1. Factors Affecting Performance

2. Objectives Identify factors affecting maximal performance.
Provide evidence for and against the central nervous system being a site of fatigue.
Identify potential neural factors in the periphery that may be linked to fatigue.
Explain the role of cross-bridge cycling in fatigue.
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.

3. Objectives
Describe the factors limiting performance in all-out activities lasting less than ten seconds.
Describe the factors limiting performance in all-out activities lasting 10 to 180 seconds.
Discuss the subtle changes in the factors affecting optimal performance as the duration of maximal performance increase from three minutes to four hours.

4. Outline Sites of Fatigue Factors Limiting All-Out Aerobic Performances
Central 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

5. Factors Affecting Performance
Sites of Fatigue
Factors Affecting Performance
Figure 19.1

6. 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

7. Possible Sites of Fatigue
Figure 19.2

8. Central Fatigue Reduction in motor units activated
Sites of Fatigue
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

9. Peripheral Fatigue: Neural Factors
Sites of Fatigue
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

10. Sites of Fatigue
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.

11. Peripheral Fatigue: Mechanical Factors
Sites of Fatigue
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

12. Sites of Fatigue
A Closer Look 19.1 Radical 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

13. Sites of Fatigue
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.

14. Peripheral Fatigue: Energetics of Contraction
Sites of Fatigue
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

15. Sites of Fatigue
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.

16. Peripheral Fatigue: Energetics of Contraction
Sites of Fatigue
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

17. Order of Muscle Fiber Type Recruitment
Sites of Fatigue
Order of Muscle Fiber Type Recruitment
Figure 19.3

18. Sites of Fatigue
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.

19. Ultra Short-Term Performances
Factors Limiting All-Out Anaerobic Performances
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

20. Factors Affecting Fatigue in Ultra Short-Term Events
Factors Limiting All-Out Anaerobic Performances
Factors Affecting Fatigue in Ultra Short-Term Events
Figure 19.4

21. Factors Limiting All-Out Anaerobic Performances
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.

22. Short-Term Performances
Factors Limiting All-Out Anaerobic Performances
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

23. Factors Affecting Fatigue in Short-Term Events
Factors Limiting All-Out Anaerobic Performances
Factors Affecting Fatigue in Short-Term Events
Figure 19.5

24. Factors Limiting All-Out Anaerobic Performances
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.

25. Moderate-Length Performances
Factors Limiting All-Out Aerobic Performances
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

26. Factors Affecting Fatigue in Aerobic Performances Lasting 3–20 Minutes
Factors Limiting All-Out Aerobic Performances
Factors Affecting Fatigue in Aerobic Performances Lasting 3–20 Minutes
Figure 19.6

27. Factors Limiting All-Out Aerobic Performances
The Winning Edge 19.1 Is 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

28. Factors Limiting All-Out Aerobic Performances
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.

29. Intermediate-Length Performances
Factors Limiting All-Out Aerobic Performances
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

30. Factors Limiting All-Out Aerobic Performances
Factors Affecting Fatigue in Aerobic Performances Lasting 21–60 Minutes
Figure 19.7

31. Factors Limiting All-Out Aerobic Performances
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.

32. Long-Term Performances
Factors Limiting All-Out Aerobic Performances
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

33. Factors Affecting Fatigue in Aerobic Performances Lasting 1–4 Hours
Factors Limiting All-Out Aerobic Performances
Factors Affecting Fatigue in Aerobic Performances Lasting 1–4 Hours
Figure 19.8

34. Factors Limiting All-Out Aerobic Performances
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.

35. 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

36. Study Questions List the factors influencing performance.
Is the limiting factor for strength development located in the CNS or out in the periphery? Support your position.
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?
When fatigue occurs, there is still ATP present in the cell. What is the explanation for this?
Describe the pattern of recruitment of muscle fiber types during activities of progressively greater intensity, and explain them.

37. Study Questions
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?
Draw a diagram of the factors limiting maximal running performances of 1,500 m to 10,000 m.
While a high VO2 max is essential to world-class performance, what role does running economy play in a winning performance?
Given that lactate accumulation will adversely affect endurance, what test might be an indicator of maximal sustained running (swimming, cycling) speed?
What is the role of environmental factors, such as altitude and heat, in very long-distance performances of one to four hours’ duration?