The Muscular System

The muscular system is made up of the muscles of the body and the tendons (tough, dense fibrous bands that join muscle to bone) that connect them to the skeleton.

What is muscle?

Muscle is a type of tissue within our bodies that can create force and change length.  Muscles do this through either a voluntary (over which we have direct control; i.e. walking) or involuntary (over which we have no direct control; i.e. heart pumping blood) contraction.

There are three different types of muscle within our bodies, these are:

1. Skeletal muscle

2. Cardiac muscle

3. Smooth muscle

What are the properties of muscle?

All muscle types share the following unique properties that allow them to function properly.

1.  Irritable – They are capable of receiving and responding to stimulation from nerves.
2.  Contractible – Once they have received stimulation, the muscle is capable of actively shortening (contracting).
3.   Extensible – With the application of force, muscle can be stretched without damage.
4.   Elastic - Whenever a muscle has been shortened or lengthened, it has the ability to return to its resting shape and length.
5.   Adaptability – Muscle will hypertrophy (enlarge) in response to increased work.  Conversely it will atrophy (waste away) if it is deprived of work.

What do muscles do?

Each type of muscle also has its own special role that it performs within our bodies.  The different types are able to perform different roles because they each have slight differences in their structural anatomy.

The roles of the three muscle types are explained in the following table.

It is also worth noting that both cardiac and smooth muscles are fatigue resistant (they don’t tire), as opposed to skeletal muscle which fatigues relatively easily.  This is quite handy really as our life expectancy would be dramatically less if our heart and blood vessels had to keep taking rest breaks!

Why is muscle important?

When considering the three types of muscle and their specific roles it is easy to see why muscle is so important.  It is muscle that; creates movement by contracting and pulling on our bones, that continuously works to pump blood and nutrients around the body and even helps with digestion.

So maintaining well conditioned muscle is a necessity if we want to live a healthy life where the body functions properly and enables us to do our daily chores and participate in all physical activities effectively.

Muscles and fitness

The muscular system and in particular skeletal muscle go hand in hand with fitness.  Whether you are training for a marathon, to be the world’s next strongest man or going through rehabilitation, you are working skeletal muscle.  This is because you are trying to improve the capacity of your muscles to run longer distances, to lift heavier loads or improve the condition and functioning of a specific muscle.

Understanding skeletal muscle and its functions is vital for all personal trainers to complete everyday tasks such as instructing exercise, assessing a clients movement, correcting exercise technique, carrying out fitness tests and designing training programmes.

For example when working with clients who want to achieve goals such as improving their upper body strength or hill running stamina, then it is vital that you have a sound knowledge of the muscles that work, what stresses to place them under, what exercises to prescribe to work them, what variables (sets, rest, load) will produce the optimal outcomes, and what adaptations to expect.

The role of skeletal muscle

When discussing the skeletal system we described the movements that can occur at joints, namely; flexion, extension, abduction, inversion etc.   Skeletal muscles produce these movements by pulling bones in different directions.

Skeletal muscle is attached to bones at either end by tendons.  The attachment end that is relatively fixed is known as the ‘origin’ of the muscle.  The origin is normally proximal (closer) to the trunk.  The attachment end that moves most and is normally distal is known as the ‘insertion’.  We see this on the adjacent diagram showing the bicep muscle which produces flexion at the elbow when it contracts and shortens.  The bicep has two (bi = two) origins, one high on the humerus, the other on the scapula.  When the muscle contracts and shortens it pulls on the insertion points on the radius causing the elbow to flex. 

Functions of skeletal muscle

Skeletal muscle has four major functions, these are:

1.  Force generation for movement:  Skeletal muscle is responsible for generating the force needed to move the body.  Walking, running, swimming, pushing, pulling etc are all movements created by the contraction of skeletal muscles. 

2.  Force generation for breathing (respiration):  The skeletal muscles of the ribs and abdomen are responsible for the movements necessary for respiration, as they systematically relax and contract to enable the lungs to fill with air and then expel the air.
 
3.  Force generation for postural support:  Skeletal muscles also stabilise the joints of the body during movement and help to maintain ideal posture.  When a person’s posture is ‘ideal’ the effects of gravity (pushing down on us) are minimal, weight is evenly distributed through the load bearing joints of the body; the joints between vertebrae, the hips, knees and ankles and injury risk is reduced.
 
4.  Heat production:  When skeletal muscles contract they produce heat.  When body temperature drops (due to exposure to cold), skeletal muscles can create heat to maintain core body temperature.  They can do this either voluntarily (doing some exercise to warm up, rubbing the hands together etc) or involuntarily by shivering.