Physiological Breakpoints and Maximal Steady-State of Cycling

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JEPonline

Physiological Breakpoints and Maximal Steady-State of Cycling

Cory Scott1, Frank Wyatt1, Jason Winchester1, Keith Williamson2, Ashleigh Welter1, Sean Brown1

1Department of Athletic Training and Exercise Physiology, Midwestern

State University, Wichita Falls, TX, 2Vinson 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. JEPonline2015;18(3):33-45.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 VelotronTM cycle ergometer, lactate meter, metabolic cart, and a heart rate monitor. Blood lactate (mM), VO2 (mL·kg-1·min-1), and heart rate (beats·min-1) data were collected every minute for both the VO2 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≤0.05. 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, VO2 max, Cycling, Threshold

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 (O2 inhaled) and carbon dioxide (CO2 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 (VE) (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 VE. 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 betweenthe 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 VO2max 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 respectivebioenegetic 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 subject qualifiedto participate in the study by ranking in the 90th percentile or higher per ACSM guidelines for VO2max.

Table 1. The Subjects’ Demographics.

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 VelotronTM 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 VelotronTM.

The instrumentation used for testing ventilation was a ParVo Medics TrueMax 2400 Metabolic Measurement SystemTM and a VelotronTM cycle ergometer. The ParVoMedicsTM 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 ParVoMedicsTM metabolic cart included, expired ventilation (VE), oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory exchange ratio (RER), and maximal oxygen uptake (VO2max). Before testing, the ParVoMedicsTM ventilation hose was affixed from the machine to the subject’s mouth using a Hans RudolfTM valve and, then, a nosepiece was applied to prevent nasal breathing. Once testing was underway, VE measures were obtained via an open spirometry system that recorded and averaged a breath-by-breath analysis every 20 sec.

A Polar RS800CXTM heart rate monitor was used to obtain HR (beats·min-1) for the duration of each VO2max test and steady-state test. The Polar WearLinkTMW.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 PlusTM was used to obtain whole blood lactate by removing 10µl from a finger during VO2max and MLSS. Extractions were taken every minute in the VO2max test and the MSS test. Lactate was determined by a reflectance photometry at a wavelength of 657nm in a colorimetric lactate-oxidase mediator reaction.

Statistical Analyses

StatSoft® Statistica 7TM was used to statistically analyze the data. Means± standard deviations (mean ±SD) were determined for each subject’s demographics (height, weight, and VO2max). Physiological thresholds (lactate, VE, 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 VO2max 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 Tukeypost hoc analysis were used to analyze the individual slopes of each subject’s blood lactate, VE, 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≤0.05.

RESULTS

Maximal Oxygen Uptake Data

The subjects’ mean VO2max was 73.59 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 VEmeasures were averaged from a breath by breath analysis every 20 sec. Table 2 shows the recorded physiological measure of each subject during the VO2max test, and the corresponding work output during each threshold.

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 VE. Heart rate was recorded beat by beat, and VEmeasures 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 VO2 max test (Table 3).

The variable found to be significantlydifferent 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).

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 VO2max test, in which a later comparison correlated the significance between thresholds and a steady-state effort. Heart rate 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-minsteady-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, VE, and blood lactate. Examining HR during the steady-state effort, an increase can be seen throughout the measured time (Figure 2).

Figure 2.Heart Rate (beats·min-1) throughout the 20-MinSteady-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 inHR 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 VO2max. 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 preventHRfrom 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 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 chemoreceptorsmay act as stimuli to increaseHR 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 VE (11). Furthermore, studies that examined VO2 during maximal lactate steady-state found a continual increase in VE, which is supported by the subsequentventilatory drift notion (9,11). The concomitant rise in VE, whichis observed with an increase in HR is referred to as the “ventilatory drift” that is associated with a gradual rise in VEdespite a constant workload (6) (Figure 3).

Figure 3.Average VO2 (mL·kg-1·min-1) during the 20-MinSteady-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 VE 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 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 VE and HR.

Blood lactate, unlike HR and VE, 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, VE, 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 steady- state. Inclusion for MLSS involves the parameters of blood lactate not increasing more than 1 mM·L-1during 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 VE, 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.