Speed and Grade Increment During Cardiopulmonary Treadmill Testing: Impact on Exercise Prescription

Abstract

Background:

Maximal oxygen uptake (VO₂max) and both first (VT₁) and second (VT₂) thresholds have been used as reference points for exercise prescription in different populations.

Objective:

We aimed to test the hypothesis that exercise prescription, based on VTs determined by treadmill cardiopulmonary exercise testing (CPET), is influenced by the rate of increase in treadmill workload.

Methods:

Nine healthy individuals underwent two CPETs, followed by two sessions of submaximal exercise, both in randomized order. For the “speed” protocol, there was an increment of 0.1 to 0.3 km.h^-1 every 15s. The “grade” incremental protocol increased 1% every 30s and 0.1 km.h^-1 every 45s. This was followed by submaximal exercise sessions lasting 40min at an intensity corresponding to heart rate (HR) between the VT₁ and VT₂.

Results:

The “speed” protocol resulted in higher VT₁ (p = 0.01) and VT₂ (p = 0.02) when compared to the “grade” incremental protocol, but there was no effect on VO₂max. The target HR for the submaximal exercise sessions was higher in the “speed” protocol compared to the “grade” incremental protocol (p < 0.01) and remained stable during the two steady-state exercise sessions. Blood lactate remained stable during the submaximal exercise sessions, with higher values observed during the “speed” protocol than those “grade” incremental protocol (p < 0.01).

Conclusions:

Compared to a grade-based protocol, a speed-based protocol resulted in higher VT₁ and VT₂, which significantly affected cardiorespiratory and metabolic responses to prescribed exercise intensity in healthy young adults.

Keywords:
Cardiovascular Diseases; Exercise Test; Exercise Tolerance; Oxygen Consunption

Introduction

Cardiopulmonary exercise testing (CPET) provides valuable diagnostic and prognostic information for healthy subjects and patients with cardiovascular disease^1,2 and has long been used in the assessment of athletic performance, as well as for various research applications.³ CPET responses have also been extensively used for the prescription of exercise intensity during aerobic training.⁴ In this context, maximal oxygen uptake (VO₂max) and, both first (VT₁) and second (VT₂) thresholds, have been used as reference points for exercise prescription among both athletes and patients with cardiovascular disease.^4-7

Tests based on ramp protocols are recommended for CPET because they generally provide a linear increase in VO₂ relative to workload, particularly when performed on the cycle ergometer.^8-11 The VT₁ and the VT₂ occur at similar VO₂, independent of the rate of increase in exercise intensity on the cycle ergometer^12,13 and the same is true for the determination of the VT₁ on the treadmill.¹⁴ Regardless of differences between cardiorespiratory fitness measured on a cycle ergometer and a treadmill, the choice of the ergometer may influence VT₁ determination.^14,15

In the Americas, the treadmill is the exercise mode of choice in clinical settings.^1,2,16 When using the treadmill, it can be difficult to achieve a linear response in metabolic rate because of the walk-run transition,¹⁷ the rate of increase in speed and grade,^18-20 or handrail support and its effect on economy.¹⁴ Moreover, there is little information available on the effects of different treadmill ramp protocol increments on the detection of the VT₁ and VT₂.^14,15

Changes in speed and/or grade may be used to develop an appropriate treadmill ramp protocol in efforts to make the work rate increments as linear as possible.²⁰ In this regard, work rate increments can have notable effects on the response to exercise due to the disproportionate interaction between muscle activation,^21-23 kinematic variables related to gait, and oxygen uptake (VO₂) kinetics;²⁴ and each of these factors can significantly influence VO₂ during exercise. Moreover, the impact of the type of increment during CPET performed on a treadmill with regard to exercise prescription has not been previously studied. Therefore, this study was conducted to compare the effects of two treadmill ramp protocols on the detection of VT₁ and VT₂. We applied one protocol mainly using speed increments and another using mainly grade increments. In addition, we evaluated the steady-state response to exercise prescription based on the measured ventilatory thresholds from the two protocols.

Material and methods

Participants

Four male and five female subjects, aged 29 ± 6 years [95% CI = 25; 33], height 170 ± 8 cm [95% CI = 165; 175], and weight 65 ± 8 kg [95% CI = 60; 71], participated in the study. All subjects were active and otherwise healthy as determined by medical history, physical examination, and resting and exercise electrocardiograms. None were taking medications. The subjects did not vary their activity levels during the testing period. All rights and privileges were honored in accordance with an established human subject’s protocol, and informed consent was obtained. The ethics committee of the institution approved the protocol.

Protocol

The protocol included two maximal incremental CPETs and two submaximal exercise sessions, performed on different days; the type of increment was chosen in random order. All tests were performed in a comfortable laboratory environment, with a minimum of 48 hours between tests. The CPET system underwent gas and volume calibration before each exercise test. No handrail support was allowed during the tests.³ The randomization of protocols was performed by an independent researcher using the software Rx 64 version 13. The protocol randomized for the first incremental CPET was the same for the first submaximal exercise session.

Incremental cardiopulmonary exercise tests

Two incremental protocols were used for determination of VT₁, VT₂, and VO₂max. The subjects were positioned on the treadmill (Inbramed, TK10200, Porto Alegre, Brazil) and initially walked 2.0 km.h^-1 and 1% grade for 2 min. The speed protocol then increased to 5.5 km.h^-1 and 1% of grade and increments of 0.1 to 0.3 km.h^-1 were added every 15s (Figure 1A), with a constant grade (Figure 1C). If the maximal speed of the treadmill (16 km/h) was attained, exercise intensity was further increased by grade increments of 0.5% per 30s. The grade protocol started at 5.5 km.h^-1 and 1% of grade, with increments of 0.1 km.h^-1 every 45s (Figure 1B) and 1% increases in grade every 30s (Figure 1D). Subjects exercised until volitional fatigue. During recovery from the incremental tests, subjects walked on the treadmill at 2 km.h^-1 for 7 min. Fingertip blood samples were collected at 1, 3, 5, and 7 min for the determination of maximal blood lactate during recovery.

Figure 1
Increments in speed and grade during the speed and grade protocols.

Cardiorespiratory variables

Heart Rate (HR) was determinate based on the R-R intervals from a twelve-lead electrocardiogram (Micromed-Biotecnologia, Brasília, Brazil). Perceived exertion using the 0 to 10 Borg scale²⁵ was obtained every 2 min. Gas exchange variables were measured breath-by-breath by a validated system (Metalyzer 3B, CPET System, Cortex, Leipzig, Germany) and expressed in 20s intervals.²⁶ VO₂max was defined as the highest value measured for a period of 20s during the CPET. VT₁ and VT₂ were determined by visual inspection. VT₁ was identified as the VO₂ or HR immediately before a systematic increase in the ventilatory equivalent for oxygen (minute ventilation [VE] / VO₂), without an increase in the ventilatory equivalent for carbon dioxide (VE / carbon dioxide output [ECO₂]).^3,27 The V-slope method was also used to confirm the VT₁. VT₂ was identified as the point immediately before a systematic increase in VE/VECO₂, usually at the same time that the end-tidal CO₂ decreased systematically.³ All ventilatory thresholds and VO₂max evaluations were determined by the same experienced researcher, who was blinded to the protocols.

Exercise prescription

The exercise intensity during the submaximal exercise sessions was based on the HR corresponding to the mean point between VT₁ and VT₂, obtained from the speed and grade protocols. During submaximal exercise sessions, the subjects underwent 10 min of walking-running in order to reach the target HR, followed by 30 min running at target HR. The treadmill was maintained at a constant level during the submaximal exercise sessions and speed was adjusted to maintain a stable HR. Rather than applying a constant workload, we chose to maintain a stable HR because the individuals exercised at intensities above the VT1, where the steady state is not established with a constant work rate.^13,28 Moreover, the target HR is frequently used to monitor aerobic training exercise sessions in practice.⁴ During the submaximal exercise sessions, HR and gas exchange responses were continuously monitored as described above. Perceived exertion and blood lactate samples were obtained at rest and every 10 min.

Blood lactate analysis

Twenty-five µL fingertip blood samples were mixed with 50µL of 1% sodium fluoride. This solution was then frozen for later analysis of blood lactate concentration using a dedicated analyzer (YSI 1500-L Sport, Yellow Springs, Ohio, USA).

Statistical analysis

Based on a previous study,²⁹ a minimum sample size was estimated to be 7 subjects, using a power of 90% and an alpha of 0.05 to detect a 10% difference (16 bpm) in the prescribed HR between protocols. Two subjects were added to the sample to account for dropouts. We used the Kolmogorov-Smirnov test to assess the normality of variables. Descriptive data are presented as mean (M) and standard deviations (SD) and 95% confidence interval (95% CI). To evaluate intra-observer reproducibility in the detection of VT₁, VT₂, and VO₂max, gas exchange curves of all tests were reviewed by the same blinded investigator twice within a one-week interval. Paired t tests, Pearson’s correlation coefficients, and Bland-Altman analyses³⁰ were used to assess intra-observer reproducibility. The responses to incremental and submaximal exercise tests were compared using paired t tests for two means and by generalized estimating equations for three or more means. When appropriate, multiple comparisons were evaluated using Bonferroni correction. The calculations for sample size, planning for randomization and Bland-Altman analyses were performed using R 3.0 (Free Software Foundation’s GNU Project), and all other analyses were performed using SPSS 21.0 software (IBM, New York, USA).

Results

All subjects completed the incremental exercise tests and the exercise sessions without complications. For the intra-observer reproducibility analysis, there were no significant differences in VO₂ (ml.kg^-1.min^-1) between the two evaluations in terms of detection of VT₁ (“speed”: 27.5 ± 7.4 [95% CI = 21.8; 33.2] vs 26.4 ± 7.2 [95% CI = 20.9; 31.9], p = 0.11; “grade”: 24.9 ± 6.1 [95% CI = 20.2; 29.6] vs 25.9 ± 6.2 [95% CI = 21.1; 30.7], p = 0.16); VT₂ (“speed”: 32.4 ± 9.2 [95% CI = 25.3; 39.5] vs 32.5 ± 9.2 [95% CI = 25.4; 39.6], p = 0.85; “grade”: 29.7 ± 6.5 [95% CI = 24.6; 34.7] vs 30.9 ± 6.9 [95% CI = 25.6; 36.2], p = 0.08), and VO₂max (“speed’: 35.8 ± 10.8 [95% CI = 27.5; 44.2] vs 35.8 ± 10.8 [95% CI = 27.2; 44.2], p = 0.99; “grade”: 34.7 ± 9.7 [95% CI = 27.2; 42.1] vs 34.7 ± 9.8 [95% CI = 27.2; 42.1], p = 0.99). Intra-observer agreement results are presented in Figure 2. The two protocols exhibited strong correlation coefficients between the first and second evaluations, varying from 0.95 to 1.00 (1^st and 3^rd columns of Figure 2). Likewise, Bland-Altman plots demonstrated values within the acceptable limits of agreement between the first and second evaluations (2^nd and 4^th columns of Figure 2).

Figure 2
Validity testing of the first and second evaluations of ventilatory thresholds and maximal oxygen uptake. The first and third columns of the graphs show scatter plots and correlation coefficients. The diagonal lines represent the identity lines. Graphs on the second and fourth columns are Bland-Altman plots. The horizontal lines represent upper limit, bias and lower limit for difference between the two evaluations. CPET: cardiopulmonary exercise test.

Table 1 shows the incremental exercise test results performed according to the speed and grade protocol. Resting HR and blood lactate were similar between the protocols. At peak exercise, HR was higher with the speed protocol, while no differences were observed between VO₂max, VCO₂max, and VEmax. Peak respiratory exchange ratio was lower with the speed protocol, as well as perceived leg exertion. Maximal blood lactate concentrations and time to maximal blood lactate concentration during recovery were similar with the two protocols.

Table 1
Ergometric data, cardiorespiratory, and metabolic results at rest and incremental exercise test with the speed and grade protocol

	Speed (n = 9) M ± SD (95% CI)	Grade (n = 9) M ± SD (95% CI)
Rest
Heart rate (beats.min-1)	72 ± 12 (65; 79)	73 ± 12 (66; 81)
Blood lactate (mmol.L-1)	1.79 ± 0.62 (1.41; 2.17)	1.69 ± 0.59 (1.33; 2.06)
Peak exercise
Time (s)	694 ± 206 (574; 821)	568 ± 151 (483; 662)*
Speed (km.h-1)	13.5 ± 2.5 (12.0; 14.9)	6.2 ± 0.3 (6.0; 6.4)*
Grade (%)	2.6 ± 2.6 (1.2; 4.3)	19.7 ± 4.9 (16.8; 22.7)*
Heart rate (beats.min-1)	189 ± 6 (184; 193)	183 ± 7 (179; 187)*
Oxygen uptake (ml.kg-1.min-1)	35.8 ± 10.8 (29.7; 43.2)	34.7 ± 9.7 (28.9; 41.0)
Carbon dioxide output (L.min-1)	2.69 ± 1.0 (2.05; 3.28)	2.88 ± 1.16 (2.16; 3.54)
Minute ventilation (L.min-1)	102 ± 33 (81; 122)	106 ± 34 (84; 127)
Respiratory exchange ratio	1.14 ± 0.11 (1.06; 1.20)	1.25 ± 0.11 (1.17; 1.31)
Perceived exertion
Respiratory	9 ± 2 (8; 10)	9 ± 1 (8-; 10)
Legs	8 ± 2 (7; 10)	9 ± 1 (9; 10)*
Maximal blood lactate during recovery
Blood lactate (mmol.L-1)	9.07 ± 2.29 (7.30; 10.83)	9.95 ± 2.25 (8.21; 11.68)
Time to maximal (min)	2 ± 2 (0.5; 3)	3 ± 2 (1; 5)

• M: mean; SD: standard deviation; 95%CI: 95% confidence interval.
• ^*
  p < 0.01.

Figure 3 shows VO₂ and HR responses to incremental exercise according to VT₁, VT₂, the mean point between VT₁ and VT₂, and VO₂max. VO₂ at these 4 intensities was higher with the speed protocol (Figure 3A). When expressed as a percentage of VO₂max, these differences were also statistically significant (Intensity: P < 0.01; Protocol: P = 0.01; Interaction: P = 0.18). The “speed” protocol resulted in higher HR (bpm) for the VT1 (153 ± 14 [95% CI = 146; 168] vs 144 ± 8 [95% CI = 137; 150], p < 0.01), the VT₂ (176 ± 7 [95% CI = 171; 182] vs 165 ± 8 [95% CI = 159; 171], p < 0.01), the mean point between VT₁ and VT₂ (169 ± 9 [95% CI = 162; 176] vs 156 ± 8 [95% CI = 150; 162], p < 0.01), and peak exercise (189 ± 8 [95% CI = 183; 195] vs 183 ± 7 [95% CI = 178; 188], p < 0.01) (Figure 3B). The responses were also significantly different when HR was analyzed as a percentage of the peak (Intensity: p < 0.01; Protocol: p < 0.01; Interaction: p = 0.01).

Figure 3
Oxygen consumption and heart rate responses at four points during the maximal exercise tests with the speed and grade protocols. Data are expressed as mean +/- SE. VT₁: first ventilatory threshold; VT₂: second ventilatory threshold. * p < 0.01 between protocols.

The submaximal exercise sessions were analyzed from 10 to 40 min, corresponding to the steady-state phase of exercise (Figure 4). Similarly to the comparison of protocols, subjects exhibited stable HR responses (Figure 4A), with higher levels observed during the session based on the speed protocol. Blood lactate concentrations (Figure 4B) were stable after 20 min, with higher concentrations observed during the speed protocol. To maintain a stable HR, speed was progressively reduced (Figure 4C), resulting in a reduction of VO₂ (Figure 4D). VE/VO₂ increased progressively (Figure 4E), but there were no significant differences between protocols. The respiratory exchange ratio decreased progressively (Figure 4F), with significant differences between the protocols. Perceived rates of respiratory (Figure 4G) and leg (Figure 4H) exertion were significantly higher during the sessions based on the speed protocol.

Figure 4
Ventilatory and metabolic results from the prescribed submaximal exercise tests according to the speed and grade protocols. Darkened circles: exercise prescription based on the speed protocol; open circles: exercise prescription based on the grade protocol. All p values were obtained from generalized estimating equations. VE/VO₂: ventilatory equivalent for oxygen. * p < 0.05 between protocols.

Discussion

The major finding of the present study was that a treadmill protocol based mainly on speed increments resulted in higher VO₂ and HR corresponding to the VT₁ and VT₂ when compared to a treadmill protocol based mainly on grade increments. Moreover, the choice of the protocol had a significant impact on exercise prescription based on ventilatory thresholds. To our knowledge, this is the first report describing the impact of the type of increment during CPET performed on the treadmill on aerobic exercise prescriptions.

Previous studies have shown that, when the cycle ergometer is used for the detection of ventilatory thresholds, the results are independent of the rate of increment in power output.^9,31,32 The detection of VT₁ is also not affected by the rate of increment in exercise intensity on the treadmill.¹⁴ Despite the fact that the treadmill is the ergometer of choice in many clinical settings, no study has previously evaluated the impact of changing grade versus speed on the detection of ventilatory thresholds using the treadmill. Kinderman et al.,¹⁵ evaluated the influence of different incremental treadmill protocols on the detection of the 4 mmol.L^-1 lactate threshold. In agreement with what had been shown for the cycle ergometer,⁹ when fixed, absolute blood lactate concentrations are used to detect thresholds, and the results were dependent on the protocol used. In the present study, VO₂ values at the ventilatory thresholds were ~7% higher on the speed-based protocol when compared to the grade-based protocol. Likewise, HR at the ventilatory thresholds was ~8% higher on the speed-based protocol when compared to the grade-based protocol. The mechanisms by which a speed-based protocol results in higher ventilatory thresholds when compared to a grade-based protocol are not readily apparent from our data. Based on the findings of Kelsey & Duffin,¹⁹ in which greater ventilatory responses to speed than grade increments were observed on the treadmill for the same VO₂, one would expect lower ventilatory thresholds with the speed protocol, assuming that greater limb movement frequency would be responsible for our findings. An additional potential explanation for the lower ventilatory thresholds for the grade-based protocol is activation of a larger muscle mass with increments in grade.²³ During uphill (+10%) running, the volume of activated limb muscles increases from 67% to 73%, and this is associated with greater oxygen deficit.²¹ Moreover, uphill running is associated with greater glycogen depletion in the lower extremities.³³ Therefore, the grade-based protocol used in the present study probably resulted in the activation of a larger muscle mass, greater glycogen utilization, and earlier blood lactate accumulation, resulting in a lower VO₂ at the ventilatory thresholds.

Some investigators have suggested that the prescription of exercise intensities for aerobic training using ventilatory (or blood lactate) thresholds as the reference is more physiologically sound than using a percentage of VO₂max or a percentage of maximal HR.^4,6,34 Despite the fact that there are few data from controlled studies to support this strategy,³⁵ the concept that individuals with different ventilatory thresholds may exhibit different metabolic and cardiorespiratory responses to exercise at a given percentage of VO₂max is well established.^4,6,13,34 Therefore, we compared metabolic and cardiorespiratory responses during exercise sessions with the intensities determined on CPET based on either speed or grade increments. As is commonly done in practice, exercise intensity was adjusted according to the HR corresponding to the mean point between VT₁ and VT₂. Our data show that the choice of protocol has a significant impact on the exercise intensity prescribed for aerobic training based on the determination of ventilatory thresholds. The speed-based protocol resulted in a higher HR, blood lactate, speed, and perceived exertion during the exercise sessions. The size of this difference (~8% for HR and ~2 mmol.L^-1 for blood lactate) is substantial. If, for instance, the exercise prescription were to be set at the HR corresponding to VT₂, the steady state blood lactate concentration would be reached using a grade-based protocol, but blood lactate would likely accumulate if a speed-based protocol was used.¹³

Our study has several limitations. Ventilatory thresholds were visually determined by one experienced investigator (JPR) blinded to the identity of the subjects and the utilized protocol. Despite the fact that we did not evaluate the inter-observer agreement for the detection of thresholds, the intra-observer agreement was appropriate,^11,27 as demonstrated by similar mean values, high correlation coefficients, and Bland-Altman analyses within acceptable limits. Moreover, the standard criteria used in clinical practice, including ventilatory equivalents, V-slope, and the end-tidal CO₂, were applied.^3,36 With this approach, the reproducibility for the detection of the ventilatory threshold was in agreement with previous studies.^31,36-38 Furthermore, to avoid the walking-running transition, which affects the linearity of VO₂ response, both protocols began at 5.5 km/h, a speed at which all subjects were jogging. Perhaps, other faster transition speeds might produce different results, which in fact may be the subject of a future experiment. Therefore, our findings cannot be extrapolated to protocols in which individuals do not run. Finally, our findings are limited to healthy young adults, and therefore may not be applicable to elderly individuals, children or those with pathological conditions.

Conclusion

A speed-based protocol results in higher ventilatory thresholds when compared to a grade-based protocol during CPET performed on a treadmill. These findings have a significant impact on cardiorespiratory and metabolic responses to prescribed exercise intensity in healthy subjects. Due to fact that exercise prescription based on CPET often requires a high degree of confidence and safety, it is necessary to keep in mind that the same protocol must be utilized when the subject is re-tested during clinical practice. Finally, the speed-based protocol was more convenient because it was more applicable for exercise prescription.

• Sources of Funding
  There were no external funding sources for this study.
• Study Association
  This article is part of the thesis of doctoral submitted by Karlyse C. Belli, from Universidade Federal do Rio Grande do Sul.
• Ethics approval and consent to participate
  This article does not contain any studies with human participants or animals performed by any of the authors.

Acknowledgements

This work was supported by the Hospital de Clínicas de Porto Alegre Fund for the Incentive of Research (FIPE/HCPA) from Porto Alegre, Brazil; the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brasília, Brazil; and the Brazilian Research Council (CNPq) Brasília, Brazil.

References

• ¹
  Guazzi M, Bandera F, Ozemek C, Systrom D, Arena R. Cardiopulmonary exercise testing: what is its value? J Am Coll Cardiol. 2017;70(13):1618-36.
• ²
  Corrà U, Agostoni PG, Anker SD, Coats AJS, Crespo Leiro MG, de Boer RA, et al. Role of cardiopulmonary exercise testing in clinical stratification in heart failure. A position paper from the Committee on Exercise Physiology and Training of the Heart Failure Association of the European Society of Cardiology. Eur J Heart Fail. 2018;20(1):3-15.
• ³
  Balady GJ, Arena R, Sietsema K, Myers J, Coke L, Fletcher GF, et al. Clinician&apos;s Guide to cardiopulmonary exercise testing in adults: a scientific statement from the American Heart Association. Circulation. 2010;122(2):191-225.
• ⁴
  Carvalho VO, Mezzani A. Aerobic exercise training intensity in patients with chronic heart failure: principles of assessment and prescription. Eur J Cardiovasc Prev Rehabil. 2011;18(1):5-14.
• ⁵
  Herdy AH, Ritt LE, Stein R, Araújo CG, Milani M, Meneghelo RS, et al. Cardiopulmonary exercise test: background, applicability and interpretation. Arq Bras Cardiol. 2016;107(5):467-81.
• ⁶
  Meyer T, Lucia A, Earnest CP, Kindermann W. A conceptual framework for performance diagnosis and training prescription from submaximal gas exchange parameters - theory and application. Int J Sports Med. 2005;26(Suppl 1):S38-48.
• ⁷
  Garber CE, Blissmer B, Deschenes MR, Franklin BA, Lamonte MJ, Lee IM, et al. American College of Sports Medicine position stand. Quantity and quality of exercise for developing and maintaining cardiorespiratory, musculoskeletal, and neuromotor fitness in apparently healthy adults: guidance for prescribing exercise. Med Sci Sports Exerc. 2011;43(7):1334-59.
• ⁸
  Zuniga JM, Housh TJ, Camic CL, Bergstrom HC, Traylor DA, Schmidt RJ, et al. Metabolic parameters for ramp versus step incremental cycle ergometer tests. Appl Physiol Nutr Metab. 2012;37(6):1110-7.
• ⁹
  Beltz NM, Gibson AL, Janot JM, Kravitz L, Mermier CM, Dalleck LC. Graded exercise testing protocols for the determination of VO2max: historical perspectives, progress, and future considerations. J Sports Med. 2016;2016:3968393.
• ¹⁰
  Myers J, Buchanan N, Walsh D, Kraemer M, McAuley P, Hamilton-Wessler M, et al. Comparison of the ramp versus standard exercise protocols. J Am Coll Cardiol. 1991;17(6):1334-42.
• ¹¹
  Hansen D, Dendale P, Berger J, Meeusen R. Low agreement of ventilatory threshold between training modes in cardiac patients. Eur J Appl Physiol. 2007;101(5):547-54.
• ¹²
  Davis JA, Whipp BJ, Lamarra N, Huntsman DJ, Frank MH, Wasserman K. Effect of ramp slope on determination of aerobic parameters from the ramp exercise test. Med Sci Sports Exerc. 1982;14(5):339-43.
• ¹³
  Loat CE, Rhodes EC. Relationship between the lactate and ventilatory thresholds during prolonged exercise. Sports Med. 1993;15(2):104-15.
• ¹⁴
  Buchfuhrer MJ, Hansen JE, Robinson TE, Sue DY, Wasserman K, Whipp BJ. Optimizing the exercise protocol for cardiopulmonary assessment. J Appl Physiol Respir Environ Exerc Physiol. 1983;55(5):1558-64.
• ¹⁵
  Kindermann W, Schrmamm M, Keul J. Aerobic performance diagnostics with different experimental settings. Int. J. Sports Med. 1980;1(3):110-4.
• ¹⁶
  Myers J, Voodi L, Umann T, Froelicher VF. A survey of exercise testing: methods, utilization, interpretation, and safety in the VAHCS. J Cardiopulm Rehabil. 2000;20(4):251-8.
• ¹⁷
  Saibene F, Minetti AE. Biomechanical and physiological aspects of legged locomotion in humans. Eur J Appl Physiol. 2003;88(4-5):297-316.
• ¹⁸
  Silva OB, Sobral Filho DC. A new proposal to guide velocity and inclination in the ramp protocol for the treadmill ergometer. Arq Bras Cardiol. 2003;81(1):48-53.
• ¹⁹
  Kelsey CJ, Duffin J. Changes in ventilation in response to ramp changes in treadmill exercise load. Eur J Appl Physiol Occup Physiol. 1992;65(5):480-4.
• ²⁰
  Porszasz J, Casaburi R, Somfay A, Woodhouse LJ, Whipp BJ. A treadmill ramp protocol using simultaneous changes in speed and grade. Med Sci Sports Exerc. 2003;35(9):1596-603.
• ²¹
  Cai ZY, Hsu CC, Su CP, Lin CF, Lin YA, Lin CL, et al. Comparison of lower limb muscle activation during downhill, level and uphill running. Isokinet Exerc Sci. 2010;18(3):163-8.
• ²²
  Vernillo G, Giandolini M, Edwards WB, Morin JB, Samozino P, Horvais N, et al. Biomechanics and physiology of uphill and downhill running. Sports Med. 2017;47(4):615-29.
• ²³
  Wall-Scheffler CM, Chumanov E, Steudel-Numbers K, Heiderscheit B. Electromyography activity across gait and incline: the impact of muscular activity on human morphology. Am J Phys Anthropol. 2010;143(4):601-11.
• ²⁴
  Swanson SC, Caldwell GE. An integrated biomechanical analysis of high speed incline and level treadmill running. Med Sci Sports Exerc. 2000;32(6):1146-55.
• ²⁵
  Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports Exerc. 1982;14(5):377-81.
• ²⁶
  Meyer T, Georg T, Becker C, Kindermann W. Reliability of gas exchange measurements from two different spiroergometry systems. Int J Sports Med. 2001;22(8):593-7.
• ²⁷
  Svedahl K, MacIntosh BR. Anaerobic threshold: the concept and methods of measurement. Can J Appl Physiol. 2003;28(2):299-323.
• ²⁸
  Roseguini BT, Narro F, Oliveira AR, Ribeiro JP. Estimation of the lactate threshold from heart rate response to submaximal exercise: the pulse deficit. Int J Sports Med. 2007;28(6):463-9.
• ²⁹
  Rusko H, Luhtanen P, Rahkila P, Viitasalo J, Rehunen S, Härkönen M. Muscle metabolism, blood lactate and oxygen uptake in steady state exercise at aerobic and anaerobic thresholds. Eur J Appl Physiol Occup Physiol. 1986;55(2):181-6.
• ³⁰
  Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res. 1999;8(2):135-60.
• ³¹
  Støren Ø, Rønnestad BR, Sunde A, Hansen J, Ellefsen S, Helgerud J. A time-saving method to assess power output at lactate threshold in well-trained and elite cyclists. J Strength Cond Res. 2014;28(3):622-9.
• ³²
  Jamnick NA, Botella J, Pyne DB, Bishop DJ. Manipulating graded exercise test variables affects the validity of the lactate threshold and VO2peak. PLoS One. 2018;13(7):e0199794.
• ³³
  Nielsen J, Holmberg HC, Schrøder HD, Saltin B, Ortenblad N. Human skeletal muscle glycogen utilization in exhaustive exercise: role of subcellular localization and fibre type. J Physiol. 2011;589(Pt 11):2871-85.
• ³⁴
  Mann T, Lamberts RP, Lambert MI. Methods of prescribing relative exercise intensity: physiological and practical considerations. Sports Med. 2013;43(7):613-25.
• ³⁵
  Londeree BR. Effect of training on lactate/ventilatory thresholds: a meta-analysis. Med Sci Sports Exerc. 1997;29(6):837-43.
• ³⁶
  Gaskill SE, Ruby BC, Walker AJ, Sanchez OA, Serfass RC, Leon AS. Validity and reliability of combining three methods to determine ventilatory threshold. Med Sci Sports Exerc. 2001;33(11):1841-8.
• ³⁷
  Aunola S, Rusko H. Reproducibility of aerobic and anaerobic thresholds in 20-50 year old men. Eur J Appl Physiol Occup Physiol. 1984;53(3):260-6.
• ³⁸
  Hunt KJ, Anandakumaran P, Loretz JA, Saengsuwan J. A new method for self-paced peak performance testing on a treadmill. Clin Physiol Funct Imaging. 2018;38(1):108-17.

Publication Dates

• Publication in this collection
  12 Aug 2019
• Date of issue
  Jul-Aug 2019

History

• Received
  01 Apr 2019
• Reviewed
  06 May 2019
• Accepted
  22 May 2019