Overview of the Muscular System
The muscular system provides movement, stability, and heat generation for the body. It works closely with the skeletal, cardiovascular, and integumentary systems to support posture, locomotion, and metabolic demands.
Pelvic Floor and Perineum
Primary Functions of the Pelvic Floor
The pelvic floor muscles support the organs of the pelvic cavity, flex the sacrum and coccyx, and control the passage of urine and feces through the urethra and anus.
Division of the Perineum
The perineum is divided by the ischial tuberosities into two triangles:
- Anterior urogenital triangle – located toward the front of the body.
- Posterior anal triangle – located toward the back of the body.
Sphincter Control
Sphincters of the pelvic outlet permit voluntary control of urination and defecation, allowing conscious regulation of these functions.
Shoulder and Arm Muscles
Major Movers of the Arm
The deltoid is the major abductor of the arm, lifting it away from the torso. The supraspinatus assists the deltoid during the initial phase of abduction.
Rotational Muscles
Medial (internal) rotation at the shoulder is produced by subscapularis and teres major. Lateral (external) rotation is produced by infraspinatus and teres minor.
Shoulder Flexion vs. Extension
At the shoulder joint, pectoralis major primarily performs flexion (bringing the arm forward), while latissimus dorsi performs extension (pulling the arm backward).
Rotator Cuff
The rotator cuff consists of four muscles: supraspinatus, infraspinatus, teres minor, and subscapularis. The biceps brachii is not part of this group.
Elbow Flexors and Extensors
Key Muscles
The major flexor of the elbow is the biceps brachii. The major extensor is the triceps brachii, whose long head originates on the scapula and inserts on the olecranon of the ulna.
Origins
Both the biceps brachii and the long head of the triceps brachii share a scapular origin, highlighting the importance of the shoulder girdle in elbow movement.
Forearm Musculature
Location of Flexors and Extensors
Forearm flexor muscles are situated on the anterior (medial) side, while extensor muscles occupy the posterior (lateral) side.
Supination and Pronation
The biceps brachii supinates the forearm, turning the palm upward. In contrast, the pronator teres pronates the forearm, turning the palm downward.
Wrist and Hand Muscles
Wrist Movements
Flexion and adduction of the wrist are performed by the flexor carpi ulnaris. Extension and abduction are performed by the extensor carpi radialis.
Retinacula
The extensor retinaculum stabilizes extensor tendons on the posterior wrist, not the flexor tendons.
Intrinsic vs. Extrinsic Hand Muscles
Intrinsic hand muscles originate from the carpal and metacarpal bones, whereas extrinsic hand muscles originate in the forearm and insert into the hand.
Hip and Thigh Muscles
Gluteal Group
The largest and most posterior gluteal muscle is the gluteus maximus, which extends and laterally rotates the hip. The tensor fasciae latae works with the gluteus maximus to tension the iliotibial tract.
Adductor Group
One example of an adductor muscle is adductor longus, which contributes to hip flexion and adduction.
Iliopsoas
The iliopsoas consists of two muscles—psoas major and iliacus—that share a common tendon insertion, making it a powerful hip flexor.
Hamstrings and Quadriceps
Hamstring muscles, such as the biceps femoris, flex the knee. Quadriceps muscles extend the knee, providing the primary force for leg straightening.
Foot Muscles
Plantar Flexion
The gastrocnemius, together with the soleus, produces plantar flexion, pointing the foot downward. The tibialis anterior opposes this action by dorsiflexing the foot.
Achilles Tendon
The Achilles tendon, also called the calcaneal tendon, is formed by the combined tendons of the gastrocnemius and soleus.
Intrinsic Foot Muscles
These muscles originate from the tarsal and metatarsal bones, helping to maintain the foot arch and providing fine motor control.
Systemic Support for Muscles
Cardiovascular Contribution
During exercise, the cardiovascular system delivers oxygen and nutrients to active muscles while removing carbon dioxide and metabolic waste, ensuring sustained performance.
Integumentary Role
The integumentary system helps disperse heat generated by muscle activity through sweating, aiding in temperature regulation.
