All About ... Muscles (Part 2)

In the Summer I wrote a basic blog about muscles, the 3 types and the main superficial muscles.  I thought this time I would look in more detail at skeletal muscle as these are the ones that can cause problems in 'working horses'. 

As I mentioned last time .... muscles are made up of muscle tissue, which consists of long cells which are called muscle fibres.  These fibres are able to change their length and so produce tension or pull.  This is the function of a muscle - to move one bone upon another and produce movement.  Muscles have an origin and an insertion (an ending).  They are attached to the bones in the same way as seaweed attaches to rock.  The muscle fibres at either end blend  with the periosteum of the bone (see my blog about bones).  

The muscle fibres make up 90% of muscle tissue, the other 10% is made up of nerves, blood vessels and connective tissue.

Structure of Skeletal Muscle

Each fibre is covered  by a sarcolemma, this is a membrane which acts as the link between the muscles and tendons and it also provides elasticity to the fibres.  At various sites along the membrane there are receptors which receive the nerve impulses (see my Nervous System blog). Each fibre is insulated from the others and is controlled directly by a branch from a voluntary nerve, which means that the horse is in control.  The fibres are arranged in bundles and the bundles are surrounded by connective tissue - endomysium - and the sheath which holds the bundles together is called the perimysium.

The entire muscle (ie: a collection of bundles) has a good blood supply and with training the blood supply can be improved further.  Muscles are attached to bones by a tendon (see next week's blog) and tendon length will vary.

As I mentioned in my last Muscles blog they act in pairs known as extensors (antagonist) and flexors (agonist).  There are also stablising muscles (synergists) which will oppose any undesired action of an antagonist! 


Microanatomy of a Muscle Fibre

Each muscle fibre (cell) is made up of hundreds of myofibrils which are arranged in parallel.  Each myofibril is made up of sarcomeres.  Each sarcomere is made up of myofilaments which are made up of actin filaments and myosin filaments, these are proteins (see my food composition blog).  

When a muscle is resting the myosin and actin are repelling each other.  When a nerve impulse stimulates the muscle there are a series of chemical reactions which remove this repelling affect and the actin filaments will slide between the myosin filaments - this results in the sarcomere contracting (ie: getting shorter).  If several sarcomeres contract this then results in contraction of the muscle.

Sarcomeres can only contract totally, they cannot contract part of the way - it is an all or nothing response.  So it is the number of sarcomere's that contract which affects the amount the muscle contracts. 

Energy is needed for muscle contraction which is provided through food sources eg: fats and glucose  (see my food composition blog).  If there is not enough energy the muscles become tired 'muscle fatigue'.

Types of Muscle Fibres

There are 2 types of muscle fibres:

Slow Twitch: contract slowly, these are the types of fibres used in slow aerobic work such as walk, trot and endurance.  These fibres can only function efficiently in the presence of oxygen.  This also means that the build up of lactic acid is avoided.  They are able to generate large amounts of energy, but not at a very fast rate.  This means that the Slow Twitch fibres are not very powerful.

Fast Twitch: rapidly contract.  There are thought to be at least 2 types of fast twitch muscle fibres; one type uses some oxygen and are brought into use for submaximal exercise eg: canter work.  Another type can function without oxygen and so are used for anaerobic work for example sprinting.  These fibres generate energy quickly.  Fast Twitch fibres can only sustain their ability to function without oxygen for a short period of time.

The proportion of the types of muscle fibres used during exercise will change accordingly.  So a horse will begin walking and using mostly slow twitch.  As the horse moves into trot a low percentage of fast twitch muscle fibres will be used - as the pace increases a higher percentage of fast twitch and a lower percentage of slow twitch will be used.

Different breeds of horses have different proportions of muscle fibre types.  A thoroughbred (bred for sprinting) will have a higher percentage of fast twitch fibres in the muscles than a horse bred for slow or endurance work.  These will also have an adjacent high concentration of blood capillaries needed to deliver enough oxygen for optimum performance and also to ensure that waste is removed efficiently - to avoid the build up of undesirable by-products of activity eg: lactose.

Unlike bone, muscles can repair to their pre-injury state with the right treatment.  Understanding about muscles and how they work helps us understand more about fitness and training too!

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Until next time!

Jo