Muscular System Anatomy Notes

Muscle Contractions and Functional Roles

Learning Objectives

  • Compare different types of muscle contractions.
  • Outline the functional roles of muscles.
  • Implement strategies for studying muscular system anatomy.

Associated Readings

  • Gray’s Anatomy for Students
    • The Body
    • Body Systems: Muscular System
  • Gray’s Atlas of Anatomy
    • The Body
    • Muscles: Anterior
    • Muscles: Posterior

Muscle Function: Tension

  • Passive Tension: Tension in a muscle when it is stretched but not actively contracting.
  • Active Tension: Tension generated by the muscle through contractile forces.

How Muscles Generate Active Tension

  • Reflexive Contractions
    • Automatic and involuntary muscle contractions.
  • Tonic Contractions
    • Slight, continuous muscle contraction that doesn't produce movement.
    • Assists in joint stability and posture maintenance.
  • Phasic Contractions
    • Voluntary contractions of skeletal muscle.
    • Two types:
      • Isotonic contractions: Involve a change in muscle length.
        • Concentric: Muscle shortens.
        • Eccentric: Muscle lengthens.
      • Isometric contractions: Involve no change in muscle length, but active tension exceeds tonic levels.

Concentric Muscle Contractions

  • Muscle shortens.
  • Movement is created.

Eccentric Muscle Contractions

  • Muscle lengthens.
  • Movement is controlled.

Isometric Muscle Contractions

  • No change in muscle length.
  • Movement is prevented.

Different Roles of Muscles in Movement

  • Agonist
    • Prime mover; responsible for creating, controlling, or preventing specific movement.
  • Fixator/Stabilizer
    • Keeps proximal parts relatively stable through joint compression while movements occur in distal parts.
    • Does not completely restrict movement.
    • Provides dynamically stable platform for coordinated movement.
  • Synergist
    • Complements the action of the agonist.
    • Provides direct assistance or acts as a fixator/stabilizer.
  • Antagonist
    • Opposes the action of the agonist.
    • Must progressively relax for smooth movement.

Muscle Anatomy and Function

  • Agonist roles are associated with large length and girth.
    • Attach relatively far from the joint(s) they cross.
  • Stabilizer roles are associated with shorter length, but with sizable girth.
    • Attach close to the joint(s) they cross.
  • Proprioceptor roles are associated with very short length with limited girth.
    • Attach very close to one or two joints which they cross.
    • Have a high density of muscle spindles.

Recommendations for Studying Muscles

  • Group muscles with similar attributes (e.g., location, layer, compartment, attachments, innervation, function).
  • Recall what attributes set a muscle apart and/or differentiate it from similar muscles.
    • Examples:
      • “What is the only muscle that can flex the interphalangeal joint of the thumb?”
      • “Which is the only muscle in the anterior abdominal wall that attaches to the xiphoid process?”
  • Think through associations between attachments and actions.
    • Examples:
      • “Where would this muscle have to attach to create this specific joint movement?”
      • “What joint movement would happen if these attachments moved closer together?”
  • Use methods to visualize muscle structure and function (e.g., live observation, digital apps).
  • Experience how muscles function through movement of your own body (e.g., concentric vs. eccentric vs. isometric actions of the quadriceps during a squat).