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The Muscular System

The Muscular System

Muscles are highly specialised to contract forcefully.  This generates the force needed to create movement.  There are over 600 muscles in the human body, they are responsible for every movement we make, for pumping blood around the, moving food through the digestive system, and many other functions as well.  Muscles are powered by muscle cells which contract individually within a muscle to generate force.  Without muscle cells we would be unable to stand, walk, talk or carry out any of the tasks that we do throughout the day.

Types of Muscle

There are three types of muscle:

  • Skeletal Muscle – responsible for body movement
  • Cardiac Muscle – Responsible for the contraction of the heart
  • Smooth Muscle – Responsible for many tasks including: movement of food along intestines, enlargement and contraction of blood vessels, size of pupils and many other contractions.

Skeletal Muscle Structure and Function

Skeletal muscles are attached to the skeleton and are responsible for the movement of our limbs, torso and head.  They are under conscious control, that means that we can consciously choose to contract a particular muscle and can regulate how strong the contraction actually is.  Skeletal muscles are made up of a number of muscle fibres.  Each muscle fibre is an individual muscle cell and may be anywhere from 1mm to 4cm in length.  When we choose to contract a muscle fibre – for instance we contract our bicep to bend our arm upwards – a signal is sent from our brain via the spinal chord to the muscle.  This signals to the muscle fibres to contract.  Each nerve will control a certain number of muscle fibres.  The nerve and the fibres it controls are called a Motor Unit. Only a small number of muscle fibres will contract to bend one of our limbs, but if we wish to lift a heavy weight then many more muscles fibres will be recruited to perform the action.  This is called muscle fibre recruitment.

Each muscle fibre is surrounded by connective tissue called an external lamina.  A group of muscle fibres are encased within more connective tissue called the endomysium. The group of muscle fibres and the endomysium are surrounded by more connective tissue called the perimysium.  A group of muscle fibres surrounded by the perimysium is called a muscle fasiculus.  A muscle is made up of many muscle fasiculus which are surrounded by a thick collagenous layer of connective tissue called the epimysium. The epimysium covers the whole surface of the muscle.

Going back to the muscle fibres; muscle fibres also contain many mitochondria which are energy powerhouses that are responsible for the aerobic production of energy molecules (ATP molecules).  Muscle fibres also contain glycogen granules as a stored energy source and myofibrils which are thread like structures running the length of the muscle fibre.  Myofibrils are made up of two types of protein: 1) Actin myofilaments, and 2) myosin myofilaments. The actin and myosin filaments form the contractile part of the muscle called the sarcomere.  Myosin filaments are thick and dark when compared with actin filaments which are much thinner and lighter in appearance.  The actin and myosin filaments lay on top of one another, it is this arrangement of the filaments that gives muscle its striated, or striped appearance.  When groups of actin and myosin filaments are bound together by connective tissue they make the myofibrils.  When groups of myofibrils are bound together by connective tissue they make up muscle fibres.

The ends of the muscle connect to bone through a tendon.  The muscle is connected to two bones in order to allow movement to occur through a joint.  When a muscle contracts only one of these bones will move.  Where the muscle is attached to the bone that moves is called the insertion.  Where the muscle is attached to the bone that remains in a fixed position is called the origin

How Muscles Contract

Muscles are believed to contract through a process called the Sliding Filament Theory.  In this theory the muscles contract when actin filaments slide over myosin filaments resulting in a shortening of the length of the sarcomeres and hence a shortening of the muscle fibres.  During this process the actin and myosin filaments do not change length when muscles contract.  But instead it is when they slide past each other that the muscles contract.

During this process the muscle fibre becomes shorter and fatter in appearance.  As a number of muscle fibres shorten at the same time the whole muscle contracts and causes the tendon to pull on the bone it attaches to.  This creates movement that occurs at the point of insertion.

For the muscle to return to normal (i.e. to lengthen) a force must be applied to the muscle to cause the muscle fibres to lengthen.  This force can be due to gravity or due to the contraction of an opposing muscle group. 

Skeletal muscles contract in response to an electric signal called an action potential.  Action potentials are conducted along nerve cells before reaching the muscle fibres.  The nerve cells regulate the function of skeletal muscles by controlling the number of action potentials that are produced.  The action potentials trigger a series of chemical reactions that result in the contraction of a muscle.

When a nerve impulse stimulates a motor unit within a muscle, all of the muscle fibres controlled by that motor unit will contract.  When stimulated these muscle fibres contract on an all or nothing basis.  The all or nothing principle means that muscle fibres either contract maximally along their length or not at all.  So when stimulated muscle fibres contract to their maximum level and when not stimulated there is no contraction.  In this way the force generated by a muscle is not regulated by the level of contraction by individual fibres but rather it is due to the number of muscle fibres that are recruited to contract.  This is called muscle fibre recruitment.  When lifting a light object such a book only a small number of muscle fibres will be recruited. But, those that are recruited will contract to their maximum level.  When lifting a heavier weight many more muscle fibres will be recruited to contract maximally.

When one muscle contracts another, opposing muscle, will relax.  In this way muscles are arranged in pairs.  An example is when you bend your arm at the elbow, you contract your bicep muscle and relax your tricep muscle.  This is the same for every movement in the body, there will always be one contracting muscle and one relaxing muscle.  If we think about this it is obvious that unless the opposing muscle is relaxed it will have a negative effect on the force generated by the contracting muscle.  A muscle that contracts and is the main muscle group responsible for the movement is called the Agonist or Prime mover.  The muscle that relaxes is called the Antagonist.  One of the effects that regular strength training has is an improvement in the level of relaxation that occurs in the opposing muscle group.

Agonist and antagonist muscle group examples

Example Agonist Muscle groups Opposing (Antagonist) Muscle groups
Latimus Dorsi (upper back) Deltoids (shoulders)
Rectus Abdominus (stomach) Erector Spinae (lower back)
Quadriceps (top of thigh) Hamstrings (Back of thigh)
Gastrocnemius (calf) & Soleus (below calf) Tibialis anterior (shin)

Smaller muscles may also assist the agonist during a particular movement.  The smaller muscle is called the synergist.  An example of a synergist would be the deltoid (shoulder) muscle during a press-up.  The front of the deltoid provides additional force during the press-up, however, most of the force is applied by the Pectoralis Major (Chest).  Other muscle groups may also assist the movement by helping to maintain a fixed posture and prevent unwanted movement. These muscle groups are called fixators. Examples of a fixator include the shoulder muscle during a bicep curl or tricep extension.

Types of muscular contraction:

  • Isometric – this is a static contraction where the length of the muscle, or the joint angle, doesn’t change.  Examples include pushing against a stationary object such as a wall.  This type of contraction is known to lead to rapid rises in blood pressure
  • Isotonic – this is a moving contraction, also known as dynamic contraction.  During this contraction the muscle fattens and there is movement at the joint. 

There are two types of Isotonic contraction:

  • Concentric – this is where the muscle contracts, and shortens, against a resistance.  This may be referred to as the lifting or positive phase.  An example would be the lifting phase of the bicep curl.
  • Eccentric – this occurs when the muscle is still contracting and lengthening at the same time.  This may be referred to as the lowering or negative phase.

Muscle Fibre Types

Not all muscle fibres are the same. In fact there are two main type of muscle fibre:

  • Type I – often called slow-twitch or highly-oxidative muscle fibres
  • Type II – often called fast-twitch or low-oxidative muscle fibres

However, type II muscle fibres can be further split into type IIa and type IIb.  Type IIb are the truly fast twitch fibres whereas type IIa are inbetween slow and fast twitch.  The interesting thing about type IIa fibres is that their characteristics can be strongly influenced by the type of training undertaken.  Following a period of endurance training they will start to strongly resemble type I fibres, but following a period of strength training they will start to strongly resemble type IIb fibres.  In fact following several years of endurance training they may end up being almost identical to slow-twitch muscle fibres.

Type I (Slow-twitch muscle fibres)

Slow-twitch muscle fibres contain more mitochondria (aerobic energy producing organelles), are smaller in size, have better blood supply, contract more slowly and are more fatigue resistant than their fast-twitch brothers.  Slow-twitch muscle fibres produce energy, primarily, through aerobic metabolism of fats and carbohydrates.  The rate of aerobic metabolism that can occur is enhanced by the large numbers of mitochondria and the enhanced blood supply.  They also contain large amounts of myoglobin – a pigment similar to haemoglobin that also stores oxygen – that provides an additional store of oxygen for when oxygen supply is limited. This along with their slow rate of contraction increases their endurance capacity and enhances their fatigue resistance.  Slow-twitch muscle fibres are recruited during continuous exercise at low to moderate levels.

Type IIb (Fast-twitch low-oxidative muscle fibres)

These fibres are larger in size, have a decreased blood supply, have smaller mitochondria and less of them, contract more rapidly and are more adapted to produce energy anaerobically (without the need for oxygen) than slow-twitch muscle fibres.  Their reduced rate of blood supply together with their larger size and fewer mitochondria makes them less able to produce energy aerobically and are therefore not well suited to prolonged exercise.  However, their faster rate of contraction, greater levels of glycogen, and ability to produce much greater amounts of energy anaerobically makes them much more suited to short bursts of energy.  Because of their greater speed of contraction and reduced blood supply they are far less fatigue resistant, than slow-twitch fibres, and tire quickly during exercise.

Numbers of slow and fast-twitch fibres

The number of slow and fast-twitch fibres varies greatly from individual to individual and is determined by your genetics.  People who do well at endurance sports tend to have a higher number of slow-twitch fibres whereas people who are better at sprint events tend to have higher numbers of fast-twitch muscle fibres.  As well as the effects that training has on type IIa muscle fibres, both the slow twitch and fast-twitch fibres can be influenced by training.  It is possible through sprint training to improve the power generated by slow twitch fibres and through endurance training it is possible to increase the endurance level of fast-twitch fibres.  This varies depending on the individual and training can never make slow-twitch fibres as powerful as fast-twitch or fast-twitch fibres as fatigue resistant as slow-twitch fibres

Cardiac Muscle Structure and Function

Cardiac muscle cells are only found in the heart.  They are elongated and contain actin and myosin filaments which form sarcomeres, these join end to end to form myofibrils.  The actin and myosin filaments give cardiac muscle a striated appearance.  The striations are less numerous than in skeletal muscle.  Cardiac muscles contain high numbers of mitochondria which produce energy through aerobic metabolism.  There is an extensive capillary network (tiny blood vessels) that supply’s oxygen to the cardiac muscle cells.  Unlike the skeletal muscle cells, the cardiac cells all work as one unit, all contracting at the same time.  In short, the sinoatrial node at the top of the heart sends an impulse to the atrioventricular node which sends a wave of polarization which travels from one heart cell to another causing them all to contract at the same time.

Smooth Muscle Structure and Function

Smooth muscle cells are variable in function and perform many differing roles within the body.  They are spindle shaped and smaller than skeletal muscle and contain fewer actin and myosin filaments.  The actin and myosin filaments are not organised into sarcomeres so smooth muscles do not have a striated appearance.  One difference between smooth muscle and other muscle types is that smooth muscle can apply a constant tension. This is called smooth muscle tone.  Smooth muscle cells have a similar metabolism to skeletal muscle.  They produce most of their energy aerobically and are not well adapted to producing energy anearobically.