This document discusses muscle mechanics across joints. It reviews muscle structure, including fibers, tendons and origins and insertions. Muscle contraction is described, noting the roles of actin and myosin. Principles of muscle physics are outlined, relating cross-sectional area and force, and length and strain. Joint movement is likened to lever systems. The roles of agonists, antagonists and synergists are defined in stabilizing joints and facilitating movement. Contraction types including concentric, isometric and eccentric contractions are also defined.

1. Mechanics of Movement II:
Muscle Action Across Joints
Review muscle force generation
Muscle Physics
--force versus cross section
--length versus strain
Lever mechanics
Stabilizing the joint—isometric and eccentric
contraction
Frolich, Human Anatomy, Mechanics of Movement

2. Muscle Structure Review
 Muscle fiber = muscle cell Fig. 10.1
 Fibers lined up = direction of pull
 Tendon attaches to bone
Frolich, Human Anatomy, Mech
 Muscle pulls on bone of Movement

3. Muscle Origin
and Insertion
 Origin
 Proximal
 Fixed
 Insertion
 Distal
 Moves
 (usually!!)
Fig. 10.3 Anatomy, Mech
Frolich, Human
of Movement

4. Mechanics of Contraction
 Muscle cell is unit
 Role of actin/myosin
 Action potential or
depolarization of
membrane makes cell
“contract”
 (motor neuron action
potential stimulates
muscle membrane
depolarization)
Fig. 10.4 Frolich, Human Anatomy, Mech
of Movement

5. Visualizing muscle contraction
How actin-
myosin
complex
(sarcomere)
shorten muscle
Fig. 10.7
Frolich, Human Anatomy, Mech
of Movement

6. Summary of Muscle Organization/Function
Frolich, Human Anatomy, Mech
of Movement

7. Summary of Muscle Organization/Function
Frolich, Human Anatomy, Mech
of Movement

8. Summary of Muscle Organization/Function
Frolich, Human Anatomy, Mech
of Movement

9. Levels of Muscle
Organization
Table 10.2
Frolich, Human Anatomy, Mech
of Movement

10. Muscle Physics: Principle I
 Cross sectional area is proportional
to Force of muscle
Frolich, Human Anatomy, Mech
of Movement

11. Muscle Physics: Principle II
 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, Mech
of Movement

12. Muscle Physics: Principle III
 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, Mech
of Movement

13. Types of fascicle arrangements
 Affects length and
cross section of
muscle
 Thus affects force
and shortening
properties of muscle
 See Muscle Physics
Principles I-III if this
doesn’t make sense
Fig. 11.3
Frolich, Human Anatomy, Mech
of Movement

14. Muscle movement across joints
is like lever system
Fig. 11.1
Frolich, Human Anatomy, Mech
of Movement

15. First-class lever
Fig. 11.2
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of Movement

16. Second-class lever
Fig. 11.2
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of Movement

17. Third-class lever
Fig. 11.2
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of Movement

18. 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 Anatomy, Mech
Frolich, Human
of Movement

19. Agonist/Antagonist
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of Movement

20. 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, Mech
of Movement