Exercise Physiology
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Exercise can be defined as “bodily exertion, especially for the sake of training, recreation, or fitness.”
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Science of processes and mechanisms of skeletal muscle contraction and the corresponding interaction of other body systems that facilitate and respond to skeletal muscle contraction
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Understanding the body’s physiologic response to repetitive skeletal muscle contraction is imperative.
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Skeletal muscle contraction that exceeds physiologic limits, is inappropriate in duration or intensity, or for which the musculoskeletal system has been inadequately prepared can lead to injury or poor performance/adaptation to training.
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Clinical relevance lies in identification of the pathology, what triggered it, and how to correct, alleviate, and/or prevent this in both individuals and populations of active people.
Terminology
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Metabolic equivalents (METs): expression of metabolic cost of exercise in terms of oxygen consumption
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Rest defined as 1 MET, which is an oxygenation of 3.5 mL O 2 /kg/min
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MET chart ( Table 17.1 )
TABLE 17.1
METs IN COMMON COMPETITIVE AND RECREATIONAL SPORTS
Basketball (comp)
7–12
Biking (rec)
3–8
Dancing
3–7
Football
6–10
Golf (cart)
2–3
Jogging (5–6 mph)
7–15
Skiing (downhill)
5–8
Soccer
5–12
Tennis
4–9
Volleyball
3–6
Modified from Mead WF, Hartwig R. Fitness evaluation and exercise prescription. J Fam Pract. 1981;13(7):1039-1050.
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max: maximal oxygen utilization
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Gold standard for evaluating cardiorespiratory fitness
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Measured in mL O 2 /kg/min
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Anaerobic threshold: point at which oxygen demand exceeds oxygen availability
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Workload: amount of energy required to complete a specific task ( Table 17.2 )
TABLE 17.2
METs IN WALKING
Workload
Miles
Minutes
5 METs
1
15–18
6 METs
1.5
21–25
8 METs
2
24–29
10 METs
4
50–54
12 METs
5
70–80
Modified from Mead WF, Hartwig R. Fitness evaluation and exercise prescription. J Fam Pract. 1981;13(7):1039-1050.
Basic Science
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The sarcomere, which is the basic unit of muscle, is composed of actin/myosin filaments ( Fig. 17.1 ).
Figure 17.1
Biochemical mechanics of muscle contraction.
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This linkage between actin and myosin is facilitated via acetylcholine release at the motor endplate, which triggers depolarization and subsequent release of calcium from the sarcoplasmic reticulum; this is followed by a series of reactions, causing formation of an adenosine triphosphate (ATP)–myosin complex, subsequent change in the conformation of the myosin unit, and ultimately, traction on the actin filament. Subsequently, this pulls on the connective tissue components of the sarcomere, resulting in contraction of the muscle ( Fig. 17.2 ), which leads to movement.
Figure 17.2
Initiation of muscle contraction by electric impulse and calcium movement.
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The motor nerve center directs timing and sequencing of motor unit recruitment and firing, which facilitates coordinated movement.
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Continued muscle contraction relies on an adequate supply of ATP in each sarcomere.
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ATP is provided by three interlinked, overlapping energy systems that synthesize ATP for both short periods of intense vigorous activity and longer periods of lower-level sustained activity.
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Exercise results in increases in protein synthesis within the muscle with long-term/cumulative changes to the steady-state level of protein synthesis with repetitive exercise.
Energy Systems
Phosphagens (ATP–Creatine Phosphate System)
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Anaerobic system used in maximum-intensity exercise lasting only seconds
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Composed of ATP and creatine phosphate stored in the cytoplasm of each sarcomere
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Phosphagens provide rapid resynthesis of ATP on the myosin head, facilitating brief high-intensity bursts of muscle activity.
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Particularly important during very-high-intensity exercise and at the beginning of exercise
Glycogen to Pyruvate (“Lactic Acid System”)
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Anaerobic process that degrades muscle glycogen to pyruvate
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Pyruvate is an essential substrate that can be oxidized to ATP via the aerobic energy system (see Oxidative System ).
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Also directly provides some ATP, which becomes part of the phosphagen pool
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Energy source in high-intensity, short-burst exercise (typically ≤3 minutes)
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Pyruvate can be reversibly converted to lactate, which can be transported out of the cell for use by other tissues.
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Also serves as a negative feedback loop because increased lactate causes metabolic acidosis, which increases the rate of ventilation and causes muscle discomfort; these reactions will eventually encourage an individual to reduce the intensity of exercise
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Oxidative System (Respiratory Response to Exercise)
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The aerobic energy system
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Quantitatively the most important system
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Typically used in activities lasting >3 minutes
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Dependent on oxygen availability at the cellular level
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Accordingly, much of the body’s physiology is structured to facilitate transport of O 2 to tissues via the cardiorespiratory system.
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Dependent on oxidation of pyruvate , acetyl-CoA (formed directly from glucose), or free fatty acids
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A low-power but extremely high-capacity energy system
Adaptations to Chronic Exercise (“Training”)
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Musculoskeletal and cardiorespiratory systems are highly adaptable to regularly performed exercise.
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Adaptation to repetitive exercise can be understood in terms of several factors, which can be represented by the mnemonic P-ROIDS .
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P rogression: Gradual increase in intensity, duration, and difficulty of physical activity to improve strength, endurance, and sport-specific skills
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R eversibility: Training is generally continuous or cyclic in nature, owing to the rapid loss of benefits from conditioning when people stop exercising
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Inactivity for as little as several days can lead to reduction in work capacity secondary to a decrease in protein synthesis from baseline with loss of exercise stimulus.
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O verload: Exercising above normal levels through combinations of type of activity, intensity, duration, and/or frequency
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I ndividual D ifferences
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S pecificity: Specific training develops specific adaptations beneficial to a particular sport/activity.
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Training should focus on the energy system that is most important to that sport (e.g., distance runners spend more time working on the oxidative system).
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Modern training programs typically incorporate cross-training because training focused on optimizing all energy systems can improve sport-specific performance (e.g., dry-land and weight training can improve the performance of skiers).
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Concurrent training: integration of both aerobic/endurance and resistance training into a training plan
Types of Exercise
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Aerobic/endurance exercise: fueled by an oxidative energy system and in place when oxygen supply/delivery to tissues is adequate for sustained; low- to moderate-intensity exercise
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Increased lipid mobilization and oxidation by skeletal muscle
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Increased number of mitochondria with adaptation
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Anaerobic exercise: typically high-intensity, shorter-duration exercise that exceeds capacity of the oxidative system and is fueled primarily by phosphagens and glycogen to pyruvate energy systems
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Resistance exercise: training for maximal contractile force and muscle hypertrophy
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Range of motion/stretching/warm-up/cool-down exercises ( Fig. 17.3 )
Figure 17.3
Physiological response to exercise and cool-down.
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Flexibility exercise: increases ability to lengthen muscle–tendon units and improve motion around specific joints
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“Lifestyle activity”: cumulative, non-structured, moderate-intensity physical activity wherein individuals partake throughout a typical day (e.g., taking the stairs and walking in from the parking lot)
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10 minutes of moderate-intensity exercise three times a day facilitates changes comparable to health benefits of 30 minutes of continuous exercise.
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Increasing lifestyle activity is as effective as structured activity for improving cardiorespiratory fitness and reducing cardiovascular disease (CVD) risk factors.
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Exercise Prescription
General Considerations
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A tool to teach, coach, and educate patients regarding opportunities to improve overall health and well-being
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Can also enhance exercise and sport-specific performance
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Exercise is safe for a majority of patients and has numerous health/fitness benefits.
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A written exercise prescription has been demonstrated to increase activity levels more compared with verbal advice alone.
Implement as Part of Clinical Practice
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Identify those who would benefit from exercise prescription
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Identify conditions amenable to exercise therapy
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Assess patient’s activity level
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Educate patient about the benefits of exercise
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Assess patient interest, motivation, and goals
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Strive to find activities that are sustainable
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Monitor progress; assess barriers
Initiation of an Exercise Program
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Start slowly and allow fitness level to improve
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Goal is long-term lifestyle change
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Initially, prescribe shorter periods of light- to moderate-intensity exercise and build in duration, frequency, and intensity over time
Safety
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Data are reassuring.
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An overwhelming majority of medical-related problems are soft-tissue and overuse injuries.
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Encourage appropriate warm-up and cool-down
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Scientific evidence to support that stretching before activity prevents injuries is limited.
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Sudden death during exercise is rare.
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A majority (approximately 80%) of instances are caused by coronary heart disease.
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The risk is greater in sedentary people who start vigorous activity.
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The relative risk of sudden death during vigorous activity is 2.1 times that of the risk during less strenuous activity.
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Regular activity is protective.
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Exercise-associated cardiovascular (CV) risks lessen as individuals become more active/fit.
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Exercise-related CV events are uncommon and frequently preceded by warning signs and symptoms.
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Encourage slow start, consistency, and progress over time
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Basic Principles of Exercise Prescription
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Tailor prescription to individual patients based on their goals, physical/financial/time considerations, medical conditions, and motivation.
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Prescreen as indicated:
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History and physical examination is the most important tool.
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Exertional chest pain, syncope, near-syncope, disproportionate dyspnea on exertion, and marked decrease in exercise capacity are red flags warranting further evaluation ( Box 17.1 ).
Box 17.1Related posts:
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