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THE MUSCULAR SYSTEM
MAJOR SKELETAL MUSCLES
 The muscular system consists of approximately
640 skeletal muscles.
Teachers Notes
 Select a volunteer to draw on the muscles with a washable pen.
 Write names of muscles on the board.
 Alternatively, go through the muscles on the picture provided using a data projector.
 Complete the worksheet ‘Major skeletal muscles’.
 Label all muscles using the list on the board or data projector.
 Correct answers.
 Play Human Bingo with the muscular system using the list on the board to assist
students. Award prizes.
STRUCTURE OF SKELETAL MUSCLES
 A tendon is the connective tissue that usually
attaches muscle to bone.
 The tendon crosses a joint and attaches itself to 1
or more bones.
Origin
 The tendon closer to the midline of the body.
Insertion
 The tendon furthest (distal) from the midline of the
body.
 It is usually attached to the bone that is moved on
contraction.
Microscopic structure of skeletal muscle
 Skeletal muscle is made up of many muscle fibres.
 Muscle fibres are arranged in bundles and are
made up of many myofibrils.
 They are arranged parallel to each other & run the
length of the muscle fibre.
 Myofibrils are made up of a chain of sarcomeres,
linking together like the carriages of a train.
A sarcomere consists of the following:
Z Lines
 Found at either end of the sarcomere.
Actin
 The thin protein filament attached to the Z line.
Myosin
 The thick protein filament attached to
crossbridges.
Crossbridges
 Tiny projections on myosin filaments that reach
towards the actin filaments.
Teachers Notes
 Go through the structures of the skeletal muscle and sarcomere using a data
projector.
 Distribute diagrams for students to complete, using the information provided to assist
them.
MUSCLES AND MOVEMENT
 The major function of the muscles is movement.
 The tendons of muscles usually cross joints and
attach to bones. The muscle contracts and the
tendon pulls on the bone, creating movement.
Reciprocal inhibition
This is the process when skeletal muscles work in
pairs to create movement.
Agonist
 The muscle that contracts, creating the movement.
 The agonist is called the ’prime mover’ and
assisting muscles are called ‘synergists’.
Antagonist
 The muscle that relaxes, allowing the movement
to occur.
For example: The bicep curl.
 The bicep originates at the scapula. It inserts at
the radius.
 During the bicep curl, the bicep contracts and
shortens.
 The tendon that inserts on the radius pulls on the
bone.
 This causes the forearm to bend.
 At the same time, the tricep relaxes, allowing the
forearm to move forward.
 The bicep is the agonist and the tricep is the
antagonist.
Fill in the following table
Muscle
Muscle pair
Bicep
Hamstrings
Abdominals
Gastrocnemius
Gluteus
maximus
Pectorals
Wrist flexors
Joint at which
the muscles
create
movement
Stabilisation
 Muscles contract isometrically to stabilise joints.
For example: The lifting of a heavy weight during the
bicep curl requires the abdominals, deltoid, pectorals
and latissimus dorsi to contract isometrically to
stabilise the shoulder joint and trunk to maintain
effective technique.
Teachers Notes
 Only the major muscle groups have been included.
 Only the obvious agonist and antagonist relationships in regards to muscle and joint
action have been included for secondary school students.
NERVOUS CONTROL OF MUSCULAR
CONTRACTION
 For movement to occur, a message is transmitted
from the brain, via the spinal cord & nerves and
then on to the muscle.
 Nerves are made up of bundles of neurons, or
nerve cells.
 The neurons that transmit impulses to muscles are
called motor neurons.
Motor Neurons
 The motor neuron consists of a cell body that
directs the neuron’s activities, branches that pick
up the impulse called dendrites and an axon that
transmits the message to the muscle.
 The terminal ends of the axon are known as motor
end plates.
Motor Unit
 A single motor neuron joins with many muscle
fibres.
 The motor neuron & the number of muscle fibres
that it innervates is called a motor unit.
 All muscle fibres will respond when the motor
neuron is activated.
 The muscle fibres will either not respond to an
impulse, or respond once the impulse reaches a
certain intensity & frequency (Maughan et.al. 2004 p24).
 A motor neuron in the eye may stimulate 4
muscles compared to a back extensor that may
stimulate 2000 muscle fibres (Watkins, 1999).
 The force produced by muscles can increase by:
 Increasing the number of motor units recruited
 Increasing the frequency at which the motor
units are stimulated
For example: When a light load is lifted by the back
extensors, few motor units are activated at a low
frequency. When a heavy load is lifted, more motor
units are activated at a higher frequency.
Teachers Notes
Distribute pictures of the motor unit. Students label and colour the 2 separate motor
units.
THE SLIDING FILAMENT THEORY
 When a message reaches the motor end plate it is
transmitted to the muscle fibre.
 This stimulates the myosin crossbridges to attach
and pull on the actin.
 Once the crossbridge exerts its pulling action, it
detaches then swings back to reattach further
along the actin filament.
 This makes the actin slide into the centre of the
sarcomere, shortening the myofibril. The actin are
the sliding filaments.
 The attaching and detaching of the crossbridges
occurs at different times so that tension is
maintained in the muscle fibre.
Teachers Notes
 Use the diagram to explain the sliding filament theory.
 Participate in a Tug Of War activity with the class. Explain the arms are acting like
crossbridges, attaching, detaching and reattaching. The rope is acting like the actin,
sliding along.
TYPES OF MUSCULAR CONTRACTION
There are 3 pure types of contraction (Maughan et.al. 2004
p34). These include:
Isometric (static) contraction
 A muscle contraction in which tension increases,
but there is no change in muscle length or the joint
angle.
For example: Gripping a tennis racquet, holding &
gripping in rockclimbing & wrestling.
Concentric contraction
 A muscular contraction that results in the
shortening of a muscle’s length.
For example: The quadriceps shorten during the
kicking of a ball & the abdominals during a sit-up.
Eccentric contraction
 A muscular contraction that results in an increased
length of the muscle.
 A muscle lengthens when resisting the force of
gravity (McArdle et.al. 2001 p510).
For example: Quadriceps when walking down a hill,
the quadriceps when lowering during a squat.
Teachers Notes
 Using a weight, perform a bicep curl.
 Show students how a concentric contraction is completed when the weight is lifted,
how an isometric contraction is completed when the weight is held and an eccentric
contraction when the weight is lowered.
TYPES OF MUSCLE FIBRES
 The basic structure of muscle fibres is the
same.
 However they vary in relation to contraction
time, the amount of tension that they produce
and resistance to fatigue.
 A motor unit consists of only one muscle fibre
type.
 There are 2 main types. These include:
Slow twitch Fibres
They are also known as red (due to the
presence of myoglobin) and Type I fibres. Their
characteristics include:
 Slow contraction time. It takes 80-100 ms to
reach maximal tension (Maughan et.al. 2004 p28).
 Fatigue slowly.
 Low force of contraction.
 They possess more aerobic characteristics
(such as more mitochondria, myoglobin,
oxidative enzymes, triglycerides, & a high
capillary density.
 They have a high capacity for use of the
aerobic energy system.
 They are endurance fibres.
 They are stimulated by smaller motor neurons.
 They are stimulated at a lower frequency.
 Small motor units are usually composed of
slow twitch fibres. They are generally used for
slow movements such as the maintenance of
posture (Maughan et.al. 2004 p33).
Studies have shown that endurance athletes
possess up to 80% of slow twitch fibres in the
quadriceps muscles.
Fast-twitch fibres
They are also known as white (much paler as
they contain little myoglobin) and Type II fibres.
Their characteristics include:
 Contract rapidly. It takes 40-90 ms to reach
maximal tension (Hawley 2000 p3).
 High force of contraction.
 Fatigue quickly.
 They possess anaerobic characteristics such
as relatively higher stores of glycogen, PCr &
glycolytic enzymes.
 They have a high capacity for use of the
anaerobic energy systems.
 They are stimulated by larger motor neurons.
 They are stimulated at a higher frequency.
 Large motor units are usually composed of
fast twitch fibres. They are generally used for
occasional fast movements such as lifting
(Maughan et.al. 2004 p33).
Studies have shown that sprinters possess up to
80% of fast twitch fibre types in the quadriceps
muscles.
Types of fast twitch fibres
Type IIa
 They possess partially aerobic
characteristics
Type IIb
 They possess purely anaerobic
characteristics
Recruitment of muscle fibres
 Type I fibres are innervated by neurons
smaller in diameter, with a slower speed of
conduction & a lower activation threshold.
 This determines the order in which muscle
fibre types are recruited during exercise of
varying intensities.
 During light exercise, mostly Type I fibres are
recruited.
 During moderate exercise Type IIa fibres are
innervated.
 In high intensity exercise, Type IIb fibres are
also recruited.