Factors Affecting Performance

Powers, Chapter 19

Factors Affecting Performance

Sites of Fatigue

• Central fatigue• Peripheral fatigue– Neural factors– Mechanical factors– Energetics of

contraction

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

Peripheral Fatigue:Neural Factors

• Neuromuscular junction– Not a site for fatigue

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

action potential • Repeated stimulation of sarcolemma can reduce size

and frequency of action potentials

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

reticulum

Peripheral Fatigue:Mechanical Factors

• Reduction in force per cross bridge• Reduction of force generated at a given Ca+

+ concentration– H+ interference with Ca++ binding to troponin

• Inhibition of Ca++ release from SR• Lack of ATP to dissociate the cross-bridge

from actin

Peripheral Fatigue:Energetics of Contraction

• Mismatch between rate of ATP production and utilization– Fatigue results in slowing of ATP utilization to

preserve homeostasis

• Muscle fiber recruitment in increasing intensities of exercise– Type I Type IIa Type IIb– Progression from most to least oxidative fiber

type• Results in increased lactate production

Pattern of Muscle Fiber Recruitment

Ultra Short-Term Performance

• < 10 seconds• Dependent of recruitment of Type II muscle

fibers– Generate great forces that are needed

• Motivation, skill, and arousal• Primary energy source– Anaerobic• Phosphocreatine

Ultra Short-Term Performance

Short-Term Performance

• 10-180 seconds• Shift from anaerobic to aerobic metabolism– 70% energy supplied anaerobically at 10s– 60% supplied aerobically at 180s

• Primary energy source– Anaerobic glycolysis

Short-Term Performance

Moderate-Length Performance

• 3-20 minutes• Increasing reliance on aerobic energy production– 60% ATP generated aerobically at 3 min– 90% ATP supplied aerobically at 20 min

• Requires energy expenditure near VO2max

– Type II fibers recruited– High levels of lactate

• Factors that interfere with O2 delivery are limiting– Altitude, anemia

Moderate-Length Performance

Intermediate-Length Performance

• 21-60 minutes• Predominantly aerobic

• Usually conducted at less than 90% VO2max

• Environmental factors are important– Heat– Humidity– State of hydration

Intermediate-Length Performance

Long-Term Performance

• 1-4 hours• Environmental factors important– Ability to deal with heat and humidity

• Muscle and liver glycogen–Maintain rate of carbohydrate utilization

• Diet and fluid ingestion influence performance

Long-Term Performance