 Basically if a stimulus is received by muscle cells (fibres) and it is great enough to cause a response – the fibres respond by contracting & relaxing (1 st glimpse at “All or None principle”).

 All muscle fibre types can operate at the same metabolic capabilities, but some are better biochemically equipped to work anaerobically or aerobically (without or with oxygen).  Anaerobic – activity of a high intensity that does not allow oxygen to be used in the metabolic process of ATP breakdown (e.g. maximal effort sprint 50m); if duration of activity of such an intensity continues, lactic acid is produced and build-up leads to fatigue (e.g. maximal effort 200m sprint)  Aerobic – activity of a lower intensity in which oxygen is used in the process of breaking down carbohydrates & fats for fuel source (e.g. 5 min + jogging)

 Slow-twitch fibres (ST)  AEROBIC (with O2)  Red in colour  Type I (slow-oxidative)  E.g. leg muscles

 Fast-twitch (FT)  ANAEROBIC (Without O2)  White in colour  Type IIa & IIb  E.g. arm muscles

 During the teen years, the number of ST and FT fibres become “fixed”; training can elicit some change, but for the most part, # of specific fibres & arrangement is pre- determined by genetics.

PROPERTIES OF MUSCLE FIBRES Characteristic TYPE ITYPE IIaTYPE IIb Speed of contractionSlowFastFastest Primary source of ATPO2Glygogen- Glucose- LA Phosphocreatine Rate of fatigueSlowIntermediateFast # of mitochondriaHighIntermediateLow Capillary densityHighIntermediateLow Glycogen capacityLowIntermediateHigh Fibre diameterSmallLargeLargest Size of Motor NeuronSmallLargeLargest Force ProductionLowHighHighest Oxidative CapacityHighIntermediateLow Activity used forLong DurationShort DurationShort Medium Duration