Different muscle types in the human body

There are three types of muscle in the body:

smooth muscle – found in the internal organs and blood vessels - this is involuntary
cardiac muscle – found only in the heart - this is involuntary
skeletal muscle – attached to the skeleton - this is voluntary

Involuntary muscles are not under our conscious control which means we can't make them contract when we think about it.

Voluntary muscles are under our conscious control so we can move these muscles when we want to.

Question

Which muscle fibre types are not under conscious control?

Muscle fibre types

There are three different types of skeletal muscles:

type I
type IIa
type IIx

Each type of muscle fibre has different characteristics which are shown in the table:

	Type I	Type IIa	Type IIx
Speed of contraction	Slow	Fast	Very fast
Force produced	Low	Medium	High
Resistance to fatigue	High	Medium	Low
Colour	Red	Pink	White

Speed of contraction
Type I	Slow
Type IIa	Fast
Type IIx	Very fast

Force produced
Type I	Low
Type IIa	Medium
Type IIx	High

Resistance to fatigue
Type I	High
Type IIa	Medium
Type IIx	Low

Colour
Type I	Red
Type IIa	Pink
Type IIx	White

Endurance cyclists need a high percentage of type I fibres so that their muscles can work for the duration of a race without getting tired. These muscles are red in colour because of the amount of capillaries that transport the oxygenated blood to the working muscles.

A sprinter would need a high percentage of type IIx fibres so that their muscles can contract very quickly and produce a lot of force so that they can run very fast over a 100 m distance. However, these muscles tire very quickly so the sprinter will not be able to run at this speed for very long.

Question

Explain which muscle fibre type a long distance runner requires in order to be successful in their event.