The Muscular System: Structure, Function, and Major Muscles

The Muscular System: Structure, Function, and Major Muscles

Muscles are the engines of human movement. As a physical therapy aide, you will assist licensed physical therapists (PTs) who evaluate and treat conditions involving muscles every day. Understanding muscle structure, how muscles produce movement, and where major muscles are located will help you set up equipment correctly, safely position patients, and communicate clearly with the PT supervising you.

How Muscle Tissue Works

Skeletal muscle is made of long cells called muscle fibers, bundled together and wrapped in connective tissue. When the nervous system sends a signal, the fibers shorten — a process called contraction. This pull moves the skeleton at joints. Skeletal muscle is voluntary, meaning we control it consciously.

Every skeletal muscle has two attachment points on bone (or occasionally cartilage or fascia):

• Origin — the more fixed, proximal attachment. Think of it as the anchor.
• Insertion — the more movable, distal attachment. This is where the pull is felt.

When the muscle contracts, the insertion moves toward the origin.

Roles Muscles Play During Movement

Rarely does a single muscle act alone. During any movement:

• Agonist (prime mover) — the muscle doing the main work (e.g., the biceps brachii during elbow flexion).
• Antagonist — the muscle on the opposite side that must relax and lengthen to allow the movement (e.g., the triceps brachii during elbow flexion).
• Synergist — a muscle that assists the agonist or fine-tunes the motion.
• Stabilizer — a muscle that holds a nearby joint steady so the prime mover can work efficiently (e.g., the rotator cuff stabilizing the shoulder while the deltoid lifts the arm).

Recognizing these roles helps you understand why a PT might exercise a muscle that seems unrelated to a patient's complaint.

Major Muscle Groups Relevant to PT Practice

Upper Extremity

Rotator cuff (SITS): Four muscles — supraspinatus, infraspinatus, teres minor, and subscapularis — originate on the scapula and insert on the humerus. Together they hold the humeral head in the glenoid socket (stabilizers) and rotate the shoulder.

Biceps brachii: Origin on the coracoid process and supraglenoid tubercle (two heads); insertion on the radial tuberosity. Primary actions: elbow flexion and forearm supination.

Triceps brachii: Origin on the infraglenoid tubercle and posterior humerus; insertion on the olecranon. Primary action: elbow extension.

Wrist flexors (flexor carpi radialis, flexor carpi ulnaris): Originate primarily from the medial epicondyle of the humerus; insert on carpal and metacarpal bones. Action: wrist flexion.

Wrist extensors (extensor carpi radialis longus/brevis, extensor carpi ulnaris): Originate from the lateral epicondyle; insert on metacarpals. Action: wrist extension.

Lower Extremity

Gluteus maximus: Origin on the posterior ilium, sacrum, and coccyx; insertion on the gluteal tuberosity and iliotibial band. Action: hip extension and external rotation.

Quadriceps (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius): Three vasti originate on the femur; rectus femoris originates on the anterior inferior iliac spine. All four insert via the patellar tendon onto the tibial tuberosity. Action: knee extension; rectus femoris also flexes the hip.

Hamstrings (biceps femoris, semitendinosus, semimembranosus): Originate primarily on the ischial tuberosity; insert below the knee on the tibia and fibula. Actions: knee flexion and hip extension.

Hip flexors (iliopsoas — iliacus + psoas major): Psoas originates on lumbar vertebral bodies; iliacus originates on the iliac fossa. Both insert on the lesser trochanter of the femur. Action: hip flexion.

Gastrocnemius: Origin on the medial and lateral femoral condyles; insertion via the Achilles tendon on the calcaneus. Actions: plantarflexion and assists knee flexion.

Soleus: Origin on the posterior fibula and tibia; insertion also via the Achilles tendon. Action: plantarflexion (does NOT cross the knee).

Tibialis anterior: Origin on the lateral tibia; insertion on the medial cuneiform and first metatarsal. Action: dorsiflexion and inversion.

Trunk

Erector spinae: A group of muscles running along the spine from the sacrum to the skull; they extend and laterally flex the vertebral column and maintain upright posture.

Abdominals (rectus abdominis, external oblique, internal oblique, transversus abdominis): Originate and insert on ribs, pelvis, and thoracolumbar fascia. Actions include trunk flexion, rotation, lateral flexion, and compression of the abdominal wall — critical for spinal stability.

Palpation: Finding Muscles by Touch

Palpation means feeling a muscle through the skin to confirm its location and assess its tension. You will palpate muscles on lab partners — never on a patient without a PT directing you. Follow these safety rules every time:

1. Explain before you touch. Tell the partner which muscle you are locating and ask permission.

2. Use appropriate pressure. Superficial muscles need only light to moderate pressure. Never press hard over bony prominences, blood vessels, or areas of reported pain without PT guidance.

3. Ask for feedback. If your partner feels sharp pain or unusual discomfort, stop immediately.

4. Maintain professional boundaries — draping and privacy apply even in a lab setting.

Useful technique: ask your partner to contract the muscle against light resistance (active contraction) to make the muscle stand out, then feel for the firming tissue. Reference Trail Guide to the Body by Biel for precise landmarks.

Connecting This to PT Aide Practice

You will never diagnose or treat, but you will set up modality equipment on specific muscle groups, assist patients with exercise programs designed by the PT, and report changes in a patient's reported sensation or movement. Knowing muscle names, locations, and actions lets you follow instructions accurately and keep patients safe.