Chapter 10

The Muscular System

J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.

The Muscular System

Interactions of Skeletal Muscles in the Body • Muscles usually work in groups, i.e. perform

“group actions”

• Muscles are usually arranged in antagonistic pairs – flexor-extensor– abductor-adductor, etc.

How Skeletal Muscles Produce Movement

• Muscles exert force on tendons that pull on bones

• Muscles usually span a joint

• Muscle contraction changes the angle or position of one bone relative to another Brachialis

flexes forearm

How Skeletal Muscles Produce Movement

• Origin: the attachment of the muscle to the bone that remains stationary

• Insertion: the attachment of the muscle to the bone that moves

• Belly: the fleshy part of the muscle between the tendons of origin and/or insertion

origin

insertion

belly

Interactions of Skeletal Muscles in the Body

• Prime Mover (agonist): the principle muscle that causes a movement – ex: biceps brachii,

flexion of forearm

• Antagonist: the principle muscle that causes the opposite movement– ex: triceps brachii,

extension of forearm

Interactions of Skeletal Muscles in the Body

• Synergists: muscles that assist the prime mover – ex: extensor carpi (wrist) muscles are synergists for

the flexor digitorum muscles when you clench your fist

• Fixators: synergists that stabilize the origin of a prime over – ex: several back muscles that stabilize scapula when

the deltoid flexes the arm

Functional Roles of Skeletal Muscles

• Group actions: most movements need several muscles working together

While the prime mover (agonist) and synergists are contracting to provide the desired movement• other muscles (antagonists) are relaxing & being stretched out

passively• agonist and antagonist change roles depending on the action• e.g., abduction versus adduction

• Synergists and fixators become agonists and antagonists in different movements

Naming Skeletal Muscles• Location of the muscle• Shape of the muscle• Relative Size of the muscle• Direction/Orientation of the muscle fibers/cells• Number of Origins• Location of the Attachments• Action of the muscle

Know the muscle names as described in Lab Guide 6!

Muscles Named by Location

• Epicranius (around cranium)

• Tibialis anterior (front of tibia)

tibialis anterior

Naming Skeletal Muscles by Shape

• Deltoid (triangle)

• Trapezius (trapezoid, 2 parallel sides)

• Serratus (saw-toothed)

• Rhomboideus (rhomboid, 4 parallel sides)

• Orbicularis and sphincters (circular)

Rhomboideusmajor

Trapezius

Deltoid

Serratus anterior

Muscles Named by Size

• Maximus (largest)• Minimis (smallest)

• Longus (longest)• Brevis (short)

• Major (large)• Minor (small)

Psoas major

Psoas minor

Muscles Named by Direction of Fibers

• Rectus (straight)-parallel to long axis

• Transverse

• Oblique

Rectusabdominis

External oblique

• Biceps (2)

• Triceps (3)

• Quadriceps (4)

Muscles Named for Number of Origins

Biceps brachii

Muscles Named for Origin and Insertion

Sternocleidomastoid originates from sternum and clavicle and inserts on mastoid process of temporal bone

origins

insertion

Muscles Named for Action

• Flexor carpi radialis (extensor carpi radialis) –flexes wrist

• Abductor pollicis brevis (adductor pollicis) –flexes thumb

• Abductor magnus – abducts thigh

• Extensor digitorum – extends fingers

Adductormagnus

Arrangement of Fascicles

• Parallel (strap-like), ex: sartorius

• Fusiform (spindle shaped), ex: biceps femoris

Arrangement of Fascicles

• Pennate ("feather shaped"), ex: extensor digitorum longus

• Bipennate, ex: rectus femoris

• Multipennate, ex: deltoid

Arrangement of Fascicles

• Convergent, ex: pectoralis major

• Circular (sphincters), ex: orbicularis oris

Arrangement of Fascicles• Range of motion:

depends on length of muscle fibers (fascicles); long fibers = large range of motion – parallel and fusiform

muscles

• Power: depends on total number of muscle fibers; many fibers = great power – convergent, pennate,

bipennate, multipennate

Lever Systems and Leverage• Lever: i.e. bones, a rigid rod that moves on

some fixed point

• Fulcrum: i.e. joint, a fixed point

• Resistance: – the force that opposes movement– the load or object (bone or tissue) to be moved

• Effort: – the force exerted to achieve a movement– the effort is provided by muscle(s)

• Motion is produced when the effort exceeds the resistance (isotonic contraction)

Lever Systems and Leverage

• Leverage: the mechanical advantage gained by a lever

• Power: muscle tension (effort) farther from joint (fulcrum) produces stronger contraction (opposes greater resistance)

• Range of motion (ROM): muscle tension (effort) closer to joint (fulcrum) produces greater range of motion.

Mechanical Advantage• Load is near fulcrum, effort is far away• Only a small effort is required to move an object• Allows a heavy object to be moved with a small effort• Example: car jack

Mechanical Disadvantage• Load is far from the fulcrum, effort is near the

fulcrum– a large effort is required to move the object– allows object to be moved rapidly, a “speed lever”– throwing a baseball

Lever Systems and Leverage• First-class lever: (EFR) Effort-Fulcrum-Resistance

Leverage Systems and Leverage• Second class lever: (FRE) Fulcrum-Resistance-Effort

Leverage Systems and Leverage• Third-class lever: (FER) Fulcrum-Effort-Resistance

Skeletal Muscles

Know the muscles, their origins and insertions as described in your Lab Guide 6.

Additional Information

• You do not need to memorize the details of the different leverage system types for Exam 3.

• Slides with some examples of the naming of muscles can be found in the PowerPoint presentation for Lab 6.

End Chapter 10