Anatomy: Muscular System

The muscular system is the body’s primary engine for movement, posture, heat production, and many internal functions that are easy to overlook until they fail. Functional anatomy focuses on what muscles do, how they work together, and how they are controlled by the nervous system. That perspective is especially useful in clinical practice, where weakness, pain, altered gait, or loss of coordination often reflect specific patterns of muscle action and innervation.

This article reviews muscle types, major muscles and their actions, how innervation is organized, and how functional groups contribute to common movements. It also connects these ideas to frequent injuries and practical assessment.

Muscle types and what they are built to do

Skeletal muscle

Skeletal muscle is voluntary, striated, and attached (directly or indirectly) to the skeleton. It produces joint motion through contraction and transmits force through tendons and aponeuroses. Functionally, skeletal muscle must balance strength and precision. Large muscles with broad attachments often generate power (for example, the gluteus maximus), while smaller muscles near joints can provide fine control and stability (for example, the rotator cuff).

A key concept is that muscles usually do not act alone. Movement results from:

• Agonists (prime movers): main contributors to an action
• Synergists: assist the agonist, add force, or refine direction
• Antagonists: oppose the action and help modulate speed and control
• Stabilizers: fix a proximal segment so distal movement can occur

Cardiac muscle

Cardiac muscle is striated but involuntary, designed for rhythmic, fatigue-resistant contraction. Its functional anatomy centers on coordinated electrical conduction rather than somatic motor innervation. Clinically, it is often considered separately from the musculoskeletal system, but it shares the basic principle that structure supports function.

Smooth muscle

Smooth muscle is involuntary and found in the walls of hollow organs and blood vessels. Its contraction regulates airflow, blood pressure, digestion, and many autonomic functions. While not central to joint movement, smooth muscle is essential to whole-body function and influences exercise tolerance and thermoregulation.

How muscles create movement: actions, joints, and leverage

Muscles generate force by shortening, lengthening under tension, or holding tension without length change:

• Concentric contraction: muscle shortens while producing force
• Eccentric contraction: muscle lengthens while resisting a load
• Isometric contraction: force without joint movement

Eccentric control is clinically important. Many injuries occur when a muscle is forced to absorb load, such as hamstring strains during sprinting or rotator cuff overload during overhead lowering.

Roles of origins, insertions, and lines of pull

A muscle’s attachments and line of pull determine which joints it crosses and what actions it can produce. If a muscle crosses the hip and knee, its net action depends on joint position and the demands of the task. Multi-joint muscles can be powerful but are also vulnerable to strain when lengthened across two joints (for example, hamstrings and rectus femoris).

Joint torque depends on force and moment arm. In simplified terms: $$\tau =F\times r$$ where $\tau$ is torque, $F$ is muscle force, and $r$ is the moment arm. Small changes in joint angle can alter the moment arm and change how hard a muscle must work.

Major muscles and their functional groups

A practical way to learn muscular anatomy is by regions and movement patterns.

Shoulder and upper limb

Scapular stabilizers create a stable base for arm motion:

• Trapezius (upper, middle, lower): elevation, retraction, upward rotation of the scapula
• Serratus anterior: protraction and upward rotation; critical for overhead reach
• Rhomboids: retraction and downward rotation

Glenohumeral movers and stabilizers:

• Deltoid: major abductor; anterior fibers flex and internally rotate, posterior fibers extend and externally rotate
• Rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis): compress the humeral head into the glenoid, controlling rotation and centering during elevation
• Pectoralis major: adduction and internal rotation; contributes to pushing
• Latissimus dorsi: extension, adduction, internal rotation; powerful in pulling and climbing

Elbow and hand:

• Biceps brachii: supination and elbow flexion; also assists shoulder flexion
• Brachialis: primary elbow flexor regardless of forearm position
• Triceps brachii: elbow extension; long head assists shoulder extension
• Forearm flexors/extensors: control wrist and finger motion; common sites of overuse syndromes

Trunk and core

The trunk transfers force between the upper and lower limbs and protects the spine.

• Rectus abdominis: trunk flexion and abdominal pressurization
• External and internal obliques: rotation and lateral flexion; contribute to anti-rotation control
• Transversus abdominis: abdominal wall tensioning and stability
• Erector spinae: spinal extension and postural endurance

Core function is less about constant “bracing” and more about matching stability to the task, whether that is lifting, throwing, or simply maintaining balance.

Hip and lower limb

Hip extensors and abductors are central to gait and pelvic control:

• Gluteus maximus: hip extension and external rotation; powerful in rising, climbing, sprinting
• Gluteus medius and minimus: hip abduction; stabilize pelvis during single-leg stance

Thigh muscles:

• Quadriceps (rectus femoris, vasti): knee extension; rectus femoris also flexes the hip
• Hamstrings (biceps femoris, semitendinosus, semimembranosus): knee flexion and hip extension; important eccentrically in running
• Adductors: hip adduction; also assist with stabilization and directional changes

Lower leg and foot:

• Gastrocnemius and soleus: plantarflexion; gastrocnemius crosses knee, soleus does not
• Tibialis anterior: dorsiflexion and inversion; important for foot clearance in swing phase
• Fibularis (peroneal) muscles: eversion; contribute to lateral ankle stability
• Intrinsic foot muscles: support arches and fine control; often weak in chronic foot pain

Innervation: how nerves map to movement

Skeletal muscle contraction depends on motor neurons from the spinal cord. Clinically, it helps to connect:

• Peripheral nerves (named nerves in the limbs)
• Spinal nerve roots (myotomes)
• Functional deficits (patterns of weakness and sensory change)

Examples that frequently matter in assessment:

• Axillary nerve (C5-C6): deltoid weakness, impaired shoulder abduction
• Radial nerve (C5-T1): wrist and finger extension; injury can cause wrist drop
• Median nerve (C6-T1): many forearm flexors and thenar muscles; carpal tunnel affects sensation and thumb function
• Ulnar nerve (C8-T1): intrinsic hand muscles; clawing with chronic compromise
• Femoral nerve (L2-L4): quadriceps; difficulty with knee extension
• Sciatic nerve (L4-S3): hamstrings and most below-knee muscles; varied patterns depending on branch involvement
• Common fibular (peroneal) nerve: dorsiflexors; foot drop is a hallmark

Myotome testing in a neurologic exam often focuses on key actions: C5 shoulder abduction, C6 wrist extension, C7 elbow extension, L4 knee extension, L5 great toe extension, S1 plantarflexion. These are not the only contributions, but they are reliable clinical anchors.

Common injuries and practical functional insights

Strains and tendinopathies

• Hamstring strain: often during high-speed running when hamstrings act eccentrically to decelerate knee extension.
• Rotator cuff tendinopathy or tears: commonly related to repetitive overhead activity, poor scapular control, or age-related degeneration. Painful arc and weakness in external rotation are frequent clues.
• Achilles tendinopathy: linked to repetitive loading, reduced calf capacity, or sudden training changes.

A useful clinical distinction: muscle pain with active contraction suggests contractile tissue involvement; pain at end range stretch can also implicate muscle-tendon units, but joint or neural structures must be considered.

Sprains and muscle imbalance patterns

Ankle sprains often involve lateral ligaments, but recovery depends heavily on restoring fibularis strength, proprioception, and calf capacity. Similarly, anterior knee pain may be influenced by hip abductor weakness and altered lower-limb mechanics rather than an isolated “knee problem.”

Functional movement and compensation

The body often preserves a task by shifting demand:

• Limited ankle dorsiflexion can increase knee valgus or stress the Achilles.
• Weak hip abductors can lead to pelvic drop and altered gait.
• Poor scapular upward rotation can overload the shoulder during overhead work.

These are not universal rules, but common patterns that guide assessment: observe the movement, identify which segment is not contributing, then test the likely muscles and their inn