Exercise 2: Aerobic Capacity

Transcription

1 Exercise 2: Aerobic Capacity Reading: Silverthorn 4 th ed, pg. 3-6, , ; Silverthorn 5 th ed, pg 4 5, , ; Silverthorn 6 th ed, pg , , The purpose of this first exercise is to illustrate the relationship between the consumption of oxygen, energy production, and physical work. Metabolism comes from the Greek word for change and refers to all the chemical and energy transformations that occur in a cell. Food is transformed into ATP in the mitochondria through aerobic respiration and the electron transport system. The overall chemical reaction for converting glucose into ATP is: C 6 H 12 O 6 + 6O 2 + ADP à 6CO 2 + 6H 2 O + ATP ATP is then transformed into mechanical energy to move our muscles and maintain our bodily functions. Oxygen is required for making ATP, so oxygen consumption is a reflection of ATP production (and useage). Energy expenditure can be indirectly measured by measuring the rate of oxygen consumption, or VO 2. The units of VO 2 are ml O 2 consumed/kg body weight/min (the units L/min are sometimes used for non-weight bearing exercise). VO 2 is the rate that oxygen is actually used by the body, not the amount of oxygen inhaled. Oxygen consumption changes as the body's activity level changes and more ATP is required. VO 2 measured at rest will be lower than VO 2 measured during physical activity. VO 2 max reflects a person s overall fitness level, which is one's maximum potential for using oxygen to make ATP. This is also known as aerobic capacity. VO 2 max is defined by the limits of ones oxygen transport system, so this value does not vary from moment to moment in the same way that VO 2 will change. Individuals who are more physically fit will be able to use oxygen more efficiently (have a higher VO 2 max), and can therefore produce more ATP and perform more work with less effort (huffing and puffing). Factors that can affect VO 2 max include general respiratory and cardiovascular health, the amount of mitochondria in the muscle cells, and vascularization of the muscle tissue. A person can increase their VO 2 max over time with physical training. We will use a method called the McArdle-Katch Bench Stepping Test to estimate VO 2 max. It is a convenient, low budget method of fitness assessment. We can measure heart rate while performing a sub-maximal amount of work, then extrapolate what our oxygen consumption would be if we were working maximally at VO 2 max. This extrapolation is based on data compiled from the VO 2 max measurements of many thousands of people of many different fitness levels (see Table 5). We will NOT be

2 measuring VO2. Rather, we will be measuring heart rate and will use this measurement to extrapolate what VO2 max would be if we were working at a maximal level. This test is based on the assumption that a defined amount of work will require a set rate of oxygen consumption, or VO2, regardless of the fitness of an individual. The amount of work performed in this test is defined by the height of the step stool and the pace and number of steps performed. Everybody will perform the same amount of work and presumably consume the same amount of O2. Differences in fitness will be reflected by the amount of effort the heart must make in order to deliver that defined amount of O2. A person that is more fit will be able to consume the same amount of oxygen with fewer heartbeats than a less fit person. In other words, if two people of the same age perform the same amount of work, the person who is more fit will have a lower heart rate than the person who is less fit. See Figure 1. In addition to estimating VO2 max, recovery time after exercise can also be used as an indicator of cardiovascular fitness. The oxygen debt incurred during exercise can be paid back more quickly if the ATP-producing apparatus in the cell is working more efficiently. Therefore, faster recovery time indicates better aerobic fitness. We will be using the 1-minute post-exercise heart rate as a second indicator of aerobic capacity. Figure 1. Relationship between heart rate and oxygen consumption As activity level decreases and oxygen debt is repaid, heart rate decreases with the decreased need for oxygen. The heart rate's response to changes in the body s demands is called homeostasis. Homeostasis is the dynamic fluctuation of a physiological process in response to changes in the body s internal environment. Homeostasis can also be described as the maintenance of a body function within set boundaries. This chain of cause and effect is part of a monitoring system known as negative feedback: as activity level decreases, less ATP is needed. This decreases oxygen consumption which then causes O2 levels to increase and CO2 levels to decrease in the blood. The change in blood gas is monitored by chemoreceptors and sent to the brain, which then sends a signal to the heart to slow it down. Thus, the heart rate will match the body's need for oxygen delivery.

3 The maintenance of homeostasis underlies most physiological processes. Homeostasis is one of the physiological themes that we will return to again and again throughout the semester. Today s Objectives 1. Use the McArdle-Katch Bench Stepping Test to predict VO 2 max for each member of your group. 2. Correlate VO 2 max to each individual's aerobic fitness and the process of cellular respiration. 3. Measure post-exercise heart rate and correlate length of recovery time to aerobic fitness. 4. Apply post-exercise recovery of heart rate to the concept of homeostasis. 5. Determine an appropriate exercise regime for each of the various fitness levels. Predicting V0 2 max The McArdle-Katch bench stepping test is used to predict VO 2 max by having the subject perform a defined amount of work and estimating their energy expenditure based on their heart rate immediately after. The subject steps at a rate of 88steps per minute (females) or 96 steps per minute (males) for 3 minutes. The bench height is inches. Validity of the test is highly dependent on the accurate measurement of pulse rate (W.D. McArdle et al. (1972) Medicine and Science in Sports, Vol. 4, p ). Before You Begin 1. Do a quick self-assessment of your fitness to predict what your fitness level might be. If you have any health concerns that would prohibit you from participating, use data from a lab partner. Table 6 is a prescription for an exercise regimen recommended by the American College of Sports Medicine. Read the description of exercise intensity in Column 2 of Table 6. This is how intensely you should feel comfortable exercising, depending on your fitness level (Column 1). Make a guess as to where you might fit. 2. Practice taking either your radial pulse in your wrist or your carotid pulse in your neck. If you use your radial pulse, place your index and middle fingers over the radial artery on the anterior side of the wrist just lateral to the tendon of your flexor carpi radialis muscle (remember any of your anatomy?) and press lightly. If you use your carotid pulse, place two fingers just below the angle of your mandible, or lower jaw, just over the carotid artery.

4 Whichever method you choose, do not press too hard or it will occlude (close) the artery and make it difficult to feel the pulse. 3. Convert the number of beats you counted in 10 seconds to Beats Per Minute (BPM), which are the standard units used to measure heart rate, by multiplying this number by 6 as per the following formula: heart beats X 60 seconds = beats per minute 10 seconds 1 minute 4. Record your Heart Rate (HR) results in Table Take your resting pulse two more times and convert to Heart Rate. How close were your three measurements? Calculate your average resting HR by adding the three numbers together and divide by 3. McArdle-Katch Bench Stepping Test 1. One lab partner will be your recorder, a second will be your timekeeper. The recorder will write down your data. The timekeeper will signal you at each of the following time points: 10 seconds before the 3-minute mark as a warning 5 seconds post-exercise when you will begin counting your pulse beats for 10 seconds Every 30 seconds for the next 7 minutes when you will continue to count pulse beats for 10 seconds 2. Be sure you are wearing skid-resistance shoes, like athletic shoes or sneakers. Warm up a bit by gently stretching your leg muscles. 3. Set the metronome to 88 beats per minute for females or 96 beats per minute for males. You can turn off the sound of the metronome and use the flashing light to keep time if you prefer. 4. With the front of the steps facing the wall, brace the stepstool against the wall to prevent it from slipping. You will be stepping up to the top step of the stool, using a four-step cadence (right foot up, left foot up, right foot down, left foot down). If you are female, you will take 88 steps each minute. If you are male, you will take 96 steps in a minute. At the signal from the timekeeper, begin stepping in time with the metronome for a period of three minutes. 5. At the end of three minutes of stepping, remain standing and immediately find your pulse, using the same method you used to take your resting pulse rate. Begin counting pulse beats 5 seconds after you stop exercising for 10 seconds. Have your lab partner record this number in Table 1 as your zero time point. Leave your fingers on your pulse. Continue to report and record your pulse beats every 30 seconds by counting for 10 seconds, then waiting for 20 seconds, then counting for 10 seconds again. You will be counting your pulse beats every 30 seconds for 7 more minutes.

5 6. Convert beats counted in 10 seconds to Beats Per Minute (BPM). Table 1. Heart Rate Data (in BPM) Beats/10 sec Heart Rate (BPM) Resting Heart Rate #1 Resting Heart Rate #2 Resting Heart Rate #3 Average Resting HR 0:00 (5 sec after exercise) 0:30 (minutes:seconds) 1:00 = 1 minute Recovery HR 1:30 2:00 2:30 3:00 3:30 4:00 4:30 5:00 5:30 6:00 6:30 7:00 *NOTE: Only the zero time point (0:00) will be used for the McArdle-Katch Test. The rest of the time points will be used to plot a graph illustrating homeostasis of recovery heart rate. The 1 minute time point (1:00) will be used to calculate fitness from Table 6.

6 Data Analysis Determine Your Aerobic Capacity & Develop an Exercise Regimen 1. Determine your estimated Aerobic Capacity, or VO 2 max (ml/kg/min). Only the zero time point (0:00) will be used to estimate as part of the McArdle-Katch Test. Use the following equations, where HR = heart rate at the 0:00 time point converted to BPM. Females: Males: VO 2 max = ( X HR) VO 2 max = (0.42 X HR) Enter aerobic capacity in line 1 of Table Once you determine your aerobic capacity, you can use this information to design a personalized exercise regimen that will help you develop and maintain your aerobic fitness: a. Use the standards provided in Table 2 for females and Table 3 for males to determine your Fitness Level based on your estimated VO 2 max. Enter your Fitness Level in line 2 of Table 4. The standard error of prediction using these equations is +/- 16% of actual VO 2 max. b. Based on your fitness level, use Table 5 to determine the appropriate Exercise Intensity for your personal exercise regimen. Exercise intensity means working out at a pace that causes your heart rate to increase a given percentage of your heart's maximal ability. For example, if you are not very fit, you should begin your exercise regimen working your heart at only 60% of its maximum ability. If you are very fit, you can work your heart at 90% of its maximum ability. Enter your recommended exercise intensity in line 3 of Table 4. c. Your heart's maximum ability, or your maximum heart rate, is referred to as HRmax. Your HRmax is estimated as 220 your age. If you are 32 years old, then your HR max would be 188 BPM. Enter your HRmax in line 4 of Table 4. d. Your personal fitness regimen will be determined by your Target Heart Rate. This is how fast your heart should be beating while you are exercising in order to safely strengthen your heart and improve your aerobic capacity. Calculate your target heart rate by multiplying your HRmax times your recommended exercise intensity. Enter this number in line 5 of Table 4.

7 e. Columns 3 and 4 of Table 5 suggests how long and how often you should exercise at your target heart rate in order to improve your aerobic fitness. Enter these recommendations in lines 6 and 7 of Table 4. Now you have developed a personalized exercise plan without the cost of a personal trainer! The American College of Sports Medicine recommends using these guidelines for about six weeks, then retesting your aerobic capacity for improvement. If your aerobic capacity has improved to the next level, then you can increase your exercise intensity accordingly. Good luck and happy exercising! Table 2. Maximal Oxygen Consumption Rates Standards (ml/kg/min) FEMALES Age (years) Low Fair Average Good High < > < > < > < > < > < > < >35 Table 3. Maximal Oxygen Consumption Rate Standards (ml/kg/min) MALES Age (years) Low Fair Average Good High < > < > < > < > < > < > < >42

8 Table 4. Calculations 1 Aerobic Capacity: VO 2 max in ml/kg/min (from formula) 2 Estimated Fitness Level (from Table 2 or 3) 3 Recommended %HRmax (from Table 5) 4 Your HRmax (HRmax = 220 age) 5 Target Heart Rate for your fitness level (%HRmax X HRmax) 6 Duration of Exercise per session 7 Frequency of Exercise per week 8 Fitness level based on the 1 minute recovery HR Table 5. Exercise Prescription Guidelines Based on Fitness Level for Healthy Young Adults Fitness Classification Based on VO 2 max Low Females: < 29 ml/kg/min Males: < 34 ml/kg/min Average Females: ml/kg/min Males: ml/kg/min High Females: > 45 ml/kg/min Males: >50 ml/kg/min 60-70% HRmax Exercise Intensity Perceived exertion: fairly light to somewhat hard Unaware of ventilation rate; Breathing and depth is comfortable; Capable of passing the talk test 70-80% HRmax Perceived exertion: somewhat hard to hard Aware of ventilation rate (i.e. increased breathing rate and depth) 80-90% HRmax Perceived exertion: hard to very hard Hyperventilatory response; respiratory distress (i.e. rapid breathing rate with deep or large breaths); Incapable of passing the talk test Exercise Duration min/session min/session min/session Exercise Frequency 3 days/wk 4 days/wk 5 days/wk

9 Questions 1. What is VO 2 and how does it differ from VO 2 max? How does VO 2 max relate to cardiovascular fitness? 2. What physiological factors contribute to an increased VO 2 max and consequently a greater efficiency in energy production (ATP production). 3. What is the relationship between physical activity, O 2 consumption, and heart rate? 4. Why can heart rate be used to estimate VO 2 max? 5. The McArdle-Katch Bench Stepping Test is only about 85% accurate. What steps could contribute to potential errors? What might be some of the advantages and disadvantages to using this test to assess aerobic fitness? 6. What is the target heart rate and why can it be used to determine how hard an individual should exercise, depending on their fitness level? What was your target heart rate and how did your assessed fitness level compare to your prediction? 7. Why should fitness level be considered when deciding on a fitness regimen? Homeostasis of your Recovery Heart Rate 1. To see how your heart recovers from exertion, make a graph of your recovery heart rate by using the 7 minutes of data from Table 1. This graph will be turned in. Put "Recovery Time" on the X-axis and "Heart Rate" on the Y-axis. Label your graph with a title, and each axis with the proper units. Plot your Resting HR across the bottom of your graph as baseline. On the same graph, plot each time point for your post-exercise heart rate. Do not use your resting HR as a zero time point or included your resting HR in the plot of your post-exercise heart rate. Label on your graph the 1-minute post-exercise heart rate. 2. Use Table 6 to determine your fitness rating based on your 1-minute recovery heart rate. Enter this number in line 8 of Table 4. How does this rating compare to the rating you determined using the McArdle-Katch method?

10 NOTE: For a review of how to construct a graph, consult Appendix A: Graphing Basics, at the end of this manual. Remember, you can graph your data by hand or with a computer spreadsheet program. If you choose to graph by hand, you must use graph paper. Do not use binder paper or unlined paper. Table 6. Ratings for 1-minute Recovery Rates (Heart rate one minute after exercise) Fitness Rating Gender Heart Rate (BPM) High Male <71 Female <97 Good Male Female Fair Male Female Low Male 118 >147 Female 143 >171 For more information on VO2 max, go to Questions: 8. Why should time be plotted on the X-axis and heart rate on the Y-axis? 9. Draw out the feedback loop that governs changes in heart rate at rest and during physical activity. Describe with words how this diagram explains why your heart rate decreases with time after you stop exercising. 10. What makes this an example of negative feedback rather than positive feedback? 11. Explain why the 1-minute recovery rate is an indicator of cardiovascular fitness. Include the concept of homeostasis in your discussion. 12. How does the fitness level you estimated using the McArdle-Katch method compare to the fitness rating estimated using the 1-minute postexercise heart rate? What, if anything, does this tell you about using different methods to obtain similar information?