Muscles Flashcards
Why does the Supraspinatus initiate abduction while the deltoid continues it?
The supraspinatus initiates abduction of the arm, while the deltoid continues it due to their different roles and mechanical advantages:
1. Supraspinatus: The supraspinatus, a rotator cuff muscle, is responsible for the first 15-20 degrees of shoulder abduction. It helps stabilize the humeral head in the shallow glenoid cavity and provides the initial force to lift the arm, especially at low angles of abduction.
2. Deltoid: Once the arm has been initiated into abduction by the supraspinatus, the deltoid (mainly the middle part) takes over to continue the movement beyond 20 degrees. The deltoid is a more powerful muscle, and its larger cross-sectional area allows it to produce greater force for continued abduction.
Thus, the supraspinatus helps start abduction, and the deltoid continues it, providing both stability and power for the full range of motion.
How does the biceps brachii act as a powerful supinator of the arm?
The biceps brachii acts as a powerful supinator of the forearm due to its anatomical structure and function:
1. Insertion on Radius: The biceps brachii tendon inserts on the radial tuberosity of the radius. When the biceps contracts, it pulls the radius, causing it to rotate around the stationary ulna during supination.
2. Action during Supination: During supination (turning the palm upwards), the biceps brachii generates a strong force that helps rotate the radius laterally, aligning the palm upwards while stabilizing the elbow joint.
3. Powerful Contraction: The biceps brachii is a large muscle, and its contraction provides the necessary force for supination, particularly when the forearm is in a flexed position, enhancing its leverage.
Thus, the biceps brachii contributes significantly to supinating the forearm, making it one of the most effective supinators.
Why does paralysis of the deltoid muscle prevent abduction beyond 15°?
Paralysis of the deltoid muscle prevents abduction beyond 15 degrees because:
1. Initial Abduction: The supraspinatus (a rotator cuff muscle) initiates the first 15-20 degrees of abduction by lifting the humerus.
2. Role of the Deltoid: The deltoid (especially the middle portion) is responsible for continuing abduction beyond 15 degrees. The deltoid generates the powerful force needed for abduction beyond the initial motion initiated by the supraspinatus.
3. Lack of Power for Continued Abduction: If the deltoid is paralyzed, the arm cannot continue abduction beyond 15 degrees, as the supraspinatus alone is not strong enough to abduct the arm further. The shoulder’s range of motion is therefore limited to the initial movement facilitated by the supraspinatus.
Thus, paralysis of the deltoid prevents the arm from being raised beyond the initial 15-20 degrees of abduction.
What is the role of the rotator of muscles in the shoulder joints stabilisation?
The rotator cuff muscles play a crucial role in shoulder joint stabilization by:
1. Maintaining Humeral Head Position: The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) secure the humeral head within the shallow glenoid cavity of the scapula, preventing dislocations or excessive movement.
2. Dynamic Stabilization: These muscles provide dynamic stability by contracting during shoulder movements, ensuring proper alignment and preventing the humeral head from shifting out of position.
3. Controlling Movements: They control the movement of the shoulder, allowing precise motion and protecting the joint from injury during dynamic activities like lifting, throwing, or overhead motions.
4. Rotational Stability: They allow for internal and external rotation of the arm while stabilizing the glenohumeral joint, particularly during activities involving rotation (e.g., throwing or swinging).
In summary, the rotator cuff muscles are essential for both static and dynamic stability, allowing for safe and controlled shoulder movement.
Why is the brachialis muscle considered the primary flexor of the elbow?
The brachialis muscle is considered the primary flexor of the elbow because:
1. Direct Attachment: The brachialis attaches to the humerus and inserts onto the ulna, bypassing the radius. This direct attachment allows it to produce strong, consistent flexion at the elbow joint regardless of the position of the forearm.
2. Constant Force: Unlike the biceps brachii, which also flexes the elbow but is more effective when the forearm is in supination, the brachialis works effectively in all positions (pronation, neutral, and supination), making it the primary flexor for all arm positions.
3. Muscle Structure: The brachialis is a pure flexor muscle with a larger cross-sectional area compared to the biceps, contributing more force in elbow flexion, especially when maximal force is needed.
Thus, the brachialis is the most efficient and consistent flexor of the elbow, especially in functional positions.
Why does injury to the serratus anterior lead to the winging of scapula?
Injury to the serratus anterior muscle leads to winging of the scapula because:
1. Role of the Serratus Anterior: The serratus anterior muscle attaches to the medial border of the scapula and the ribs. It plays a key role in protracting the scapula (moving it forward) and holding it securely against the thoracic wall.
2. Weakness or Paralysis: When the long thoracic nerve, which innervates the serratus anterior, is damaged, the muscle becomes weak or paralyzed.
3. Lack of Protraction: Without the serratus anterior, the scapula is no longer stabilized against the rib cage and can move away from the thoracic wall. This results in the medial border of the scapula protruding outward, creating the “winging” appearance.
In summary, injury to the serratus anterior causes winging of the scapula due to the loss of its stabilizing and protracting function, leading to scapular instability.
How does the flexor retinaculum prevent bow-stringing of tendons at the wrist?
The flexor retinaculum (also known as the transverse carpal ligament) prevents bow-stringing of the tendons at the wrist by:
1. Securing Flexor Tendons: The flexor retinaculum forms a fibrous tunnel at the wrist, also known as the carpal tunnel, through which the flexor tendons (including the flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus) and the median nerve pass.
2. Preventing Bow-stringing: By acting as a superficial ligament, the flexor retinaculum holds the tendons in place against the wrist and palm. Without this structure, the tendons would be subject to excessive movement or bow-stringing (where the tendons would move away from their proper alignment during flexion).
3. Maintaining Efficiency: The flexor retinaculum ensures that the tendons move smoothly and efficiently without losing their mechanical advantage, particularly during activities that require wrist and finger flexion.
Thus, the flexor retinaculum maintains the stability of the tendons and prevents them from bow-stringing, ensuring efficient and controlled wrist movement.
Why is the Parmar longus muscle often used in tendon graft surgeries?
The palmaris longus muscle is often used in tendon graft surgeries because:
1. Absent Function: The palmaris longus muscle has a minimal or no significant functional role in wrist and hand movements. Many people have a redundant palmaris longus, so its removal or use for grafting does not impact the function of the hand significantly.
2. Tendon Properties: The tendon of the palmaris longus is long, strong, and easily accessible in the wrist area, making it ideal for use as a graft in reconstructive surgeries.
3. Ease of Harvesting: The palmaris longus tendon is relatively easy to harvest without significant risk of functional impairment, as it does not play a major role in wrist or finger movements.
Thus, its minimal functional impact, ease of accessibility, and favorable tendon characteristics make the palmaris longus a preferred choice in tendon graft surgeries.
Why do the lumbricals contribute to the precision grip of the hand?
The lumbrical muscles contribute to the precision grip of the hand due to their unique function and anatomical positioning:
1. Flexion at the MCP Joints: The lumbricals originate from the tendons of the flexor digitorum profundus and insert into the extensor expansions of the fingers. They flex the metacarpophalangeal (MCP) joints while extending the interphalangeal (IP) joints. This motion helps in fine-tuning finger positioning during precision tasks.
2. Independence of Finger Movements: The lumbricals enable the fingers to move independently, which is crucial for controlled, delicate movements required for precision grip (e.g., holding a pencil or manipulating small objects).
3. Stabilizing the Finger: By stabilizing the MCP joints, the lumbricals allow the fingers to maintain a stable position while the thumb performs its role in gripping, thus contributing to a stronger and more controlled grip.
In summary, the lumbricals aid in finger flexibility, fine control, and stabilization, making them essential for precision grips in the hand.
What is the role of the interossei muscles in the hand movements?
The interossei muscles play a crucial role in hand movements, specifically in fine motor control and grip:
1. Dorsal Interossei (4 muscles): These muscles are responsible for abduction (spreading the fingers apart). They move the fingers away from the middle finger (which acts as the axis), aiding in tasks like holding objects or grasping items with a spread-out hand.
2. Palmar Interossei (3 or 4 muscles): These muscles are responsible for adduction (bringing the fingers together). They move the fingers toward the middle finger, essential for gripping or pinching motions.
3. Assisting in Flexion: Both the dorsal and palmar interossei assist in flexing the metacarpophalangeal (MCP) joints while extending the interphalangeal (IP) joints. This action helps in forming a powerful grip and precision grip by stabilizing the fingers.
4. Coordination and Fine Movement: The interossei muscles work with the lumbricals to allow for independent and coordinated finger movements, which are crucial for fine motor tasks like writing, typing, or picking up small objects.
In summary, the interossei muscles enable finger abduction, adduction, and stabilization during grasping, contributing to both precision and power grips.
Why does a subscapularis muscle provide media rotation of the arm?
The subscapularis muscle provides medial rotation (internal rotation) of the arm due to its anatomical position and direction of its fibers:
1. Origin and Insertion: The subscapularis originates from the subscapular fossa on the anterior surface of the scapula and inserts on the lesser tubercle of the humerus.
2. Direction of Fibers: The fibers of the subscapularis run laterally and superiorly from the scapula to the humerus. When the muscle contracts, it pulls the lesser tubercle of the humerus forward and medially.
3. Effect on the Humerus: The medial pull of the subscapularis on the humerus causes the humeral head to rotate toward the body, resulting in medial rotation of the arm.
Thus, the subscapularis is a primary muscle responsible for medial (internal) rotation of the arm, especially when the arm is in an adducted or extended position.
How does the extensor retinaculum maintain the alignment of the extensor tendons?
The extensor retinaculum plays a key role in maintaining the alignment of the extensor tendons at the wrist by:
1. Serving as a Stabilizing Structure: The extensor retinaculum is a fibrous band that forms a roof over the extensor tendons as they pass from the forearm to the hand, holding them in place and preventing them from bow-stringing (moving away from their intended path) during wrist and finger movements.
2. Creating a Tunnel: It forms a series of compartments (sheaths) that keep the extensor tendons in their correct anatomical positions as they cross the wrist joint. This compartmentalization ensures that the tendons can glide smoothly without deviating from their paths, even under high mechanical stress.
3. Preventing Tendon Slippage: The extensor retinaculum helps to prevent the tendons from bowing out (i.e., becoming displaced) during wrist extension or flexion, thus maintaining the mechanical efficiency of the tendons.
In summary, the extensor retinaculum serves to stabilize and restrain the extensor tendons, ensuring efficient and aligned movement during wrist and hand function.
