There are three types of muscle within the human body:
Skeletal Muscle (voluntary muscle)
These are the muscles which are attached to our skeletons and cause us to move our body parts. They are called voluntary muscles as they are under our control. They are sometimes also called striated as they have a stripy appearance!
Smooth Muscle (involuntary muscle)
This muscle is not under our direct control and contracts of its own accord. It is situated in the walls of many of our organs, such as the stomach and blood vessels. It is called smooth as it does not share the same stripy appearance as skeletal muscle.
As the name suggests, this muscle is found in the heart and nowhere else. It is a specialised type of muscle which works continuously and is not under our control.
Muscles are made of bundles of fibres. These can be either Fast Twitch or Slow Twitch
Fast twitch fibres are used for powerful, fast movements. This however means they get tired quickly. Athletes who are good at short events which require speed and power will have a higher number of fast twitch fibres
Slow twitch fibres are good for endurance activities. They contract slowly with less force, but do not tire so easily. Long distance runners tend to have more of these fibres.
Everyone has a similar number of muscle fibres. People with larger muscles have larger fibres, not more of them.
In order for a muscle to cause a movement, it crosses a joint. It is attached to two bones which form the joint by tendons. Just one of these bones will move when the muscle contracts. For example, when the quadriceps muscles contract, the Tibia of the lower leg is pulled forwards to straighten the knee.
Muscles have two ends, one is called the origin, and is the end that attches to the stationary bone (in the example of the quadriceps - the end attaching to the Femur). The other end is called the Insertion and is attached to the moving bone (the Tibia).
Because muscles only pull, they cannot push, they work in pairs. These pairs are called antagonistic pairs. As one muscle contracts (shortens), the other relaxes (lengthens). The muscle which is contracting is called the agonist, or prime mover. The relaxing muscle is called the antagonist. There is a third muscle type within this model, called a synergist. These muscles help to stabilise the bone which isn't moving.
For example - when performing a bicep curl:
Agonist - Biceps Brachii
Antagonist - Triceps Brachii
Synergist - Deltoid
Muscles can contract in two different ways:
Isometric - the muscle length does not change and there is no movement. For example when you are carrying a bucket of water
Isotonic - the muscle length changes, causing movement at a joint. There are 2 types of Isotonic contractions:
Concentric - the muscle shortens as it contracts ( a bicep curl.)
Eccentric - the muscle lengthens as it contracts (downwards phase of press up)
Performing exercises and being active in our daily life can cause our muscles to get stronger. As already mentioned, strong people have larger muscle fibres. This growth in muscle size is called hypertrophy.
If we do not use our muscles regularly, the opposite can happen and the muscles reduce in size. This is called atrophy.
Muscles are always slightly under tension, to enable us to hold a position, such as sitting upright. This small amount of muscle tension, is known as muscle tone. Exercise improves muscle tone.