The Physiological Effects of Endurance Aerobic Training
Your ability to sustain long efforts can be attributed to all of the base miles that you put in at the beginning of the season. As you transition from the gym back to the bike the endurance training you do begins the process of building fitness, capacity and the energy systems needed for sub threshold work. Generally speaking, “endurance” can be defined as the “ability to maintain a specific power level, involving muscular contractions, for a given period of time”. There is aerobic endurance, threshold endurance and anaerobic endurance. The intensity of the work will define the limits of the efforts and limits you have at certain intensities define your endurance for that level of work.
Endurance Training and Muscular Endurance
There are a few specific body systems that directly affect endurance. Among them are the cardiovascular system, respiratory system and most notably the muscular system. Without a doubt, a large percentage of the endurance gains that you see in your training are due to muscular adaptation. Muscle is where energy metabolism takes place and muscular contraction is the end result if a series of a very complicated physiological and biological process.
There are basically three types of muscle fibers that produce the different ranges of power you use on the bike:
“Slow twitch” slow glycolytic (endurance)
“Fast twitch” fast-oxidative glycolytic (sustained power, maximal aerobic efforts)
“Fast glycolytic” (non oxygen use used for bursts power).
Your genetic make up strongly influences the composition of the muscle you use on the bike. With specific training, you can increase the size and capacity (hypertrophy) of all the cycling muscles. However, it is virtually impossible to convert say “slow” twitch fibers into “fast” twitch and vice versa. However, you can train each of these groups of muscles to be more efficient and effectively re-balance the ration between the fibers. The ratio of fibers will be a strong determiner of your event specialty in cycling, from track sprinter, to ITT specialist to high endurance road racer.
Along with hypertrophy effects, aerobic training will cause many chemical adaptations in the muscles. Sustained aerobic training has been shown to increase the size and number of mitochondria in the muscle cells by as much as 200%. Mitochondria are the “power plants” of all living cells and where energy metabolism takes place. The mitochondria receive the oxygen carried in your blood and mixed it with glycogen (from either the blood, or stores in the muscle or liver) to produce adenosine tri-phosphate (ATP), the basic unit of energy for the cell that fuels muscular contraction to create movement. Another chemical adaptation is the increase in production and efficiency of metabolic enzymes in the mitochondria. Metabolic enzymes fuel glycolysis (that breaking down carbohydrates) and beta-oxidation (the breaking down fats) to produce ATP more efficiently during exercise.
Endurance Training and the effects on Blood Flow and Oxygen
The delivery of fuel to the cells in your body and the processes by which your body maximizes the use of this energy is a truly remarkable process. To put it simply, when you breathe air into your lunges, oxygen is exchanged with carbon dioxide in the blood through the small air sacs in the lunges called alveoli. When you exhale you are expelling the carbon dioxide along with water vapor (both are byproducts of aerobic energy metabolism in the muscles) with each breath.
The next breath delivers a fresh load of oxygen to the lungs for another round of gas exchange and the blood stream delivers it to the cells in your body for energy production. This involuntary process continues 24 hours a day 7 days a week.
Oxygen is transported in the blood as a gas attached to a carrier compound called hemoglobin. Hemoglobin is carried by the red blood cells. The amount of red blood cells is regulated by the amount of oxygen that reaches the tissues in the body. If a lack of oxygen is detected, the kidneys will produce the compound erythopoetin (EPO), which stimulates the bone marrow to produce more red blood cells. The amount of red blood cells in the blood is measured in terms of hematocrit levels (the % of blood volume made up of red blood cells). Normal hematocrit levels can range from 39-54% for men and 35-48% for women.
Upon delivery to the muscle oxygen is transferred from the hemoglobin to the carrier compound in the muscle cell called myoglobin. The stresses of aerobic training cause adaptations that make this process more efficient by increasing the level of hemoglobin in the blood and increasing the total blood volume.
The increase in blood volume causes another adaptation in the heart muscle. The heart must now develop the capacity to pump more blood per stroke (heart beat). Hypertrophy (increased size) of the heart muscle allows more blood to be pumped per stroke. This is called increasing cardiac output. Cardiac stroke volume (the pumping capacity of the heart) is represented in the following equation.
Cardiac Output = Stroke Volume x Heart Rate (in beats per minute)
So, as the stroke volume increases, the heart has to beat fewer times per minute to maintain blood flow during all levels of activity. This is why during the off-season, your resting heart rate might be 56 beats per minute, but as your stroke volume increases as a result of increased aerobic training your resting heart rate may be somewhere in the range of 45-50 beats per minute (or lower!). This adaptation is also felt during high-level exercise when the heart has to do less work to keep the muscles properly fueled and flushed out and you see a lower HR response to the work being done.
Note: artificially produced EPO is a banned substance that increases the hemoglobin concentrations in the blood allowing more oxygen to be carried and enhancing performance but at great health risks. As the percentage of hemoglobin increases, the viscosity (thickness) of the blood increases. This puts great strain on the heart and increases the risks of clotting.
One last adaptation in the cardiovascular system is the growth of new blood vessels and capillaries, known as “neovascularization”. Every year that you train, your body develops new networks of blood vessels to carry blood and nutrients and fuels to your muscles, specifically the ones we use the most during training. Once these vessels are “built” they are easy to maintain with light exercise in the off-season that keeps the pathways open and flowing. The bonus of neovascularization is that every season your body remodels and builds onto the vascular network, enhancing your capacity for endurance and power.
Endurance Training and Energy
Remember ATP, the main molecule of energy for muscle contraction? It can be derived from three different sources in your diet, Carbohydrates (sugars), Protein and Fat (fatty acids). Carbohydrates (sugars) are converted to a more easily consumed form called glucose. Glucose is in turn converted to ATP through a metabolic process called glycolysis (breaking down the glucose molecule). This produces 19 ATP molecules per molecule of glucose. In contrast, one molecule of fatty acid goes through a different process called beta-oxidation (breaking down the fatty acid molecule) and produces 441 molecules of ATP.
Endurance level training increases both the activity and efficiency of beta-oxidation and increases the use of fatty acids for fuel at sub-maximal intensities. The dramatic differences in yield of ATP molecule makes fatty acids the preferable choice of fuel for sustainable endurance level work.
Aerobic training also effects lactic acid, the chemical that causes your legs to burn and your strength to fade during extended hard efforts. Lactate is produced in the muscles, in large concentrations, during anaerobic level efforts. When the levels of lactate are too high in the muscles and blood stream for the body to deal with you get the burn and have to back off to recover. It has been shown that an enzyme called lactate dehydrogenase increases with aerobic level training. This enzyme helps your body convert Lactate into a chemical called (AcetyLcoA) that easily produces more ATP, allowing you to beat the burn. This process is called lactate turnover and can be trained effectively through aerobic/endurance exercise.
Putting it all Together
Generally speaking, effective aerobic training takes place anytime you exercise enough to raise your heart rate to within 70% of your maximal HR. As a competitive cyclist, you will want to maximize the benefits of aerobic conditioning by training at the most effective levels for your current fitness. One way to monitor your effort is to exercise at a level that allows you to carry on a conversation. This allows your body to utilize fatty acids for energy, while minimizing the production of lactic acid and enhancing all of the systems and processes mentioned earlier. For those of you training with a heart rate monitor this means using a HR limit that is about 85-90% your maximal aerobic capacity (MAC) heart rate for long periods of time. This level of effort will yield consistent benefits to your aerobic fitness and provide many hours of high quality social training time.