Aerobic Exercise: Chapter 6

• Chapter 6 discusses the principles of aerobic exercise

Objectives of the chapter

• Physical activity
• Exercise
• Physical Fitness
• Endurance
• Adaptation
• Deconditioning
• Energy Systems and Energy Expenditure
• Physiological Response to Aerobic Exercise

Resources

• Information on endurance training can be found for athletes, healthy young people, and individuals with chronic diseases like heart disease and diabetes
• The American College of Sports Medicine (ACSM) has published guidelines for common chronic conditions
• Physical therapists can use aerobic-type activity with healthy or patient individuals with a variety of conditions
• Information on cardiovascular and respiratory parameters for children, the elderly, young, and middle-aged adults is presented to help physical therapists treat individuals of all ages

Key Terms and Concepts

Physical Activity

• Physical activity is any bodily movement produced by skeletal muscle contraction
• It results in a substantial increase over resting energy expenditure, as defined by ACSM and CDC

Exercise

• Exercise is structured physical activity designed to improve or maintain physical fitness

Physical Fitness

• Fitness is a general term describing the ability to perform physical work

Performing Physical Work

• Requires cardiorespiratory functioning
• Requires muscular strength
• Requires endurance
• Requires musculoskeletal flexibility
• Optimum body composition is also included when describing fitness

Training for physical fitness

• Individuals must participate regularly in physical activity
• Use large muscle groups
• Challenge the cardiorespiratory system
• Individuals of all ages can improve fitness with activities like walking, biking, running, swimming, stair climbing, cross-country skiing, or training with weights
• Fitness levels range from poor to superior
• Based on energy expenditure during physical work
• Ratings are often based on direct or indirect measurement of maximum oxygen consumption
• Oxygen consumption is influenced by age, gender, heredity, inactivity, and disease

Maximum Oxygen Consumption

• Maximum oxygen consumption (VO2 max) measures the body's capacity to use oxygen
• It is measured during exercises using large muscle groups
• Swimming
• Walking
• Running
• Maximum oxygen consumption is the maximum amount of oxygen consumed per minute when the individual reached maximum effort
• It is expressed relative to body weight, as milliliters of oxygen per kilogram of body weight per minute
• It depends on the transport of oxygen
• It depends on the oxygen-binding capacity of the blood
• It depends on the cardiac function
• It depends on oxygen extraction capabilities
• It depends on muscular oxidative potential

Endurance as a Measure of Fitness

• Endurance is the ability to work for prolonged periods of time

• Endurance is the ability to resist fatigue

• It includes muscular endurance

• It includes cardiovascular endurance

• Muscular endurance refers to the ability of an isolated muscle group to perform repeated contractions over a period of time

• Cardiovascular endurance refers to the ability to perform large muscle dynamic exercise

• Examples of activities include walking, swimming, and/or biking for long periods of time

Aerobic Exercise Training

• Aerobic exercise training, or cardiorespiratory endurance training, is augmentation of the muscle energy utilization by means of exercise

• 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

• Increased muscle fiber capillary supply

• Aerobic Exercise Training depends on sufficient exercise frequency, intensity, and time

• Aerobic Exercise Training depends on muscular adaptation and is reflected in an individual's endurance

• The specificity principle states the individual improves in the exercise task used for training

• The specificity principle states one may not improve in other tasks.

• Swimming may enhance one's performance in swimming events

• Swimming may not improve one's performance in treadmill running

Adaptation

• The cardiovascular system and the muscles 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
• Adaptation is dependent on the ability of the organism to change
• Adaptation is dependent on 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
• 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

Coronary Occlusion

• Persons who have coronary occlusion may not present with any type of chest pain/symptoms (angina) until they need to exert themselves
• This occurs because when the body works harder, the heart rate increases, diastolic filling time decreases, thus affecting the oxygenation to myocadiac muscle
• Without an adequate blood supply by coronary artery, the underlying cardiac tissue no longer receives the oxygen needed for metabolic activity, resulting in anginal pain/symptoms

Deconditioning

• Deconditioning occurs with prolonged bed rest
• The effects of Deconditioning are frequently seen in the patient who has had an extended, acute illness or long-term chronic condition
• Decreases in: VO2 max, cardiac output (stroke volume), and muscular strength occur rapidly
• These effects are also seen, although to a lesser degree, in the individual who has spent a period of time on bed rest without any accompanying disease process
• They are also seen in the individual, who is sedentary because of lifestyle and increasing age

Effects Associated with Bed Rest

• Decreased Muscle mass
• Decreased Strength
• Decreased Cardiovascular function
• Decreased Total blood volume
• Decreased Plasma volume
• Decreased Heart volume
• Decreased Orthostatic tolerance
• Decreased Exercise tolerance
• Decreased Bone mineral density

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 adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and phosphate (P) to perform metabolic activities
• Muscle cells use this energy for actin-myosin crossbridge formation when contracting

Influence of Intensity and Duration

• There are three major energy systems
• The intensity and duration of activity determine when and to what extent each metabolic system contributes

The Three Major Energy Systems:

Phosphagen or ATP-PC System

• The Phosphagen or ATP-PC System has many characteristics
• PC and ATP are stored in the muscle cell
• PC 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
• The maximum power of the system is great
• 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 also has many characteristics, including:
• 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
• The maximum power of the system is intermediate
• 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 many characteristics, including:
• Glycogen, fats, and proteins are fuel sources
• Utilizing 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
• The maximum power of the system is small
• 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 Muscle Fiber (Type I)

• A slow contractile response
• Rich in myoglobin and mitochondria
• Have a high oxidative capacity
• Low anaerobic capacity
• Recruited for activities demanding endurance
• These fibers supplied by small neurons with a low threshold of activation
• Used preferentially in low-intensity exercise

Fast-Twitch Muscle Fiber (Type IIB)

• A fast-contractile response
• Have a low myoglobin content and few mitochondria
• Have a high glycolytic capacity
• Recruited for activities requiring power

Fast-Twitch (Type IIA)

• Have characteristics of both type I and type IIB fibers
• Recruited for both anaerobic and aerobic activities

Functional Implications

• Bursts of intense activity lasting only seconds, develop muscle strength and stronger tendons and ligaments, uses ATP supplied by the phosphagen system
• Intense activity lasting 1 to 2 minutes repeated after 4 minutes of rest or mild exercise enhances anaerobic power, uses ATP 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, uses ATP supplied by the phosphagen, anaerobic glycolytic, and aerobic systems
• Activity of sub-maximum intensity lasting 20 to 30 minutes or more that taxes a high percentage of the aerobic system and develops endurance

Energy Expenditure

• Energy is expended by individuals engaging in physical activity
• Energy is expressed in kilocalories
• Activities are categorized as light, moderate, or heavy
• Based on 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

Measuring Energy Expended

• Energy expended is computed from the amount of oxygen consumed
• Units used to quantify energy expenditure are METs and kilocalories
• A MET is defined as the oxygen consumed (milliliters) per kilogram of body weight per minute (mL/kg)
• Equal to approximately 3.5 mL/kg per minute
• 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)

Formula to convert METs to kcal per minute:

• [(METs x 3.5 mL/kg per minute x body weight in kg) ÷ 1000)] × 5

Classification of Activities by METs

• Activities are classified as light, moderate, or vigorous according to the energy expended or the oxygen consumed while accomplishing them

Levels of Activities

• Light activity is 1.0 to 2.9 METs
• Moderate activity is 3.0 to 5.9 METs
• Vigorous activity is 6 to 8.8 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

Energy Expenditure of Daily Tasks:

• Light Activity: Sitting, standing, self-care, making the bed, food shopping, walking less than 2.5 mph
• Moderate Activity: Walking downstairs, walking 2.5 mph to less than 3.5 mph, mowing the lawn (walking) with a power mower, playing golf
• Vigorous Activity: Walking faster than 3.5 mph, swimming laps (moderate effort), jogging, running at 5.0 mph, shoveling snow

Physiological Response to Aerobic Exercise

• The rapid increase in energy requirements during exercise requires equally rapid circulatory adjustments to meet the need for oxygen
• Nutrients to remove the end products of metabolism, such as carbon dioxide, water, and lactic acid, and to dissipate excess heat
• The shift in body metabolism occurs through a coordinated activity of all the systems of the body
• Neuromuscular, respiratory, cardiovascular, metabolic, and hormonal
• Oxygen transport and its utilization by the mitochondria of the contracting muscle are dependent on adequate blood flow in conjunction with cellular respiration

Acute Response to Exercise

1. Generalized peripheral vasoconstriction in non-exercising muscles allows blood to be shunted from the nonworking muscles, kidneys, liver and spleen, to the working muscles
2. Increased myocardial contractility
3. Increased heart rate
4. Marked increase and redistribution of the cardiac output
5. Increase in systolic blood pressure

• The increase in systolic blood pressure is the result of the augmented cardiac output

6. Increased muscle metabolism during exercise results in more O2 extracted from arterial blood

• Resulting in an increase in venous PCO2 and H+

7. The production of more CO2 causes the tissue to become acidic, increases the temperature of the tissue
8. Increased epinephrine
9. Increased stimulation of receptors of the joints and muscles.

Oxygen Consumption

• Factors determining how much of the oxygen is consumed are

1. Vascularity of the muscles
2. Fiber distribution
3. Number of mitochondria
4. Oxidative mitochondrial enzymes present in the fibers