PHYSIOLOGICAL RESPONSES TO SUB-MAXIMAL EXERCISE
BY: BRETT R. TAYLOR
The purpose of this study was to measure ones aerobic capacity (VO2 max) and find the physiological responses to sub-maximal exercise. ‘VO2 max testing is currently the most reliable and valid indicator for measuring one’s aerobic capacity’ (Gamberale, F. (1972). We focused on the physiological responses that coincide with determining our subjects VO2 max which include, ventilatory (VE) drive, oxygen consumption (VO2), carbon dioxide production (VCO2), blood lactate level and HR.
Our subject was a young, healthy male PE student. His age was 20, weight 81.1kg and height 185.3cm tall. A multistage protocol was used to monitor physiological responses over five different phases of the test, using a cycle ergometer.
Results demonstrated that a rise in HR and blood lactate is highly correlated with RPE and VO2 max. Our subject had a higher than normal ability to use fat as a fuel as intensity was rising. Criteria to achieve VO2 max was not met, and a correct VO2 max result was not established, instead we found our subjects VO2 peak.
Key words: VO2 max, physiological responses, criteria, aerobic capacity
The purpose of this study was to measure ones aerobic capacity (VO2 max) and find the physiological responses to sub-maximal exercise. ‘VO2 max testing is currently the most reliable and valid indicator for measuring one’s aerobic capacity’ (Gamberale, F. (1972). It is based on the maximum amount of oxygen in milliliters, one can use in one minute per kilogram of body weight. Numerous studies show that one can increase their VO2 max by working out at an intensity that raises their heart rate to between 65 and 85% of its maximum for at least 20 minutes three to five times a week. A mean value of VO2 max for male athletes is about 3.5 liters/minute and for female athletes it is about 2.7 liters/minute.
There are numerous cardiovascular, respiratory and muscular responses to the body during and after exercise, however, when studying the integration of these components during a VO2 max test, ‘researchers typically examine the relative contribution of central or respiratory-metabolic and peripheral or local cues in the determination of ratings of perceived exertion (RPE).’ (Pandolf, K.B., Burse, R. L., & Goldman, R. F. (1975). For the purpose of this study we shall focus on the physiological responses that coincide with determining our subjects VO2 max which include, ventilatory (VE) drive, oxygen consumption (VO2), carbon dioxide production (VCO2), blood lactate level and HR. By also measuring VCO2 students can calculate the Respiratory Exchange Ratio, which provides an indication of the type of molecules, either carbohydrates or fats, which are being used to fuel the body.
We analyzed one subject who was a young, healthy male PE student. His age was 20, weight 81.1kg and height 185.3cm tall. A multistage protocol was used to monitor physiological responses over five different phases of the test. Using a cycle ergometer (Monark 818), we took all data at rest, followed by the 2’45 – 3’45 minute mark, 6’45-7’45, 10’45-11’45, 14’45-15’45 minute marks, and the final minute. The initial load was set at 1.5 then every phase after, the load was increased by .5 (kp). The test was conducted at 60 revs per minute (RPE). Exercise intensity increased until maximal voluntary effort was attained. The test duration was 19’45 minutes. Data for pulmonary ventilation (VEATPS, L/min), oxygen uptake (VO2, L/min), carbon dioxide production (VCO2, L/min), temperature expired air (Â°C) and blood lactate levels were collected. Strong verbal encouragement was given to facilitate the best possible effort. The criteria to reach VO2 max were as follows; a plateau in oxygen uptake must occur as the work load is increasing, a respiratory exchange ratio must exceed 1.15, and the heart rate must be within 5 beats of the age predicted maximum heart rate. Criteria for test termination, which were considered consistent with an exhaustive effort, included extreme fatigue, hyperpnoea, and profuse sweating.
An oxygen and carbon dioxide analyzer (Metex) was used to measure selected respiratory gas exchange measures for VE, VO2, VCO2, and RER. The analyzers were calibrated before each test according to manufacturer guidelines. Heart rate (HR) was monitored continuously with a polar heart rate monitor. The HR was recorded during the last 10 seconds of each stage. Blood samples were collected 10 seconds prior to the end of each stage by finger stick and analyzed for lactate via lactate analyzer (Schmide electronic).
Ratings of perceived exertion were assessed during the last 20 seconds of each minute using the 6-20 point-category scale. Other apparatus included;
- Blood glucose meter (Bayer Diagnostics)
- Blood lancet (softclix)
- Lancet (Accu-chek, softclix)
- Alcohol preps (Kendall, Webcol)
- Latex examination gloves (RMS)
- Boehringer Mannheim (Accusport)
- Stop watch (Casio)
- Height measuring apparatus (Holtan Limited)
Our subjects VO2 max was 57.81 ml/kg/min. Below is a chart showing VO2 Max norms in U.S.A
Graph 1.2 HEART RATE
Graph 1.3 RPE
Graph 1.4 RPE COMPARED TO BLOOD LACTATE LEVELS
Graph 1.5 FeO2 Vs FeCO2
Graph 1.6 VO2 VERSUS VCO2
Graph 1.7 RESPIRATORY EXCHANGE RATIO (RER)
Graph 1.8 CHO VERSUS FAT METABOLISM
Throughout this experiment, physiological responses were highly evident as the intensity rose. HR peaked at around 18 min. 45 sec. which is just short of 5 beats of age predicted maximum heart rate, one of our VO2 max criteria. Chart 1.2 shows the HR rising steadily due to the increase of oxygen supply needed for working muscles and tissues throughout the body. As the intensity rose, the blood needed to be pumped at a much higher rate.
When the heart rate rises, it seems that the RPE does also. Linda M. LeMura, Serge P. von Duvillard, Frank Stanek (2001) stated that ‘perception of effort during exercise was dependent on two factors; namely, peripheral factors related to feelings of strain in the joints and muscles, and respiratory-metabolic factors related to sensations from the cardiopulmonary system’. This is clearly demonstrated in Graph 1.3 and 1.4. As lactate levels and HR grew, so did our RPE as indicated.
Our Lactate tester was not working well throughout the middle region of the test and failed to register 2 phases. However, we can still see that the trend was quite equivalent to the RPE. Blood lactate levels rose when our subject was reaching his lactate threshold, at around the 17 minute mark, indicating that the aerobic system could no longer work under this type of intensity and energy systems had to change to anaerobic in order to keep supplying ATP.
As intensity rose, so did the VO2 and VCO2. As we can see from graph 1.6, VCO2 is looks as if it is catching up with VO2 but never makes it. We needed this to happen in order to reach our criteria for respiratory exchange ratio shown in graph 1.7.
The respiratory exchange ratio represents the amount of carbon dioxide produced divided by the amount of oxygen consumed. The respiratory exchange ratio generally ranges from 0.7 to 0.85 at rest and is dependent in part on the predominant fuel used for cellular metabolism. At high levels of exercise, CO2 production exceeds VO2, and a respiratory exchange ratio greater than 1.0 often indicates that the subject is giving a near-maximal level of effort. Unfortunately, our subject did not manage to exceed past 1.1.
As expected the Carbohydrate usage rose throughout the test as it is our major form of energy. CHO provides a quick form of energy by its end product, glucose, which via glycolysis creates pyruvic acid, and ATP. What was unexpected was that Fat was utilized more as the intensity reached higher levels. This is a contradiction to recent criteria for body fat reduction during sub maximal exercise recommended by government health officials. They have created guidelines and asked communities to work at a lower intensity to work in a fat burning zone, whereas in this experiment we see that our subject uses more fat as a fuel during more intense phases of the test. This could, however, mean that our subject has adapted to using fat as a fuel as a result of earlier aerobic training.
Unfortunately, we had not reached any of the three criteria that we needed in order to have a correct VO2 max result. ‘Most clinical studies report peak VO2 rather than maximal VO2 because the latter is often difficult to determine precisely’ (Gamberale, F. (1972)). Peak VO2 is the highest VO2 attained during graded exercise testing, but the term does not imply that a plateau in measured VO2 is reached, which is one of the criteria in establishing a correct VO2 max result. Respiratory exchange ratio had only reached a high of 0.97 and our final criterion for VO2 max was also 17 beats short of reaching at least 5 beats away from his age predicted maximum. Although our subjects blood lactate level had risen to 11.1(Usually over 8 when VO2 max is recorded), we must conclude that we had not successfully pushed the subject to total fatigue levels and our reading of 57.81 ml/kg/min is a VO2 peak rather than a VO2 max.
- Pandolf, K.B., Burse, R. L., & Goldman, R. F. (1975). Differentiated ratings of perceived exertion during physical conditioning of older individuals using leg-weight loading.
- Gamberale, F. (1972). Perception of exertion, heart rate, oxygen uptake, and blood lactate in different work operations.
- Lemur, L. M., Devilled, D., Stank, F (2001). Official Journal of The American Society of Exercise Physiologists (ASEP)
- Wilmore, J.H., Costill, D.L. (2004). Physiology of sport and exercise – Third edition.
- Robertson, R.J. (1982). Central signals of perceived exertion during dynamic exercise. Med Sci Sports Exerc