10: Skeletal muscle 3

• Type I slow oxidative fibers are active for all intensity activity/activities. This is because they are used for continuous things, such as maintaining posture.
• Fatigue is defined as decreased tension development in a muscle fiber or whole muscle that is caused by previous contractile activity, despite the same motor neuron input.
• Muscle fiber fatigue occurs before ATP depeltion, and so may be protective against rigor-type contractions.
• The two main types of fatigue are:

  • fatigue of fast fibers (due to high intensity activation)
  • prolonged low frequency force depression
• Major contributors to prolonged low frequency force depression are probably depletion of glycogen, low blood glucose, dehydration (electrolyte imbalances, volume depletion) and damage to muscle fibers.
• Some contributing factors to fast fiber fatigue include…

  • a decrease in ATP (but not complete depletion)
  • increased Pi and ADP
  • increased Mg2+ levels
  • increased H+ due to lactic acid production
  • Individually or due to complex interactions, these factors decrease…
  • force production
  • Ca2+ release from the SR
  • Ca2+ sensitivity of the thin filament
• Whole muscle tension is determined both by the tension developed by each individual fiber AND the number of active fibers.
• When muscle fibers are being recruited, slow fibers are activated first due to their smaller diameters requiring less input from the motor neuron.