Types of Muscle

Skeletal Muscle

Skeletal Muscles are those which attach to bones and have the main function of contracting to facilitate movement of our skeletons. They are also sometimes known as striated muscles due to their appearance. The cause of this 'stripy' appearance is the bands of Actin and Myosin which form the Sarcomere, found within the Myofibrils.

Skeletal muscles are also sometimes called voluntary muscles, because we have direct control over them through nervous impulses from our brains sending messages to the muscle. Contractions can vary to produce powerful, fast movements or small precision actions. Skeletal muscles also have the ability to stretch or contract and still return to their original shape.

Fibre Type

Not all fibres within Skeletal muscles are the same. Some contract at different speeds and vary in colour depending on their Myoglobin (an oxygen carrying protein) content. These include:

Type I: Type I fibres are also known as slow twitch fibres. They are red in colour due to the presence of large volumes of myoglobin and so oxygen and high numbers of Mitochondria. Due to this fact they are very resistant to fatigue and are capable of producing repeated low-level contractions by producing large amounts of ATP through an aerobic metabolic cycle. For this reason the muscles containing mainly type I fibres are often postural muscles such as those in the neck and spine due to their endurance capabilities Also, athletes such as marathon runners have a high number of this type of fibre, partly through genetics, partly through training.

Type IIa: Type IIa fibres are also sometimes known as fast oxidative fibres and are a hybrid of type I and II fibres. These fibres contain a large number of mitochondria and Myoglobin, hence their red colour. They manufacture and split ATP at a fast rate by utilising both aerobic and anaerobic metabolism and so produce fast, strong muscle contractions, although they are more prone to fatigue than type II fibres. Resistance training can turn type IIb fibres into type IIa due to an increase in the ability to utilise the oxidative cycle.

Type IIb: Often known as fast glycolytic fibres they are white in colour due to a low level of myoglobin and also contain few mitochondria. They produce ATP at a slow rate by anaerobic metabolism and break it down very quicky. This results in short, fast bursts of power and rapid fatigue. As mentioned above, this type of fibre can be turned into type IIa fibres by resistance training. This is a positive change due to the increased fatigue resistance of type IIa fibres. These fibres are found in large quantities in the muscles of the arms.

Smooth Muscle

Smooth muscle is also sometimes known as Involuntary due to our inability to control its movements, or Unstriated as it does not have the stripy appearance of Skeletal muscle. Smooth muscle is found in the walls of hollow organs such as the Stomach, Oesophagus, Bronchi and in the walls of blood vessels. This muscle type is stimulated by involuntary neurogenic impulses and has slow, rhythmical contractions used in controlling internal organs, for example, moving food along the Oesophagus or contricting blood vessels during Vasoconstriction.

Cardiac Muscle

This type of muscle is found solely in the walls of the heart. It has similarities with skeletal muscles in that it is striated and with smooth muscles in that its contractions are not under conscious control. However this type of muscle is highly specialised. It is under the control of the autonomic nervous system, however, even without a nervous imput contractions can occur due to cells called pacemaker cells. Cardiac muscle is highly resistant to fatigue due to the presence of a large number of mitochondria, myoglobin and a good blood supply allowing continuous aerobic metabolism.