Cardiopulmonary exercise testing (CPET) provides valuable diagnostic and prognostic information for healthy subjects and patients with cardiovascular disease1^,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 ergometer12^,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.

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

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.

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

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

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.

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.