Lec 7 Factors Affecting Human Performance

Chapter 19: Factors Affecting Human Performance

General Overview

  • Focus on performance factors for the remainder of the semester, exploring:
      - Limiting factors
      - Training
      - Nutrition and body composition
      - Assessments
      - Special populations
      - Ergogenic aids

Lecture Outline

  • Topics to be covered include:
      - Fatigue:
        - Definition
        - Sites of Fatigue
          - Central
          - Peripheral
      - Factors limiting performance based on activity duration:
        - All-out anaerobic performances
        - All-out aerobic performances
      - Concept of Athlete as Machine

Fatigue

  • Definition: Fatigue is defined as an inability to maintain power output or force during repeated muscle contractions.

  • Two primary types of fatigue:
      - Central Fatigue: Central nervous system (CNS).
      - Peripheral Fatigue: Neural factors, Mechanical factors, Energetics of contraction

Central Fatigue

  • What is 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) is associated with:

  • Reduced performance, prolonged fatigue, etc.

    • Related to brain serotonin levels, but also, dopamine and norepinephrine.

Central Fatigue

  • “Central Governor” model

    • Conscious and subconscious brain, not spinal cord or motor unit

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    • While central fatigue is certainly a factor it is estimated to only contribute about 10% to total fatigue

Peripheral Fatigue: Neural Factors

  • Neuromuscular junction

    • Not the site of fatigue    

  • Sarcolemma and transverse tubules

    • Altered ability of muscle membrane to conduct an action potential

      • Inability of Na+/K+ pump to maintain action potential amplitude and frequency

        • This doesn’t seem to cause fatigue

        • Can be improved by training

    • An action potential block in the T-tubules

      • Can happen under certain conditions

        • Reduction in Ca++ release from sarcoplasmic reticulum leads to reduced force production

Peripheral Fatigue: Mechanical Factors

  • Cross-bridge cycling and tension development depends on:

    • Arrangement of actin and myosin

    • Ca++ 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++ concentration

    • Inhibit Ca++ release from SR

  • Longer “relaxation time” is a sign of fatigue

    • Due to slower cross-bridge cycling

Peripheral Fatigue: Energetics of contraction

  • Imbalance between ATP requirements and ATP generating capacity

    • Accumulation of Pi

      • Inhibits maximal force

      • Reduces cross-bridge binding to actin

      • Inhibits Ca++ release from SR

  • Rate of ATP utilization declines faster than the rate of ATP generation

    • Maintains ATP concentration

    • The cell does not run out of ATP

Summary Sites of Fatigue § The cross-bridge's ability to “cycle” is important in continued tension development. § Fatigue may be related, in part, to the effect of a high H+ concentration and the inability of the sarcoplasmic reticulum to rapidly take up Ca++. § The end result may be a longer relaxation time, which affects the rate of muscle contraction.– Extends the time it takes for the muscle to return to resting length § 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

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

    • Least fatigable because they’re highly aerobic

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

    • These can do Aerobic & Anaerobic pretty well

  • Exercise >75% VO2 max requires IIx fibers

    • These are predominantly anaerobic

      • Results in increased lactate and H+ production

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Radical Production During Exercise Contributes to Muscle Fatigue During Prolonged Exercise

  • Exercise promotes free radical production in muscles

    • Radicals are molecules that contain an unpaired electron in outer orbital

    • Capable of damaging proteins and lipids in muscle

  • Can contribute to fatigue during exercise > 30 minutes long

    • Damage contractile proteins (myosin and troponin)

      • Limits the number of cross-bridges in strong binding state

    • Depress sodium/potassium pump activity

  • Supplementation with most antioxidants does not prevent fatigue and high antioxidant doses can impair muscle performance

Summary of fatiguing issues § Central fatigue (≤10% of fatigue) § Sarcolemma conduction and t-tubule block § Accumulation of H+ and Pi § Inability to produce ATP at a rate equal to its utilization during heavy exercise § Reactive oxidative species production during prolonged exercise

Factors Limiting Performance

  • Several factors contribute to performance limitations, which can be categorized into:
      1. Diet:
         - Carbohydrate intake
         - Water intake
      2. Environment:
         - Altitude
         - Heat
         - Humidity
      3. Strength/Skill:
         - Practice
         - Natural endowment
         - Body type
         - Muscle fiber type
      4. CNS Function:
         - Arousal
         - Motivation
      5. Energy Production:
         - Anaerobic sources (Phosphocreatine, Glycolysis)
         - Aerobic sources (VO2 max, Cardiac output, O2 delivery)
           - Related metrics include hemoglobin content, partial pressure of O2 (PO2), O2 extraction, and mitochondrial function.

Sites of Fatigue

  • Central Fatigue:
      - Causes include:
        - Decreased motor unit activation
        - Reduced motor unit firing frequency
        - Factors affecting CNS arousal which include motivation, physical or mental diversion, and effects of excessive training.

  • Peripheral Fatigue: Factors here could be:
      - Altered ability of muscle membranes (sarcolemma) to conduct action potentials.
      - Mechanisms of fatigue include disruptions in ionic balance (K+, Na+) and reduction in calcium (Ca++) release.

Detailed Mechanisms of Fatigue

  • Potential Sites of Fatigue:
      1. Spinal cord
      2. Peripheral nerve
      3. Muscle membranes (sarcolemma, transverse tubular system)
      4. Calcium release from the sarcoplasmic reticulum (SR)
      5. Actin-myosin interactions, leading to changes in cross-bridge tension and heat production.

Specific kinds of performance

  • Now we’ve talked about particular sources of fatigue which may limit performance

    • Central

      • Changes in motor recruitment

        • Due to afferent information, brain metabolism, changes in neurotransmitters, and motivation

    • Peripheral

      • Altered neuromuscular coupling

      • Changes in metabolites that interfere with muscle mechanics

      • Mismatched ATP requirement and ATP generation

  • These are the general points where fatigue seems to occur

  • However, the source of fatigue may differ based on the activity being performed (specificity)

    • Short-term High intensity

    • Long-term “high” intensity

Performance Factors Related to Fiber Type

  • Muscle fiber recruitment increases with intensity:
      - Type I fibers recruited at lower intensities (~40% of VO2 max)
      - Type IIa fibers between 40–75% of VO2 max
      - Type IIx fibers for performances >75% of VO2 max, predominantly anaerobic, leading to increased lactate production.

Exercise Duration Categories

Ultra Short-Term Performances

  • Less than 10 seconds (e.g.,high power events 100-meter dash):
      - Rely primarily on Type II muscle fibers and anaerobic energy sources (ATP-PC system and glycolysis).
      - Motivation, skill, and arousal play key roles in force generation
      - Creatine supplementation may enhance performance.

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Short-Term Performances

  • Lasting 10-180 seconds:
      - Transition from anaerobic to aerobic metabolism.

  • (~70% energy supplied anaerobically at 10s )

  • (60% supplied aerobically at 180s)
      - Significant contributions from anaerobic glycolysis; H+ accumulation

  • impacts performance (interferes with Ca++ binding with toponin & Interferes with glycolytic ATP production )

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Short-Term Events: can vary based on their duration because this is a transitory period beyond the capacity of the ATP/PC system, but too short to reach beyond oxygen deficit.

About 50/50 at ~2 minutes of exercise

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Moderate-Duration Performances

  • 3-20 Minutes (Moderate):
      - Vo2 max is critical, with a reliance on aerobic metabolism.

    • 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

Moderate-Duration Performances

These activities require an energy expenditure near VO2 max, with type II fibers being recruited, in addition to type I fibers.

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 H+ accompany these types of activities

Intermediate-Duration Performances

  • Events lasting 21–60 minutes

  • Predominantly aerobic

    • Usually conducted at <90% VO2 max

    • High VO2 max is important

  • Other important factors

    • Running economy or exercise efficiency

      • High percentage of type I muscle fibers

    • Environmental factors

      • Heat

      • Humidity

    • State of hydration

    • Lactate threshold

~7,400m → ~20:00 (est.) ~6m/s

~21,000m → ~60:00(est.) ~5.9m/s

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Long-Term Performances

  • 1-4 Hours (Long-Term): clearly aerobic
      - Carbohydrate availability and hydration become crucial. Environmental factors (heat/humidity) significantly impact performance.

    • 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

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

      • A 2:15 marathon requires sustaining a VO2 of ~ 60 ml•kg-1•min-1

      • However they cannot perform at max for 135 min. So their maximal aerobic capacity must be higher

      • = At 80% VO2 max, this requires VO2 max of 75 ml•kg-1•min-1

  • Performance determined by:

    • %VO2 max at which runner can maintain performance

      • Estimated by the lactate threshold

    • Running economy

Factors Affecting Performance in Ultra-Endurance Events

  • Examples

    • 166 km mountain run, Triple Iron Triathlon, 24 hour run

  • Most important variables

    • VO2 max

    • %VO2 max that can be sustained

  • Metabolic responses

    • Fat oxidation is 3.5x higher after event

      • Consistent with exercise at <60% VO2 max

    • 50% reduction in muscle glycogen stores

  • Potential for hyponatremia

    • Only affects 4% athletes

    • Excessive sweating & fluid replacement without electrolytes

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