Introduction To Exercise Physiology And Basic Body Systems

Introduction To Exercise Physiology And Basic Body Systems


The purpose of this section is to present a brief overview of the workings of the human body and its various responses to exercise and activity. Fitness Expert feels that people are more prone to follow an exercise program if they possess a basic understanding of why a particular activity or exercise is beneficial and how it will help them achieve their goals.

Sources of Energy

The human body requires energy to function. The body receives this energy from the carbohydrates, fats and proteins in the food we eat and the beverages we drink. Once these nutrients have been broken down and assimilated, the body needs access to these energy sources to function. We also require energy to exercise and work.

The body has access to two different types of energy systems: anaerobic and aerobic.

The most immediate energy source available is anaerobic. It uses a substance called creatine phosphate (CP) to produce adenosine triphosphate (ATP). ATP is located in our muscles and is the body's main energy source, allowing us to move and function. This substance can be compared to an automobile's fuel source, or gasoline. This immediate energy source is only utilized for brief, high intensity bursts of activity. Such activities usually last ten seconds or less due to the fact that muscles store only limited amounts of CP and ATP.

The next available source of energy is also anaerobic in nature. This short-term energy system "kicks" into place after the available CP stores are exhausted. The body's cells begin to break down a stored substance called glucose, using glucose and glycogen to produce more ATP. This process is known as glycolysis. Glucose is a basic sugar that comes from the carbohydrates that we eat every day. The process of glycolysis provides enough energy for high intensity work that can last from one to three minutes in duration or until fatigue sets in. Such fatigue is due to lactic acid build-up in the muscles. Lactic acid is a by-product of the glycolytic process.

The body's long-term energy source is aerobic in nature, meaning the presence of oxygen is necessary. This energy system relies on the chemical breakdown of muscle glycogen, blood glucose, plasma-free fatty acids and stored intramuscular fats to produce ATP. The aerobic energy system is suited for long-duration, sub-maximal activity levels. Such activity usually lasts longer than three minutes and continues until fatigue sets in. Fatigue will occur when all available muscle glycogen and sugars have been used and when insufficient transport of oxygen to the muscles takes place. These factors will bring a halt to ATP production and thus an end to the activity.

Many activities involve a mix of both aerobic and anaerobic energy systems to a certain degree. The more intense an activity and the closer it is to your maximum work output, the greater the amount of energy that is derived from anaerobic sources as opposed to aerobic energy systems used with sub-maximal activities.

The Cardiopulmonary System

This system includes the heart, lungs and circulatory system and serves as the body's transportation network.

The blood vessels themselves form a vast transportation system throughout the human body. They carry the blood from the heart, bringing it to the various body tissues before returning to the heart. The flow of blood is controlled by blood pressure and resistance, setting up an established pressure gradient of higher to lower pressure.

Blood Pressure represents the pressure exerted by blood on the wall of an artery when the left ventricle of the heart undergoes systolic and then diastolic contraction. Resistance represents the opposing forces that result from the friction between the blood and the blood vessel walls.

The Heart

Aorta Superior vena cava Pulmonary artery (to lungs) Pulmonary veins (from lungs) Right atrium Left atrium Left ventricle Inferior vena cava Right ventricle

The body's circulatory mechanisms involve the function of arteries, arterioles, capillaries, venules, and veins. Arteries are the vessels that carry blood away from the heart. The blood is then carried to arterioles. These smaller arteries influence the regulation of blood flow into the capillaries, or microscopic vessels which control the exchange of material between blood and tissue on a cellular level. Venules then drain the blood from the capillaries. These small blood vessels eventually merge into veins, which are the vessels that control the flow of blood back to the heart. Veins have valves that work to prevent any blood flowing back into the venules.

These blood vessels are all involved in the various circulatory routes. Systemic circulation is the largest route and contains both the hepatic portal circulation and the coronary (cardiac) circulation route. The systemic circulation route involves movement from the heart's aorta to large arteries to arterioles to capillaries to venules to veins to the venae cavae to the heart's right atrium.

The heart is made up of a right and left side. Each side contains an upper portion, or atrium, and a lower portion, or ventricle. After the right side of the heart receives blood from the venous system, the heart muscle contracts and pumps blood from the right ventricle through the pulmonary arteries to the lungs. While in the lungs, the blood picks up fresh oxygen and gives off carbon dioxide in the pulmonary capillaries. The carbon dioxide is expelled from the body during exhalation, while the newly oxygenated blood returns to the left atrium by way of the pulmonary veins and then moves into the left ventricle.

The left ventricle then contracts, pumping blood to the rest of the body through its largest artery, the aorta. This contraction occurs at the same time as the contraction of the right ventricle, which is how the blood was transported to the lungs in the first place. Thus, the cardiopulmonary system works in a never-ending cycle.

Cardiorespiratory fitness is directly linked to oxygen consumption, since the transport of oxygen to the body's tissues is inherently dependent on heart and lung function. Respiration is the process whereby oxygen is taken into the lungs from the atmosphere and then transported by the blood to the tissues. Simultaneously, carbon dioxide is taken by the blood from the tissues to the lungs and exhaled back into the atmosphere. Pulmonary ventilation refers to the amount, or volume, of air that is inhaled or exhaled per minute. Pulmonary ventilation normally increases during exercise.

Oxygen consumption is measured by the difference between the amount of oxygen exhaled and the amount inhaled during activity. Maximal oxygen consumption is the greatest volume of oxygen someone can consume during exercise. This maximal volume, or VO2 Max, also represents a person's maximum aerobic capacity. As such, it is often used as an index of cardiorespiratory fitness. VO2 Max delineates the upper limit that the cadiovascular system can deliver oxygenated blood to working muscles. This figure thus represents the fastest rate at which the body can aerobically produce ATP.

VO2 Max figures can be improved by 5-25% through endurance exercise programs. The degree of improvement is dependent on one's initial fitness level. Those people with low VO2 Max figures can actually make the greatest improvement. Age and gender also have an influence on VO2 Max. As one grows older, there occurs a systematic decrease in the measurement over time. In addition, females generally have VO2 Max figures that are 15% less than those of males. People suffering from pulmonary or cardiovascular disease usually exhibit decreased VO2 Max values. Yet, as stated above, their low functional capacities are also capable of showing the greatest degree of improvement as a result of participating in endurance training regimens.

The VO2 Max value is influenced by the body's maximum cardiac output, or the amount of blood pumped by the heart per minute. This is because the oxygenated blood needs to be delivered to the working muscles. Cardiac output is the product of heart rate, the number of times the heart beats per minute, and stroke volume, the amount of blood pumped from each ventricle every time the heart beats. The stroke volume will normally increase during the early stages of exercise and then level off. From that point, the heart rate is solely responsible for further increases in blood flow from the heart and lungs to the working muscles.

The Lungs

Nasal septum Respiratory bronchioles Bronchiole Larynx Pharynx Trachea Terminal bronchioles Bronchioles Respiratory bronchioles Larynx Bronchioles Alveolar sac Primary bronchi Secondary bronchi Tertiary bronchi Alveoli

Heart rate values can also serve as a vehicle for monitoring intensity (see "Principles of Fitness: Intensity"). A person's maximum heart rate can be estimated by the following formula:

Age-predicted maximum heart rate = 220 ­ Age

The maximum heart rate represents the highest heart rate one can attain during exercise. As with any formula, there exist exceptions to the rule. This is due to the fact that there is variation in heart rate values at any age.

Heart rate can also be used to express a person's physical work capacity (PWC), or the amount of work a person can endure until he achieves a specified level of physiological response. A PWC Max value represents the work rate that induces maximum heart rate.

The Physiology and Anatomy of Muscle

The human body contains three distinct types of muscle: cardiac, smooth, and skeletal. Cardiac muscle cells are unique in structure and function. They are located only in the heart. Smooth muscle cells are located in artery and intestinal walls. These cells are responsible for the ability of the blood vessels to constrict and dilate. Skeletal muscle cells are those muscles that exert force on the skeletal system, allowing body and limb movement. Skeletal muscle is made up of about 75% water, 20% protein, and small amounts of inorganic salts, pigments, enzymes, fats and carbohydrates.

Skeletal muscle is surrounded by several wrappings of connective tissue. These layers are joined into a tendon, which in turn is attached to the bone. This attachment system sets the stage for the muscles to exert force, thus producing tension that can become movement.

Skeletal muscles are divided into two primary types: slow-twitch, or Type I, and fast-twitch, or Type II. Each fiber type possesses characteristics of its own. Slow-twitch muscle fibers, also referred to as red muscle fibers, are better suited for activities that require a high component of aerobic energy. Similarly, these type of muscle fibers are called upon for activities requiring less than maximal forces. Endurance athletes, like runners and cyclists, often possess a majority of slow-twitch fibers within their muscles.


Trapezius Sternodeidomastoid Anterior deltoid Medial deltoid Pectoralis major Serratus anterior Biceps brachii Rectus abdominus Brachioradialis External oblique Wrist flexors Iliacus Psoas major Tensor fasciae latae Pectineus Sartorius Adductor magnus Gracilis Quadriceps Adductor longus Rectus femoris Vastus lateralis Tibialis anterior Vastus medialis Gastrocnemius Vastus intermedius (deep) Soleus
Shoulder joint Tendons Biceps brachii muscle Tendon Radius bone Ulna bone Bone Scapula Scapula Ligament Humerus bone Cartilage Elbow joint Bone

On the other hand, fast-twitch muscle fibers are used for activities requiring maximal force exertion. These fibers, known as white fibers, are larger and have a higher anaerobic component than the slow-twitch type. Thus, they are better suited for short-duration activity. Olympic weight lifters, jumpers, and sprinters normally possess a greater amount of fast-twitch muscle fibers compared to the slow-twitch variety.


Sternocleidomastoid Trapezius Medial deltoid Infraspinatus Posterior deltoid Teres minor Teres major Triceps brachii Latissimus dorsi External oblique Brachioradialis Erector spinae (deep) Wrist extensors Gluteus medius Tensor fasciae latae Flexor carpi ulnaris Gluteus maximus Hamstrings Vastus lateralis Biceps femoris Adductor magnus Semitendinosus Gracilis Semimenbranosus Plantaris Soleus Gastrocnemius Soleus Achilles tendon

These fast-twitch fibers can also be sub-divided into the categories Type IIa and Type IIb. The Type IIa fibers are referred to as fast-twitch oxidative. In addition to their high anaerobic component, Type IIa fibers have a greater aerobic potential than Type IIb, although not as large as slow-twitch fibers. Type IIb muscle fibers are also called fast-twitch glycolytic. These fibers have high anaerobic potential, but poor aerobic potential. There is also a third category of fibers, Type IIc fibers, which possess characteristics similar to both IIa and IIb. However, these muscle fibers are usually rare in human beings.

A person usually possesses varying distributions of fiber types for each muscle in their body. In other words, your front arm muscles, or biceps, could be composed of mostly slow-twitch muscle fiber types while your back thigh, or hamstring muscles, might be made up of a majority of fast-twitch fibers.

During activities requiring maximum force production, both major types of fibers, fast-twitch and slow-twitch, are activated. If the force required is of a sub-maximal nature, then the slow-twitch fibers will be activated first, followed by the fast-twitch types, if necessary.

Muscle fibers are always activated in groups. These groups are known as motor units and consist of a motor nerve and all the muscle fibers that it activates. Motor units come in different sizes. The nerves may stimulate a relatively small number of muscle fibers, as in eye movements, or hundreds of fibers, such as extending the leg. Thus, when we pick up a pen, we may utilize a small number of the arm muscles' motor units. On the other hand, the curling of a dumbbell would require a greater number of motor units.

Muscle contraction occurs when the central nervous system sends a nervous impulse to the muscle cells, causing the simultaneous shortening of many fibers in a muscle. While the individual fibers shorten, a contraction doesn't always involve a shortening of the muscle's length.

A concentric contraction occurs when the muscle itself shortens while developing tension. This is the most common type of muscle contraction and occurs in most sports and activities. An example of a concentric contraction would take place when you bend an extended arm, bringing the hand toward the shoulder.

An eccentric contraction takes place when a muscle lengthens while exerting force. This usually takes place when the resistance on the muscle is greater than the generated force. Eccentric contraction occurs when you lower a dumbbell from a flexed arm position to an extended arm position. These types of contractions are sometimes referred to as negatives or negative training.

An isometric contraction occurs when there is no change in the length of the muscle during contraction. This happens when the muscle does not exert sufficient force against the resistance to cause movement. It is commonly referred to as static resistance training.

Regardless of the type of contraction, ATP is used as the energy source. Neuromuscular contractions involve a specific force. The force of contraction is regulated by the size of the fibers that are contracting as well as the number of fibers contracting.

Force is also affected by the speed of the movement that results in the contraction. The faster the speed of movement, the lower the generated muscular force.

The human body normally protects itself from skeletal muscle damage that can result from too great an amount of muscle tension or muscle stretching. The golgi tendon organs are located within the tendons and respond to excessive amounts of muscular tension, inhibiting the contracting muscle. This process is not always successful. The effects of the golgi tendon organ are often minimized as a result of intense resistance training. This may be why elite weight lifters and power lifters sometimes experience injuries during maximal lifts.

In a similar fashion, muscle spindles are sensitive to excessive amounts of muscular stretch. When this occurs, the spindle causes the muscle to contract, thus reducing the risk of injury. These spindles are located in the muscles themselves.

Besides increases in muscular strength and endurance, resistance training has profound effects on the skeletal muscle system. When muscles are progressively overloaded, they can grow larger. This process is called hypertrophy. This muscular growth occurs as a result of increased size of the individual muscle fibers.

There are also conflicting views as to whether or not muscle growth is influenced by a process called hyperplasia, or an increase in the number of muscle fibers themselves. As of this time, hyperplasia has not been totally accepted by the scientific community as a viable explanation for muscle growth.

Regardless of the specific manner in which it occurs, increased muscle tissue has a positive effect on one's body composition and resting metabolic rate. Since muscle tissue requires more energy than fat, the additional lean body mass actually allows you to burn more calories at rest than you were previously able to. On the other hand, atrophy, or loss of muscle size, can occur with disuse or immobilization. Such a loss of muscle may also accompany the aging process.

Connective tissue is also affected by resistance training. Those connective tissues associated with muscle will grow thicker and stronger in order to cope with increased muscular contraction forces. In turn, this improved thickness contributes to the overall strength gains associated with resistance training.

Cartilage provides a cushion between the bones that meet at a joint. This type of tissue can act as a shock absorber between the joint's bone surfaces.

Ligaments provide the connection of bone to bone at the body's joints. It is believed that physical activity can help strengthen ligaments.

Tendons, as mentioned earlier, connect the skeletal muscles to the bones. The tendons are an extension of the connective tissue that wraps around the muscle itself, thus creating stability and support for the individual muscle fibers.

Along with bone and joint structure and muscle and skin elasticity, the tendons and ligaments that are located at the joint also influence flexibility. As explained elsewhere in the manual, stretching activity can be an effective method of improving flexibility.

The Skeletal System

The skeletal system consists of an extensive network of bones and cartilage. As previously mentioned, our bones act as the levers to which muscles are attached. In this way, the skeletal system provides a means for movement. Moreover, the skeletal bones serve as protection for many of the body's internal organs. In addition, our bones form a network of support for the body's soft tissues and muscles. An additional function of the skeletal system involves storage. The bones store minerals, tissues responsible for production of blood cells, and chemical energy in the form of stored lipids. The human skeletal system is divided into two sections: the axial skeleton and the appendicular skeleton. Together, they consist of 206 named bones.


Cranium Nasal box Clavicle Maxilla Sternum Mandible Xiphoid process Humerus Vertebral column Sacrum Coccyx Ulna Ilium Radius Greater trochanter Lesser trochanter Ischium Femur Pubis Patella Tibia Fibula Tarsals Metatarsals Phalanges Calcaneus

The axial skeleton is made up of the skull, vertebral column, or backbone, sternum, or breast bone, ribs and the hyoid bone, which is located in the neck between the lower jaw and the voicebox.

The appendicular skeleton is made up of the shoulder girdles, pelvic girdle, and the bones of the upper and lower extremities, which include the limbs, feet and hands.

Skeletal bones are classified by shape. These include long, short, flat and irregular. Short bones are usually almost equal in length and width. Examples include the wrist and ankle bones. Long bones are longer than they are wide and have a slight curvature for purposes of strength. They are designed to withstand stress. Examples include the femur (upper leg bone) and the phalanges (finger bones). Flat bones are usually thin and provide space for the attachment of muscle. They also protect vital organs of the body. Examples include the ribs and the scapulas, or shoulder blades. Irregular bones are characterized by complex shapes that don't fit into any of the other categories described above. An example would be the vertebrae.

Sutural bones, also called Wormian bones, are classified by location. They are small bones that occupy the spaces between certain cranial bone joints. Sesamoid bones are small bones that are located in the area of tendons where pressure usually occurs. An example would be the patella, or kneecap. Both sutural and sesamoid bones vary in number from person to person.

The bones of the skeletal system attach at joints, which have cartilage acting as pads in between them.


Parietal bone Occipital bone Cervical vertebrae Acromian process Humerus Scapula Thoracic vertebrae Olecranon Ilium Sacrum Coccyx Femur Ischium Tibia Fibula Talus Calcaneus

Skeletal bones are not totally solid, as many people believe. There are spaces between the harder parts of the bone. Besides making the bones lighter, these spaces serve as a pathway for blood vessels to transport nutrients to the bone cells. Compact bone tissue forms a layer over the less dense, or spongy bone tissue. The compact tissue contains few spaces and provides support and protection, thus helping the bone resist the stresses placed upon it.

Compact bone is also referred to as dense bone, and makes up the bulk of tissue located in the main portion, or shaft, of the skeleton's long bones. Spongy bone tissue is made up of an irregular network of thin bone plates. The spaces within these plates are filled with red marrow, which is responsible for producing blood cells. Some spaces also contain yellow marrow, which consists of fatty tissue and doesn't contribute to blood production. Spongy bone forms the majority of short, flat and irregular bones in the skeletal system. In long bones, spongy tissue is located in the ends of the bone. Spongy bone tissue is also called cancellous bone.

Regular exercise, especially resistance training, can help bones grow stronger. Stronger bones are more resistant to fractures. In addition, the increased bone mass that results from a regular training program is helpful in preventing degenerative bone diseases such as osteoporosis, which is characterized by a loss of the amount and strength of bone tissue. This loss of bone mass is sometimes accompanied by a decreased hormone output. The aging process also contributes to osteoporosis.

The vertebral column can be another source of skeletal-related pain for people. Exaggerated curvature or a lateral bend of the column can lead to abnormal spinal conditions like scoliosis, kyphosis and lordosis.

In addition, protruded intervertebral discs, such as herniated or slipped discs, are also a great source of lower back pain for many individuals.

The importance of keeping this area's surrounding musculature strong and balanced cannot be overemphasized from a protective standpoint.

For this reason, regular exercise and flexibility work should play a major role in the prevention and management of lower back pain. An increased range of motion combined with improved strength of the lower back and abdominal core muscles may help minimize the risks of future lower back problems.

The Nervous System

In relation to exercise, the nervous system's main function is the contraction of muscles that in turn control movement and activity.

The motor cortex is the portion of the brain that is responsible for movement. The nervous system stimulates muscle contraction by utilizing the spinal cord to send nervous impulses from the motor cortex to the individual motor nerves. Thse nervous impulses are conducted by nerve cells called neurons. A neuron is the basic unit of the human nervous system. A motoneuron is the nerve that sends these impulses to the muscle fibers.

As mentioned before, motor nerves and the muscle fibers they activate are referred to as motor units. The number of muscle fibers that a single motor nerve can stimulate ranges from a few fibers to thousands. When the brain activates a motor nerve, every single one of the muscle fibers stimulated by that specific nerve are contracted.

The nervous impulses can also be inhibited, or cancelled, by spinal cord reflex areas or other areas of the brain. Such inhibitory devices are used to protect the muscles, joints, and connective tissue that could be damaged by excessive forces.

The nervous system thus works hand-in-hand with the other systems of the human body to promoted movement and activity.

Fitness Facts home

© 1992-2017, All rights reserved. May not be reproduced in any medium without written permission.