muscle fibre types

(redirected from fast muscle fibres)

muscle fibre types

categories of muscle fibre adapted for various modes of use. Normally taken to refer to skeletal muscle unless specified otherwise. The main types found in stable human skeletal muscles after infancy are shown in the table. Intermediate 'hybrid' forms also occur, commonly in non-stable states such as during intensive or recently changed training regimes, or in recovery from injury. Types termed 1, 2A and 2B, containing myosins similarly designated, have been recognized since about 1970; however, the separate existence in many mammals of type 2X (initially also known as 2D), distinguishable from 2B only by sophisticated techniques, was recognized during the 1990s and it is now accepted that most, if not all, human fibres formerly called 2B in fact have 2X myosin. The slower-contracting muscles have a majority of type 1 fibres (typically approaching 90% in the extreme instance, soleus), while the faster-contracting ones have rather more, and often rather larger, type 2. Only some small specialist muscles (e.g. extraocular) have 80-85% fast fibres; in large limb muscles above 60% fast is reported only for sprint athletes. See also muscle enzymes. See table below .
Table 1: Characteristics of the three main types of stable human skeletal muscle fibres
Fibre type and myosin type12A2X (formerly '2B')
DescriptionSlow (-twitch) redFast (-twitch) redFast (-twitch) white
Principal energy supply systemOxidativeOxidative and glycolyticGlycolytic (anaerobic)
Abbreviated descriptionSOFOGFG
Mitochondrial densityHighMedium to very highLow
Motor neuron sizeSmallMediumLarge
Contraction speedSlowFairly fastFast
Fatigue resistanceHighMedium to highLow
Myosin ATPase activity after pH ∼ 10.3 pre-treatmentLowFairly highHigh
Ditto, after pH 4.6 pretreatmentHighLowMedium
Ditto, after pH 4.3 pretreatmentHighLowLow