Skeletal muscles are responsible for directed voluntary movement under the control of the nervous system.
Muscles are contractile organs and produce motion by their connection with such structures as bones, cartilage, ligaments, fascia and skin. Usually muscles are attached to these structures at two points: one relatively fixed point known as the Origin, and one relatively moveable point called the Insertion. In most cases, the two points are a distance apart and functioning of the muscle is across one or more joints.
A muscle is a fibrous organ possessing considerable elasticity. At the points of origin and insertion, we find attachments where the muscle has become very tough and tendonous. These are called Tendons, and are inserted in the bones and other structures (mentioned above).
Muscles of the skeleton work in groups, and each group works in opposition to another group known as its Antagonist. For instance, taking the biceps and triceps muscles as an example, the biceps muscle, when stimulated by impulses via the nervous system, contracts (shortens and thickens), drawing its point of insertion in the radius towards its origin in the shoulder joint, thus bending the arm. To straighten the arm, the biceps must give way to the opposing pull of the contracting triceps. So from this, we see that the biceps muscle works in opposition to the triceps, and vice versa, thus making one muscle the antagonist of the other.
Muscles have various functions and are classified accordingly. We have Extensors for straightening and extending limbs etc., and for bending the limbs etc .. we have their antagonists called Flexors. Further to these we have other antagonistic pairs such as Abductors and Adductors and Pronators and Supinators.
Following is a list of some of the important skeletal muscles together with the Origin (O), Insertion (I) and Action (A) of each muscle.
STERNO-MASTOID O. Sternum and Clavicle
I. Mastoid process of the temporal bone
A. Turn and flex the neck
TRAPEZIUS O. Occiput and spines of the thoracic vertebrae
I. Clavicle and spine of the scapula
A. Extends neck. Braces Shoulders. The upper part lifts the shoulders and the lower part draws the shoulders down.
DELTOID O. Scapula
I. Radius
A. Raises the arm to right angles with the scapula and draws the arm backwards and forwards.
BICEPS O. Scapula
I. Radius
A. Flex elbow joint and supinate forearm
TRICEPS O. Scapula and humerus.
I. Ulna
A. Extend arm
SERRATUS ANTERIOR O. The 8 upper ribs
I. Scapula
A. Draws scapula forward and is antagonistic to the trapezius
LATISSIUMS DORSI O. Lower thoracic vertebrae, lumbar vertebrae and crest of ilium
I. Humerus
A. Internal rotation of the shoulder. Adduction of the shoulder, drawing the arm backwards and downwards
PECTORALIS MAJOR O. Clavicle and 7 upper costal cartilages
I. Humerus
A. Internal rotatio of the shoulder. Adduction of the shoulder and draws the arm across the chest.
RECTUS ABDOMINUS O. Pubic crest
I. 5th, 6th and 7th costal cartilages
A. Flexes body. Compresses abdominal viscera
OBLIQUE EXTERNUS ABDOMINIS O. The lower 8 ribs
I. Iliac crest and pubis
A. Assists in the elimination of faeces and urine
GLUTEUS MAXIMUS O. Iliac crest
I. Femur
A. Extends the hip joint. Rotates inwards and abducts the thigh
SARTORIUS O. Spine of the ilium.
I. Inner surface of the tibia
A. Abducts the thigh and rotates it inwards and outwards
ADDUCTOR LONGUS O. Pubis
I. Femur
A. Adducts thigh
QUADRICEPS FEMORIS The quadriceps femoris consists of 4 musces:-
1. RECTUS FEMORIS 2. VASTUS LATERALIS 3. VASTUS MEDIALS 4. VASTUS INTERMEDIUS (a deeper muscle than the other 3)
O. Ilium and femur
I. Patella and is joined to the tibia by the patella ligament
A. Flexion of the hip and extension of the knee
ADDUCTOR MAGNUS O. Pubis and ischium
I. Femur
A. Adducts and rotates the thigh inwards
GRACILIS O. Pubis and ischium
I. Femur
A. Rotates leg inwards. Adducts and flexes the thigh
HAMSTRINGS O. Ischium and femur
I. Tibia and fibula
A. Extension of hip and flexion of knee
GASTROCNEMIUS O. Femur
I. The tendon of this muscle unites with the tendon of the soleus and together they form the Tendo Achillis.
The tendon is inserted into the calcaneum.
A. Flexes the knee and extends the foot
SOLEUS O. Fibula and tibia
I. The tendon combines with that of the gastocnemius muscle.
A. Extends the foot
TIBIALIS ANTERIOR O. Tibia
I. Tarsal and metatarsal bones at the inside of the instep
A. Flexes the foot on the leg. Raises the inner border of the foot
when studying the muscles of the skeleton, it is useful to know some of the terms applied to various features and contours of bones to which the muscles are attached
SPINE – This is not necessarily a reference to the vertebral column. It is also the name given to a bony ridge such as the spine of the scapula
ILIUM – The ilium is the broad, flat upper part of the innominate bone. Its upper edge ends in a broad ridge called the Iliac Crest
ISCHIUM – This is the lower rear portion of the innominate bone. It carries the Ischial Tuberosity (a protusion) which gives attachment for muscles
PUBIS – The pubis is the front lower part of the innominate bone
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