Maximus Gage
The human body has over 600 muscles, and muscle insertion refers to the location where a muscle attaches to a moving bone or structure, allowing movement when the muscle contracts. The place on the stationary bone that is connected to the muscle is called the origin, while the place on the moving bone that is connected to the muscle is called the insertion. For example, the biceps connect to the forearm, and the sternocleidomastoid connects the sternum and clavicle to the mastoid process of the skull.
| Characteristics | Values |
|---|---|
| Muscle insertion definition | The location where a muscle attaches to a moving bone or structure |
| Muscle insertion examples | Biceps connecting to the forearm, sternocleidomastoid, latissimus dorsi, deltoid muscle |
| Muscle insertion points | Used in biomechanical studies to understand lever systems during movement |
| Muscle movement | Occurs when muscles shorten their length, pull on tendons, and move bones closer together |
| Types of muscle movement | Voluntary (controlled by the nervous system) and involuntary (automatic) |
| Types of voluntary movement | Flexion, extension, abduction, and adduction |
| Types of involuntary movement | Breathing, speaking, swallowing, digesting food, pumping blood, giving birth |
| Prime mover or agonist | The muscle that provides the primary force driving an action |
| Antagonist | The muscle that provides resistance and/or reverses a given movement |
| Synergist | Muscles that help stabilise a movement and reduce extraneous movements |
| Fixator | Muscles that assist in movement by holding the origin stable |
| Strongest muscle by weight | The myometrial layer of the uterus during childbirth |
| Strongest muscle overall | Quadriceps femoris or gluteus maximus |
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What You'll Learn
Sternocleidomastoid
The sternocleidomastoid (SCM) is a large, easily recognisable muscle that divides the neck into anterior and posterior triangles. The SCM has dual innervation and multiple functions, including head rotation, head tilt, and pointing the chin toward the breastbone. It also assists in breathing, maintains neck posture, and helps the temporomandibular joint (TMJ) function. The SCM is innervated by the accessory nerve (cranial nerve XI) and direct branches of the cervical plexus (C2-C3). The blood supply to the SCM is through the superior thyroid artery, a branch of the external carotid artery.
The SCM is involved in the formation of the triangles of the neck. The anterior triangle is delimited by the posterior border of the SCM, the inferior border of the mandible, and the medial line of the neck. The posterior triangle is delimited by the SCM anteriorly, the clavicle inferiorly, and the trapezius muscle posteriorly. The supraclavicularis muscle arises from behind the SCM and passes to the upper surface of the clavicle. The SCM is a unique muscle in terms of variations at its origin, and it has a variable innervation arrangement.
The SCM is a superficially palpable muscle that acts as an anatomical landmark within the neck region. It is involved in how the neck feels and moves. Examination of the SCM forms part of the examination of the cranial nerves. It can be felt on each side of the neck when a person moves their head to the opposite side. Contraction of the SCM gives rise to a condition called torticollis or wry neck, which can have rheumatic, reflex, or congenital causes. Torticollis can be treated with physiotherapy exercises to stretch and strengthen the neck muscles.
The SCM has been observed to fuse with the trapezius muscle in some cases, resulting in the absence of a posterior triangle. This phenomenon is described by Sinohara's law of separation, which states that two muscles with a common nerve supply are derived from a common muscle mass. The SCM can also be affected by lesions, which can cause atrophy and impair the function of the muscle, leading to difficulties in turning the head and bending the neck. Treatment for SCM lesions involves strengthening exercises and repairing the nerve if possible.
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Occipitofrontalis
The occipitofrontalis muscle is a wide, two-bellied muscle that covers parts of the skull. It is involved in creating facial expressions and moving the scalp and eyebrows. The occipitofrontalis is made up of two parts or bellies: the occipital belly and the frontal belly. The occipital belly originates on the lateral two-thirds of the highest nuchal line and on the mastoid process of the temporal bone. It inserts into the epicranial aponeurosis. The frontal belly, on the other hand, originates from an intermediate tendon that connects to the occipital belly and inserts into the fascia of the facial muscles and the skin above the eyes and nose.
The two bellies of the occipitofrontalis muscle are connected by a thick fibrous sheath called the epicranial aponeurosis (also known as galea aponeurotica), which covers the upper part of the cranium. The occipital belly inserts into the epicranial aponeurosis posteriorly to the lambdoid suture, while the frontal belly inserts anteriorly to the coronal suture of the skull. The occipital belly receives blood from the occipital artery, while the frontal belly is supplied by the supraorbital and supratrochlear arteries.
The occipitofrontalis muscle has several functions depending on which of its attachments is fixed. When its aponeurotic attachment is fixed, the frontal belly elevates the eyebrows and wrinkles the skin of the forehead, creating a facial expression of shock or surprise. When its forehead attachment is fixed, the frontal belly aids the procerus, orbicularis oculi, and corrugator supercilii muscles in frowning by pulling the scalp forwards and wrinkling the forehead. The occipital belly, on the other hand, draws the scalp backwards.
Damage to the facial nerve can cause atony of the occipitofrontalis muscle, affecting facial expressions.
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Biceps and forearm
The biceps brachii is a two-headed muscle that lies on the front of the upper arm between the shoulder and the elbow. It is one of four muscles in the upper arm, the others being the brachialis, coracobrachialis, and triceps brachii. The biceps brachii is a large, thick muscle consisting of two heads, the short head and the long head, which originate at the coracoid process and supraglenoid tubercle of the scapula, respectively. The two heads unite to form a single muscle belly, which attaches to the radial tuberosity and the fascia of the forearm via the bicipital aponeurosis.
The biceps brachii is a powerful supinator of the forearm, meaning it turns the palm upwards or forward. It also flexes the forearm, bending it towards the upper arm, and is thus used in lifting and pulling movements. This muscle also has an important role in supporting the humeral head within the shoulder joint. The long head of the biceps crosses both the shoulder and elbow joints, and its main function is at the elbow, where it flexes and supinates the forearm.
The brachialis muscle lies deep to the biceps brachii and is found more distally. It originates from the medial and lateral surfaces of the humeral shaft and inserts into the ulnar tuberosity, just distal to the elbow joint. The brachialis muscle flexes the elbow and, along with the brachioradialis and supinator muscles, helps to flex the forearm when it is in pronation (the palm facing the ground).
The forearm contains many muscles, including several superficial muscles on the underside of the forearm that start below the elbow and extend to the wrist. These include the flexor carpi ulnaris, palmaris longus, flexor carpi radialis, flexor digitorum superficialis, and pronator teres. There are also superficial muscles on the top of the forearm, including the brachioradialis, extensor carpi radialis longus and brevis, extensor digitorum communis, extensor digiti minimi, and extensor carpi ulnaris.
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Triceps brachii and anconeus
The triceps brachii is a three-headed muscle of the arm, with long, medial, and lateral heads. It is the only constituent of the posterior muscle group of the arm, spanning almost the entire length of the humerus. The triceps brachii muscle fibres converge onto a common tendon, which inserts onto the olecranon of the ulna. The medial head of the triceps brachii does not attach to the scapula, and therefore has no action on the glenohumeral joint. However, the medial head is active during extension of the forearm at the elbow joint when the forearm is supinated or pronated. The triceps brachii is the primary extensor of the elbow joint and is the major muscle responsible for elbow extension.
The anconeus is a small, triangular muscle located at the elbow, attaching the humerus and ulna. It is considered a continuation of the triceps brachii muscle, as their fibres are often partially or completely blended together. The anconeus assists in the extension of the forearm and provides support for both the dorsal capsule of the humeroulnar joint and the ulna itself. The anconeus muscle originates by a tendon on the dorsal aspect of the lateral epicondyle of the humerus, just proximal to the common extensor tendon. The anconeus tendon inserts at the lateral surface of the olecranon of the ulna and the adjoining posterior surface of the ulnar shaft.
The triceps brachii and anconeus muscles are both involved in elbow extension, with the triceps brachii being the primary extensor and the anconeus assisting in this movement. The triceps brachii is innervated by the radial nerve, with contributions from the deep brachial artery and the superior ulnar collateral artery. Ruptures of the triceps muscle are rare and typically occur due to a fall on an outstretched hand or a direct blow to the triceps tendon.
The anconeus muscle is supplied by a motor branch of the radial nerve, which arises at the radial sulcus of the humerus and continues through the medial head of the triceps. An anatomical variation of the anconeus, known as the anconeus epitrochlearis muscle, has been found in approximately one-third of all humans. This variation originates at the medial epicondyle of the humerus, crossing the ulnar groove where the ulnar nerve lies. In cases of hypertrophy, such as in weightlifters, this variation can compress the ulnar nerve, leading to numbness in the ulnar side of the hand and pain in the elbow, a condition known as cubital tunnel syndrome.
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Deltoid muscle
The deltoid muscle is a thick, triangular shoulder muscle. It gets its name from its shape, which resembles the inverted Greek letter delta (Δ). The muscle has a wide origin, spanning the clavicle, acromion, and spine of the scapula. It then passes inferiorly, surrounding the glenohumeral joint on all sides, and inserts onto the humerus. The deltoid is made up of three distinct sets of muscle fibres, each with a different origin:
The first set, known as the anterior or clavicular part (commonly called the front delt), originates from the superior surface and anterior border of the lateral third of the clavicle. These fibres help move the arm forward and are used when reaching for an object on a shelf, for example.
The second set, the intermediate or acromial part (commonly known as the side delt), arises from the superior surface of the acromion process of the scapula. This is the largest and strongest part of the deltoid muscle. It is a multipennate muscle, arising as four intramuscular septa that interdigitate with three tendons at the insertion site.
The third set, the posterior or spinal part (commonly referred to as the rear delt), originates from the lateral 1/3 of the spine of the scapula, on its posterior border. These fibres help move the arm out to the side, up, and down, and are used in activities such as jumping jacks.
The fibres from all three sets converge to insert into the deltoid tuberosity located on the lateral aspect of the shaft of the humerus. The deltoid muscle is a powerful muscle used in many everyday activities and athletic pursuits. It is involved in flexion, internal rotation, and horizontal adduction, as well as extension, external rotation, and horizontal abduction of the shoulder joint.
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Frequently asked questions
The triceps brachii.
The biceps.
The occipitofrontalis.
The occipitofrontalis.
The tibialis anterior.
