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					   Mechanics of Movement I:
   Muscle Force and Action Across Joints

Review   muscle force generation
Muscle Mechanics
    --force versus cross section
    --length versus strain
Lever mechanics and agonist/antagonists
Stabilizing the joint—isometric and eccentric
 contraction


                                Frolich, Human Anatomy, Mechanics of Movement
         Muscle Structure Review




 Muscle fiber = muscle cell
 Fibers lined up = direction of pull
 Tendon attaches to bone
 Muscle pulls on bone
                                        Frolich, Human Anatomy, Mechanics of Movement
Muscle Origin
and Insertion

 Origin
    Proximal

    Fixed

 Insertion
    Distal

    Moves

 (usually!!)




    Frolich, Human Anatomy, Mechanics of Movement
           Mechanics of Contraction
   Muscle fiber is one cell
    made up of myofibrils,
    each filled with
    myofilament proteins
    actin and myosin, all lined
    up along length of muscle
    cell.
   Action potential or
    depolarization of
    membrane releases
    calcium
   Calcium changes shape
    of actin so myosin cross-
    bridges form and “row” or
    pull in to center of
    sarcomere.

                                  Frolich, Human Anatomy, Mechanics of Movement
Visualizing muscle contraction

                   How actin-
                   myosin complex
                   (sarcomere)
                   shortens muscle




                   Frolich, Human Anatomy, Mechanics of Movement
Summary of Muscle Organization/Function




                         Frolich, Human Anatomy, Mechanics of Movement
Summary of Muscle Organization/Function




                         Frolich, Human Anatomy, Mechanics of Movement
Summary of Muscle Organization/Function




                         Frolich, Human Anatomy, Mechanics of Movement
Levels of Muscle
Organization




 Frolich, Human Anatomy, Mechanics of Movement
Muscle Mechanics:
Cross section versus force




           Cross sectional area is proportional
            to Force of muscle
                              Frolich, Human Anatomy, Mechanics of Movement
       Muscle Mechanics:
       length versus force
 Force generation depends
  on current length of
  muscle or overlap in
  actin/myosin of
  sarcomeres
 Muscle force strongest
  between 80-120% of
  normal resting length—
  WHY? (don’t forget role of cross-bridges)
 Most muscles arranged to
  work in this range

                                              Frolich, Human Anatomy, Mechanics of Movement
         Muscle Mechanics:
         length versus total shortening
   Length of muscle is proportional to
    ability to shorten (strain)
       Number of sarcomeres in series gives
        shortening ability
   Short, fat muscles
       Lots of force
       Less shortening range
   Long, skinny muscles
       Less force
       More shortening range


                                               Frolich, Human Anatomy, Mechanics of Movement
Types of fascicle arrangements
                 Affects length and
                  cross section of
                  muscle
                 Thus affects force
                  and shortening
                  properties of muscle
                 See Muscle
                  Mechanics if this
                  doesn’t make sense



                    Frolich, Human Anatomy, Mechanics of Movement
             Long thin straight muscle versus
             short fat pinnate muscle
Sartorius
    Tailor’s or hackey-sac
     muscle
    Longest muscle in
     body’
    Thin and straight fibers
    Low force, great
     shortening distance



                                   Gastrocnemius (calf muscle)
                                       Short and bulky
                                       Pinnate fibers
                                       Great force, low shortening distance
                                       Pushes off each step—”spring-
                                        loaded”

                                                Frolich, Human Anatomy, Mechanics of Movement
Muscle movement across joints
is like lever system




                   Frolich, Human Anatomy, Mechanics of Movement
Agonist/Antagonist muscles




                 Frolich, Human Anatomy, Mechanics of Movement
    Stabilization and Control Around Joint

Agonist               Main Mover          E.g. biceps

Antagonist            Opposite            E.g. triceps
                      motion
Synergist             Aids agonist        E.g. brachialis


   Antagonist often “fires” or contracts or is stimulated
    simultaneously with agonist to stabilize around joint
    during movement
   NOTE: Muscle “contraction” or stimulus to “fire”
    does not always result in muscle shortening
                                          Frolich, Human Anatomy, Mechanics of Movement
  Relation between muscle contraction
  (or “firing”) and shortening

 Concentric contraction—muscle contracts and
  shortens to cause movement across joint
 Isometric contraction—muscle contracts but
  stays same length to hold joint or body in
  same position
 Eccentric contraction—muscle contracts while
  lengthening to stabilize joint during movement
  (most common in antagonist to slow
  movement caused by agonist)

                                Frolich, Human Anatomy, Mechanics of Movement