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The physiology of being in shape: adaptation at its best.

Every person who engages in prolonged vigorous movement realizes that with continuous training something happens over time that allows the body to withstand increasingly-demanding physical challenges. What would have sunk your ship in stormy seas just a few weeks ago now presents with calmer water towards your destination. This destination is what every serious athlete and dedicated exercise participant should be striving for throughout their training experience: to be in the kind of shape that allows for a positive adaptive transformation to occur that matches the physical potential Nature has granted each of us, and then a little more. Climbing the formidable mountain of being in shape should be the goal of everyone wanting to be labeled an athlete.

Getting in "shape" is neither easy to attain nor to explain. As you know, the human body is a wondrous machine with complicated systems able to produce great quantities of energy both quickly and over extended periods of time. This permits the body to adapt to whatever is physically challenging it, and enlarge its capacity to handle increasingly more vigorous exercise. Unlike an automobile engine which has the exact parts needed to produce a certain amount of predicted energy and power, the human body's components can be made to produce more by having them induced more through use of specific physiologic protocols over time. In every instance of adapting to exercise, three main elements, the holy grails of athletic training as 1 see it, are brought into the picture and must be addressed to a greater or lesser extent depending upon the venue and sport. These elements are endurance, strength, and power. Following the correct pathway to physiologic condition is like a professional concert or jazz musician mastering the three main woodwind instruments in proper order: clarinet, saxophone, and flute. The athlete should build endurance, then go for increased strength, and finally work to capture power.


Endurance is where it all starts if the coach/trainer and athlete correctly approach getting in shape. It takes the greatest amount of time and the most effort to develop all the physiological changes the body needs to build endurance. It is not a simple goal; it is an ongoing process. When speaking of endurance, we must include both the muscular and cardiorespiratory systems. In dealing with muscular tissue, endurance becomes specific to individual muscle groups. When dealing with cardio-respiratory endurance, we speak more of the body as a whole and its ability to sustain extended vigorous physical exercise. This becomes the more important aspect of physical fitness. If physical condition is suspect and fatigue sets in too quickly, muscular strength diminishes, as does neuromuscular coordination, concentration, and alertness. To prevent this and to correctly train the athlete, we need to increase the mechanisms required to harvest and utilize energy supplies for prolonged bouts of movement and to concomitantly increase the distribution of nutrients and oxygen throughout the body to sustain total body involvement.

Although fast-twitch muscle fibers are usually larger in size than slow-twitch, slow-twitch fibers can become up to 22% larger than fast-twitch fibers with effective endurance training. What this causes, however, is increased development of endurance fibers at the expense of pure power. Even the subtype of fast-twitch fibers (Fiia), which has more oxidative capacity than the absolute all-out fast twitch fibers (Fiib), develops more with endurance training. Consequently, the athlete ends up sacrificing all-out power for enhanced endurance.

Specificity of training will enhance one ability at the cost of another. A sprinter who trains mostly endurance will cause some fast twitch fibers to switch over to fire more slowly; this will lessen power and all-out speed, but will add the ability to perform longer. Along with the change in fiber type, a second adaptation occurs: an increase of more than 15% in the number of capillaries innervating muscle fibers which allows for greater exchange of O2 and CO2, heat, wastes, and nutrients between the blood and active muscle tissue. This is an important adaptation; the muscles are then able to contract more efficiently over an extended period of time to delay fatigue.

A third muscular adaptation to endurance training is the increased formation of the iron-containing protein, myoglobin. With appropriate aerobic training, muscle myoglobin can be increased in situ by up to 80%, allowing for a much better oxygen supply.

The fourth adaptation to aerobic training is an increased number of muscle mitochondria, allowing for increased energy production throughout the working muscles. Again, specificity of training: only those muscles being called upon regularly will produce more mitochondria. This is the goal of much of our training: work the main muscle groups needed to power the athlete through the event's requirements, but don't ignore the ancillary groups that can be used to support the whole body through various movements. Total body development is key to superior athletic performance.

It takes vigorous exercise to better induce mitochondria to enlarge, multiply, and perform efficiently. But it's important to remember that as the production of mitochondria progresses to where they split and double in amount, those doing the splitting temporarily lose their ability to provide energy. During this time, the athlete may feel sluggish and fatigue more easily. This is only a temporary condition until all the new mitochondria are able to contribute to the aggregate energy supply.

The fifth adaptation to occur in aerobically-trained muscle is the enhanced ability to utilize free fatty acids (FFAs) for energy, sparing more of the carbohydrate stores until later in the event or training session for fueling speed.

The cardio-respiratory system's response to endurance-type training is even more encompassing as the major systems adapt to deliver oxygen and energy in greater supply per unit time. The heart's left ventricle gets larger and the wall thickens to increase the stroke volume, thus the "athletic heart." And, of course, the heart rate decreases during rest and in sub-maximal activity because of the heart's increased efficiency. Several studies have shown that an average of one beat per minute per week is dropped as cardiac condition improves. After six months or more of training, some responsive athletes can drop their resting heart rates by 20 to 30 beats per minute or more.

The conditioned athlete also benefits from increased VO2max; however, in fully-matured athletes, the highest attainable VO2max is reached within eight to 18 months. This indicates that athletes have genetic limits to maximal oxygen consumption.

The respiratory system can be enhanced to a greater percentage increase than cardiovascular function. Though respiration at rest or with easy movement does not increase in functionality from aerobic training, the tidal volume rises consistently at maximum aerobic effort, as does the respiratory rate. This is due to increased usage of respiratory tissue, its flexibility in function, increased activity of the intercostal muscles, and increased vascularity for enhanced O2 and CO2 respiration.


The athlete may ask the trainer or coach, "Why do 1 need more strength? Why should I devote time and effort to weight training when my focus is endurance?" Of course the answer to that athlete would be, "Because a stronger athlete can perform the same tasks with less effort and this translates into less fatigue over time." And as you know, the athlete must also work on areas that are ultimately involved with muscle. Connective tissue must be made to adapt to handling increasing resistance from resistance training; tendons must be forced to adapt to handle stronger and thicker muscle fibers that result from this training. Within the first 10 weeks of strength training, the nervous system also adapts by producing more motor-neuronal units.


Powerful athletes can move through their sport or activity-specific requirements with speed and grace, and training for power should be as sport-specific as possible.

Moving heavy resistance is not enough. Moving heavy resistance quickly but under control is what develops power (with sufficient rest and recovery between power-training bouts, of course).

When pushing the body through bouts of power-generating activity, past the "comfort zone," the athlete is also intentionally creating chemical buffers at the cellular level, e.g., bicarbonate forms to absorb lactic acid and delay paralyzing acidosis.

Some athletes are just genetically gifted, having a greater percentage of fast-twitch muscle fibers to produce more power. Being the largest fibers in muscle, fast-twitch react the quickest when voluntarily asked to contract; however, without a blood supply, the ability to produce energy and remove waste is hindered, and thus limited. The slower type of fast-twitch fiber can be induced to have increased blood innervation which allows two things to occur: 1) an increase in the power of the athlete to be able to be hold for a longer time, but 2) the absolute amount of potential power able to be generated is somewhat diminished. The positive aspect of this is that the increased power produced, though not at an absolute maximum, can be held over a greater period of time. The athlete then has the ability to pursue the power event longer and stronger to the finish. Fast-twitch fibers also retain their ability to produce power much longer, up to six months, during detraining than slow-twitch fibers, which lose their functional aerobic endurance capacity within approximately two weeks of inactivity.

Getting in good condition is a relative thing. Almost everyone has an innate ability to rise to their optimum level, but most who commit to enhancing their physiologic condition will stop short of this. They may fail to see the importance of capturing the elements of endurance, strength, and power. As a result, they will miss the opportunity to achieve an optimally conditioned state.

Coach Ed Nessel, a frequent contributor to the AMAA Journal, is the United States Masters Swimming (USMS) National Resource Librarian and an active member of the Sports Medicine and Coaches Committees. He was selected USMS Coach of the Year in 1998 and was invited to coach at the Olympic Training Center in 2002.

By Edward H. Nessel, RPh, MS, MPH, PharmD
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Author:Nessel, Edward H.
Publication:AMAA Journal
Geographic Code:1USA
Date:Mar 22, 2009
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