Scott K. Powers Edward T. Howley Theory and Application to Fitness and Performance SEVENTH EDITION Chapter Copyright ©2009 The McGraw-Hill Companies, Inc.

Scott K. Powers • Edward T. HowleyScott K. Powers • Edward T. HowleyScott K. Powers • Edward T. HowleyScott K. Powers • Edward T. Howley

Theory and Application to Fitness and PerformanceTheory and Application to Fitness and PerformanceSEVENTH EDITION

Chapter

Presentation prepared by:

Brian B. Parr, Ph.D.

University of South Carolina Aiken

Copyright ©2009 The McGraw-Hill Companies, Inc. Permission required for reproduction or display outside of classroom use.

Factors Affecting Performance

Chapter 19

Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.

Objectives

1. Identify factors affecting maximal performance.

2. Provide evidence for and against the central nervous system being a site of fatigue.

3. Identify potential neural factors in the periphery that may be linked to fatigue.

4. Explain the role of cross-bridge cycling in fatigue.

5. Summarize the evidence on the order of recruitment of muscle fibers with increasing intensities of activity and the type of metabolism upon which each is dependent.

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Objectives

6. Describe the factors limiting performance in all-out activities lasting less than ten seconds.

7. Describe the factors limiting performance in all-out activities lasting 10 to 180 seconds.

8. Discuss the subtle changes in the factors affecting optimal performance as the duration of maximal performance increase from three minutes to four hours.

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Outline

Sites of FatigueCentral Fatigue

Peripheral Fatigue

Factors Limiting All-Out Anaerobic Performances

Ultra Short-Term Performances (Less than Ten Seconds)

Short-Term Performances (10 to 180 Seconds)

Factors Limiting All-Out Aerobic Performances

Moderate-Length Performances (Three to Twenty Minutes)

Intermediate-Length Performances (Twenty-One to Sixty Minutes)

Long-Term Performances (One to Four Hours)

Athlete as Machine

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Factors Affecting Performance

Figure 19.1

Sites of Fatigue

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Sites of Fatigue

• Fatigue– Inability to maintain power output or force during

repeated muscle contractions• Central fatigue

– Central nervous system• Peripheral fatigue

– Neural factors– Mechanical factors– Energetics of contraction

Sites of Fatigue

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Possible Sites of Fatigue

Figure 19.2

Sites of Fatigue

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Central Fatigue

• Reduction in motor units activated• Reduction in motor unit firing frequency• Central nervous system arousal can alter the state

of fatigue– By facilitating motor unit recruitment

Increasing motivation Physical or mental diversion

• Excessive endurance training (overtraining)– Reduced performance, prolonged fatigue, etc.– Related to brain serotonin activity

• “Central Governor” model – Conscious and subconscious brain, not spinal cord

or motor unit

Sites of Fatigue

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Peripheral Fatigue: Neural Factors

• Neuromuscular junction– Not a site for fatigue

• Sarcolemma and transverse tubules– Ability of muscle membrane to conduct an action

potential Inability of Na+/K+ pump to maintain action potential

amplitude and frequency– Can be improved by training

– An action potential block in the T-tubules Reduction in Ca+2 release from sarcoplasmic reticulum

Sites of Fatigue

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In Summary

Increases in CNS arousal facilitate motor unit recruitment to increase strength and alter the state of fatigue.

The ability of the muscle membrane to conduct an action potential may be related to fatigue in activities demanding a high frequency of stimulation.

Repeated stimulation of the sarcolemma can result in a reduction in the size and frequency of action potentials; however, shifts in the optimal frequency needed for muscle activation preserve force output.

Under certain conditions an action potential block can occur in the t-tubule to result in a reduction in Ca+2 release from the SR.

Sites of Fatigue

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Peripheral Fatigue: Mechanical Factors

• Cross-bridge cycling and tension development depends on:– Arrangement of actin and myosin– Ca+2 binding to troponin– ATP availability

• High H+ concentration may contribute to fatigue– Reduce the force per cross-bridge– Reduce the force generated at a given Ca+2

concentration – Inhibit Ca+2 release from SR

• Longer “relaxation time” is a sign of fatigue– Due to slower cross-bridge cycling

Sites of Fatigue

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A Closer Look 19.1Radical Production During Exercise Contributes to Muscle Fatigue • Exercise promotes free radical formation

– Molecules that contain unpaired electron in outer orbital

– Capable of damaging proteins, lipids, and DNA• Can contribute to fatigue

– Damage contractile proteins (myosin and troponin) Limits the number of cross-bridges in strong binding state

– Depress sodium/potassium pump activity Disruption of potassium homeostasis

• Optimal levels of antioxidants can postpone fatigue– High doses can impair muscle function

Sites of Fatigue

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In Summary

The cross-bridge ability to “cycle” is important in continued tension development. Fatigue may be related to the effect of a high H+ concentration on the ability of troponin to bind to Ca+2, the inability of the sarcoplasmic reticulum to take up Ca+2, or the lack of ATP needed to dissociate the cross-bridge from actin.

Sites of Fatigue

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Peripheral Fatigue: Energetics of Contraction

• Imbalance ATP requirements and ATP generating capacity

– Accumulation of Pi

Inhibits maximal force Reduces cross-bridge binding to actin Inhibits Ca+2 release from SR

• Rate of ATP utilization is slowed faster than rate of ATP utilization– Maintains ATP concentration

Sites of Fatigue

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In Summary

Fatigue is directly associated with a mismatch between the rate at which the muscle uses ATP and the rate at which ATP can be supplied.

Cellular fatigue mechanisms slow down the rate of ATP utilization faster than the rate of ATP generation to preserve the ATP concentration and cellular homeostasis.

Sites of Fatigue

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Peripheral Fatigue: Energetics of Contraction

• Muscle fiber recruitment in increasing intensities of exercise– Type I Type IIa Type IIx

– Up to 40% VO2 max type I fibers recruited

– Type IIa fibers recruited at 40–75% VO2 max

– Exercise >75% VO2 max requires IIx fibers

– Results in increased lactate production

Sites of Fatigue

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Order of Muscle Fiber Type Recruitment

Sites of Fatigue

Figure 19.3

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In Summary

Muscle fibers are recruited in the following order with increasing intensities of exercise: Type I Type IIa Type IIx

The progression moves from the most to the least oxidative muscle fiber type. Intense exercise (>75% VO2

max) demands that type IIx fibers be recruited, resulting in an increase in lactate production.

Sites of Fatigue

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Ultra Short-Term Performances

• Events lasting <10 seconds• Dependent on recruitment of Type II muscle fibers

– Generate great forces that are needed• Motivation, skill, and arousal are important• Primary energy source is anaerobic

– ATP-PC system and glycolysis Creatine supplementation may improve performance


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Factors Affecting Fatigue in Ultra Short-Term Events


Figure 19.4

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In Summary

In events lasting less than ten seconds, optimal performance is dependent on the recruitment of appropriate type II fibers to generate the great forces needed.

Motivation or arousal is required, as well as the skill needed to direct the force.

The primary energy sources are anaerobic, with the focus on phosphocreatine.


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Short-Term Performances

• Events lasting 10–180 seconds• Shift from anaerobic to aerobic metabolism

– 70% energy supplied anaerobically at 10s– 60% supplied aerobically at 180s

• Anaerobic glycolysis is primary energy source– Results in elevated lactate and H+ levels

Interferes with Ca+2 binding with troponin

• Ingestion of buffers may improve performance


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Factors Affecting Fatigue in Short-Term Events


Figure 19.5

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In Summary

In short-term performances lasting 10 to 180 seconds, there is a shift from 70% of the energy supplied anaerobically at 10 seconds to 60% being supplied aerobically at 180 seconds.

Anaerobic glycolysis provides a substantial portion of the energy, resulting in elevated lactate levels.


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Moderate-Length Performances

• Events lasting 3–20 minutes– 60% ATP generated aerobically at 3 min– 90% ATP supplied aerobically at 20 min

• High VO2 max is important– High maximal stroke volume– High arterial oxygen content

Hemoglobin content Inspired oxygen

• Requires energy expenditure near VO2 max– Type IIx fibers recruited

High levels of lactate and H+ accumulation


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Factors Affecting Fatigue in Aerobic Performances Lasting 3–20 Minutes

Figure 19.6


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The Winning Edge 19.1Is Maximal Oxygen Uptake Important in Distance Running Performance?

• VO2 max sets the upper limit for ATP production in endurance events– Even though race is not run at 100% VO2 max

• Performance also determined by:– %VO2 max at which runner can perform

Estimated by the lactate threshold

– Running economy


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In Summary

In moderate-length performances lasting three to twenty minutes, aerobic metabolism provides 60% to 90% of the ATP, respectively.

These activities require an energy expenditure near VO2

max, with type II fibers being recruited. Any factor interfering with oxygen delivery (e.g., altitude

or anemia) would decrease performance, since it is so dependent on aerobic energy production. High levels of lactate accompany these types of activities.


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Intermediate-Length Performances

• Events lasting 21–60 minutes• Predominantly aerobic

– Usually conducted at <90% VO2 max– High VO2 max is important

• Other important factors– Running economy

High percentage of type I muscle fibers

– Environmental factors Heat Humidity

– State of hydration


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Factors Affecting Fatigue in Aerobic Performances Lasting 21–60 Minutes

Figure 19.7


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In Summary

Intermediate-length activities lasting twenty-one to sixty minutes are usually conducted at less than 90% VO2

max, and are predominantly aerobic. Given the length of the activity, environmental factors

such as heat, humidity, and the state of hydration play a role in the outcome.


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Long-Term Performances

• Events lasting 1–4 hours– Clearly aerobic

• Environmental factors more important• Maintaining rate of carbohydrate utilization

– Muscle and liver glycogen stores decline– Ingestion of carbohydrate

Maintain carbohydrate oxidation by the muscle

• Consumption of fluids and electrolytes• Diet also influences performance


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Factors Affecting Fatigue in Aerobic Performances Lasting 1–4 Hours

Figure 19.8


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In Summary

In long-term performances of one to four hours duration, environmental factors play a more important role as the muscle and liver glycogen stores try to keep up with the rate at which carbohydrate is used.

Diet, fluid ingestion, and the ability of the athlete to deal with heat and humidity all influence the final outcome.


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Athlete as Machine

• Continuing goal to improve performance• Potential to treat elite athletes like machines

– Collection of parts evaluated by specialists– Implementation of research to improve performance– May be exposing athletes to risk

In research or in implementation of techniques

• Institutional Review Boards– Minimize risk to subjects being studied

Athlete as Machine

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Study Questions

1. List the factors influencing performance.

2. Is the limiting factor for strength development located in the CNS or out in the periphery? Support your position.

3. Tracing the path the action potential takes from the time it leaves the motor end plate, where might the “weak link” be in the mechanisms coupling excitation to contraction?

4. When fatigue occurs, there is still ATP present in the cell. What is the explanation for this?

5. Describe the pattern of recruitment of muscle fiber types during activities of progressively greater intensity, and explain them.

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Study Questions

6. As the duration of maximal effort increases from less than ten seconds to 10 to 180 seconds, what factor becomes limiting in terms of energy production?

7. Draw a diagram of the factors limiting maximal running performances of 1,500 m to 10,000 m.

8. While a high VO2 max is essential to world-class performance, what role does running economy play in a winning performance?

9. Given that lactate accumulation will adversely affect endurance, what test might be an indicator of maximal sustained running (swimming, cycling) speed?

10. What is the role of environmental factors, such as altitude and heat, in very long-distance performances of one to four hours’ duration?

Scott K. Powers Edward T. Howley Theory and Application to Fitness and Performance SEVENTH EDITION Chapter Copyright ©2009 The McGraw-Hill Companies, Inc.

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