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Original Research

Identification of a o2 Deflection Point Coinciding With the Heart Rate Deflection Point and Ventilatory Threshold in Cycling

Grazzi, Giovanni; Mazzoni, Gianni; Casoni, Ilario; Uliari, Simone; Collini, Gabriella; Heide, Larja van der; Conconi, Francesco

Author Information
Journal of Strength and Conditioning Research: July 2008 - Volume 22 - Issue 4 - p 1116-1123
doi: 10.1519/JSC.0b013e318173936c
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Abstract

Introduction

In 1982 the authors described an incremental test for determining the speed-heart rate (HR) relationship in running (13). The relationship was linear up to submaximal speeds and curvilinear in the final phases of the test, when HR increments were no longer directly proportional to speed increments and thereby caused a HR deflection. The HR deflection point coincided with the start of a sharp accumulation of blood lactate (13). The same relationship between work rate (WR) and HR was subsequently found in incremental cycling tests (8,14,22,24).

The hypothesis of this study was that because HR increases linearly with o2 during incremental exercise (40), the WR-o2 relationship should be similar to the WR-HR relationship. Therefore, the first purpose of this study was to compare the WR-o2 relationship with the WR-HR relationship in an incremental cycling test and to verify the possible occurrence of a o2 deflection point (o2def) analogous to the HR deflection point (HRdef) demonstrated in several other studies (8-10,13,14,22-25,30).

During incremental exercise, the work intensity above which adenosine triphosphate (ATP) aerobic synthesis is supplemented by anaerobic glycolysis is referred to as the anaerobic threshold, a term coined by Wasserman and McIlroy in 1964 (41). The anaerobic threshold is commonly assessed during incremental exercise as either lactate threshold or ventilatory anaerobic threshold (VT).

The second purpose of this investigation was to determine whether o2def, if present, coincides with VT and, therefore, with the activation of the anaerobic mechanisms.

Anaerobic threshold determination can be used to prescribe optimal training intensity (6). The demonstration of a coincidence of HRdef with o2def and of o2def with VT would further emphasize that the simple determination of the WR-HR relationship provides information useful for training.

Methods

Experimental Approach to the Problem

The WR-o2, WR-HR, and o2 -co2 relationships were determined following a test protocol previously described by the authors (14, 22). This protocol follows the physiological principle that humans and other mammals increase work intensity primarily by increasing the frequency of an action (11,21,26,29). The test protocol for cycling is based, therefore, on increments in pedalling cadence.

Subjects

This study was based on the data collected in 24 world-class professional cyclists referred to the authors' research center for testing. Among them were winners of international 1-day races (e.g., World Cup) and of individual stages of major 3-week races (e.g., Giro d'Italia, Tour de France, and Vuelta a España). All subjects had been declared eligible for professional cycling competition following medical examination carried out elsewhere and were examined after having given written informed consent. The testing protocol was approved by the Institutional Review Board of the University of Ferrara. The subjects' characteristics (mean ± SD) were 26 ± 3.2 years in age, 179 ± 5 cm in height, and 70.3 ± 6.0 kg in weight. Each subject was well rested and had not performed any races or hard training sessions within 48 hours of testing. Tests were carried out during the racing season in the intervals between races and were performed 2 to 3 hours after breakfast. None of the subjects consumed any drugs that could influence the HR, including caffeine. All tests were performed under similar environmental conditions (room temperature, 21-22°C; humidity, 45-55%).

Procedures

Wind-Load Simulator.

The subject's bicycle was mounted on the wind-load simulator previously described (22). The rear tubular tire was inflated to a pressure of 8 atm (i.e., 117.6 psi) and put in contact with a freely rotating axle with 2 fans attached to either end. The axle of the wind-load simulator could be moved in a horizontal direction with a micrometric screw and pressed against the tire to standardize rolling resistance. Before each test, the pressure of the roller against the tire was regulated to obtain a resistance of 100 W at 60 rpm, with a gear ratio of 52 × 15.

Power Output, Cadence, and Heart Rate-Measuring Apparatus.

A crank set with a built-in power measuring system (SRM Training System; Ingenierbüro Schoberer, Jülich-Welldorf, Germany) was mounted on the athlete's bicycle. This instrument measures torque and angular velocity. The validity of this power measuring system has been verified (36). The average power output and cadence of every pedalling revolution are transmitted and stored in a handlebar-mounted computer, which also receives the heartbeat signals from a pulse transmitter (Polar Electro, Kempele, Finland) and stores the cyclist's HR values.

Equipment for Respired Gas Analysis.

A breath-by-breath mass spectrometer (AMIS 2000; Innovision A/S, Odense, Denmark) was employed for gas analysis and calibrated before each test. The subjects breathed through a Rudolph face mask. Gas flows were measured by a pneumotachometer, and O2 and CO2 concentrations were determined continuously. o2 and co2 were calculated and stored by the spectrometer for subsequent analysis.

Warm-Up Procedure.

The warm-up was carried out on the athlete's bicycle, mounted on the wind-load simulator. After 15 minutes of easy cycling at a WR varying from 100 to 150 W, a cadence-guided incremental warm-up was started. Cadence was increased from 60 to 90 or 95 rpm, with increments of 1 rpm every 30 seconds. The warm-up was concluded by 5 to 10 minutes of easy cycling followed by 3 bouts at near-maximal WR for 10 seconds, with 1 minute of recovery at lower revolutions per minute in between. This prolonged warm-up allows the athlete to reach his or her maximal HR (14,22) in the subsequent test and is the warm-up often utilized by professional cyclists prior to time-trial races. The face mask for gas analysis was then applied, and the test started after a few additional minutes of easy cycling.

Testing Procedure.

All subjects were acquainted with the testing procedure by having followed the same incremental protocol for determining the WR-HR relationship (22) on several occasions during previous months. The gear ratios used in this study (52 × 15 in 6 subjects and 52 × 14 in the other 18 subjects) were those utilized by the athletes in previous testing sessions.

Tests were started at a cadence of 60 rpm and a WR of 100 or 120 W (gear ratio of 52 × 15 or 52 × 14, respectively). Cadence was increased by 1 rpm every 30 seconds. The cadence to be maintained at any given time as well as the one kept by the athlete were displayed on a monitor. Regular increments in cadence were made until the exertion perceived by the subject was close to maximum. As detailed in an article on test methodology (14), the athlete was instructed as to when to begin the final acceleration, that is, when he felt a burning sensation in his muscles and started breathing heavily, phenomena that occur after having reached anaerobic threshold (18). In the final acceleration, the cadence was increased progressively up to the maximal exercise intensity.

Data Analysis

The WR, HR, cadence, o2, and co2 data collected during the test were downloaded to a personal computer. These data were averaged for 5-second intervals and smoothed with a width of 15 seconds.

Determination of o2def.

o2def was determined by using the following mathematical analysis of the WR-HR relationship: identification of the point where the slope of the WR-o2 relationship decreased, which allowed division of the data points into 2 segments; and determination of the best regression equations obtained by moving, in both directions, the limit dividing the 2 segments. o2def was identified at the conjunction of the lines best fitting the data of the 2 segments.

Analysis of the WR-HR relationship and identification of HRdef were carried out as for the WR-o2 relationship.

Ventilatory Threshold.

This determination was made by using the V-slope method described by Beaver et al. (4).

Statistical Analyses

Data are presented as mean ± SD. The data obtained at o2def, HRdef, start of final acceleration (SFA), and maximum WR were compared by a t-test analysis. A t-test analysis was also used to compare the WR-to-o2 ratios below and above o2def, to compare the slope of the lower and upper lines of the o2-co2 relationships for the determination of the VT, and to compare the values of o2 at VT and at o2def. The association between the WRs at HRdef and at o2def and between the o2 values at VT and at o2def was evaluated by interclass correlation coefficient determination, by Passing and Bablok linear regression analysis, and by Bland and Altman analysis (7).

Statistical analyses were performed with the MedCalc statistical package 7.3.0 (MedCalc Software, Mariakerke, Belgium).

Results

Preliminary data analysis showed that the tests performed by the 24 cyclists complied with the criteria of acceptability previously reported (14). In particular, HR increments were below 8 b·min−1·min−1 (3.9 ± 0.6 b·min−1·min−1); the HR deflection was identifiable by visual inspection; the straight section of the WR-HR relationship had an R greater than 0.98 (R2 = 0.99 ± 0.01); and the final acceleration was started minutes after reaching HRdef (average, 4 minutes and 10 seconds).

Comparison of the Work Rate-o2 and Work Rate-Heart Rate Relationships

Figure 1 shows the WR-o2 relationship obtained in 1 of the 24 cyclists examined. The relationship is linear up to submaximal WRs and curvilinear in the final phases of the test. The point at which the linearity is lost, o2def, identified mathematically at the conjunction of the linear and curvilinear lines, occurred at a WR of 305 W and a o2 of 3800 mL·min−1.

F1-13
Figure 1:
Work rate (WR)-Figure 1o2 relationship in 1 of the 24 cyclists examined. Final acceleration started at 27 minutes and 40 seconds and at a cadence of 115 rpm. The test duration was 28 minutes and 40 seconds. Gearing was 52 × 15. Data below the submaximal WR are best-fitted by a straight-line equation (y = 11x + 460; R 2 = 0.98) and above the submaximal WR by a polynomial equation (y = -0.01x2 + 16.2x - 146; R 2 = 0.96). The last 6 points (i.e., final acceleration phase) were excluded from the calculation.

An identical pattern was found for the WR-HR relationship recorded in the same subject (Figure 2). The point at which the linearity is lost, HRdef, identified mathematically at the conjunction of the 2 lines, occurred at a WR of 305 W and a HR of 166 b·min−1.

F2-13
Figure 2:
Work rate (WR)-heart rate relationship in the same cyclist as in Figure 1. The data below the submaximal WR are best-fitted by a straight-line equation (y = 0.3x + 68; R 2 = 0.98) and above the submaximal WR by a polynomial equation (y = -0.0001x2 + 0.3x + 91; R 2 = 0.98). The last 6 points (i.e., final acceleration phase) were excluded from the calculation.

The pattern of the WR-o2 and WR-HR relationships found in the 24 subjects was identical to that shown in Figures 1 and 2, that is, linear below o2def and HRdef and curvilinear above them. o2def and HRdef were evident and mathematically determined in all subjects.

Table 1 summarizes the data obtained in the 24 athletes. The values of WR, o2, HR, cadence and testing time at start, o2def, HRdef, SFA, and maximum WR are presented. At o2def and HRdef, the values of the variables considered are not significantly different. o2 at o2def and HR at HRdef were 82% and 88%, respectively, of the values reached at maximum WR.

T1-13
Table 1:
Work rate, Figure 1o2, heart rate, cadence, and testing time recorded in the 24 subjects at 5 relevant testing points.

At maximum WR, WR values ranged from 458 to 632 W, HR values from 164 to 204 b·min−1, o2 values from 3900 to 5850 ml·min−1, and cadence from 107 to 127 rpm.

Figure 3A shows the highly significant correlation between the WR values recorded at HRdef and o2def in the 24 subjects. The Passing and Bablok regression analysis showed no deviation from linearity (p > 0.10). The Bland and Altman analysis (Figure 3B) demonstrates that the WR values registered at HRdef and o2def are in good concordance.

F3-13
Figure 3:
(A) Correlation between work rate (WR) at the heart rate deflection point (HRdef) and WR at the Figure 1o2 deflection point (Figure 1o2def) in the 24 subjects examined. Regression equation (dashed line) y = 21.9 + 0.94x (R 2 = 0.96; p < 0.0001). (B) Difference compared to the mean of the WR at Figure 1o2def versus WR at HRdef (Bland and Altman plot) in the 24 subjects examined. Work rate mean bias, 0.7%; limits of agreement from -4.7% to 3.2%; 95% confidence interval from -1.5% to 0.1%).

Association of o2def With Ventilatory Threshold

The V-slope analysis was successfully carried out in all subjects. The slope increment from the lower to the upper lines was highly significant (p < 0.0001) and above the 0.1 acceptance limit established by Beaver et al. (4).

The correlation between the o2 values recorded at VT and at o2def in the 24 subjects (Figure 4A) was highly significant. The Passing and Bablok regression analysis showed no deviation from linearity (p > 0.10). The Bland and Altman analysis (Figure 4B) demonstrated that the o2 values recorded at VT and at o2def are in strong agreement.

F4-13
Figure 4:
(A) Correlation between Figure 1o2 at ventilatory anaerobic threshold (VT) and at the Figure 1o2 deflection point (Figure 1o2def) in the 24 subjects examined. Regression equation (dashed line) y = 12.1 + 1.0x (R 2 = 0.99; p < 0.0001). (B) Difference compared to the mean of Figure 1o2 at Figure 1o2def versus VT (Bland and Altman plot) in the 24 subjects examined. Figure 1o2 mean bias, -0.4%; limits of agreement from -1.9% to 1.0%; 95% confidence interval from -0.7% to -0.1%.

The work rate generated per unit o2 consumption (ΔWR·Δo2−1) below and above o2def for each of the 24 subjects is shown in Table 2.

T2-13
Table 2:
Increments of work rate and Figure 1o2 and work rate-to-Figure 1o2 ratios recorded in the 24 subjects below and above the Figure 1o2 deflection point.

The values of the ΔWR·Δo2−1 ratio below deflection are significantly lower than those above deflection (90 ± 11 versus 133 ± 35, p < 0.0001).

Discussion

This study demonstrated that in incremental cycling tests on a wind-load ergometer, the WR-o2 relationship is identical to the WR-HR relationship. In the 24 professional cyclists tested, both of these relationships were linear up to submaximal WR and curvilinear thereafter, due to a levelling off of O2 consumption and HR.

The curvilinear pattern of the WR-o2 relationship observed at high work intensities allows the identification of a o2def that occurs at the same WR value as the HRdef (Figure 3).

A similar levelling off of o2, starting at submaximal exercise intensities, has been reported by several authors (1,2,5,19,27,32,39,43), most notably by Lucia et al. (32) in a group of 12 professional cyclists. Nevertheless, with the exception of Bickham et al. (5), these studies did not consider the exact point at which the o2def occurred, and none of them considered its possible association with the HRdef.

The second purpose of this study was to determine the association between o2def and VT. It has been shown that o2def occurs at VT (Figure 4); it follows that above o2def, the lactacid mechanisms are activated. An analogous association of o2def and lactate threshold was demonstrated by Bickham et al. (5) in runners. Several studies have established that HRdef is also associated with the activation of the anaerobic mechanisms (8,9,13,14,23,38). Only in the study by Carey (12) was this association not demonstrated.

The activation of anaerobic mechanisms above o2def generates anaerobic synthesis of ATP, which is added to the ATP synthesized aerobically. Because of this extra ATP, the ΔWR·Δo2−1 ratio increases, as clearly indicated by the data in Table 2. As can be seen, increments of 1 L of O2 consumption are associated with WR increments of 90 W below o2def and with increments of 133 W above o2def. No other hypothesis (e.g., improved biomechanics (16,20), improved efficiency of the muscles (15-17,31,33), or aerobic metabolism (35,43)) could explain this 48% increase in WR relative to O2 consumption observed above o2def.

The activation of anaerobic mechanisms occurring above both o2def and HRdef may depend on reductions in myocardial functioning, as proposed by Bodner et al. (8,9). These reductions may be due to decreased left ventricular ejection fraction (23), reduced stroke volume (30), or reduced HR increments mediated by catecholamine saturation of the cardiac β1-adreno receptors (25). A similar suggestion was made by Bassett and Howley (3), according to whom o2 attenuation “represents a levelling off in cardiac output and arterial-venous O2 difference that may be seen toward the end of a graded exercise testing. Since the o2 fails to keep pace with the increasing oxygen demand, there is an increased reliance on oxygen-independent pathways (i.e., anaerobic glycolysis).”

The hypothesis that impairments in cardiovascular functioning may activate muscle anaerobic metabolism is confirmed by the progressive reduction in VT observed when muscle O2 flow is reduced artificially by increasing CO-Hemoglobin concentration with carbon monoxide breathing (28). HRdef and o2def should represent, therefore, physiological signals indicating both reductions in myocardial functioning and the activation of anaerobic glycolysis.

In summary, the WR-o2 relationship was identical to the WR-HR relationship (i.e., linear up to submaximal WR and curvilinear thereafter). o2def coinciding with the HRdef was found. o2def coincided with VT. Taken together, these findings provide further evidence that anaerobic glycolysis is activated above o2def and HRdef.

Practical Applications

For the purposes of this study, the incremental test with simultaneous measurement of HR and respiratory gases was entrusted to professional cyclists who were used to carrying out incremental tests with a respiratory face mask and who, notwithstanding the discomfort, were able to follow precisely the increases in cadence called for by the protocol.

When performed without a mask and following the experimental protocol previously described (22), the test can be performed by all subjects and allows for identification of the HRdef, even in individuals of a low fitness level. In these cases, an experienced test assistant should determine the proper gearing for the subject in question, such that neither during the warm-up nor during the test will the HR rise more than 8 b·min−1·min−1 (14). The average HR increments in the 24 subjects were 4 b·min−1·min−1, and the average WR increments were 6.3 W every 30 seconds from start to HRdef and 10.2 W every 30 seconds above HRdef.

The identification of a o2def and its coincidence with HRdef and VT provide strong evidence that this test relating exercise intensity and HR allows the noninvasive identification of the anaerobic threshold. The findings obtained in this study confirm the practical utilization of HR monitoring during incremental tests for the easy, and inexpensive determination of anaerobic threshold, and is of advantage with respect to determination through blood lactate measurements or gas analysis (10,42). This method of identifying anaerobic threshold offers coaches and trainers data for formulating individualized training programs with workouts that can be purely aerobic or a mixture of aerobic and anaerobic. Subsequently, the test can be used to monitor the training effect and to modify training programs accordingly.

In the 24 subjects considered, the SFA began on the average 4 minutes and 10 seconds after HRdef and o2def. The duration of the curvilinear phase and the final sprint are directly connected to the anaerobic capacity of the subject. Additional information obtained at the end of the final acceleration regards the HR, which reaches the same maximum value obtained in competition.

Acknowledgments

The authors thank Patricia Ennis for editing the manuscript.

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Keywords:

anaerobic threshold; testing in cycling

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