Journal of Exercise Physiologyonline

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1 33 Journal of Exercise Physiologyonline June 2015 Volume 18 Number 3 Editor-in-Chief Official Research Journal of Tommy the American Boone, PhD, Society MBA of Review Exercise Board Physiologists Todd Astorino, PhD Julien Baker, ISSN PhD Steve Brock, PhD Lance Dalleck, PhD Eric Goulet, PhD Robert Gotshall, PhD Alexander Hutchison, PhD M. Knight-Maloney, PhD Len Kravitz, PhD James Laskin, PhD Yit Aun Lim, PhD Lonnie Lowery, PhD Derek Marks, PhD Cristine Mermier, PhD Robert Robergs, PhD Chantal Vella, PhD Dale Wagner, PhD Frank Wyatt, PhD Ben Zhou, PhD Official Research Journal of the American Society of Exercise Physiologists ISSN JEPonline Physiological Breakpoints and Maximal Steady-State of Cycling Cory Scott 1, Frank Wyatt 1, Jason Winchester 1, Keith Williamson 2, Ashleigh Welter 1, Sean Brown 1 1 Department of Athletic Training and Exercise Physiology, Midwestern State University, Wichita Falls, TX, 2 Vinson Health Center, Midwestern State University, Wichita Falls, TX ABSTRACT Scott C, Wyatt F, Winchester J, Williamson K, Welter A, Brown S. Physiological Breakpoints and Maximal Steady-State of Cycling. JEPonline 2015;18(3): The aim of this study was to examine various physiological thresholds and their association with maximal steady-state exercise. Elite level cyclists (n = 15) participated in the study. All data were collected on a Velotron TM cycle ergometer, lactate meter, metabolic cart, and a heart rate monitor. Blood lactate (mm), VO 2 (ml kg -1 min -1 ), and heart rate (beats min -1 ) data were collected every minute for both the VO 2 max test and the maximal steady-state test. Statistical analysis included the descriptive mean (±SD) of the subjects and the test results, the comparison of each physiological measurement and the mean (±SD) values, and t-tests analyzing the wattage corresponding to the said variable and physiological test. Statistical significance was set at P There was no noticeable trend of one threshold being indicative of another threshold. Heart rate and ventilatory thresholds displayed strong correlations with their respective values at steady-state intensity, while blood lactate values at threshold exhibited weak, but significant correlations to values obtained at steady-state efforts. The correlation shown in the study between heart rate threshold and ventilatory threshold and their respective values at steady-state can help provide evidence-based training information, which can lead to an increase in athletic performance by training the proper bioenergetic system. Key Words: Steady State, VO 2 max, Cycling, Threshold

2 34 INTRODUCTION In any physical activity, it is safe to assume that as the workload increases there is a resulting increase in the body s metabolic responses. Specifically, in cycling, it has been found that as wattage or work increases, there is an increase in heart rate, blood lactate, and ventilation (18). At a point in these physiological responses, a threshold is established which is marked by a rapid change despite the proportionate workload (18). These changes can be seen in the respiratory exchange ratio (RER) when the ratio of the volume of oxygen (O 2 inhaled) and carbon dioxide (CO 2 exhaled) is 1.0 or higher (4). During an incremental test of maximal oxygen uptake, thresholds are established for the parameters mentioned above. These threshold changes, or physiological breakpoints, are exhibited when a rapid change occurs despite the proportionate change in workload volume. Furthermore, these thresholds are synonymous with the term maximal lactate steady-state (MLSS) when referencing blood lactate, and maximal steady-state (MSS) when referring to heart rate (HR) and ventilation (V E ) (2). Maximal steady-state or its derivative (MLSS) can be defined as the highest point at which a physiological response and workload can be sustained, and in the context of MLSS, blood lactate values do not accrue (3). At rest and below threshold, there is a correlation between lactate production and removal, HR response, and V E. Understanding bioenergetic responses can translate into greater performances, and training at MLSS and MSS has been shown overtime to increase an individual s thresholds. Based on the literature, it can be proposed that the concept of anaerobic threshold and maximal steady-state efforts vary widely. It is understood that variations in the circadian rhythm from hour to hour and day to day have an effect on physiological reactions and, consequently, the comparison of studies. Thresholds and steady-states within the same person have been shown to fluctuate based on the time of day, amount of rest, when the last bout of exercise occurred, and diet. In the current study, these limitations were taken into account. According to past studies, there is a breach in the literature between the relationship of physiological breakpoints and the accompanied reactions as a whole. Theoretically, each one of the physiological thresholds mentioned above should occur at approximately the same time and workload in a VO 2 max test. However, exercise at a maximal steady-state has the potential to display dissimilar threshold points through different bioenergetic responses. Thus, this study examined and compared the following conditions: (a) lactate threshold and maximal lactate steady-state (MLSS); (b) ventilatory threshold and maximal steady state (MSS); and (c) heart rate threshold and MSS. Moreover, it examined the three observed parameters as a whole and their relationship to steady-state efforts. It was hypothesized that each physiological threshold strongly correlates to the respective bioenegetic response at steady-state, and the three combined physiological responses would display a significant association with maximal steady-state. METHODS Subjects A total of 15 subjects (13 males and 2 females) with an age range of 19 to 43 participated in a graded exercise test to maximal exertion and/or volitional fatigue as well as a 20-min maximal steady-state self-selected effort. Refer to Table 1 for the subjects demographics. Prior to the testing, all subjects filled out a medical questionnaire to assure the absence of asymptomatic neuromuscular and cardiovascular disorders. Each subject also signed an Internal Review Board informed consent form approved at Midwestern State University. All subjects were familiarized with a bicycle, and each

3 subject qualified to participate in the study by ranking in the 90th percentile or higher per ACSM guidelines for VO 2 max. Table 1. The Subjects Demographics. 35 Variable Mean ± SD Age (yrs) 24.9 ± 6.3 Height (cm) ± 5.0 Weight (kg) 72.7 ± 8.1 Body Fat (%) 11.9 ± 6.1 Procedures Each subject s height, weight, blood pressure, and body fat were measured. Instrumentation for determining the values included the mechanical beam scale with stadiometer, sphygmomanometer and stethoscope, and Lange skin fold calipers measure on the right pectoral, right abdominal, and right thigh for males and right tricep, right mid-axillary, and right thigh for females. Equipment The Velotron TM cycle is a computer controlled, chain driven, fixed ratio, electronic ergometer used for obtaining and/or manipulating workload in watts. The cycle was configured with a cadence sensor and magnet to monitor the subject s revolutions per minute (rev min -1 ). Furthermore, preceding the exercise test the following measurements and equipment (saddle height, saddle setback, reach, stack, crank arm length, and pedals) from the subject s bike were extracted and transferred to the Velotron TM. The instrumentation used for testing ventilation was a ParVo Medics TrueMax 2400 Metabolic Measurement System TM and a Velotron TM cycle ergometer. The ParVo Medics TM flow meter was calibrated before each trial with the ambient data entered for temperature, humidity, and atmospheric pressure. Gas calibration was performed to analyze the percentage of oxygen and carbon dioxide. Obtained measurements from the ParVo Medics TM metabolic cart included, expired ventilation (V E ), oxygen consumption (VO 2 ), carbon dioxide production (VCO 2 ), respiratory exchange ratio (RER), and maximal oxygen uptake (VO 2 max). Before testing, the ParVo Medics TM ventilation hose was affixed from the machine to the subject s mouth using a Hans Rudolf TM valve and, then, a nosepiece was applied to prevent nasal breathing. Once testing was underway, V E measures were obtained via an open spirometry system that recorded and averaged a breath-by-breath analysis every 20 sec. A Polar RS800CX TM heart rate monitor was used to obtain HR (beats min -1 ) for the duration of each VO 2 max test and steady-state test. The Polar WearLink TM W.I.N.D. transmitter was the chest strap device, which allowed for the subject s HR signal to be transmitted to the training computer. Heart rate data were recorded at an R-R interval, allowing for thousands of data points per test. Lastly, a Nova Biomedical Lactate Plus TM was used to obtain whole blood lactate by removing 10 µl from a finger during VO 2 max and MLSS. Extractions were taken every minute in the VO 2 max test and the MSS test. Lactate was determined by a reflectance photometry at a wavelength of 657 nm in a colorimetric lactate-oxidase mediator reaction.

4 36 Statistical Analyses StatSoft Statistica 7 TM was used to statistically analyze the data. Means ± standard deviations (mean ± SD) were determined for each subject s demographics (height, weight, and VO 2 max). Physiological thresholds (lactate, V E, and HR) were compared for association with MSS through a Pearson Product R Correlation Coefficient. A dependent sample t-test was used for comparison between physiological parameters obtained during both VO 2 max efforts and averaged steady-state efforts. Moreover, a dependent sample t-test was used for comparison of workload at each physiological parameter. An ANOVA test and a Tukey post hoc analysis were used to analyze the individual slopes of each subject s blood lactate, V E, and HR. Additionally, a means with error plot examined the individual mean and standard deviation for each observed physiological threshold. Statistical significance was set at P RESULTS Maximal Oxygen Uptake Data The subjects mean VO 2 max was ml kg -1 min -1. Blood lactate was obtained every minute starting at the 5th-min of the test, and stopped once the subject reached volitional fatigue. Heart rate was recorded beat by beat, and V E measures were averaged from a breath by breath analysis every 20 sec. Table 2 shows the recorded physiological measure of each subject during the VO 2 max test, and the corresponding work output during each threshold. Table 2. VO 2 Max Physiological Threshold Data and Work Output of Corresponding Threshold. Subject VO 2 Max (ml kg -1 min -1 ) Max HR (beats min -1 ) HRT (beats min -1 ) BLaT (mm L -1 ) Vent T (ml kg -1 min -1 ) HRT (Watts) BLa (Watts) VentT (Watts) In the maximal steady-state test, subjects self-selected a workload at which they completed a 20-min steady-state effort to their highest achievable ability. Physiological responses were noted every minute, including blood lactate, HR, and V E. Heart rate was recorded beat by beat, and V E measures

5 were averaged from a breath-by-breath analysis every 20 sec. For the following data analysis, bioenergetic responses were averaged over the 20-min steady-state period and, then, compared with each threshold value determined from the VO 2 max test (Table 3). 37 Table 3. Mean Physiological Measurements during a 20-Min Maximal Steady-State Test. Subject HRSS (Mean) VO 2 SS (Mean) BLaSS (Mean) WattsSS (Mean) Figure 1 presents the percentage of the physiological parameter during steady-state exercise compared to the maximal value of the respective physiological variable. Thus, the percentages are indicative of how close to maximal exercise an individual is working during a 20-min steady-state effort. From the Table it can be noted that HR per subject at steady-state is typically in the 90% range of maximal heart rate, with the average being 90.1%. Ventilation at a steadystate effort was consistently at the lower percentage compared to HR, with the average being 82.9%, while blood lactate was significantly lower at 58.5% A dependent samples t-test (P 0.05, see Table 4) examined the measured variables in the study. Two sets of variables were not found to be significantly different (e.g., ventilatory threshold and the average VO 2 at steady-state (P=0.765) and the heart rate threshold and the average HR at steady-state (P=0.263) BL Work HR work VO2 work Mean Mean±0.95 Conf. Interval Figure 1. Percentages of Mean Steady-State Values Related to Maximal Physiological Values.

6 The variable found to be significantly different was blood lactate, when comparing the lactate threshold and the average lactate at steady-state. Moreover, additional dependent samples t-test were implemented (P 0.05) for the analysis of threshold power output at each measured variable (Table 5), and for the comparison of the threshold wattage derived from each variable, with the average wattage during the steady-state effort (Table 6). Table 4. Dependent Samples t-test Comparing Threshold and Steady-State Values. Variable Mean ± SD N t p BLa Threshold 2.83 ± 0.89 BLaSS avg 6.76 ± Vent Threshold ± 7.70 VO 2 SS avg ± HRT ± HRSS avg ± Table 5. Dependent Samples t-test Comparing Threshold Wattage. Variable Mean ± SD N t p BLa T Watts ± Vent Threshold Watts ± BLa T Watts ± HRT Watts ± Vent Threshold Watts ± HRT Watts ± Table 6. Dependent Samples t-test Comparing Threshold Variable Watts with Steady-State Watts. Variable Mean ± SD N t p Bla T Watts ± WattsSS ± Vent Threshold Watts ± WattsSS ± HRT Watts ± WattsSS ± DISCUSSION The main purpose of this study was to examine the differences in physiological thresholds and their association with maximal steady-state. Analysis of the results compared: (a) lactate threshold and maximal lactate steady-state (MLSS); (b) ventilatory threshold and maximal steady state (MSS); (c) heart rate threshold and MSS; and (d) the relationship between each of the measured bioenergetic responses to steady-state exercise. Thresholds were established from a VO 2 max test, in which a later comparison correlated the significance between thresholds and a steady-state effort. Heart rate

7 threshold values and ventilatory threshold values were found to be strongly correlated to their respective values at a steady-state effort. Meanwhile, blood lactate was found to be significantly different and showed no correlation to its steady-state value. Furthermore, there was a strong correlation between each measured physiological threshold wattage and average wattage exerted at steady-state intensities. Thus, the values obtained at heart rate threshold and ventilatory threshold are a good indicator of the achievable physiological magnitude of work an individual is capable of during a 20-min steady-state effort. Conversely, blood lactate values at threshold are not indicative of the values obtained during a 20-min steady-state effort. During the 20-min timed steady-state effort, numerous physiological responses occurred that led to the resulting relationships between HR, V E, and blood lactate. Examining HR during the steady-state effort, an increase can be seen throughout the measured time (Figure 2) Steady State HR Heart Rate (b*min. -1 ) y = x Time (minutes) Figure 2. Heart Rate (beats min -1 ) throughout the 20-Min Steady-State Test. This is not an unfamiliar concept, and is a profuse finding throughout the literature. Most studies examining the physiological effects of steady-state report an increase in HR as time is increased (15,17). This physiologic augmentation is a phenomenon known as the cardiovascular drift (CVD). Cardiovascular drift is known to occur around 10 min of moderate intensity exercise or between 60 to 75% of VO 2 max. Commonalities seen with CVD are a decrease in stroke volume (SV) as well as a decrease in mean arterial pressure (MAP). If cardiac output (Q) continues to increase across time, it is due to the gradual increase HR while the SV stays the same (13). In a study using a β-blockade to prevent HR from increasing, CVD was found to be attributed to other physiological variables (5). The researchers concluded that CVD can also be a resultant of blood volume, cutaneous blood flow, esophageal core temperature, and skin temperature (5). Another study examining CVD and the role of muscle activation found that a greater amount of muscle recruitment can lead to a greater increase in HR (8). Like the current study displaying CVD, this could be due in part from the spatial summation of muscle fibers in the working muscle groups. As a muscle fiber fatigues, more fibers are needed in

8 order to maintain the same workload, leading to larger activation and, therefore, an increase in HR (1). Additionally, different central and peripheral chemoreceptors including carotid bodies and aortic bodies detect metabolites being discharged, and transduce a signal causing an action potential. The action potential transmitted from the different chemoreceptors may act as stimuli to increase HR during steady-state exercise by increasing the rate of ventilation. Therefore, when exercising at a high intensity, oxygen consumption is increased, which may eventually lead to a decreased oxygen concentration and an increased carbon dioxide concentration. These fluctuations are sensed by the chemoreceptors causing vasodilation and, additionally, resulting in an increase the rate of blood flow to the heart. The larger volume of blood causes the heart to stretch and pressure to increase. This stretch activates mechanoreceptors in the heart initiating a cascade of events leading to an increase in HR. During continuous steady effort exercise, low muscle efficiency has been shown to increase the need for V E (11). Furthermore, studies that examined VO 2 during maximal lactate steady-state found a continual increase in V E, which is supported by the subsequent ventilatory drift notion (9,11). The concomitant rise in V E, which is observed with an increase in HR is referred to as the ventilatory drift that is associated with a gradual rise in V E despite a constant workload (6) (Figure 3) Steady State Ventilation Average VO2 (ml*kg -1 *min. -1 ) y = x Time (minutes) Figure 3. Average VO 2 (ml kg -1 min -1 ) during the 20-Min Steady-State Test. Potential causes for this drift are speculated to be from an increase in body temperature, and are thought to be both unproductive and advantageous. To an extent, the drift is considered to be uneconomical because of the amount of V E for the given workload and, conversely, it is beneficial for gas exchange and the maintenance of the acid-base balance (6). The ventilatory drift displayed in this study could be a result partially from muscular fatigue in addition to low economy of muscle working at steady-state. High intensity exercise is known to increase the intramuscular temperature as well as adjust the muscle metabolism leading to fatigue, which has previously been thought to be a casual effect of the upward drift in ventilation (9). Muscular fatigue is

9 brought upon by insufficient energy supply and oxygen deficiency to working skeletal muscles. A type of fatigue called peripheral fatigue is comprised of the peripheral nervous system and relates to force reduction in working muscles as a result from the lessening of muscle action potentials (7,12). As mentioned previously, a larger recruitment of fibers would be needed in order to maintain the same work output, leading to a drift in both V E and HR. Blood lactate, unlike HR and V E, was found to have significant differences when comparing the average threshold value with the average steady-state value. However, the workload determined to be the threshold wattage strongly correlated to the average wattage exerted during the 20-min steady-state test. Moreover, like the trend seen in HR, V E, and RPE, blood lactate was found to have an upward slope for the entire period of the steady-state effort, with small oscillations fluctuating up and down between individual subjects. These fluctuations are not evident in the average slope displayed in Figure 4. Given this slope and the changes in blood lactate throughout the steady-state effort, the workload at a 20-min maximal steady was reasoned to not be the maximal lactate steadystate. Inclusion for MLSS involves the parameters of blood lactate not increasing more than 1 mm L -1 during a steady-state effort, or exercising at the highest steady-state level that balances the rate of lactate appearance and lactate disappearance. Consequently, as seen with HR and V E, a proposed term for the purpose of this study is lactate drift. This drift in lactate is from exercising at an intensity slightly above the MLSS, causing a disequilibrium between the rate at which lactate appears in the blood and the rate at which it is removed from the blood Steady State Blood Lactate Average Blood Lactate (mm*l. -1 ) y = x Time (minutes) Figure 4. Average Blood Lactate during the 20-Min Steady-State test. It is important to recall that the human body burns a mixture of energy systems during exercise and for several reasons, exercising at a higher intensity utilizes a greater amount of carbohydrates. One of the end products of this is lactate, which does not cause fatigue. Instead, it is metabolized and used as an energy source. During steady-state exercise, cells are using glycolysis to make energy in addition to consuming muscle glycogen. One of the culminating steps in breaking down glucose is the production of 2 molecules of pyruvate. During low intensity exercise, pyruvate goes directly into the mitochondria, but in strenuous activity the energy demands exceed the electron transport chain causing a cascade of events to reduce pyruvate to lactate. The main indication is that because

10 metabolic acidosis coincides with an increased lactate production, the measurement of blood lactate is principally an indirect marker for the metabolic state of a cell. As noted earlier, the main producer of lactate in the body is skeletal muscle. The main proponent in the transportation of lactate across the plasma membrane of a cell is a variety of protein linked monocarboxylate transporters (MCT), notably aiding metabolism and the regulation of homeostatic ph within muscles. The MCTs within skeletal muscles that assist in this transport of lactate are known to be MCT1 and MCT4. Fundamentally, MCT1 is found in higher concentrations in muscle identified to be more oxidative, while MCT4 is predominately in muscles that are primarily glycolytic, although it is found in all muscles. A high glycolytic rate in skeletal muscle is correlated to an increased rate of lactate production, which is taken up by the heart, kidney, liver, brain, and skeletal muscle for metabolism into a fuel source. MCT1 is recognized for its acceptance of lactate into a cell, while MCT4 is acknowledged for its export properties of lactate out of a cell. As glycolysis exceeds oxidative capacity, excess lactate is produced and begins to drift. It can be expected that during a high intensity steady-state effort, the rate of blood lactate production and removal is high. Lactate metabolism can be trained via endurance and high intensity exercise, and through these exercise activities the removal of lactate has been shown to increase (10). In regards to this study, the general trend for the superiorly fit subjects (VO 2 max = 80+ ml kg -1 min -1 ) exhibited the estimated blood lactate threshold determined from the VO 2 max test to be higher than either the ventilatory threshold or heart rate threshold, or both. A potential reasoning behind this is that highly trained athletes may have greater mitochondrial density and muscle economy, leading to a higher, more sustainable equilibrium of lactate production and removal. One study that examined humans with a vast assortment of fitness abilities reported a wide array of lactate transport capacity, with the higher fit subjects displaying the greatest potential (14). Thus, the rate of lactate metabolism providing oxidative substrates and gluconeogenic precursors is related to the expression level of MCTs. This is supported by Thomas and colleagues who found (16) an inverse relationship to MCT1 expression with the fatigue index during intermittent supramaximal exercises (16). A probable explanation for blood lactate being related to fatigue is when a high accumulation of lactate occurs, ph levels decrease leading to a greater inhibition of glycolysis, consequently decreasing the bioenergetics and decelerating work to a greater oxidative capacity allowing for MCT1 to recycle the excess lactate (i.e., turning it into pyruvate and oxidizing it in the mitochondria for ATP production). In addition to the physiological measurements of HR, V E, and blood lactate, a measurement of RPE was obtained every minute for the duration of the maximal steady-state test. As indicated in the results in Figure 5, an increasing slope was existent in the average rating of all subjects. Going back to the central command model, the RPE slope is associated to the trend of the increasing slopes found in HR, V E, and blood lactate. This hypothetically demonstrates that not only were the constant changes on a physiological level during steady-state efforts, but also on the psychological level. This can relate back to understanding the pre-anticipatory effect, which would cause a possible release of supplementary catecholamines to match with the RPE. 42

11 43 Borg RPE Steady State Average Borg RPE Time (minutes) Figure 5. Average Perceived Exertion for steady-state Exercise using the Borg RPE Scale. CONCLUSIONS The findings from this study indicate that the physiological thresholds of HR and V E are significantly correlated to the average measurements obtained during a 20-min maximal steady-state test. In regards to training and exercise prescription, these data can assist in understanding the duration and intensity for cyclists. Blood lactate levels at threshold showed to be an inaccurate predictor of the levels acquired from a maximal steady-state effort. In the field, blood lactate measurements are difficult to acquire and an invasive process, therefore leading to laboratory testing being the only efficient and practical way of obtaining measurements. Furthermore, like blood lactate, ventilatory measurements are very difficult to attain in a field setting leaving this measurement for laboratory purposes. Heart rate is an excellent and noninvasive measurement, and it can be easily acquired from both the laboratory and field. Additionally, aside from power measurements obtained from a variety of power devices made for bicycles, HR is the most popularly used physiological variable for exercise prescription. What needs to be considered when prescribing short, intense exercises is the well-known physiological occurrence of heart rate drift. When exercising at the same workload for ~10 min, HR will begin to increase slightly over time despite of the steady-state workload. Taking into account for this drift, an individual should not adjust their workload, but rather should maintain their wattage as much as possible. In addition, findings from the RPE using the Borg scale suggest that psychologically the workload and effort increased with time despite no change in exercise intensity. Thus, the decrease in workload during interval training should be eradicated and the increase in HR and perceived exertion should be overlooked but not unnoticed.

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