The Muscle System

Relationship Between Body Movement and Muscle Action

The muscular system is responsible for producing movement by contracting and pulling on bones. Muscles work in groups to coordinate complex actions, and their activity is essential for maintaining health and function.

• Aerobic (endurance) exercise increases muscle capillaries, mitochondria, and myoglobin synthesis, resulting in greater endurance, strength, and resistance to fatigue.

• Resistance (anaerobic) exercise leads to muscle hypertrophy (increase in fiber size), increased mitochondria, myofilaments, glycogen stores, and connective tissue, resulting in increased muscle strength and size.

• Disuse atrophy is the degeneration and loss of muscle mass due to immobilization or loss of neural stimulation, with muscle strength declining rapidly.

Force of Muscle Contractions

• Force depends on the number of cross bridges attached, affected by:

  • Number of muscle fibers stimulated (recruitment)

  • Relative size of fibers (hypertrophy increases tension)

  • Frequency of stimulation

  • Degree of muscle stretch (optimal sarcomere length: 80–120% of resting length)

Velocity and Duration of Contraction

• Muscle fiber type (slow or fast fibers)

• Load (muscles contract fastest with no load; greater load shortens duration and slows contraction)

• Recruitment (more motor units = faster, more prolonged contraction)

Characteristics of Smooth Muscle Tissue

Smooth muscle is found in the walls of most hollow organs (except the heart) and is responsible for involuntary movements such as peristalsis and regulation of organ diameter.

• Contains two layers:

  • Longitudinal layer: fibers run parallel to the organ's long axis, shortening the organ when contracted.

  • Circular layer: fibers run around the organ's circumference, constricting the lumen when contracted.

• Alternating contractions mix and propel substances through the organ.

• Innervated by the autonomic nervous system; contains varicosities instead of neuromuscular junctions.

• No striations or sarcomeres, but overlapping thick and thin filaments.

• Slow, synchronized contractions; cells are electrically coupled by gap junctions.

• Regulation of contraction is controlled by nerves, hormones, or local chemical changes.

Muscle Attachments: Origin and Insertion

Muscles attach to bones at two points: the origin (stationary) and the insertion (movable). During contraction, the insertion moves toward the origin.

• Origin: attachment point that does not move during contraction; usually more proximal.

• Insertion: attachment point that moves during contraction; usually more distal.

• Muscles can only pull, never push. What one muscle group does, another undoes.

Functional Groups of Muscles

• Prime mover (agonist): major responsibility for producing a specific movement.

• Antagonist: opposes or reverses a particular movement.

• Synergist: helps prime movers by adding extra force or reducing unnecessary movement.

• Fixator: immobilizes bone or muscle's origin, providing a stable base for the prime mover.

Naming Muscles

Muscles are named based on several criteria:

• Location (e.g., temporalis over temporal bone)

• Shape (e.g., deltoid = triangle)

• Size (e.g., maximus = largest, minimus = smallest)

• Direction of fibers (e.g., rectus = straight, transversus = right angles)

• Number of origins (e.g., biceps = two origins)

• Location of attachments (origin named first)

• Action (e.g., flexor, extensor)

Fascicle Arrangements

Fascicle arrangements determine muscle shape and function:

• Circular: fascicles arranged in concentric rings (e.g., orbicularis oris)

• Convergent: broad origin, fascicles converge toward single tendon insertion (e.g., pectoralis major)

• Parallel: fascicles parallel to long axis (e.g., sartorius)

• Pennate: short fascicles attach obliquely to central tendon (e.g., rectus femoris)

Levers in the Muscular System

Most skeletal muscles move bones using leverage. The lever system consists of:

• Lever: rigid bar (bone) that moves on a fulcrum (joint)

• Effort: force applied by muscle contraction

• Load: resistance moved by the effort

Basic principle: Effort farther than load from fulcrum = mechanical advantage Effort nearer than load to fulcrum = mechanical disadvantage

Major Muscles: Points of Attachment and Function

Muscles of Facial Expression

Facial muscles insert into skin, not bone, and are important for nonverbal communication. They are divided into muscles of the scalp and face.

Muscles of Mastication and Tongue Movement

• Prime movers of jaw closure: temporalis and masseter

• Grinding movements: pterygoids

• Chewing role: buccinator

• Tongue movements: genioglossus, hyoglossus, styloglossus

Neck and Back Muscles

• Sternocleidomastoid divides neck into anterior and posterior triangles.

• Erector spinae (iliocostalis, longissimus, spinalis) and quadratus lumborum extend trunk and maintain posture.

Muscles of Breathing

• Inspiratory muscles: diaphragm and external intercostals enlarge rib cage.

• Expiration: relaxation of inspiratory muscles and contraction of internal intercostals.

• Forced expiration: abdominal muscles assist.

Abdominal Wall Muscles

• Four paired muscles: rectus abdominis, external obliques, internal obliques, transversus abdominis

• Actions: lateral flexion, rotation of trunk, and assistance in bodily functions (urination, defecation, childbirth, etc.)

Pelvic Diaphragm Muscles

• Functions: seals inferior outlet of pelvis, supports pelvic organs, lifts pelvic floor, resists increased intra-abdominal pressure.

Shoulder Muscles

• Extrinsic shoulder muscles fix and move the scapula to increase range of arm movements.

• Actions: elevation, depression, rotation, lateral and medial movements, protraction, retraction.

• Groups: anterior thorax and posterior thorax muscles.

Arm Muscles

• Prime movers: pectoralis major, latissimus dorsi, deltoid

• Rotator cuff muscles: supraspinatus, infraspinatus, teres minor, subscapularis, coracobrachialis, teres major

Forearm Muscles

• Anterior compartment: biceps brachii, brachialis, brachioradialis (flexes and supinates forearm)

• Posterior compartment: triceps brachii (powerful forearm extensor)

Thigh Muscles

• Grouped as anterior, medial, or posterior

• Anterior: flex femur at hip, extend leg at knee

• Posterior: extend thigh, flex leg

• Medial: adduct thigh

• Thigh flexors: iliopsoas, tensor fasciae latae, rectus femoris

• Thigh extensors: hamstring muscles

Adductor Muscles

• Adductors (also medially rotate thigh): adductor magnus, adductor longus, adductor brevis, pectineus, gracilis

Quadriceps Femoris

• Four heads: rectus femoris, vastus lateralis, vastus medialis, vastus intermedius

• All insert into quadriceps tendon, patella, and tibial tuberosity

• Powerful knee extensor

Gluteal Muscles

• Abductors and lateral rotators: gluteus maximus, gluteus medius, gluteus minimus, piriformis, obturator externus, obturator internus, gemellus

Hamstring Muscles

• Posterior thigh: biceps femoris, semitendinosus, semimembranosus

• Prime movers of thigh extension and knee flexion

• Innervated by sciatic nerve

Leg Muscles

• Anterior compartment: tibialis anterior, extensor digitorum longus, extensor hallucis longus (toe extensors and ankle dorsiflexors)

• Posterior compartment: soleus, tibialis posterior (superficial), gastrocnemius (deep) (plantar flex the ankle)

Summary Table: Major Muscle Groups and Functions

Key Equations

• Mechanical advantage (levers):

• Force of contraction:

Additional info:

• Muscle structure and function are closely related; understanding origin, insertion, and lever systems is essential for analyzing movement.

• Muscle groups work together to produce coordinated actions, and their arrangement determines the efficiency and range of movement.