Evaluation of Exercise Tests Used to Examine Aerobic Capacity
Aerobic exercise was developed by Dr. Kenneth Cooper in 1969, to study why some individuals with superior muscular strength, scored poorly in long distance running, cycling or swimming regimes (Cooper, 1969). Dr. Cooper’s research involved using a bicycle ergometer to measure sustained human performance, in terms of an individual’s ability to utilize oxygen (Cooper, 1969). The data Dr. Cooper collected for his research is the foundation for the development of all modern aerobic tests and programs (Cooper, 1969). Aerobic exercise describes a physical activity performed at moderate levels of intensity for extended periods of time; ultimately increasing heart rate (Donatelle, 1969). This includes exercises like long distance jogging but not sprinting (Donatelle, 1969). In aerobic exercise, glycogen is decomposed to produce glucose; however, when glucose levels are minimal, fat is broken down (Donatelle, 1969). The ability an individual’s respiratory and cardiovascular systems can meet the oxygen demands of exercising muscles is called aerobic capacity (NYCFD, 2006). It is the maximum volume of oxygen muscles will intake during exercise (Donatelle, 1969). High aerobic capacity translates to better performance (Donatelle, 1969), not only because oxygen is efficiently delivered to and used by muscles, but also because recovery from extreme efforts by the muscles is faster (NYCFD, 2006). To boost aerobic capacity, heart rate needs to be worked up to 70% to 85% of its maximum; this is known as the heart’s “Target Heart Rate” or “Training Sensitive Zone” (NYCFD, 2006). This can be done by participating in aerobic training that recruits large muscle groups, such as those in the legs (NYCFD, 2006).
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As exercise intensity increases, oxygen consumption (VO2) linearly relates to workload, but only to a certain point, where VO2 plateaus, even as exercise intensity rises (McArdle et al, 2001). This plateau value, known as the maximal oxygen consumption (VO2 max), is synonymous with aerobic power (Peterson, 2004), which is defined as the extent at which aerobic capacity, the peak aerobic energy strength, is exploited (Sports Resource Group, 2003). However, VO2 max is not the only predictor of aerobic capacity (Peterson, 2004); expressed as a fraction of VO2 max (% VO2 max ) (Peterson, 2004), the physiological value (LT), typically known as lactate threshold or anaerobic threshold (Peterson, 2004), also provides valuable information. LT defines situations when glycogen decomposition does not meet the required energy demands of the exercising muscles (Donatelle, 2005), and measures the degree of muscular and metabolic stress during exercise (Peterson, 2004). It is the point above resting level, when light elevating levels of exercise causes glycogenolysis increases (Peterson, 2004), and lactate begins to accumulate in active muscles and blood (Farrell et al, 1993). Although lactic acid build up is removed gradually by slow oxidative muscle fibers, lactic acid often builds up in muscles before LT is reached (Peak Performance, 2006). Since there is no definitive start point in lactic acid build up, a set lactate accumulation value of 4 mmol/L of lactic acid in the blood is usually used as the point of onset of blood lactic acid (OBLA) (Peterson, 2004).
LT is considered to be a more accurate predictor of aerobic capacity than VO2 max, however, it pertains only to the local muscles’ training state (Peterson, 2004). The LT phenomenon helps to explain why individuals with similar VO2 max can differ in endurance performance times (Peterson, 2004); those with a higher LT exhibit better ability to exercise because they use a larger fraction of their aerobic capacity (85-90% VO2 max) (Peterson, 2004). Factors leading to these results include examining whether subjects have undergone previous strength and endurance training in the muscles being analyzed and the percentage of Type 1 muscle fibers in the targeted muscles (Peterson, 2004).
Exercise tests that informatively examine aerobic capacity, should help describe the overall changes in the cardiovascular system, as well as the local changes in the muscles, active in exercise (Peterson, 2004). Theses changes, collectively known as adaptations (Peterson, 2004), include increase in plasma volume, blood flow redirection to active muscles, heart size enlargement, heart rate reduction (HR), stroke volume elevation (SV), increase in cardiac output (Q), total muscle blood flow increase during maximal exercise and blood pressure reduction (Peterson, 2004). When capillary density is increased because of aerobic exercise, muscles take in more oxygen (Peterson, 2004). The local adaptations of the muscles that should be regarded as a part of examining aerobic capacity include: increase in the number and size of mitochondria, more carbohydrate oxidation because of increased oxidative enzyme activity, improved fat metabolism, the amount of muscle and type of muscle fiber evident during the exercise (Peterson, 2004).
Today, common forms of aerobic capacity testing are the treadmill (TM), walk tests and cycle ergometer (CE) tests (Peterson, 2004). These methods are effective because they require the use of large muscle groups and are also cheap and straightforward enough for subjects to handle (Peterson, 2004). In all incidences, the clinical exercise testing protocols would involve initial warm-up; gradual increasing loads of uninterrupted exercise, with adequate duration per level; and finally a recovery period (Fletcher et al, 1995).
Endurance running can be defined as maintainable velocity over a given distance (Peterson, 2004). Measuring aerobic capacity using treadmill facilitated tests require subjects to walk at a light pace, then gradually pickup the workload at set time intervals (Donatelle, 1969). The equipment used is an accurately calibrated, standard treadmill with variable speed and grade capability (Fletcher et al, 1995). Subjects should refrain from tightly grasping handrails on the treadmill during the test, as this results in decrease in VO2 and increase in muscle exertion and exercise time (Fletcher et al, 1995). The duration of an average protocol is 6 to 12 minutes (Fletcher et al, 1995), but a number of different protocols exist, varying in the increments of time or amount of increase in workload (Fletcher et al, 1995). The ideal protocol however, should be tailored to the type of subject being tested (Fletcher et al, 1995).
The values usually measured in TM tests, VO2 and peak cardiac power output (CPOmax) , (Fletcher et al, 1995), give an idea about an individual’s aerobic capacity, and are used as data for many different studies. For example, studies have used TM tests to determine aerobic capacity of subjects (Williams et al, 2001 and Cooke et al, 1998). Their data gave evidence that CPOmax during exercise, was significantly related to aerobic capacity and also correlated to exercise duration (Williams et al, 2001 and Cooke et al, 1998). CPOmax was found to be an independent mortality predictor (Williams et al, 2001), and using TM cardiopulmonary exercise testing is beneficial because it is non-invasive, therefore less stressful for patients to participate (Williams et al, 2001 and Cooke et al, 1998). This finding was consistent in a population of normal subjects and individuals with heart disease (Cooke et al, 1998), and ultimately gave a more definitive idea about the extent of cardiac impairment of patients with heart failure (Williams et al, 2001 and Cooke et al, 1998).
A lower impact alteration of the TM test is the six minute walk test (6MWT), which is cheaper and simpler to conduct (American Thoracic Society, 2002). This test is usually used on patients with health problems, and measures the distance of hard, flat surface subjects can briskly over in 6 minutes (American Thoracic Society, 2002). The test is useful for evaluating the body’s overall and local adaptation responses involved in exercise (American Thoracic Society, 2002). This includes pulmonary and cardiovascular systems, systemic circulation, peripheral circulation, blood, neuromuscular units, and muscle metabolism (American Thoracic Society, 2002). However, since the 6MWT evaluates the submaximal level of functional capacity (American Thoracic Society, 2002), information generated is not specific about the causes of limitation (Johnson, 2004).
Measuring performance at submaximal levels of exertion, the 6MWD gives a good indication of the level of functional exercise in daily physical activities (American Thoracic Society, 2002). However, for patients with severe chronic obstructive pulmonary disease (COPD), the test generates a similar stress as a maximal test (Johnson, 2004). In studies conducted using a population of patients with cardiac and or respiratory problems (Solway et al, 2001), the 6MWT was established to be the easiest to administer, most tolerated by patients and most reflective of daily activities, out of 2-min walk tests (2MWT), 12-min walk tests (12MWT), self-paced walk tests (SPWT), and shuttle walk tests (SWT) (Solway et al, 2001). In order to study the effects aerobic and strength training have on improving aerobic endurance and muscle strength in female cardiac transplant recipients (Haykowsky et al, 2005), the 6MWT was administered to measure cardiac transplant patients’ aerobic endurance before and after placing them in aerobic training programs (Haykowsky et al, 2005). However, firm conclusions on the usefulness of the test in clinical practice, are still lacking (Opasich et al, 2001). In a study to investigate the correlation between walk test performance, cardiac function and exercise capacity (Opasich et al, 2001), it was found that for moderate to severe chronic heart failure patients, the 6-min walk test is not related to cardiac function and only moderately related to exercise capacity (Opasich et al, 2001). Therefore, the paper deemed the test to have only limited usefulness as a decisional indicator in clinical practice (Opasich et al, 2001).
Some disadvantages of the 6MWT is that being a time controlled test, the only way a subject can show improved aerobic capacity in subsequent testing, is by walking faster (Johnson, 2004). However, for some COPD patients, walking faster is difficult due to factors such as stride length (Johnson, 2004); a TM test on the other hand can accommodate for a steeper grade, hence allowing patients to show improvements in their overall condition (Johnson et al, 2002). TM testing can be deemed better than 6MWT as it is more versatile; it can be used with or without advanced monitoring such as continuous electrocardiography or expired gas analysis (Johnson, 2004). However, TM tests are more expensive and require more expertise (Johnson, 2004). Although 6MWT is a good test to repeat for the purposes of documenting decline in exercise tolerance (Johnson, 2004), TM tests are better at documenting improvements in function because they test at constant workload, and is therefore more sensitive (Johnson, 2004).
For individuals untrained in cycling, VO2 max is higher when tested on TM compared to CE (Peterson, 2004), while trained cyclists generated only slightly higher VO2 max values when tested via CE compared to TM (Peterson, 2004). VO2 values from TM tests are generally higher than those attained from CE protocols (McArdle et al, 2001). The cause is because most individuals are more comfortable walking or running, as oppose to cycling (McArdle et al, 2001). It could also be because CE testing causes discomfort and fatigue of the quadriceps muscles (Fletcher et al, 1995). Leg fatigue of an inexperienced cyclist causes subjects to stop before reaching a true VO2max (Fletcher et al, 1995), making the value 10% to 15% lower in CE than TM tests (Fletcher et al, 1995).
In cycling terms, endurance performance is the power output maintained for a given time (Peterson, 2004). CE tests require an initial power output of about 10 or 25W, followed by a 25W increase in 2 to 3 minute increments (Fletcher et al, 1995). Arm ergometry would require a similar approach, but with a smaller initial power output and lower incremental increases; usually every 2 minutes (Franklin, 1985 and Balady et al, 1985). Studies have also shown that it is possible to measure actual aerobic capacity in a single session by continuously increasing the load (Birkhorst & Leeuwen, 1963).
The equipment used for CE tests can either be mechanical or electrically braked cycles with adjustable variable force on the pedals (Fletcher et al, 1995). The highest values of VO2 and heart rate can usually be obtained with pedaling speeds of 50 to 80 rpm (Fletcher et al, 1995). The cycles are calibrated in kilopounds (kp) or watts, where 1 W corresponds to about 6 kilopound-meters per minute (kpm/min) (Fletcher et al, 1995). This can be converted to oxygen uptake in milliliters per minute for aerobic capacity measurement purposes (Fletcher et al, 1995). A cycle ergometer is usually less expensive, more space efficient and less noisy than a treadmill (Fletcher et al, 1995). Other advantages of CE tests are that upper body movement of subjects’ are reduced, which facilitates measurements in blood pressure and making ECG recordings (Fletcher et al, 1995). CE tests also give precise quantization of external work, ultimately facilitating the calculation of certain parameters for aerobic capacity examination (Johnson, 2004). Like the TM test, it is important that subjects refrain from exercising their arms in a resistive fashion, because this results in attaining inaccurate data for aerobic capacity (Fletcher et al, 1995).
In studies of patients with COPD, in addition to being inconsistent with patients’ normal activities, CE tests are less commonly used because they produced significant respiratory differences when compared with walking tests (Johnson, 2004). However, CE tests have been useful for identifying that the pathology of Gulf War veterans (GV) with chronic fatigue syndrome (CFS), do not show a decreased aerobic capacity like most normal CFS patients (Nagelkirk et al, 2003).
Overall, although TM, 6MWT and CE tests are all used to examine aerobic capacity, there are fundamental differences between the three tests (Peterson, 2004); these include differences in the muscle group exerted, pattern of muscle use, contraction speed and time of muscles and the metabolic processes (Peterson, 2004). In summary, TM and walking tests are more likely to identify oxygen desaturation (Turner et al, 2004). 6MWT is the easiest test to perform (Turner et al, 2004), and is adequate for most purposes (Johnson, 2004). However, depending on how and what the data from the aerobic capacity testing is used for, an important factor for choosing the most suitable test is the population of subjects being observed.
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