Marathon Physiology
Physiologic principles and adaptations form the basis for marathon training requirements.
There are two basic principles which underlie all types of athletic training: overload and
specificity.
Overload
Overload means exercising at a level which causes the body to make specific adaptations to
function more efficiently. Overload does not mean overtraining. Think of a rubberband - as
more pull is applied it stretches more and becomes easier to stretch, but too much pull can
cause it to snap. To keep the runner from breaking, i.e., becoming injured or ill, overload
must always be used in conjunction with rest. Overload and rest form the basis for what is
known as the "hard/easy" training approach. This technique uses variations in frequency,
intensity and duration to achieve cycles of overload and rest. Frequency is how often you
run, intensity is the pace at which the workout is conducted and duration is the time spent on
an individual run. In a program of increasing mileage or of building basic endurance, "hard"
may be a long slow run while "easy" may be a shorter distance run at the same effort. For
the experienced runner with an established mileage base, "hard" might be a shorter workout
of increased intensity such as hill work, fartlek or some kind of interval training. After a hard
workout, rest or an easy workout is important because it allows the muscles and other
tissues a chance to rebuild and adapt to the stress. This is the basis of overload training.
Specificity
Specificity refers to adaptations of both metabolic and physiologic systems, depending on the
type of overload used. Specific exercise brings about changes in those systems used in that
particular exercise. Running is obviously the specific training for running. Different
adaptations result from different kinds of running using variations of frequency, intensity,
duration and terrain to utilize different sources of energy. This is where long term goal
setting is so important: you need a running program designed for the specific type of races
you want to run. This approach will assist you to maximize performance and eliminate
wasted effort. Specific endurance training with its resultant physiologic adaptations is
essential for marathons.
Energy Sources
Energy to perform work comes from the generation of a substance called ATP, (adenosine
triphosphate). The breakdown of the food provides the sources of this energy. The main
energy sources used in endurance running are carbohydrates, (stored in the body as
glycogen), and fat. When these materials are broken down in the presence of oxygen, the
metabolism or energy used is termed aerobic. The waste products of this aerobic
metabolism are water (H20) and carbon dioxide (CO2). Metabolism is termed anaerobic
when the glycogen is broken down to form pyruvate and, lacking oxygen, further breaks
down into lactic acid.
The body can endure only limited amounts of lactic acid. If lactic acid accumulates, fatigue
occurs faster and glycogen breaks down rapidly, depleting your energy source. Anaerobic
metabolism provides energy for short intense exercise such as sprinting or for bursts of
speed in sports like soccer and basketball. In prolonged exercise, the major metabolic
pathway used is the aerobic one. When aerobic metabolism cannot meet the requirements
for energy due to either increased intensity or prolonged time, anaerobic metabolism may be
called in for short term assistance.
Carbohydrates are the most readily available source of energy in the muscle and are utilized
mainly in faster continuous running, especially for shorter distances. Fat is the major supplier
of energy at slower paces or for longer distances. More oxygen is required to produce an
equal amount of energy when fat rather than glycogen is used as the energy source. In
addition fat requires some breakdown of carbohydrates to be taking place simultaneously.
The body can not accumulate adequate glycogen stores to provide enough energy for the
duration of the marathon. Many marathoners have experienced a severe energy loss around
the 20 mile point in a marathon. The effect is referred to affectionately as "Hitting the Wall".
Hitting the wall is thought to be the depletion of muscle glycogen. Fat stores (even in the
leanest runners) are almost inexhaustible. However, when muscle glycogen is gone, fat is
probably unable to be used because it needs some carbohydrate as a primer for its
metabolism.
Marathon Requirements
Running the marathon requires a combination of both carbohydrate and fat metabolism. The
physiologic goals of a marathon training program are to provide enough endurance training to
optimize aerobic metabolism; to have the ability to metabolize fat while running faster paces
and to facilitate increased storage of carbohydrates.
These goals require adaptations to allow greater consumption and utilization of oxygen.
Some of these changes occur at the local muscle level and include better utilization of
oxygen through increased size and number of mitochondria (little energy factories within the
cells) and an increase in their aerobic enzymes. These muscles can more easily mobilize
and use fat for energy, which helps to preserve the carbohydrate stores. The body also
develops a greater ability to store and utilize carbohydrates. Some muscle fibers can be
adapted for aerobic or anaerobic metabolism exercise. For the marathon, you want to adapt
these convertible muscles for aerobic or endurance work. Through training there will be an
increase in the number of capillaries for better nutrient supply as well as an increase in the
amount of muscle tissue.
Adaptations of the cardiovascular and respiratory systems are also important results of
training. The heart muscle increase in size, weight and the amount of blood increases.
Resting and submaximal exercise heart rates are decreased. The amount of blood that the
heart pumps, called the stroke volume, increases which aids in increasing the amount of
oxygen that can be extracted from the blood through better distribution of blood to the
working muscles. As the blood goes through the muscles, increased utilization of oxygen
from the blood results in increased consumption of oxygen.
VO2 max
One of the most important training adaptations is an increase in the maximal oxygen uptake,
called VO2 Max, which is a quantitative measure of a person"s capacity for aerobic energy
transfer (the ability to do work). An improvement in VO2 Max thus increases the amount of
work you can do, that is you can run faster and/or farther. Variables that determine VO2 Max
are heredity, sex, body composition (the amount of lean body tissue), age and training.
Obviously you can do nothing about several of these variables, but improvements of 20-25%
in maximal oxygen uptake because of training have been observed. VO2 max peaks within 6
months to 2 years after starting an endurance training program. However, even after it has
levelled off, it is still possible to improve performance. Typical marathoners are able to
maintain their pace using approximately 75-80% of VO2 Max for the well over 2 hours
required. Some athletes, notably ultramarathoners, are able to work for prolonged periods at
levels approaching 90% of VO2 Max. The ability to run at higher percentages of VO2 Max
may be explained by the concept of anaerobic threshold. Anaerobic threshold is the point
when increasing intensity of exercise causes lactic acid to accumulate and impair
performance. Improving the anaerobic threshold means that you can run harder for a longer
period of time without going into anaerobic metabolism and accumulating much lactic acid.
Studies have shown that anaerobic threshold can be increased by endurance training.