There are numerous sources from which to obtain information on training for endurance in athletes and healthy young people and for individuals with coronary heart disease. Information or emphasis on endurance training and the improvement of fitness in the individual who has other types of chronic disease or disability is beginning to emerge. Using the most recent research, the American College of Sports Medicine published basic guidelines for several of the more common chronic conditions. This chapter uses information from well known sources to demonstrate that the physical therapist can use aerobic type activity when working with either healthy individuals or patients with a variety of conditions. In addition, some fundamental information about cardiovascular and respiratory parameters in children and the elderly, as well as the young or middle aged adult, is presented so the physical therapist can be prepared to treat individuals of all ages.

KEY TERMS AND CONCEPTS

Fitness
Fitness is a general term used to describe the ability to perform physical work. Performing physical work requires cardiorespiratory functioning, muscular strength and endurance, and musculoskeletal flexibility. Optimum body composition is also included when describing fitness.

To become physically fit, individuals must participate regularly in some form of physical activity that uses large muscle groups and challenges the cardiorespiratory system. Individuals of all ages can improve their general fitness status by participating in activities that include walking, biking, running, swimming, stair climbing, cross-country skiing, and/or training with weights.

Fitness levels can be described on a continuum from poor to superior based on energy expenditure during about of physical work. These ratings are often based on direct or indirect measurement of the body’s maximum oxygen consumption ( O2 max). Oxygen consumption is influenced by age, gender, heredity, inactivity, and disease.

Maximum Oxygen Consumption
Maximum oxygen consumption is a measure of the body’s capacity to use oxygen. It is usually measured when performing an exercise that uses many large muscle groups such as swimming, walking, and running. It is the maximum amount of oxygen consumed per minute when the individual has reached maximum effort. It is usually expressed relative to body weight, as milliliters of oxygen per kilogram of body weight per minute (mL/kg per minute). It is dependent on the transport of oxygen, the oxygen binding capacity of the blood, cardiac function, oxygen extraction capabilities, and muscular oxidative potential.

Endurance
Endurance (a measure of fitness) is the ability to work for prolonged periods of time and the ability to resist fatigue. It includes muscular endurance and cardiovascular endurance. Muscular endurance refers to the ability of an isolated muscle group to perform repeated contractions over a period of time, whereas cardiovascular endurance refers to the ability to perform large muscle dynamic exercise, such as walking, swimming, and/or biking for long periods of time.

Aerobic Exercise Training (Conditioning)
Aerobic exercise training, or conditioning, is augmentation of the energy utilization of the muscle by means of an exercise program. The improvement of the muscle’s ability to use energy is a direct result of increased levels of oxidative enzymes in the muscles, increased mitochondrial density and size, and an increased muscle fiber capillary supply.
• Training is dependent on exercise of sufficient intensity, duration, and frequency.
• Training produces cardiovascular and/or muscular adaptation and is reflected in an individual’s endurance.
• Training for a particular sport or event is dependent on the specificity principle; that is, the individual improves in the exercise task used for training and may not improve in other tasks. For example, swimming may enhance one’s performance in swimming events but may not improve one’s performance in treadmill running.

Adaptation
The cardiovascular system and the muscles used adapt to the training stimulus over time. Significant changes can be measured in as little as 10 to 12 weeks.

Adaptation results in increased efficiency of the cardiovascular system and the active muscles. Adaptation represents a variety of neurological, physical, and biochemical changes in the cardiovascular and muscular systems. Performance improves in that the same amount of work can be performed after training but at a lower physiological cost.

Adaptation is dependent on the ability of the organism to change and the training stimulus threshold (the stimulus that elicits a training response). The person with a low level of fitness has more potential to improve than the one who has a high level of fitness.

Training stimulus thresholds are variable. The higher the initial level of fitness, the greater the intensity of exercise needed to elicit a significant change.

Myocardial Oxygen Consumption
Myocardial oxygen consumption is a measure of the oxygen consumed by the myocardial muscle. The need or demand for oxygen is determined by the heart rate (HR), systemic blood pressure, myocardial contractility, and afterload. Afterload is determined by the left ventricular wall tension and central aortic pressure. It is the ventricular force required to open the aortic valve at the beginning of systole. Left ventricular wall tension is primarily determined by ventricular size and wall thickness.

The ability to supply the myocardium with oxygen is dependent on the arterial oxygen content (blood substrate), hemoglobin oxygen dissociation, and coronary blood flow, which is determined by aortic diastolic pressure, duration of diastole, coronary artery resistance, and collateral circulation. In a healthy individual, a balance between myocardial oxygen supply and demand is maintained during maximum exercise. When the demand for oxygen is greater than the supply, myocardial ischemia results.

Deconditioning
Deconditioning occurs with prolonged bed rest, and its effects are frequently seen in the patient who has had an extended, acute illness or long-term chronic condition. Decreases in maximum oxygen consumption, cardiac output (stroke volume), and muscular strength occur rapidly. These effects are also seen, although possibly to a lesser degree, in the individual who has spent a period of time on bed rest without any accompanying disease process and in the individual who is sedentary because of lifestyle and increasing age.

Energy Systems, Energy Expenditure, and Efficiency

Energy Systems
Energy systems are metabolic systems involving a series of biochemical reactions resulting in the formation of adenosine triphosphate (ATP), carbon dioxide, and water. The cell uses the energy produced from the conversion of ATP to adenosine diphosphate (ADP) and phosphate (P) to perform metabolic activities. Muscle cells use this energy for actin-myosin cross-bridge formation when contracting. There are three major energy systems. The intensity and duration of activity determine when and to what extent each metabolic system contributes.

Deconditioning Effects Associated With Bed Rest
Muscle mass
Strength
Cardiovascular function
Total blood volume
Plasma volume
Heart volume
Orthostatic tolerance
Exercise tolerance
Bone mineral density

Phosphagen, or ATP-PC, System
The ATP-PC system (adenosine triphosphate-phosphocreatine) has the following characteristics.
• Phosphocreatine and ATP are stored in the muscle cell.
• Phosphocreatine is the chemical fuel source.
• No oxygen is required (anaerobic).
• When muscle is rested, the supply of ATP-PC is replenished.
• The maximum capacity of the system is small (0.7 mol ATP).
• The maximum power of the system is great (3.7 mol ATP/min).
• The system provides energy for short, quick bursts of activity.
• It is the major source of energy during the first 30 seconds of intense exercise.

Anaerobic Glycolytic System
The anaerobic glycolytic system has the following characteristics.
• Glycogen (glucose) is the fuel source (glycolysis).
• No oxygen is required (anaerobic).
• ATP is resynthesized in the muscle cell.
• Lactic acid is produced (by-product of anaerobic glycolysis).
• The maximum capacity of the system is intermediate (1.2 mol ATP).
• The maximum power of the system is intermediate (1.6 mol ATP/min).
• The systems provide energy for activity of moderate intensity and short duration.
• It is the major source of energy from the 30th to 90th second of exercise.

Aerobic System
The aerobic system has the following characteristics.
• Glycogen, fats, and proteins are fuel sources and are utilized relative to their availability and the intensity of the exercise.
• Oxygen is required (aerobic).
• ATP is resynthesized in the mitochondria of the muscle cell. The ability to metabolize oxygen and other substrates is related to the number and concentration of the mitochondria and cells.
• The maximum capacity of the system is great (90.0 mol ATP).
• The maximum power of the system is small (1.0 mol ATP/min).
• The system predominates over the other energy systems after the second minute of exercise.

Recruitment of Motor Units
Recruitment of motor units is dependent on the rate of work. Fibers are recruited selectively during exercise.
• Slow-twitch fibers (type I) are characterized by a slow contractile response, are rich in myoglobin and mitochondria, have a high oxidative capacity and a low anaerobic capacity, and are recruited for activities demanding endurance. These fibers are supplied by small neurons with a low threshold of activation and are used preferentially in low-intensity exercise.
• Fast-twitch fibers (type IIB) are characterized by a fast contractile response, have a low myoglobin content and few mitochondria, have a high glycolytic capacity, and are recruited for activities requiring power.
• Fast-twitch fibers (type IIA) have characteristics of both type I and type IIB fibers and are recruited for both anaerobic and aerobic activities.

Functional Implications
• Bursts of intense activity (seconds) develop muscle strength and stronger tendons and ligaments. ATP is supplied by the phosphagen system.
• Intense activity (1 to 2 minutes) repeated after 4 minutes of rest or mild exercise enhances anaerobic power. ATP is supplied by the phosphagen and anaerobic glycolytic system.
• Activity with large muscles, which is less than maximum intensity for 3 to 5 minutes repeated after rest or mild exercise of similar duration, may develop aerobic power and endurance capabilities. ATP is supplied by the phosphagen, anaerobic glycolytic, and aerobic systems.
• Activity of submaximum intensity lasting 20 to 30 minutes or more taxes a high percentage of the aerobic system and develops endurance.

Energy Expenditure
Energy is expended by individuals engaging in physical activity and is often expressed in kilocalories. Activities can be categorized as light, moderate or heavy by determining the energy cost. The energy cost of any activity is affected by mechanical efficiency and body mass. Factors that affect both walking and running are terrain, stride length, and air resistance.

Quantification of Energy Expenditure
Energy expended is computed from the amount of oxygen consumed. Units used to quantify energy expenditure are kilocalories and METs.
• A kilocalorie is a measure expressing the energy value of food. It is the amount of heat necessary to raise 1 kilogram (kg) of water 1°C. A kilocalorie (kcal) can be expressed in oxygen equivalents. Five kilocalories equal approximately 1 liter of oxygen consumed (5 kcal = 1 liter O2).
• A MET is defined as the oxygen consumed (milliliters) per kilogram of body weight per minute (mL/kg). It is equal to approximately 3.5 mL/kg per minute.

Classification of Activities
Activities are classified as light, moderate, or heavy according to the energy expended or the oxygen consumed while accomplishing them.
• Light work for the average male (65 kg) requires 2.0 to 4.9 kcal/min, or 6.1 to 15.2 mL O2/kg per minute, or 1.6 to 3.9 METs. Strolling 1.6 km/hr, or 1.0 mph, is considered light work.
• Heavy work for the average male (65 kg) requires 7.5 to 9.9 kcal/min, or 23.0 to 30.6 mL O2/kg per minute, or 6.0 to 7.9 METs. Jogging 8.0 km/hr, or 5.0 mph, is considered heavy work.
• Jogging 8.0 km/hr, or 5.0 mph, requires 25 to 28 mL O2/kg per minute and is considered heavy work. The energy expended is equivalent to 8 to 10 kcal/min, or 7 to 8 METs.
• The energy expenditure necessary for most industrial jobs requires more than three times the energy expenditure at rest.
• Energy expenditure of certain physical activities can vary, depending on factors such as skill, pace, and fitness level.

Efficiency
Efficiency is usually expressed as a percentage.

Work output equals force times distance (W = F × D). It can be expressed in power units or work per unit of time (P = w/t). On a treadmill, work equals the weight of the subject times the vertical distance the subject is raised walking up the incline of the treadmill. On a bicycle ergometer, work equals the distance (which is the circumference of the flywheel times the number of revolutions) times the bicycle resistance.

Work input equals energy expenditure and is expressed as the net oxygen consumption per unit of time. With aerobic exercise, the resting volume of oxygen used per unit of time (VO2 value) is subtracted from the oxygen consumed during 1 minute of the steady-state period.
• Steady state is reached within 3 to 4 minutes after exercise has started if the load or resistance is kept constant.
• In the steady-state period, VO2 remains at a constant (steady) value.

Efficiency Expressed as a Percentage


Total net oxygen cost is multiplied by the total time in minutes the exercise is performed. The higher the net oxygen cost, the lower the efficiency in performing the activity. Efficiency of large muscle activities is usually 20% to 25%.

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