This was shamelessly stolen from the Team Performance Website, and is copywrited etc. etc.

Endurance, as defined in McArdle, Katch and Katch (1991), is the "time limit of a person's ability to maintain a specific power level involving muscular contractions". In other words, the intensity of the effort that one puts out directly determines the duration of the effort. The higher the intensity, the shorter the endurance.

ENDURANCE and MUSCLE

There are a few specific body systems that directly affect endurance and readily adapt to aerobic training. Among them are the cardiovascular system, respiratory system and most notably the muscular system. Without a doubt, a large percentage of the endu rance gains that we see with training are due to muscular adaptations. This is because there are so many changes that take place simultaneously in the muscles.

In the preceding article (Power to the Pedals) we discussed three types of muscle fibers: the "slow twitch" slow glycolytic (endurance), and the "fast twitch" fast-oxidative glycolytic (sustained power) and fast glycolytic (burst power). With training, we can hypertrophy (increase the size and capacity of) our slow glycolytic fibers in the muscles of our legs. Unfortunately, our genetic make-up determines the percentage of "slow" versus "fast" twitch muscle fibers that we have, and it is relatively imposs ible to turn "slow" fibers into "fast" fibers and visa-versa. But you can train each group to be more efficient, balancing the ratio between the fibers. This ratio of muscle fiber type directs us towards a cycling specialty, like the powerful track and cr iterium riders, or the high endurance road racer and time trial specialists.

Along with hypertrophy effects, there are many chemical adaptations in muscles. First, there is an increase in the size and number of mitochondria in the muscle cells, up to 200 percent! Mitochondria are the "power houses""of all living cells. They utiliz e the oxygen that our blood carries, mix it with the glucose (from the blood or stored in the muscle and liver) and produces adenosine tri-phosphate (ATP), the basic unit of energy for the cell. There is also an increase in the metabolic enzymes in the mi tochondria, allowing them to "run" more efficiently.

ENDURANCE, BLOOD and OXYGEN

When we inhale air into our lungs, oxygen passes through the small sacs (alveoli) into the blood stream, and upon expiration the carbon dioxide and water vapor (cellular exhaust) is forced out. This respiration refuels the blood with oxygen to carry to al l of our cells. Oxygen is carried in the blood in its gas form, but more predominantly it is bound to a carrier compound called hemoglobin. At the same time, myoglobin, the muscle's oxygen carrier compound, can increase as much as 80 percent, making for a n efficient machine! With aerobic training, our bodies adapt by increasing the level of hemoglobin and total blood volume. As the body produces more hemoglobin, and the volume of the blood increases, there is an increased volume of blood to be pumped with each stroke of the heart. To accommodate for this, there is an increase (hypertrophy) of the heart muscle, allowing the heart to pump more blood per beat, essentially increasing the cardiac stroke volume. This concept can be better understood with the fo llowing equation: Cardiac output = stroke volume x heart rate

So, as the stroke volume increases, the heart rate decreases (when the cardiac output remains the same, like at rest). This is why during the off-season, your resting heart rate might be 56 beats per minute, but as your stroke volume increases during the season, your resting hear rate may be somewhere in the range of 45-50 beats per minute (or lower!).

Note: the banned substance erythropoiten (EPO) increases the hemoglobin concentrations in the blood, allowing more oxygen to be carried and enhancing performance but at a potentially fatal price. As the percentage of hemoglobin increases, the viscosity (thickness) of the blood increases, putting a strain on the heart. This could ultimately lead to heart failure or a blockage of blood vessels.

One last adaptation in the cardiovascular system is the growth of new blood vessels and capillaries, termed neovascularization. Every year that we train, our body develops new networks of blood vessels to carry blood and nutrients to our muscles, specific ally 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. The bonus to neovascularization is that every year our bodies remodel and build onto the vast vascular network, enhancing our capacity for endurance every year we ride.

ENDURANCE and ENERGY

The main molecule of energy, as stated earlier, is ATP. It can be derived from carbohydrates, protein, and fat. With aerobic training, the body can more efficiently utilize fat (free fatty acids) for energy. As glucose goes through a process called glycol ysis, it produces 19 ATP per molecule of glucose. In contrast, one molecule of free fatty acid goes through beta-oxidation, which produces an amazing 441 ATP! So, by increasing the metabolic enzymes that fuel beta-oxidation and ATP production, you can sig nificantly enhance your level of endurance.

Aerobic training also effects lactic acid, the evil chemical that causes your legs to burn and your strength to fade. It has been shown that an enzyme called lactate dehydrogenase increases with training, and converts lactate into a chemical (Acetyl-CoA) that easily produces more ATP, allowing you to beat the burn.

PUTTING it all TOGETHER

Generally speaking, aerobic training takes place anytime you exercise enough to raise your heart rate. As a competitive cyclist, you will want to perform a high volume of aerobic training. One way to self-monitor your effort is to exercise at a level tha t allows you to carry on a conversation. This allows your body to utilize free fatty acids for energy, while enhancing all of the systems listed above. For those of you training with a heart rate monitor, this means staying 10 beats or so below your anaer obic threshold, or the point where you begin to loose your breath and lactate begins to accumulate in the blood.

Credentials: Brian Adams has a Master of Science in Physical Therapy, and a Bachelor of Science in Human Physiology, with special emphasis on Exercise Physiology and Athletic Training. Brian has been ra cing for eight years, and is a USCF Category I and NORBA Expert racer. Highlight finishes include '98 Michigan State Criterium Champion (Cat I,II) and a third at the '98 Collegiate Cyclocross Nationals, earning All-American honors.