B1 - Fatigue

• We cannot continue to exercise indefinitely because we will eventually fatigue
• Fatigue means tiredness from physical exertion
• Fatigue occurs as a result of a number of factors including:

  • depletion of energy sources
  • accumulation of waste products
  • depletion of acetylcholine
• Depletion of Energy Sources:

  • In order to exercise we must break down the energy stored in our body and turn in into ATP.
  • Sources of energy include phosphocreatine, glucose and glycogen
  • We have only enough phosphocreatine to last us for 10 seconds of maximal exercise
  • We then switch to glucose for energy production
  • We have around 15-20 g of glucose in our blood stream, around 345 g glycogen in our muscles and 90-110 glycogen stored in our liver
• Depletion of Energy Sources:

  • When our blood sugar levels are low, the liver converts either its store of glycogen into glucose or the skeletal muscles store of glycogen into glucose
  • However, we have only enough glycogen stores to last us for around 2 hours
  • Once the body's stores of glucose and glycogen are used up, we become fatigued and/or have to exercise at a lower intensity
• Accumulation of waste products:

  • Lactate is the main by-product of anaerobic glycolysis
  • Blood always contains a small amount of lactate; however, during high intensity exercise this increases greatly
  • The increased production of lactate results in the pH of the blood decreasing
  • A blood pH of 6.4 or lower affects muscle and neural function and eventually prevents continued exercise
• Accumulation of waste products:

  • Carbon dioxide is also produced as a waste product of the aerobic energy system
  • This is converted into carbonic acid as it is carried in the blood back to the lungs ready to be exhaled
  • However, when blood is carrying carbon dioxide in the form of carbonic acid it does lower the pH of the blood as carbonic acid is acidic
• Depletion of Acetylcholine:

  • The effect of the depletion of acetylcholine results in neuromuscular fatigue, this means that the muscles are either not able to receive signals from the CNS that stimulates the muscle to contract, or that the muscle tissue is unable to function properly
  • High-intensity exercise or exercise for long periods of time can eventually interfere with the availability of acetylcholine
• Depletion of Acetylcholine:

  • Acetylcholine is required to transmit nervous impulses between the nerves and the muscle fibres
  • If this neurotransmitter is not available, then this prevents the nervous stimulation from the CNS reaching the muscle tissue/motor unit, so the muscle is not able to contract