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Muscle Actions & Antagonistic Pairs

Antagonistic Muscle Pairs

  • All skeletal muscles are organized in antagonistic pairs, meaning they work in direct opposition to each other.
    • One muscle of the pair contracts (shortens & thickens) while the opposing muscle relaxes (lengthens & thins).
    • These opposite actions produce smooth, controlled movement.
  • Muscle fibers are specialized cells capable of changing length/shape through contraction and relaxation.

Muscle Contraction vs. Relaxation

  • Contraction
    • Tightening of a muscle belly.
    • Muscle becomes shorter, thicker; belly enlarges.
  • Relaxation
    • Return to original length & shape.
    • Muscle becomes longer, thinner; belly no longer enlarged.

Upper-Arm Example (Figures 4.3 A & B)

  • Pair: Biceps brachii vs. Triceps brachii.
  • Extension of elbow
    • Triceps = contracted.
    • Biceps = relaxed.
  • Flexion of elbow
    • Biceps = contracted.
    • Triceps = relaxed.

Categories of Opposing Movements (Fig. 4.4–4.8)

Abduction vs. Adduction (Fig. 4.4)

  • Abduction: moves limb away from body’s midline.
    • Word roots: "ab" = away from; "duct" = to lead; "-ion" = action.
    • Abductor muscle performs this action.
  • Adduction: moves limb toward body’s midline.
    • Roots: "ad" = toward; "duct" = to lead; "-ion" = action.
    • Adductor muscle performs this action.

Flexion vs. Extension (Fig. 4.5)

  • Flexion: decreases angle \theta between two bones (e.g. bending elbow/knee).
    • Roots: "flex" = bend.
    • Flexor muscle bends limb at joint.
  • Extension: increases angle \theta or straightens limb.
    • Roots: "ex" = away from; "tens" = stretch.
    • Extensor muscle straightens limb.
  • Hyperextension: extension beyond normal limit (e.g.
    head thrown far backward/forward).

Rotation vs. Circumduction (Fig. 4.6)

  • Rotation: circular movement around an axis (e.g.
    humerus rotating in shoulder socket).
  • Circumduction: circular sweep made by distal end of limb (e.g.
    drawing a cone with the hand while shoulder remains relatively fixed).

Pronation vs. Supination (Fig. 4.7 A & B)

  • Pronation: forearm rotates so palm faces downward/posteriorly.
  • Supination: forearm rotates so palm faces upward/anteriorly.

Dorsiflexion vs. Plantar Flexion (Fig. 4.8)

  • Dorsiflexion: foot bends upward at ankle (toes toward shin).
  • Plantar flexion: foot bends downward at ankle (pointing toes).

Elevation vs. Depression (Facial & Other Muscles)

  • Elevation: raises or lifts a body part.
    • Example: levator anguli oris elevates corners of mouth (smile).
    • "Levator" = muscle that raises.
  • Depression: lowers a body part.
    • Example: depressor anguli oris lowers corners of mouth (frown).
    • "Depressor" = muscle that lowers.

Terminology & Functional Roles

  • Antagonist: muscle whose action directly opposes that of another.
  • Agonist/Prime mover (implied): muscle chiefly responsible for a movement while antagonist relaxes.
  • Prefix/Root patterns to memorize:
    • "ab-" (away), "ad-" (toward), "ex-" (out/away), "flex" (bend), "tens" (stretch), "lev-" (lift), "depress-" (lower).
  • Understanding these linguistic building blocks aids in decoding unfamiliar muscle names and actions.

Practical & Clinical Implications

  • Muscle balance between antagonistic pairs is crucial for joint stability and injury prevention.
  • Hyperextension injuries often involve overstretching antagonists beyond physiological range.
  • Rehabilitation protocols typically strengthen both members of an antagonistic pair to restore proper biomechanics.

Visual References Mentioned

  • Figure 4.3 – Antagonistic pair of upper arm.
  • Figure 4.4 – Abduction vs. Adduction.
  • Figure 4.5 – Flexion vs. Extension (including hyperextension).
  • Figure 4.6 – Rotation & Circumduction.
  • Figure 4.7 – Pronation & Supination.
  • Figure 4.8 – Dorsiflexion & Plantar Flexion.
  • Figure 4.9 – Superficial facial muscles illustrating elevation & depression actions.

Study Tips

  • Practice naming the antagonist for any movement; e.g.
    when testing "flexion of knee," immediately recall "quadriceps femoris (extensor)" vs.
    "hamstrings (flexor)."
  • Use flashcards matching term → definition & associated root meanings.
  • Perform the motions physically to kinesthetically reinforce definitions (e.g.
    pronate/supinate forearm while studying).