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

< 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 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 VO 2max –Type II fibers recruited –High levels of lactate Factors that interfere with O 2 delivery are limiting –Altitude, anemia

Moderate-Length Performance

Intermediate-Length Performance minutes Predominantly aerobic Usually conducted at less than 90% VO 2max 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