7.3 Major Muscle Groups and Their Functions

The muscular system is a complex network of tissues that enables movement, maintains posture, and supports vital bodily functions. Understanding how major muscle groups are organized, where they attach, and how they interact during movement is foundational for anatomy and has direct applications in exercise science and physical therapy.

Major Muscle Groups and Locations

Head, Neck, and Trunk Muscles

The muscles of the head and neck handle everything from facial expression to chewing and head positioning.

• Facial muscles (orbicularis oculi, zygomaticus, orbicularis oris) control expressions like smiling, squinting, and frowning
• Mastication muscles (masseter, temporalis) elevate the mandible for chewing. The masseter is one of the strongest muscles in the body relative to its size.
• Neck muscles (sternocleidomastoid, scalenes) control head movement. The sternocleidomastoid is easy to identify: when one side contracts, it laterally flexes the head to the same side and rotates it to the opposite side. When both sides contract together, they flex the neck forward.

The trunk muscles form the structural core of the body, divided into anterior and posterior groups:

• Anterior trunk: pectoralis major (adducts and medially rotates the arm), rectus abdominis (flexes the vertebral column), and external/internal obliques (rotate and laterally flex the trunk)
• Posterior trunk: trapezius (elevates, retracts, and depresses the scapula depending on which fibers contract), latissimus dorsi (extends, adducts, and medially rotates the arm), and erector spinae group (extends the vertebral column and maintains upright posture)

Upper Extremity Muscles

• Shoulder: The deltoid caps the shoulder and has three parts (anterior, middle, posterior) that flex, abduct, and extend the arm, respectively. The rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis, often remembered as SITS) stabilizes the glenohumeral joint during movement.
• Arm: Biceps brachii on the anterior side flexes the elbow and supinates the forearm. Triceps brachii on the posterior side extends the elbow.
• Forearm: Anterior compartment muscles flex the wrist and fingers; posterior compartment muscles extend them.
• Hand: Intrinsic muscles (within the hand itself) control fine movements like gripping and pinching. Extrinsic muscles originate in the forearm and control gross finger and wrist movements via long tendons.

Lower Extremity Muscles

• Hip: The gluteal muscles (gluteus maximus, medius, minimus) extend and abduct the hip. The iliopsoas (iliacus + psoas major) is the primary hip flexor, originating on the lumbar vertebrae and iliac fossa and inserting on the lesser trochanter of the femur.
• Thigh: The anterior compartment houses the quadriceps femoris (four muscles that extend the knee). The posterior compartment contains the hamstrings (three muscles that flex the knee and extend the hip). The medial compartment holds the adductor group, which adducts the thigh.
• Leg (below the knee): The gastrocnemius (superficial, two-headed) and soleus (deep) make up the calf and plantar flex the ankle. Together they insert via the calcaneal (Achilles) tendon. The tibialis anterior on the front of the leg dorsiflexes the ankle.
• Foot: Intrinsic muscles support the arches and control toe movements. Extrinsic muscles originate in the leg and move the foot and toes via tendons.

Muscle Origin, Insertion, and Action

Attachment Points and Movement

Every skeletal muscle has at least two attachment points connected to bone by tendons:

• The origin is the more stationary attachment, typically proximal
• The insertion is the more movable attachment, typically distal
• When the muscle contracts, it pulls the insertion toward the origin, producing movement at the joint between them

A helpful way to remember: the origin stays put, the insertion moves.

Key Upper Body Muscle Attachments and Actions

Key Lower Body Muscle Attachments and Actions

Note that the rectus femoris is the only quadriceps muscle that crosses both the hip and knee joints, which is why it can flex the hip in addition to extending the knee. This is a detail that commonly shows up on exams.

Muscle Roles in Movement

Agonist-Antagonist Relationship

Muscles don't work in isolation. For any given movement, muscles take on specific functional roles:

• Agonist (prime mover): The muscle primarily responsible for producing the desired movement. It contracts concentrically (shortens) to generate force.
• Antagonist: The muscle on the opposite side of the joint that opposes the agonist's action. During movement, the antagonist relaxes to allow motion but can also contract eccentrically to control the speed of the movement and prevent overshooting.

This coordination is called reciprocal inhibition: when the nervous system activates the agonist, it simultaneously inhibits the antagonist, allowing smooth motion.

Synergists and Fixators

• Synergists assist the agonist by adding force or reducing unwanted movements at the joint. They fine-tune the action so it's efficient and precise.
• Fixators are synergists that specifically stabilize the origin of the agonist or stabilize a nearby joint so the agonist can work effectively. For example, the scapular stabilizers (rhomboids, serratus anterior) hold the scapula in place during arm movements.

Movement Examples

These examples show how the roles play out in real movements:

• Elbow flexion: biceps brachii (agonist), triceps brachii (antagonist), brachialis and brachioradialis (synergists)
• Knee extension: quadriceps femoris (agonist), hamstrings (antagonist), tensor fasciae latae and sartorius (synergists)
• Shoulder abduction: middle deltoid (agonist), latissimus dorsi and pectoralis major (antagonists), supraspinatus (synergist), rotator cuff and scapular stabilizers (fixators)

Applying Muscle Knowledge to Exercise

Upper Body Exercises

Push-ups target the pectoralis major, triceps brachii, and anterior deltoid.

• During the upward (pushing) phase, these muscles contract concentrically.
• During the downward (lowering) phase, the same muscles contract eccentrically to control the descent. The core muscles act as fixators to keep the trunk rigid.

Pull-ups primarily work the latissimus dorsi, biceps brachii, and posterior deltoid.

• The upward pulling motion involves concentric contraction of these muscles.
• The controlled lowering phase involves eccentric contraction of the same muscles.

Bench press engages the same primary movers as push-ups (pectoralis major, triceps brachii, anterior deltoid) with added external resistance. The rotator cuff muscles play a critical stabilizing role at the shoulder throughout the lift.

Lower Body Exercises

Squats work the quadriceps femoris, gluteal muscles, and hamstrings.

• Descending phase: quadriceps contract eccentrically to control knee flexion while gluteals and hamstrings eccentrically control hip flexion.
• Ascending phase: quadriceps concentrically extend the knee; gluteals and hamstrings concentrically extend the hip.
• The erector spinae and core muscles act as fixators to maintain an upright trunk.

Lunges target the quadriceps, hamstrings, and gluteal muscles. The forward leg's quadriceps concentrically contract during the upward phase, while the gluteals and hamstrings extend the hip. Lunges also demand significant stabilization from the hip abductors (gluteus medius) to maintain balance.

Calf raises engage the gastrocnemius and soleus. These muscles concentrically contract to plantar flex the ankle and raise the heel, then eccentrically contract to control the lowering phase.

Functional Movements

Walking and running involve coordinated lower extremity action:

• Quadriceps and hamstrings alternately extend and flex the knee
• Gluteal muscles and hip flexors (iliopsoas) control hip movement
• Gastrocnemius and soleus plantar flex the ankle for push-off propulsion
• Tibialis anterior dorsiflexes the foot during the swing phase to clear the ground

Throwing a ball integrates shoulder, arm, and trunk muscles:

1. Wind-up: trunk rotation is initiated by the obliques and spinal rotators, storing elastic energy
2. Acceleration: pectoralis major and latissimus dorsi generate power at the shoulder; the deltoid and rotator cuff stabilize the joint
3. Release: triceps brachii extends the elbow to release the ball
4. Follow-through: rotator cuff and posterior deltoid eccentrically decelerate the arm to protect the shoulder joint

Lifting and carrying objects engages multiple groups simultaneously:

• Leg muscles (quadriceps, hamstrings, gluteals) generate the lifting force from the ground
• Core muscles (rectus abdominis, obliques, erector spinae) stabilize the trunk and protect the spine
• Back muscles (erector spinae, latissimus dorsi) maintain posture
• Arm muscles (biceps, forearm flexors) control and hold the load