Chapter 03: PRINCIPLES OF EXERCISE PHYSIOLOGY
PRINCIPLES OF EXERCISE PHYSIOLOGY
Phillip Bishop, Ed. D.
Professor Emeritus of Exercise Science, University of Alabama
THE HUMAN BODY is composed of many integrated systems. The excellent working of all these systems is essential to good health and great sports performance. Some of these systems play a key role in sports performance and will be introduced in this chapter. This is not comprehensive of any of these systems necessary to body function.
The Cardiovascular System
The cardiovascular system is composed of the heart and blood vessels. For us to remain alive, we need this system to deliver vital oxygen and nutrients throughout our entire body. The number-one cause of death in the USA in coaches, retired sportsmen and the general public is cardiovascular malfunction.
In sports, where prolonged muscle contractions are required, training the cardiovascular system is essential. For example, long distance runners, cyclists, skiers and swimmers must be able to pump large quantities of oxygen to provide energy for muscle contractions over the entire length of their event. In these athletes, the heart is doing two things – it is pumping large quantities of blood each beat and it is beating very fast for the duration of the event.
If you examine a healthy heart in any athlete – or even from a pig, large goat or cow – you will notice a few of the same details. The heart is actually a large left chamber wrapped around a smaller right chamber. You have to look closely to see the two small chambers (the atria) sitting on top of the left and right heart. There are thin valves that often function over 70 years without external maintenance. The right side of the heart is small, relative to the size of the left side of the heart because the right heart only needs to move blood a few inches to fill the lungs since the heart is located in the upper-middle of the left lung. If the lungs are healthy and functioning well, the blood pumped there will be exposed to oxygen.
The hemoglobin in the blood picks up a full supply of oxygen for future use to produce energy for the muscles. The oxygenated blood from the lungs is pumped back to the left heart and the left heart has the task of pumping blood to the muscles all over the body – from foot to brain. Moving large amounts of blood requires a large, healthy left-heart muscle. Training the heart requires gradually exposing it to high workloads in training.
All athletes need a good supply of hemoglobin to carry oxygen. Low hemoglobin, more common in women, can hurt the performance of any athlete. Utilize your team physician to evaluate the hemoglobin levels of your endurance athletes and recommend proper treatment for those who have low hemoglobin levels. Blood doping, a common phrase used in professional sports, is an illegal manipulation of the blood to supply it with extra hemoglobin thereby raising the oxygen-carrying capacity of the blood.
The pumping action of each side of the heart is accomplished by valves. The mitral valve in the left heart must close and remain sealed when the left-heart powerfully contracts, or squeezes. Consequently, in some athletes, often long-distance runners, the valve leaks. This creates a heart murmur which reduces the heart’s maximal output to the muscles. Athletes with leaky valves or valves that are pushed down into the heart chamber will not be able to perform at their best. Again, your sports physician will be able to detect this and may be able to offer some treatment in severe cases. The blood vessels are distributed throughout the body and blood flow amounts are controlled by making these vessels bigger or smaller. The radius. i.e., the length from the center of a vessel to its inside perimeter, impacts the volume of flow by the fourth power. This means the radius of a blood vessel doubles the flow four times. For example, if the radius increases by a factor of two, the flow increases 2x2x2x2 or 16 times as much flow.
The left heart responds to endurance training by increasing in size over time. The more endurance training an athlete does, the larger the size of the left heart, which allows it to pump more blood with each beat. The amount of blood pumped in each beat (or stroke) is called stroke volume. Not all the blood is emptied from the left heart when it contracts. The percentage of blood ejected is called the ejection fraction. A strong left ventricle is able to eject most of the blood delivered to it from the lungs and has a high ejection fraction. The heart of a well-trained endurance athlete has a maximal beating rate slightly lower than when untrained, probably due to the increase in muscle mass. But the slight reduction in rate is overcompensated by the very high stroke volume, resulting in a very high amount of total blood pumped per minute. This is called cardiac output. Each year of life the maximal heart rate in humans tends to drop by about one beat per minute. This results in the maximal cardiac output lowering by about 0.5% for a 20-year-old getting a year older and partly explains some of the decline in performance when athletes age, especially past the mid-30’s.
Training the Cardiovascular System
The cardiovascular system, like most body systems, adapts to the stressors placed on it. If the heart is stressed time and time again to pump more blood, the left chamber muscle will enlarge and the heart muscle will get stronger. This increases oxygen delivery capacity by the blood which increases energy output capacity and improves endurance performance. To train the cardiovascular system for maximum endurance, athletes should gradually increase the length and intensity of their training.
The blood volume of the heart also increases to meet the body’s demands. If the heart is stressed to supply blood to more muscles, the response is greater. So, for example, cross-country skiing uses both the legs and the arms over long durations which requires more blood. Over time, this results in larger adaptive increases than running or cycling alone, which only use your legs. Swimming strokes requiring vigorous leg muscle engagement (e.g., breaststroke, butterfly) would produce larger cardio improvements than strokes with less leg muscle engagement. When practical, coaches may want to experiment during the off-season with different types of training utilizing more muscle groups, which will reduce monotony and joint stress and may produce better cardiac performance over time.
In very short, intense forms of exercise such as competitive weightlifting, the need for oxygen is low and oxygen delivery is not a limitation to exercise. Even though these types of athletes may not need endurance training for their performance, some endurance training will be beneficial to their overall health since heart disease claims many lives.
The Respiratory System
The respiratory system is chiefly made up of the trachea and lungs. When we inhale, or take a breath, the lungs receive air from the atmosphere by creating a lower pressure. The trachea has cartilage in it, making it stiff, such that it remains open at all times. In the breathing cycle, the low pressure is created by increasing the volume of the chest cavity. This is achieved by flattening the diaphragm (which is curved upwards into the chest) or by lifting the ribs with the respiratory muscles to make them rounder. When heavy breathing is needed, both approaches maximize the pressure drop and increase the amount of air with each respiratory cycle.
Exhaling happens when the diaphragm is relaxed, or the respiratory muscles relax to reduce chest volume, thereby making the pressure in the lungs higher than outside in the atmosphere. Fortunately, our respiratory capacity exceeds our ability to pump blood, so desaturation due to respiratory insufficiency does not usually occur in healthy athletes at or near sea level.
The role of the respiratory system is to move oxygen from the atmosphere into the blood for ultimate use by muscles (mostly) and other organs. It also transports the gaseous products of metabolism, carbon dioxide, from the site of energy production to the blood then to the lungs. Gas exchange happens when the gas exists in different concentrations on two sides of a membrane that allows the gas to pass through. For example, our hard breathing whilst running, mechanically delivers air from the atmosphere with an oxygen concentration of ~21% to the little chambers of the lungs, where, in heavy exercise, oxygen concentration can be almost 0%. But the oxygen level doesn’t ever get this low in our bodies because we have blood returning not only from muscles working very hard (e.g., the leg muscles of a runner or the arm muscles of a rower) but also blood from unused muscles of the body, which ideally are relaxed and using only a little of the oxygen supplied to them.
God designed the lung tissue and the walls of the blood vessels which surround the lung cells to easily allow passage of oxygen into the blood and carbon dioxide out of the blood. So exercise situations requiring lots of energy for lots of muscles, such as cross-country skiing will use up much of the oxygen in the blood and produce lots of carbon dioxide. The limitation to performance in an endurance sport is the heart’s ability to pump blood to the muscles and lungs for energy production in the muscles and delivery of carbon dioxide and pickup of oxygen in the lungs. In healthy athletes, the lungs will meet their demands readily. It is the heart and muscles that need to be trained to increase their capacities.
Altitude Training
As a person ascends to a higher altitude, the pressure of the atmosphere on the lungs diminishes simply because there is less air above us pushing down. Consequently, the partial pressure of oxygen declines as we ascend. So, if we take an endurance runner, cyclist, swimmer or skier to high altitude, the stressors on their respiratory and cardiovascular systems will rise, due to less oxygen delivery to the blood. The effects of pressure change become more noticeable at above 2,100 meters. At altitudes higher than this, the body will make short-term and longer-term adaptations in an attempt to deliver more oxygen to itself. Historically, the problem with taking athletes who have been training at altitudes below 2,100 meters to higher altitudes to train is that few can sustain their training volume and intensity at the higher altitudes and gains in system adaptation are overwhelmed by the detraining they experience.
This has led to a training concept of “Live-high, train low.” Whereas this approach is impractical for most people, attempts have been made to use altitude chambers, oxygen-reduction equipment (not the same as pressure, but it works the same in our respiratory system) and other approaches to induce performance adaptations without having to commute up and down tall mountains. If this is of interest to you, we recommend you investigate this training possibility, since it is beyond the scope of this introductory text.
Regardless of your interest in altitude training, keep in mind that competitions held at altitudes greater than 2,100 meters will be especially challenging for some endurance athletes. If a competition is to be held at an altitude higher than what the athlete is typically training in, expose the athlete to the competition altitude for as long as practical prior to the competition. Altitude not only reduces oxygen delivery, it can also impact nutrition and sleep, both of which are vital to good performance.
Muscle Training
Muscle training is essential to improving most sports performance. The muscles of the body all work by shortening, or contracting. With few exceptions for skeletal muscle, the shortening of the muscle causes a bone to move, which allows us to lift a weight, throw an object, move the body or manipulate a ball or other object.
Muscles are connected to bones by tendons and bones are connected to other bones by ligaments. Where the muscle attaches to the bone via the tendon, in relation to the bone’s attachment to another bone via the ligament determines the relative leverage force, or speed, of limb movement. In humans most agree that our anatomy generally favors speed over force.
The muscles are composed of actin and myosin filaments which ratchet past each other in one direction during contraction to exert muscle force. It is logical and accurate that the more fibers that ratchet, the stronger the force. In general, the larger the cross-section of the muscle, the more force produced. For example, the quadriceps muscles on the front of the leg have larger cross-sections than the biceps in the arms and can exert more force. It is also obvious that for a muscle to become stronger after the initial nervous adaptation, the increase in strength will be marked by an increase in muscle cross-section. Therefore, very strong athletes tend to be very large-muscled.
Basics of Muscle Training
Although we can make muscle training very complex, the basics are simple. Muscle groups, like all of the body’s systems, are trained by stressing them, then providing nutrients and allowing them to respond during a recovery period. The adaptation the muscles make is directly related to the type of stress they experience. That is, muscles exposed to very short bouts of high contraction respond by increasing size and force production capacity. Muscles exposed to repeated lighter contractions over longer time intervals develop increased energy processing capacity and endurance increases. We use training cycles to facilitate recovery and adaptation.
The muscles, like all the adaptable systems of the body, respond to stress by changing such to reduce that stressor. This is called The General Adaptation Syndrome. This Syndrome says that if a stressor is applied (e.g., a resistance training workout), and nutrients and recovery are available, the part that is stressed will adapt in such a way to reduce the stress. Specificity means that the adaptation is specific to the type of stressor applied. Frequency of training is determined by how long it takes an athlete to recover and adapt which varies by the stressor and individual characteristics of each person. Intensity refers to how hard someone has to work to create stress. Duration, the length of a workout, likewise, is dependent upon how much stress is required to produce more adaptation. Mode of training is deter- mined by what adaptations are desired.
For there to be an optimal adaptive response (e.g., increased sport performance) the training stim- ulus must be at a greater level of intensity and frequency than the person is accustomed to, which is basically the principle of overload. However, the overload stimulus must be applied in a gradual progression to allow recovery and to avoid injury and over-training responses.
Perhaps the most important principle of training is specificity. The specificity principle states that an athlete will get specific outcomes based on the type of exercise they perform. Simply stated, specificity says that an athlete will, “reap what they sow,” with long-term training. If exercise is solely lifting heavy weights, the athlete will increase her ability to lift heavy weights that is, strength training results in strength. If a coach has an athlete spend all her time running or cycling long distances, she will become very good at running and cycling long distances. If the coach wants to improve both muscular and cardiovascular fitness, then a combination of training approaches is required.
In general, each sport requires a unique training program that is designed to develop optimal levels of sport fitness within all parameters: body composition, flexibility, cardiovascular fitness, muscular strength and muscular endurance. A wise coach will GRADUALLY increase training. Too much over- load too quickly vastly increases the possibility of injury.
Recovery plays an important factor in the athlete’s ability to adapt to stress. If no recovery is allowed, the body is likely to over-train and become injured. The amount of recovery that may be needed is different for everyone and is related to many factors such as a person’s sleep quality, diet, physical fitness level and psychological stress. An optimal training program must be structured to allow for sufficient recovery, even though it requires exercise on most days of the week. Typically, the type of workouts are alternated to allow for recovery of a given body part or physiological system, while continuing to train. Our experience and research demonstrates that some people need a longer recovery than others and, as much as possible, recovery should be tailored to the individual more than to the team as a unit.
What goes up must come down. This quote certainly applies to sport training. Physical adaptations achieved through training will disappear when training is reduced. Most of us coaches, in our enthusiasm, do not allow sufficient recovery, and consequently our athletes’ bodies are too tired to perform as well as they could.
So, in summary, on the most basic level, muscle training programs must stress the muscles in the same manner that we hope to produce increased muscle function. Strength increases occur in tandem with size and are obtained by exposure to very high loads. Endurance adaptation occurs when endurance loads are applied. Regardless of the stressor, recovery is vital to allow muscles to adapt and to avoid over-stress and muscle injury and whole-body illness. Chapter 15 is devoted to recovery; you can learn more in that chapter.
Muscle Genetics
There are four factors that must align for an elite athlete to reach their potential:
- Genetics—Optimal genetics for a specific sport or activity are vital for us to excel.
- Opportunity—There are many people who never discover their capabilities due to lack of opportunity.
- Desire—Despite favorable genetics and opportunity there is still much work required; some people do not have a desire to do what is required for sport success.
- Coaching—Optimal training is usually a complex process of development, practice and recovery with skilled supervision; it is vital to achieve optimal performance in sport.
Genetic gifting is manifest in the muscles. To over-simplify, the muscles can be thought of as existing in two extremes fast muscle and slow muscle. Whereas fast muscle can be trained to act with slower characteristics, slow muscle cannot be trained to act in the way fast muscles do. While muscles display a continuum of capabilities, we are merely discussing the extremes.
Fast-twitch muscle fibers can contract very fast. These fibers tend to be innervated by large and myelinated nerves (see section on the nervous system on page 21) which allow for fast nervous transmission of the signal to contract. They are able to rapidly contract and their fibers are large in diameter. Energy capabilities of fast twitch muscles are based on non-oxidative means such as glycolysis. Because their metabolic capacity for processing oxygen is limited, and because their larger size slows the diffusion of oxygen, they have high power capacity but relatively low endurance.
At the other end of the spectrum is the slow-twitch muscle fiber. As the name suggests, these fibers contract more slowly than the fast-twitch fibers. Slow-twitch muscle fibers are smaller in diameter to facilitate the diffusion of oxygen so that their oxygen-based metabolism can produce a lot of energy sustained over a long time. The nerves supplying these muscle fibers tend to be smaller.
You have probably noticed athletes who are rich in fast-twitch fibers and those who are rich in slow-twitch fibers. Those with an abundant population of fast-twitch fibers are weightlifting and sprinting sport competitors, for example. They tend to be muscular, can add muscle mass relatively quickly and are fast-moving but do not have high endurance. In contrast, athletes rich in slow-twitch fibers tend to be smaller, with smaller muscles and less ability to add muscle mass. They do not have high speed movement capabilities but respond very readily to endurance training, such as long distance runners and cyclists.
Many sports, like soccer or basketball, need both speed and endurance. The better players in these types of sports are not extremely fast- or slow- twitch but have a good mixture of both. As people age, the fast-twitch fibers become more slow-twitch. The peak performance age for fast-twitch sports (e.g., sprinting, weightlifting) tends to be younger than for slow-twitch (e.g., endurance) sports.
The body’s ability to utilize oxygen to produce energy for sport performance is measured as the oxygen uptake expressed as volume of oxygen (VO2) per minute. The maximal capacity is the VO2max. The single best predictor of endurance sport performance is the VO2max. The VO2max is primarily limited by the body’s ability to pump blood to the working muscle. The higher the slow-twitch fiber percent- ages, the higher the VO2max is in response to training. And vice versa, a high percentage of fast-twitch fibers will result in lower VO2max.
The Nervous System
The nervous system is the body’s communication system. It coordinates all the actions and sensory information throughout each part. The nervous system controls:
- Movement, balance and coordination
- Sensations such as touch or hearing
- Breathing and heartbeat
- Interpretations of sensory information (e.g., vision and sounds)
- Mental function including learning and memory
- Sleep, healing and recovery
- Stress and responses to stress
- Body temperature
- Hunger, thirst and digestion
- And others (this list is not all encompassing)
The nervous system, through the brain, spinal cord and nerves, provides the signal for muscles to contract. In fact, when a new athlete or young person is initially exposed to resistance training, it is the nervous system that adapts first and provides the initial increase in strength.
The nervous system is extremely complex. It functions by using charged chemicals (ions) and allowing ions to move inside and outside the nerve to report sensations such as hot or cold, or to send a signal to a muscle to contract. Contractions can be both voluntary and reflex (or involuntary). In a voluntary contraction, nerves in our brain decide to contract a muscle. For example, to move our biceps in the way we want, a signal from the brain travels down the spinal cord where other nerves are stimulated and passed from nerve cell to nerve cell until the signal reaches the biceps, causing the muscle to contract. An example of a reflex contraction, is when you make a movement based on a sensory input without telling your brain to signal your body to do it – like the reflex causing you to withdraw your hand from a hot stove. The sensory nerves in your fingers send a signal to the spinal cord. Since we have a God-given reflex, it doesn’t have to travel to the brain. Instead, to save time, the spinal cord sends a signal back to the muscles causing you to quickly withdraw your hand before incurring a serious burn.
The speed at which a nerve can conduct a signal is dependent on two factors: 1) The diameter of the nerve the larger the faster and 2) whether the nerve is covered in a myelin sheath which acts to isolate the conducting part of the nerve to speed signal flow. A highly coordinated skill, like throwing a fast-breaking curveball, requires training the neuromuscular system to apply the proper forces to the baseball to generate high speed and the appropriate spin. Anytime we are coaching skills, essentially we are training the nervous system to work with the muscles in an effective fashion.
The nervous system is also one of three integrated systems that control our heart. Nervous stimulation is what causes the heart to contract in a coordinated fashion, and if part of the heart’s nervous system is damaged, the coordination will be disrupted to some extent. As we train endurance athletes, the left chamber of their heart enlarges and becomes more powerful, the nervous system adapts to slow the heart more at rest because the body’s resting need for blood is mostly unchanged and more blood is ejected with every heartbeat. So, slower resting heart rate can be a good indicator of the adaptation of the heart to endurance training.
It should be clear that we cannot train athletes without training the nervous system. To develop motor skills or muscular power, muscle excitation and relaxation must be trained. To some extent, all training requires nervous system training.
The Endocrine System
In addition to the messages and control the body gets from the nervous system, the endocrine system is another signaling system within the body. For example, in addition to nerves controlling our heart rate as we mentioned, our endocrine system also influences heart rate by changing the level of the hormones called adrenaline and noradrenaline.
Hormones are produced by glands. Each gland produces one or more hormones, which communicate messages to our body. Some of the key glands include:
| Gland | Location | Hormone | Function in the Body |
| hypothalamus | two triangular bodies which sit on top of each kidney | corticosteroid epinephrine (i.e., adrenaline) | controls salt and water balance in the body, the body’s response to stress, metabolism, the immune system, and sexual development and function increases blood pressure and heart rate when the body is under stress, such as immediately before and during competition. |
| pituitary | base of brain | produces many hormones that control many other endocrine glands | |
| growth hormone | stimulates the growth of bone and other body tissues and plays a role in the body’s handling | ||
| of nutrients and minerals, thus is key to training adaptations | |||
| thyrotropin (pronounced: thy-ruh-TRO-pin) | stimulates the thyroid gland to make thyroid hormones | ||
| corticotropin (pronounced: kor-tih-ko-TRO-pin) | stimulates the adrenal gland to make certain hormones | ||
| antidiuretic hormone | helps control body water balance through its effect on the kidneys | ||
| the pituitary also secretes endorphins which | |||
| reduce feelings of pain. It also controls ovulation and the menstrual cycle in women. | |||
| thyroid | in the front part of the lower neck | thyroxine and triiodo- thyronine | control the rate at which cells burn fuels from food to make energy. Higher thyroid hormone cause faster chemical reactions. Thyroid hormones play a role in the development of the bones, brain and nervous system. |
| parathyroids | just behind the thyroid glands | parathyroid hormones | control blood calcium levels |
| adrenals | on top of each kidney | corticosteroids (e.g. aldosterone) | controls fluid levels and blood pressure |
| glucocorticoids (e.g. cortisol) | regulate metabolism | ||
| adrenal androgens (eg. testosterone) | impact muscle growth and sexual development | ||
| catecholamines (e.g. adrenaline) | impact the “flight or fight” responses. | ||
| ovaries | side of women’s uterus | estrogen | there are three types of estrogen (estradiol, estrone, and estriosuch) that control the menstrual cycle, influence reproduction, body weight, and learning and memory. |
| testes | male testicles | testosterone | testosterone is produced in the testes, and plays an important role in increasing muscle size and strength. It has been illegally supplied and administered to athletes in an artificial form. |
Table 3.1
Another important part of the endocrine system is the pancreas. It makes insulin and glucagon which control the level of glucose (blood sugar). Insulin moves glucose from the blood to the cells. This glucose, as we said in the energy section above, is essential in supplying energy. When insulin levels are insufficient, diabetes (Type I) occurs, and when it is ineffective, Type II diabetes.
Acclimatization
In this chapter we have learned about various body systems that are important for athletic performance. One aspect that must be considered in training athletes is acclimatization – acclimating their body systems to various environments. Earlier we discussed acclimatization to high altitudes. Acclimatization to heat is perhaps even more important because many athletes compete in hot environments. The good news is that our bodies will adapt to improve performance in hot environments.
Muscle contraction generates a great deal of heat. In cold situations, this is positive for our bodies. In hot situations, this heat generation will reduce physical performance and expose athletes to the possibility of deadly heat stroke. Because of the dangers of heat stroke, coaches must monitor their athletes when exercising in the heat, and work intensity and volume will need to be reduced, at least initially. Heat acclimation, at a minimum, takes five to seven days of exposure and may take 10-14 days for full acclimatization for some. Heat acclimatization must be done slowly and carefully to protect people’s health. Even a single sudden day of unseasonably warm temperatures can precipitate heat injuries. As the season turns from cool to warm, coaches must monitor the environment to protect their athletes.
Humans cool themselves by evaporating sweat from the surface of the skin. This requires two things:
1) sweat on the skin, and 2) an environment of sufficiently low humidity to allow evaporation to occur. Clothing causes air to be trapped under it, which raises the humidity of that air, and may also raise the temperature. In hot environments, the less clothing covering the skin, the better the heat removal.
Hypohydration (taking in less water than is lost from the body) is an added risk for exercise when sweating. Hypohydration reduces the blood volume (blood provides fluid to the sweat glands) which reduces oxygen delivery and also reduces blood flow to the skin. This blood flow is a major source of delivery of heat to the skin (where evaporation occurs) and the delivery of cooling to the muscles and internal organs. Hypohydration also contributes to heat injury. Coaches must ensure athletes consume sufficient water and electrolytes to replace the water and electrolytes lost in sweat, urination and breathing.
Sweat rates can be as high as just under four liters (or one gallon) per hour, depending on the size of the person. Some fluids ingested will be lost to urine production, so to maintain fluid levels most people will need to consume at least one liter per hour. Weighing athletes prior, during and after exercise and recording those weights is a simple and useful way to ensure athletes do not lose too much fluid by providing an indication of how much fluid needs to be replaced. This is NOT an issue to take lightly. People die every year from hypohydration and heat stroke.
The good news is that cold exposure, to a degree, is much less of an issue for sports. In general, for athletes the key danger in cold weather is frostbite. Frostbite is when the cells freeze and are damaged. This typically occurs in areas with large surfaces relative to their mass, and lower supplies of warmth-giving blood. The ears, tips of noses, toes and fingers are most susceptible to frostbite. Good clothing has been developed which provides adequate protection against frostbite. Coaches, as in warm weather, must also monitor and educate athletes about frostbite, about monitoring their teammates for frostbite and about wearing protective clothing.
