When viewed under a microscope, skeletal muscle has a striped appearance and is therefore sometimes called striated muscle.
This striped appearance is caused by thousands of long, narrow fibres or cells that are able to contract (shorten in length).
These fibres are enclosed in a sheath of connective tissue, known as the epimysium.
Muscle fibres are made up of very small threads called myofibrils and each myofibril consists of rows of protein molecules, actin and myosin. It is the interaction between the actin and myosin molecules that causes the muscle to contract
Remember: muscles can only pull they cannot push.
There are two types of muscle fibre: slow and fast twitch.
They are physiologically different.
The amounts of fast twitch or slow twitch fires in the muscle will determine their suitability to certain sporting activities.
The amount of each fibre type in our muscles is also determined genetically, that is, we are born with it.
Training will have little effect on altering the ratio of fast twitch to slow twitch fibre in the muscle.
This is why there are people who are 'natural' endurance athletes; they have greater amounts of slow twitch fibre which contracts slowly with little force but do not tire easily.
Other people have greater amounts of fast twitch fibres and are 'naturally' good at sprinting and throwing. These fibres produce larger forces but tire quickly.
Skeletal muscle contractions are stimulated by electrical signals transmitted along motor nerve fibres that have been sent from the central nervous system. (See 'Nervous system').
Cross-bridges are formed between the myosin and actin molecules. The cross-bridges originate on the myosin molecule and attach themselves to the actin molecule. They then drag the actin molecule towards the origin of the muscle. It is the actin molecule that is active and moves the myosin molecules are fixed.
Every time a cross-bridge is formed energy is required.
This energy is provided in the form of a chemical known as Adenosine Triphosphate (ATP).
When the nervous stimulus arrives at the muscle cell ATP breaks down into a chemical known as Adenosine Diphosphate (ADP).
During this break down energy is released which is used to form one cross-bridge.
Duration of the nervous stimulus will determine the duration of the muscle contraction.
The strength of the stimulus will determine the force that the muscle contraction exerts.
There is a limited quantity of ATP in the muscle and for muscle contraction to continue over a long period of time.
ATP has to be re-built from ADP + P. The re-building of ATP is known as muscle respiration and described in detail in Muscle Respiration.