Energy systems in muscles

• Fast-twitch muscle fibre


  Type of muscle fibre used in short bursts of activity
• Glucose


  Sugar which is the main respiratory substrate in cells
• Glycogen


  Main storage form of glucose in the body, stored in liver and muscle
• Lactate


  Pyruvate is converted to lactate in muscle cells when there is insufficient oxygen for the electron transport chain to take place
• Mitochondrion


  Cell organelle in which glycolysis and the citric acid cycle occur
• Muscle fatigue


  Caused by accumulation of lactate in muscle cells
• Myoglobin


  Protein in muscle tissue that can bind with oxygen
• Oxygen debt


  Builds up in muscle cells when there is insufficient oxygen
• Slow-twitch muscle fibre


  Type of muscle fibre used in endurance activities
• Lactate metabolism


  1. Pyruvate is converted to lactate

  2. Hydrogen ions from NADH are transferred to pyruvate to produce lactate

  3. Regenerates NAD needed to maintain ATP production through glycolysis
• Lactate accumulates and muscle fatigue occurs
• Oxygen debt is repaid when exercise is complete (i.e. is reversible)
• Lactate metabolism is a very inefficient process as it results in only 2 molecules of ATP being produced during glycolysis
• Slow twitch muscle fibres

  • Contract relatively slowly but can sustain contractions for longer
  • Useful for endurance activities such as long-distance running, cycling or cross-country skiing
  • Rely on aerobic respiration to generate ATP
  • Have many mitochondria, a large blood supply and a high concentration of the oxygen-storing protein myoglobin
  • Major storage fuel is fats
• Fast twitch muscle fibres


  • Contract relatively quickly, over short periods
  • Useful for activities such as sprinting or weightlifting
  • Can generate ATP through glycolysis only
  • Have fewer mitochondria and a lower blood supply compared to slow-twitch muscle fibres
  • Major storage fuel is glycogen
• Most human muscle tissue contains a mixture of both slow and fast twitch muscle fibres
• Athletes show distinct patterns of muscle fibres that reflect their sporting activities