Structure and function of skeletal muscle

Question 1

Muscle tone and posture

Answer 1

• Skeletal muscle are contracted in a graded, asynchronous recruitment of motor units until enough force is generated
• Muscles are physiologically silent when at rest
• Muscles are spared when ligament suffice: posture is maintained primarily by ligaments and CT
• Only the upper fibers of the trapezius and one lower limb muscle (triceps surae) display low-level muscle activity during posture

Question 2

Posture of upper limb 1

Answer 2

• Isometric contractions of upper fibers of trapezius keep the scapula in place
• At the glenohumeral joint there is no muscle activity. The joint is maintained by a locking mechanism (prevents the humerus from sliding down) due to the architecture of the glenoid fossa
• The locking mechanism is formed by “ligaments” which are the rotator cuff muscles (mostly supraspinatus), but they aren’t contracted
• These muscles, along w/ the joint capsule simply keep the humerus in the GH joint w/o needing to contract
• They prevent the humeral head from moving laterally

Question 3

Posture of upper limb 2

Answer 3

• Elbow joint ligaments are sufficient to maintain posture (when elbow is extended)
• The muscles of the wrists and forearm are silent and the ligaments maintain posture

Question 4

Types of contraction

Answer 4

• Shortening (concentric) contraction brings the two muscle attachments closer together (muscle shortens)
• Isometric contraction: the whole muscle remains at the same length (no work being done)
• Legnthening (eccentric) contraction: the muscle becomes longer when contraction

Question 5

Force vs movement

Answer 5

• Force is proportional to cross-sectional area (larger x-sectional are the more force)
• Movement is proportional to the length of a muscle’s fibers (a longer muscle can move a greater distance)
• Muscles can contract up to 30% of their total length

Question 6

Fiber architecture

Answer 6

• Parallel fibered muscles: have long but few muscle fiber are adapted for movement, not force
• Adding tendons to parallel fibers decreases the amount of movement generated, but does not change the amount of force
• Pennate fibered muscles: have short but numerous muscle fibers are adapted for force, not movement
• Adding tendons to pennate fibers decreases force generated but doesn’t change movement generated

Question 7

Tendons

Answer 7

• So strong they rarely rupture
• Have the advantages of small size where they cross joints and can reach a required length at low metabolic cost
• Reduce the movement generated in parallel fibers
• Reduce the force generated in pennate fibers

Question 8

Roles of muscles

Answer 8

• Prime mover (agonist): activates and maintains a particular movement
• Antagonist: muscle capable of opposing or resisting the action of an antagonist
• Fixator (stabilizer): stabilizes joint position and integrity (agonists + antagonists working together, like in GH joint)
• Synergist: muscle that eliminates unwanted movements caused by prime movers

Question 9

Role of gravity

Answer 9

• Sometimes acts as a prime mover

- Inertia can contribute to prime movements by maintaining them w/o needing further muscle help

Question 10

What to know about muscles

Answer 10

• Name, form, shape, and general disposition
• Attachments: only those that are to important topographic features of the bone they are related to
• Innervation
• Anatomical actions (only the primary actions)
• Muscle function (what tasks the muscle is used for)
• Relationships and blood supply
• For innervation and blood supply, just know what compartment the muscle is in and what artery/nerves are in that compartment (also the exceptions)

Question 11

Deltoid muscle

Answer 11

• Anterior fibers: cause flexion at the GH joint, medial rotation of the humeral head, and adduction (since the anterior most fibers pass medially to the AP axis of the humeral head)
• Lateral fibers: have pennate arranged fibers that pass lateral to the shoulder. Very powerful abductors of the arm
• Posterior fibers: cause extension at the GH joint, lateral rotation of the humeral head, and adduction (since the most posterior fibers pass medially to the AP axis of the humeral head)
• Primary function of deltoids: Abduction of arm

Question 12

Biceps brachii

Answer 12

• Arises from radial tuberosity on radius and has 2 heads that insert at 2 different parts of the scapula
• The short head runs tot he coracoid process and the long head runs to the supraglenoid tubercle
• Has 3 functions: flex arm at shoulder joint, flex forearm at elbow joint, supinates forearm from prone position
• The supination function is because the radial tuberosity is on the ventral side of the proximal radius
• Thus when the radius is in the prone position, the tendon of the biceps is wrapped around the radius to the tuberosity and when it pulls it flips the forearm back over to a supine position, resulting in the tuberosity back on the ventral side

Question 13

Muscles of pronation

Answer 13

• To turn the arm from supine (ventral side up) to prone (ventral side down) there are 2 main muscles acting
• The pronator teres passes from the medial epicondyle of the humerus to the lateral side of the radial shaft
• Therefore when this contracts it will pull the radius medially and the radius will cross over the ulna, resting in a pronated hand
• The pronator quadratus is attached to the distal unla and radius, w/ horizontal fibers
• This also pulls the radius over the ulna (medially) and results in pronation

Question 14

Primary muscles of supination

Answer 14

• The biceps brachii supinate the forearm, as discussed earlier
• The supinator muscle arises from the lateral epicondyle and inserts on the proximal end of the radial shaft
• Therefore this muscle counteracts the action of the pronator teres muscle
• The fibers of the supinator become wrapped around the radius during pronation
• Contraction of these fibers result in lateral movement of the radius back to its original position (supine)

Question 15

Secondary muscles of supination

Answer 15

• Supinator muscle is prime mover for supination
• Biceps are recruited for supination when speed or power are needed (not activated when elbow is extended)
• Whenever the biceps are activated, the triceps are usually recruited as synergists to stop the unintended action of elbow flexion caused by the biceps
• The shoulder muscles (deltoid + rotator cuff) are also recruited when the triceps/biceps are active as stabilizers at the GH joint to prevent arm movement around the GH joint

Question 16

Sternoclavicular joint

Answer 16

• Articular disk acts as a ligament that tethers clavicle to sternum
• The joint has considerable ball-and-socket mobility
• Clavicle can be elevated 60 degrees from rest

Question 17

Acromioclavicular joint

Answer 17

• The joint appears weak but is made strong by the coracoclavicular ligament connects the coracoid process to the underside of the distal clavicle
• This tethers the scapula and clavicle together, thus any movement that moves the scapula also moves the clavicle and vice versa
• Shoulder separation is usually due to tearing the coracoclavicular ligament, which results in a sag of the shoulder since this joint is no longer allowing the clavicle to hold up the scapula at the distal end of the clavicle

Question 18

Glenohumeral joint and scapulothoracic joint

Answer 18

• GH joint (shoulder) is a ball-and-socket joint that provides more mobility than any other joint (but this movement requires movements from other joints)
• Scapulothoracic: the scapula can move along the surface of the rib cage in any direction (elevation, depression, protraction, retraction, and rotation)

Question 19

Actions of the trapezius muscles

Answer 19

• The upper fibers elevate the clavicle and thus the scapula (due to the coracoclavicular joint)
• Secondary action: retract clavicle/scapula
• The middle fibers retract the scapula/clavicle
• The lower fibers depress the scapula and rotate the glenoid fossa upwards (by pulling down on the proximal end of the acromion process?)
• This allows the arm to have a larger range of motion once the glenoid fossa is facing upwards
• Secondary action of lower fibers: retract the scapula

Question 20

Supraspinatus and serratus anterior

Answer 20

• Supraspinatus: part of rotator cuff and is an abductor of the humerus at the GH joint
• Serratus anterior: arises from the lateral body wall by slips from each of the upper 8 ribs. These join to cover the ribs laterally and forms the medial wall of the axilla
• The fibers attach to the medial border of the scapula to act as a strong protractor of the scapula (pulls the scapula anteriorly around the body wall
• The lower fibers play an important role in scapular rotation

Question 21

Adbuction of arm above head

Answer 21

• Requires many muscles and actions
• Abduction of humerus (deltoid and supraspinatus)
• Rotation of scapula (trapezius and serratus anterior)
• Lateral rotation of the arm required to abduct above 90 degrees

Question 22

Deltoid and supraspinatus in abduction of arm above head

Answer 22

• Both of these humeral abductors remain active for the entire 180 degrees of arm abduction
• Only these two are active for the first 90 degrees of abduction
• The remaining 90 degrees requires rotation of the scapula (humerus comes into contact w/ acromion at 90 degrees abduction)

Question 23

Trapezius and serratus anteiror in abduction of arm above head 1

Answer 23

• Both of these scapular rotators are only active for the last 90 degrees of arm abduction, to allow for further action of the deltoid/supraspinatus muscle by rotating the acromion process (GH joint) superior
• The lower fibers of the trapezius pull on the medial side of the scapular spine (pull in a posterior, inferior direction)
• The lower fibers of the serratus anterior pull on the most inferior part of the scapula (pull in a anterior direction)

Question 24

Trapezius and serratus anteiror in abduction of arm above head 2

Answer 24

• Since the upper part of the scapula is pulled posterior/inferior and the lower part is pulled anterior, the overall effect is rotating the GH joint 90 degrees upwards
• The upper fibers of the trapezius assist by raising the lateral end of the clavicle. This lifts the acromion process by the tethering of the coracovlacivular ligament

Question 25

Lowering the arm from the abducted position

Answer 25

• Gravity is the prime mover
• The same four muscles used to raise the arm above the head act as antagonists to maintain a smooth descent (contract eccentrically)
• Adductors of the arm (pec major/lat) are only active if there is resistance