Chapter 9BGross Anatomy and

Functions of Skeletal Muscles

Back Muscles

• These muscles extend, laterally flex, and rotate the vertebral column. They also hold the vertebral column erect

• A superficial group of muscles, the Erector spinae, runs from the pelvis to the skull, extending from the vertebrae to the ribs– Consist of three subgroups on each side of the

vertebrae: Iliocostalis, Longissimus, and Spinalis– Lateral bending of the back is accomplished by

unilateral contraction of these muscles

Fig. 9.10

Tab. 9.6

Abdominal Wall Muscles

• The abdominal wall is composed of four paired muscles (Internal and External obliques, Transversus abdominis, and Rectus abdominis), their fasciae, and their aponeuroses

• Fascicles of these muscles run at right and oblique angles to one another, giving the abdominal wall added strength

• In addition to forming the abdominal wall, these muscles:– Are involved with lateral flexion and rotation of the

trunk– Aid in functions such as forced expirations (coughing

and screaming), vomiting, defecation, urination, and childbirth

Fig. 9.11

Tab. 9.7

Pelvic Floor and Perineum

• The pelvic diaphragm is composed of two paired muscles: Levator ani and Coccygeus– These muscles:

• Close the inferior outlet of the pelvis• Support the pelvic floor• Elevate the pelvic floor to help release feces • Resist increased intra-abdominal pressure

• Two sphincter muscles allow voluntary control of urination (External urethral sphincter) and defecation (External anal sphincter)

• The Ischiocavernosus and Bulbospongiosus assist in erection of the penis and clitoris

Fig. 9.12

Tab. 9.8

Thoracic Muscles

• Mainly involved in the process of breathing

• Diaphragm: most important muscle in respiration

• External intercostals: more superficial layer that lifts the rib cage and increases thoracic volume to allow inspiration

• Internal intercostals: deeper layer that aids in forced expiration

Fig. 9.13

Tab. 9.9

Scapular Muscles and Movements

• The scapula is attached to the rest of the skeleton only by the clavicle

• Six muscles attach the scapula to the trunk and enable the scapula to function as an anchor point for the muscles and bones of the arm– Trapezius – Levator scapulae– Rhomboideus (major and minor) – Serratus anterior– Pectoralis minor

• Prime movers of shoulder elevation are the trapezius and levator scapulae

Fig. 9.14

Tab. 9.10

Upper Limb Muscles and Arm Movements

• Nine muscles attach the humerus to the scapula. Two additional muscles attach the humerus to the trunk

• Trunk muscles moving the arm:– Pectoralis major: flexes the extended shoulder and

extends the flexed shoulder– Latissimus dorsi: adducts and medially rotates arm;

shoulder extension• Muscles located in the shoulder moving the arm:

– Posterior fibers of the deltoid: shoulder extension– Anterior fibers of the deltoid: shoulder flexion– Lateral fibers of the deltoid: arm abduction and

rotation– Teres major: shoulder extension

Fig. 9.15

Fig. 9.16

Tab. 9.11

Upper Limb Muscles and Arm Movements

• Muscles located in the shoulder that move the arm (cont):– Rotator cuff muscles: Supraspinatus, Infraspinatus,

Teres minor, and Subscapularis (mnemonic SITS)• Function mainly to reinforce the capsule of the shoulder by

holding the head of the humerus in the glenoid cavity• Secondarily act as synergists and fixators

• Muscles located in the arm that move the arm– Coracobrachialis – Biceps brachii– Triceps brachii

• Actions of the trunk, shoulder, and arm muscles on the shoulder and arm are summarized in Table 9.12

Fig. 9.17

Tab. 9.12

Upper Limb Muscles and Forearm Movements

• Flexion and extension of the elbow are accomplished by three muscles located in the arm and two in the forearm

• Most anterior muscles are flexors, and posterior muscles are extensors– Forearm flexion

• Brachialis and Biceps brachii are the chief forearm flexors• The Brachioradialis acts as a synergist and helps stabilize the elbow

– Forearm extension• The Triceps brachii is the prime mover of forearm extension• The Anconeus is a weak synergist

• Supination and pronation are accomplished primarily by forearm muscles– The Supinator muscle is a synergist with the Biceps brachii in

supinating the forearm– The Pronator teres and Pronator quadratus pronate the forearm

Fig. 9.18

Forearm Muscles and Wrist, Hand, and Finger Movements

• Most anterior muscles are flexors, and posterior muscles are extensors

• Forearm muscles– Muscles that originate on the medial epicondyle are responsible

for flexion of the wrist and fingers– Muscles extending the wrist and fingers originate on the lateral

epicondyle– Forearm muscles moving the wrist, hand and fingers are

summarized in Table 9.14

• Extrinsic hand muscles are in the forearm– Retinaculum: covers the flexor and extensor tendons and holds

them in place around the wrist

• Intrinsic hand muscles are in the hand– Thenar muscles, hypothenar muscles, midpalmar muscles

Fig. 9.19

Fig. 9.20

Hip and Lower Limb Muscles

• Most anterior compartment muscles of the hip and thigh flex the femur at the hip and extend the leg at the knee

• Posterior compartment muscles of the hip and thigh extend the thigh and flex the leg

• The medial compartment muscles all adduct the thigh

• These three groups are enclosed by the fascia lata

Hip Muscles and Thigh Movements

• Gluteus maximus extends the hip• Gluteus medius and minimus help hold the

hip level while walking or running• Deep hip muscles laterally rotate the thigh• Anterior hip muscles flex the hip• The thigh can be divided into three

compartments– Anterior muscles flex the hip– Posterior muscles extend the hip– Medial muscles adduct the thigh

Fig. 9.21

Thigh Muscles and Leg Movements

• Anterior thigh muscles– Quadriceps femoris: extends the knee– Sartorius: flexes the knee– Tensor fasciae latae: stabilizes the knee

• Posterior thigh muscles flex the knee

• One medial thigh muscle flexes the knee: Gracilis

• Summarized in Table 9.16

Fig. 9.22

Tab. 9.16

Fig. 9.23

Tab. 9.17

Lower Limb Muscles and Ankle, Foot, and Toe Movements

• The leg is divided into three compartments– Muscles in the anterior compartment cause

dorsiflexion, inversion, or eversion of the foot and extension of the toes

– Muscles of the lateral compartment plantar flex and evert the foot

– Muscles of the posterior compartment flex the leg, plantar flex and invert the foot, and flex the toes

• Intrinsic foot muscles flex or extend and abduct or adduct the toes

• Summarized in Table 9.18

Fig. 9.25

Fig. 9.26