Muscle Fibres

Type I fibres

Relatively slow in generating peak force (fast twitch = 10x faster)

Adapted for slow rythmical contractions but still contract fast enough for all sporting activities

Smaller in size to fast twitch

Produce less overall force than fast twitch

More energy efficient than fast twitch - producing more force for the same energy input, this making them well edapted for prolonged/endurance exercise (gets ATP from primarily aerobic energy pathways)

High aerobic capacity = greater no. of blood capillaries for the delivery of O2/fibre than fast twitch

Slow twitch contain more myoglobin (red pigment in blood that combines with O2) sometimes called 'red fibres'

They contain a lot more mitochondria & their associated oxidative enzymes than fast twitch

Not as easily fatigued as fast twitch muscle fibres

Examples: Distance runner - 79% slow twitch fibres in gastrocnemius and 21% fast twitch

Type II

Poor aerobic capacity/endurance

Best suited to anaerobic exercise = produce their energy primarily from anaerobic pathways

Well adapted to short intense bursts of effort

They ahve a larger diameter as they have more myosin filaments in them. This myosin is thicker than that found in slow twitch fibres

Able to produce more force due to the thicker myosin arrangement in the myofibrils. This greater force of contraction is helped by having a more complex arrangement of Sarc Ret = Ca+ can be released & returned to storage quicker than slow twitch

Have a different myosin-ATPase that releases energy quicker than the type found in slow-twitch fibres

Anaerobic enery system - Fast twitch have laregr stores of Phosphocreatine than slow fibres 

Example - Sprinter - 76% fast twitch muscle in gastrocnemius 24% slow twitch

Fast Oxidative Glycolytic 

Fast Contraction

Large force

Fatigues easily

Example - 400m swim, 1-mile run

Maximum contraction force & velocity = between type 1 & type IIb

IIa - component of myosin

When activity is absent so are type IIa  fibre types

Heavy training can turn type IIb into type IIa when heavy training stops they rveert back, however the body overcompensates converting more type IIa

Fast Twitch Glycolytic

Used infrequently in normal low intensity exerciseused in highly explosive events

Examples - 100m sprints, 50m swims (speed dependent activities)

Max contraction velocity = 10x that of a slow twitch

Very high anaerobic capacity

Fast contraction speed

Low fatigue resistence 

IIb - myosin filament

Large motor neurone size - fast

Fast Myosin-ATPase

Overtraining (in strength & power) converts IIb to IIa, converts back after, converts more