Limits to human performance

Question 1

What is fatigue?

Answer 1

failure of the working musculature to maintain force or power output during a series of contractions. it is caused by a complex mix of central and peripheral mechanisms

Question 2

What is central fatigue?

Answer 2

Reduction in the function of motor units that are involved in contraction or a reduction in motor unit firing frequency

Question 3

What causes central fatigue?

Answer 3

• CNS takes signals from around the body and integrates them to affect the rate of motor recruitment
• possibly based on serotonin and dopamine levels
• Muscles themselves are also likely to signal the CNS
  
  • peripheral factors send signals via afferents which will inhibit the CNS, limit the output of spinal motor neurons and limit the exercise

Question 4

What is peripheral fatigue?

Answer 4

• Most common form of fatigue

- Caused by changes in the working musculature

Question 5

What causes peripheral fatigue?

Answer 5

• Neural: anywhere in the excitation-coupling process
• Muscular: mechanics of muscle function - cross bridging cycle - accumulation in H+ and decrease pH
• Energetics: increased rate of ATP demand = build up of phosphate and H+

Question 6

What factors limit VO2max?

Answer 6

• Pulmonary system: possible large Q with reduced transit time in endurance athletes
• Cardiovascular sytstem: cardiac output cannot deliver O2 fast enough
• Oxygen carrying capacity of blood: amount of Hb to deliver oxygen
• Peripheral: possibly the uptake of O2 into the muscle
• Functional threshold: Efficiency and Lactate threshold -> fatigue

Question 7

What factors limit short and long endurance?

Answer 7

• VO2 max and lactate threshold (velocity at VO2 for shorter distances)
• glycogen depletion (less likely for shorter duration)
• hyperthermia due to increased metabolism (hydrolysis), increased heat production and environmental heat. Core temp limit 40 deg.
• dehydration: sweating decreases plasma volume
• accumulation of metabolites: H+ increase = pH decrease in shorter sprints

Question 8

What factors limit short duration high-intensity efforts?

Answer 8

• Muscular: CSA, number of type 2 fibres
• Central factors: ability to maximally activate the muscle; training, motivation and arousal, skill and technique
• Energy systems: PCr store depletion, decrease in pH due to increase in H+, affects PFK, PHOS, Ca2+ peripherally, accumulation of Pi and K+.

Question 9

Explain how H+ accumulation and decreased pH may cause fatigue

Answer 9

Anaerobic glycoloysis causes an accumulation of H+ and a decrease in pH. This peripherally affects PFK, PHOS and Ca2+. The decrease in pH makes the body acidic and causes fatigue. PFK and PHOS affect glycolysis and require the aerobic system. Ca2+ affects the muscle cross bridging cycle and slow muscle relaxation rates.

Question 10

What are the common movement patterns observed during match play in team-sports

Answer 10

Typically bouts of 4-7 x 2-3s sprints with short active recovery between.

Question 11

Describe the timeline for PCr recovery?

Answer 11

PCr stores are almost fully recovered after 3 mins rest following a single sprint. After a bout of sprints only 84% is recovered.
Initially there is a fast recovery of PCr controlled by the rate of oxidative ATP resynthesis then the rate slows following pH recovery (3-5 mins)

Question 12

What affects PCr recovery?

Answer 12

Low pH and low ATP possibly affect PCr recovery. O2 is required for rapid repletion of PCr stores

Question 13

Why is aerobic fitness important to team-sport players?

Answer 13

Team sports use repeated single sprints with minimal recovery.

• not enough time for PCr to recover
• VO2 max could be reached during final sprints of a bout of repeated sprints

Question 14

How is energy supplied for a single short sprint?

Answer 14

ATP + PCr

Question 15

How is energy supplied for repeated sprints with only short recovery?

Answer 15

Anaerobic glyoclysis and aerobic glycolysis

Question 16

Factors limits repeated sprints and team sport peformance

Answer 16

• ability to recover PCr between bouts of sprints
• build up of H+ and decrease pH: decreased PCr stores
• Glycogen depletion
• availability of O2 (Aerobic fitness): ATP needs to be reformed via ETC and requires PCr repletion.