2rest was determined using the last 30 s of a 5 min phase applied as reference^{6,19,21,27,28}.

The metabolism of W_Gly was calculated using the difference (∆La^–) between resting La^– before both ramp tests and the highest value (peak La^–) obtained from 5-min interval blood samplings (every minute blood sampling) after the ramp tests^19,20,21,27, assuming that a level of 1 mmol·L^{– 1} was equivalent to 3 mL·kg^{– 1} of body mass²⁹. During the second and third blood samplings, peak La^– levels for most participants were observed during the second and third blood samplings. The decreased value of La^– after peak La^– during the fourth and fifth samplings was ≥ 0.5 mmol·L^{– 1}, approximately.

The W_PCr level was calculated using 6-min $\:\dot{\text{V}}$O₂ data after ramp tests and the fast component of excess post-exercise (EPOC_fast)^{6,19,20,21,28,30}. Values of Off $\:\dot{\text{V}}$O₂ kinetics after ramp tests were fitted by mono-exponential and bi-exponential models using OriginPro 2021 software (OriginLab Corp, Northampton, MA, USA). The slow component of the bi-exponential model was negligible. Therefore, $\:\dot{\text{V}}$O₂ data after ramp tests were calculated using a mono-exponential model and W_PCr was obtained by integrating the exponential domain^{6,18,19,20,22,23,−24,30}.

A caloric quotient of 20.92 kJ was applied in all energetic contributions³⁰. The total energy expenditure was estimated as the sum of the three energy systems in kJ^{18,19,22,23,25,30}.

Fat and carbohydrate oxidations during ramp tests

Values of $\:\dot{\text{V}}$O₂ and $\:\dot{\text{V}}$CO₂ during ramp tests were utilized to calculate fat and carbohydrate oxidations (FAT_Ox and CHO_Ox) using Frayn’s stoichiometric Eq³¹. During the present study, values of urinary nitrogen could not be measured. Therefore, equations assumed that urinary nitrogen (n) excretion was zero according to previous studies^6,22,32,33. Finally, averaged values of FAT_Ox and CHO_Ox (mean FAT_Ox and mean CHO_Ox) during both ramp tests were determined.

(Frayn’s equations):

$${\text{FA}}{{\text{T}}_{{\text{Ox}}}}{\text{(g}}\cdot{\text{mi}}{{\text{n}}^{{\text{-1}}}}{\text{) = 1}}{\text{.67}}\cdot{{\text{O}}_{\text{2}}}{{\text{(L}}\cdot{\text{mi}}{{\text{n}}^{{\text{--1}}}}{\text{)}}^{}}{\text{- 1}}{\text{.67}}\cdot{\text{C}}{{\text{O}}_{\text{2}}}{\text{(L}}\cdot{\text{mi}}{{\text{n}}^{{\text{-1}}}}{\text{) - 1}}{\text{.92}}\cdot n{\text{(g}}\cdot{\text{mi}}{{\text{n}}^{{\text{-1}}}}{\text{)}}$$

$${\text{CH}}{{\text{O}}_{{\text{Ox}}}}{\text{(g}}\cdot{\text{mi}}{{\text{n}}^{{\text{-1}}}}{\text{) = 4}}{\text{.55}}\cdot{\text{C}}{{\text{O}}_{\text{2}}}{{\text{(L}}\cdot{\text{mi}}{{\text{n}}^{{\text{-1}}}}{\text{)}}^{}}{\text{- 3}}{\text{.21}}\cdot{{\text{O}}_{\text{2}}}{\text{(L}}\cdot{\text{mi}}{{\text{n}}^{{\text{-1}}}}{\text{) - 2}}{\text{.87}}\cdot n{\text{(g}}\cdot{\text{mi}}{{\text{n}}^{{\text{-1}}}}{\text{)}}$$

Statistical analyses

All collected data were analyzed using GraphPad Prism version 10.3.1 (GraphPad Prism Software Inc., La Jolla, CA, USA). Normal distribution of parameters was evaluated using the Shapiro-Wilk test. Normal and non-normal distributed parameters are presented as mean ± standard deviation (SD), and 95% confidence interval for medians, respectively. The strength of the relationship between the cycling and CLIMIT ramp tests was assessed using two-tailed Pearson’s correlation for the following variables: absolute and relative $\:\dot{\text{V}}$O_2peak, physiological parameters (resting La^–, peak La^–, ∆La^–, RER, HR_peak, HR_mean, mean FAT_Ox, mean CHO_Ox), energetic contributions, and ramp test durations. Bland-Altman plots assessed bias and the 95% limits of agreement in absolute and relative $\:\dot{\text{V}}$O_2peak between the two tests. Paired comparisons of absolute and relative $\:\dot{\text{V}}$O_2peak, physiological parameters (resting La^–, peak La^–, ∆La^–, RER, HR_peak, HR_mean, mean FAT_Ox, mean CHO_Ox), energetic contributions, and test durations between both ramp tests were conducted using a paired t-test. Additional relationships among all energetic contributions, absolute and relative $\:\dot{\text{V}}$O_2peak, and test times of cycling and CLIMIT ramp tests across the entire 202 data were analyzed using two-tailed Pearson’s correlation. Averaged 1-min intervals were used to perform in-depth analyses of absolute and relative $\:\dot{\text{V}}$O₂ values and RER during both ramp tests. Data were statistically compared up to the 5th minute using a paired t-test due to the differing sample sizes between both tests from the 6th minute onward. Furthermore, 101 absolute and relative $\:\dot{\text{V}}$O_2peak data were separated into 3 parts (high, middle, and low groups) according to upper and lower limits of 95% confidence interval (in absolute $\:\dot{\text{V}}$O_2peak: 3.2–3.5 L·min^{– 1}, in relative $\:\dot{\text{V}}$O_2peak: 44–48 mL·kg^{– 1}·min^{– 1}). For paired comparisons and relationships, a paired t-test and two-tailed Pearson’s correlation were used, or a Wilcoxon signed rank test (absolute $\:\dot{\text{V}}$O_2peak in the low group, RER at absolute $\:\dot{\text{V}}$O_2peak in middle and low groups, relative $\:\dot{\text{V}}$O_2peak in high and low groups, and RER at relative $\:\dot{\text{V}}$O_2peak in the low group) and Spearman’s rank correlation (absolute $\:\dot{\text{V}}$O_2peak in the middle and low groups and relative $\:\dot{\text{V}}$O_2peak in the high and low groups) if the normality assumption was violated. The alpha level of significance was set at P < 0.05. Effect sizes of paired comparisons were calculated for non-parametric tests ($\:\frac{Z}{\sqrt{N}}$) and parametric tests (Cohen’s d [d]). Effect size thresholds for small, medium, and large effects were at ≥ 0.1, ≥ 0.3, and ≥ 0.5 for $\:\frac{Z}{\sqrt{N}}$ and ≥ 0.2, ≥ 0.5, and ≥ 0.8 for d, respectively³⁴.

Results

Pearson’s correlations and Bland-Altman plots of $\:\dot{\mathbf{V}}$O_2peak and physiological parameters between cycling and CLIMIT ramp tests

Strong positive interclass correlations of absolute and relative $\:\dot{\text{V}}$O_2peak (L·min^{– 1} and mL·kg^{– 1}·min^{– 1}) between cycling and CLIMIT ramp tests were observed (r = 0.91, R² = 0.84, 95% CI = 0.87–0.94, and P < 0.0001; r = 0.87, R² = 0.76, 95% CI = 0.81–0.91, and P < 0.0001, respectively). The Bland-Altman plot indicated that the bias of absolute $\:\dot{\text{V}}$O_2peak between cycling and CLIMIT ramp tests was 0.069 ± 0.366 and 95% limits of agreement were − 0.64 to 0.78. Relative $\:\dot{\text{V}}$O_2peak between cycling and CLIMIT ramp tests showed that the bias and 95% limits of agreement were 0.77 ± 5.07 and − 9.17 to 10.73, respectively (Fig. 2A-D). Figure 2E presents a correlation matrix of physiological parameters such as resting La^–, peak La^–, ∆La^– (mmol·L^{– 1}), RER, HR_peak, HR_mean (beats·min^{– 1}), mean FAT_Ox, mean CHO_Ox (g·min^{– 1}), W_PCr, W_Gly, W_Oxi, W_Total in kJ or %, and test duration.

Fig. 2

Pearson’s correlations of (A) absolute $\:\dot{\text{V}}$O_2peak between cycling and CLIMIT ramp tests and (B) relative $\:\dot{\text{V}}$O_2peak between cycling and CLIMIT ramp tests. (C and D) Bland-Altman plots of differences between absolute and relative $\:\dot{\text{V}}$O_2peak on cycling and CLIMIT tests, with dashed lines representing biases and dotted lines representing 95% limit of agreements. (E) Heat map exhibiting relationships of several physiological parameters and test durations. HR_peak, peak heart rate; HR_mean, mean heart rate; mean FAT_Ox, mean fat oxidation; mean CHO_Ox, mean carbohydrate oxidation; peak La^–, peak blood lactate concentrations; resting La^–, resting blood lactate concentrations; ∆La^–, difference between resting and peak blood lactate concentrations; r, correlation coefficient; R², coefficient of determination; RER, respiratory exchange ratio; $\:\dot{\text{V}}$O_2peak, peak oxygen uptake; W_PCr, phosphagen contribution; W_Gly, glycolytic contribution; W_Oxi, oxidative contribution; W_Total, total energy expenditure.

Mean absolute and relative $\:\dot{\mathbf{V}}$O₂ and RER data during ramp tests on cycling and CLIMIT

In detail, Fig. 3A shows the mean values of absolute $\:\dot{\text{V}}$O₂ 1-min intervals between cycling and CLIMIT ramp tests. Absolute $\:\dot{\text{V}}$O₂ values during the CLIMIT test from the 3rd to the 5th minute were significantly higher than absolute $\:\dot{\text{V}}$O₂ values during the cycling test (3rd, P = 0.0062 and d: 0.20; 4th, P < 0.0001 and d: 0.62; 5th, P < 0.0001 and d: 2.33, respectively). Mean values of relative $\:\dot{\text{V}}$O₂ indicated that relative $\:\dot{\text{V}}$O₂ values during the CLIMIT test from the 2nd to the 5th minute were significantly higher than $\:\dot{\text{V}}$O₂ values during the cycling test (2nd, P = 0.0226 and d: 0.28; 3rd, P = 0.0049 and d: 0.31; 4th, P < 0.0001 and d: 0.99; 5th, P < 0.0001 and d: 2.54, respectively) (Fig. 3B). Additionally, levels of RER during the CLIMIT test from the 3rd to the 5th minute were significantly increased compared to RER levels during the cycling test (3rd, P = 0.0249 and d: 0.25; 4th, P < 0.0001 and d: 0.52; 5th, P < 0.0001 and d: 0.71, respectively) (Fig. 3C).

Fig. 3

Averaged 1-min intervals of absolute (A) and relative (B) $\:\dot{\text{V}}$O₂ and RER (C) during cycling and CLIMIT ramp tests. $\:\dot{\text{V}}$O₂ and RER were only statistically compared until the 5th minute due to different sample sizes between both tests from the 6th minute to the end. $\:\dot{\text{V}}$O₂, oxygen uptake; RER, respiratory exchange ratio; %, percentages of total exercise duration (highest values) between tests. Significant difference = *P < 0.05; **P < 0.01; ****P < 0.0001.

$\:\dot{\mathbf{V}}$O_2peak and RER in separated groups (high, middle, and low) between cycling and CLIMIT ramp tests

Among the separated groups, the low group showed the strongest relationships in both absolute and relative $\:\dot{\text{V}}$O_2peak values between the cycling and CLIMIT ramp tests. The absolute $\:\dot{\text{V}}$O_2peak values in high and middle groups were not significantly different between cycling and CLIMIT tests (P > 0.05) (Fig. 4A, C). Values of RER at absolute $\:\dot{\text{V}}$O_2peak in high and middle groups were significantly higher during the cycling test than those during the CLIMIT test (high, P < 0.0001 and d: 1.62; middle, P < 0.0001 and $\:\frac{Z}{\sqrt{N}}$: −0.88, respectively) (Fig. 4B, D). Absolute $\:\dot{\text{V}}$O_2peak and RER at absolute $\:\dot{\text{V}}$O_2peak in the low group were significantly higher and lower during the CLIMIT test than those during the cycling test ($\:\dot{\text{V}}$O_2peak, P < 0.0001 and $\:\frac{Z}{\sqrt{N}}$: −0.58; RER, P < 0.0001 and $\:\frac{Z}{\sqrt{N}}$: −0.71) (Fig. 4E, F). No significant relationship of absolute $\:\dot{\text{V}}$O_2peak between cycling and CLIMIT tests in the middle group was observed (r = 0.34, R² = 0.11, 95% CI = −0.22–0.73, and P = 0.2067) (Fig. 4H). Significant positive relationships of absolute $\:\dot{\text{V}}$O_2peak between cycling and CLIMIT tests in high and low groups were observed (high, r = 0.39, R² = 0.15, 95% CI = 0.11–0.62, and P = 0.0073; low, r = 0.87, R² = 0.75, 95% CI = 0.77–0.93, and P < 0.0001, respectively) (Fig. 4G, I). Furthermore, the relative $\:\dot{\text{V}}$O_2peak and RER of the high group during the cycling test were significantly higher than those during the CLIMIT test ($\:\dot{\text{V}}$O_2peak, P = 0.0028 and $\:\frac{Z}{\sqrt{N}}$: −0.48; RER, P < 0.0001 and d: 1.55, respectively) (Fig. 4J, K). Furthermore, RER values of middle and low groups and relative $\:\dot{\text{V}}$O_2peak of the low group showed significant differences between cycling and CLIMIT tests (RER of the middle group, P < 0.0001 and d: 1.30; RER of the low group, P < 0.0001 and $\:\frac{Z}{\sqrt{N}}$: −0.74, $\:\dot{\text{V}}$O_2peak, P < 0.0001 and $\:\frac{Z}{\sqrt{N}}$: −0.70, respectively) (Fig. 4M, N, O). No significant difference in relative $\:\dot{\text{V}}$O_2peak of the middle group and no correlation of relative $\:\dot{\text{V}}$O_2peak between cycling and CLIMIT tests in the middle group were observed (Fig. 4L, Q). Relative $\:\dot{\text{V}}$O_2peak between cycling and CLIMIT tests in high and low groups also showed significant positive associations (high, r = 0.69, R² = 0.47, 95% CI = 0.46–0.83, and P < 0.0001; low, r = 0.70, R² = 0.49, 95% CI = 0.52–0.82, and P < 0.0001, respectively) (Fig. 4P, R).

Fig. 4

Paired comparisons and relationships of absolute and relative $\:\dot{\text{V}}$O_2peak and RER between cycling and CLIMIT ramp tests among high, middle, and low absolute and relative $\:\dot{\text{V}}$O_2peak groups according to lower and upper limits of 95% confidence intervals. (A-I) Paired comparisons and relationships of absolute $\:\dot{\text{V}}$O_2peak and RER between cycling and CLIMIT ramp tests among high, middle, and low groups. (J-R) Paired comparisons and relationships of relative $\:\dot{\text{V}}$O_2peak and RER between cycling and CLIMIT ramp tests among high, middle, and low groups. r, correlation coefficient; CI, confidence interval; R², coefficient of determination; RER, respiratory exchange ratio; $\:\dot{\text{V}}$O_2peak, peak oxygen uptake. Significant difference = **P < 0.01; ****P < 0.0001.

Energetic contributions, test durations, physiological parameters, and performances during cycling and CLIMIT ramp tests

Regarding energetic contributions in kJ and %, three energy systems (W_Oxi, W_Gly, and W_PCr) and total energy expenditure indicated significant differences between cycling and CLIMIT ramp tests (W_Oxi, P < 0.0001 and d: 1.17; W_Gly, P < 0.0001 and d: 0.33; W_PCr in kJ, P < 0.0001 and d: 0.35; W_Total, P < 0.0001 and d: 0.99; W_Oxi, P < 0.0001 and d: 1.31; W_Gly, P < 0.0001 and d: 1.36; W_PCr in %, P < 0.0001 and d: 0.89, respectively) (Fig. 5A-G). Figure 5H showed that the CLIMIT ramp test duration was significantly longer than the cycling ramp test duration (P < 0.0001 and d: 1.97). In addition, W_Total, W_Oxi, W_Gly, W_PCr, test time, absolute $\:\dot{\text{V}}$O_2peak, and relative $\:\dot{\text{V}}$O_2peak showed positive relationships in all measurements (n = 202) (W_Total vs. test time, r = 0.85, R² = 0.73, 95% CI = 0.81–0.89, and P < 0.0001; W_Oxi in kJ vs. test time, r = 0.86, R² = 0.75, 95% CI = 0.83–0.89, and P < 0.0001; W_Gly in kJ vs. test time, r = 0.28, R² = 0.08, 95% CI = 0.15–0.41, and P < 0.0001; W_PCr in kJ vs. test time, r = 0.59, R² = 0.35, 95% CI = 0.49–0.67, and P < 0.0001; absolute $\:\dot{\text{V}}$O_2peak vs. W_Oxi in kJ, r = 0.83, R² = 0.69, 95% CI = 0.78–0.87, and P < 0.0001; relative $\:\dot{\text{V}}$O_2peak vs. W_Oxi in kJ, r = 0.65, R² = 0.43, 95% CI = 0.57–0.72, and P < 0.0001) (Fig. 5I-N). Paired comparisons of absolute and relative $\:\dot{\text{V}}$O_2peak between both tests showed no significant differences (P > 0.05). Further paired differences in physiological parameters and external performances between cycling and CLIMIT tests are summarized in Table 3.

Fig. 5

Paired comparisons of energy system contributions, ramp test durations, and relationships among energy system contributions, $\:\dot{\text{V}}$O_2peak, and test durations between cycling and CLIMIT ramp tests in all measured data (n = 202). (A-D) Paired comparisons of absolute oxidative, glycolytic, phosphagen system contributions in kJ, and total energy expenditure between cycling and CLIMIT ramp tests. (E-G) Paired comparisons of relative oxidative, glycolytic, and phosphagen system contributions in % between cycling and CLIMIT ramp tests. (H) Paired comparison of cycling and CLIMIT test durations in minutes. (I-N) Relationships among energy system contributions, $\:\dot{\text{V}}$O_2peak, and test time. r, correlation coefficient; R², coefficient of determination; $\:\dot{\text{V}}$O_2peak, peak oxygen uptake; W_PCr, phosphagen contribution; W_Gly, glycolytic contribution; W_Oxi, oxidative contribution; W_Total, total energy expenditure. Significant difference = ****P < 0.0001.

Table 3 Absolute and relative $\:\dot{\text{V}}$O_2peak, physiological parameters, and external performances during cycling and CLIMIT ramp tests (n = 101).

Discussion

As an alternative exercise test and format of cardiovascular fitness in potential popularity, the present study evaluated the validity of $\:\dot{\text{V}}$O_2peak between a developed ramp protocol test on a similar stair whole-body climbing-based machine CLIMIT and a standardized ramp protocol on the cycling ergometer in physically active adults. To the best of our knowledge, this was the first study to validate $\:\dot{\text{V}}$O_2peak values using a ramp test protocol of stair whole-body climbing-based exercise.

Our main findings confirmed strong interclass correlations of absolute and relative $\:\dot{\text{V}}$O_2peak values between cycling and CLIMIT ramp tests (84% and 76%) without significant differences in $\:\dot{\text{V}}$O_2peak between tests. The measured $\:\dot{\text{V}}$O_2peak of the CLIMIT ramp test yielded a high and acceptable validity compared to the reached $\:\dot{\text{V}}$O_2peak of the standardized cycling ramp test. Bland-Altman plots of absolute and relative $\:\dot{\text{V}}$O_2peak also suggested moderate biases and percentage errors (< 30% acceptable³⁶, 20.5% for absolute $\:\dot{\text{V}}$O_2peak and 25.9% for relative $\:\dot{\text{V}}$O_2peak) (Fig. 2C, D). Such acceptable limits of $\:\dot{\text{V}}$O_2peak suggest that the CLIMIT test method could be equivalent to the referenced cycling test method^35,36.

Absolute and relative $\:\dot{\text{V}}$O₂ on CLIMIT during initial periods of the ramp test were significantly higher than those during the cycling ramp test (Fig. 3A, B). These results might be due to different skeletal muscle activation patterns between the two tests^12,37. During the CLIMIT test, movements of upper and lower extremities included small and large muscle groups, where involvement of the leg, back, and core muscles might amplify total muscle mass activation compared to initial phases of the cycling test^12,37. Accordingly, higher levels of RER during initial phases of the CLIMIT test than RER values during the cycling test support $\:\dot{\text{V}}$O₂ differences observed (Fig. 3C). In contrast, absolute and relative $\:\dot{\text{V}}$O₂ and RER data of the cycling test from the 6th minute to the end tended to be higher than those of the CLIMIT test. This tendency seemed to be caused by a greater activation of a large muscle mass in the lower extremity with increased exercise intensities during the cycling test^12,37. Furthermore, in the group with high absolute and relative $\:\dot{\text{V}}$O_2peak values were higher during the cycling test than those during the CLIMIT test. Participants of this group focused more on cycling exercise sessions, with six of these participants trained with load bicycles (range: 65.8–73.8 mL·kg^{– 1}·min^{– 1}). Their skeletal muscles in the lower extremity might have been optimized to perform during the cycling ramp test compared to the CLIMIT ramp test. Their RER levels were also higher than those in the CLIMIT test (Fig. 4B, J-K). On the other hand, in the group with low $\:\dot{\text{V}}$O_2peak, $\:\dot{\text{V}}$O_2peak levels during the CLIMIT test were higher than those during cycling. In the group with low $\:\dot{\text{V}}$O_2peak, $\:\dot{\text{V}}$O_2peak values showed the highest relationships between the two tests, although RER levels in middle and low groups during cycling were increased compared to those during CLIMIT test. In this regard, the abovementioned reasons (different activations and optimization of skeletal muscle mass) might also influence these outcomes^12,37.

Additionally, diverse physiological parameters, energy system contributions, and test durations showed high positive relationships between both ramp tests (Fig. 2E). However, paired comparisons of these parameters showed that absolute W_Oxi and W_PCr values during the CLIMIT test were increased compared to those during the cycling test while the absolute W_Gly value during the CLIMIT test was lower than that during the cycling test, which might be related to lower peak La⁻ and ΔLa⁻ in CLIMIT test (Table 3). Previous studies have reported that longer exercise duration can increase oxidative system contribution^{18,19,22,38,39}. Therefore, elevated W_Oxi as shown by HR_peak, HR_mean, and mean FAT_Ox and CHO_Ox could be explained by a longer CLIMIT test duration than the cycling test duration (Fig. 5; Table 3). With increased W_Oxi, values of W_Gly during the CLIMIT test seemed to be decreased because CLIMIT movements might affect adenosine triphosphate (ATP) re-synthesis from glycolysis-accumulated La⁻ via substrate-level/oxidative phosphorylation reactions such as increased elimination rate of La⁻ more than cycling movements^{6,18,19,21,22,28,39}. On the other hand, previous studies have suggested that smaller muscles such as movements of arms do not have a high impact on total lactate production or W_Gly^18,19. In contrast, the finding of a higher W_PCr value during the CLIMIT test than that during the cycling test agreed with previous studies^40,41. A higher aerobic contribution/performance might induce a higher phosphocreatine (PCr) re-synthesis during exercises, which might have influenced our results^40,41.

The relative value of W_PCr during the cycling test was higher than that during the CLIMIT test (Fig. 5G). This result might be due to a relatively higher value of W_Oxi during the CLIMIT test than that during the cycling test. Moreover, $\:\dot{\text{V}}$O₂ kinetics are influenced by the type and configuration of the ramp protocol⁴². Therefore, differences in workload may explain the longer exercise duration and lower glycolytic contribution observed during the CLIMIT ramp test. Additionally, the participants’ familiarization with CLIMIT may have affected their performance and physiological responses, despite a 10-minute warm-up on CLIMIT conducted before the test.

Further analyses of relationships among energetic contributions, test time, and absolute $\:\dot{\text{V}}$O_2peak of the two combined test data (n = 202) supported that W_Oxi values occupied the largest part of the total energy expenditure and $\:\dot{\text{V}}$O_2peak compared to W_Gly and W_PCr during both ramp tests. These results were associated with test duration (Fig. 5I-N).

In the present study, the developed ramp test protocol for attaining $\:\dot{\text{V}}$O_2peak and several physiological parameters using a similar stair climbing-based machine CLIMIT was acceptably validated as an alternative $\:\dot{\text{V}}$O_2peak test protocol to a standardized ramp test on a cycling ergometer. As another form of climbing-based exercise, stair climbing is an easy and accessible method to increase physical activity that can improve cardiovascular health and reduce the risk of cardiometabolic diseases^16,17. This finding leads to another indicator of cardiovascular fitness with a different exercise variation for providing physically active adults with individual climbing-specific exercise prescriptions.

This study has some limitations. Only one test for each exercise modality was performed because the focus of this study was on its validity as the first step. Therefore, it is difficult to interpret the variability within exercise modes. Additionally, the rate of increase in energy demand during the test protocols may not have been perfectly aligned. This is evident from the higher $\:\dot{\text{V}}$O₂ values observed in the CLIMIT test compared to the cycling test up to the fifth minute. Achieving a more precise alignment between the two protocols could enhance the reliability and comparability of the findings. As for future studies, the reliability of our developed and validated ramp protocol on CLIMIT should be determined based on reproducibility of $\:\dot{\text{V}}$O_2peak. It is also important to determine which muscles are predominantly activated during the CLIMIT ramp test using electromyography. Furthermore, this ramp protocol should be performed with higher performance levels in elite athletes such as climbers. Due to popularity of a similar stair climbing-based machine such as CLIMIT in the general population, creating an algorithm for determining $\:\dot{\text{V}}$O_2peak through multiple regression analyses on a stair climbing-based machine CLIMIT might be helpful if it is not possible to be available for expensive spiroergometry equipment.

Conclusion

Cycling and stair climbing-based CLIMIT ramp protocols demonstrated high validity of absolute and relative $\:\dot{\text{V}}$O_2peak as well as high interclass correlations, acceptable biases, and percentage errors, showing similar and acceptable $\:\dot{\text{V}}$O_2peak data, $\:\dot{\text{V}}$O_2peak-related physiological parameters (peak La^–, ∆La^–, RER, HR_peak, HR_mean, mean FAT_Ox, and mean CHO_Ox), and energy system contributions (W_Oxi, W_Gly, and W_PCr) in physically active adults. Therefore, stair climbing-based CLIMIT test protocol and $\:\dot{\text{V}}$O_2peak data might be useful for guiding another feasible way of cardiovascular/-metabolic fitness test as an indicator and reference to exercise prescriptions in physically active adults.