1. Interactions of skeletal muscles
Muscles only pull – never push.
As a muscle shortens, its insertion (attachment on the more movable bone) moves toward its origin (fixed point of attachment).
Muscles are generally arranged in pairs so that what one muscle does, the other undoes.

2. 4 functional groups of muscles
1. Prime movers (agonists)
2. Antagonists
3. Synergists
4. Fixators

3. Prime movers (agonists)
Agonist = “leader”
The agonist is the muscle that has the major responsibility to cause a certain movement.
Ex. Biceps is the agonist for elbow flexion.

4. Antagonist Antagonist = “against the leader”
Antagonists are the muscles that undo what the agonist has done.
Ex. Triceps extends the elbow after the biceps has flexed it.
When the prime mover is active, the agonist is generally relaxed and the muscle may be stretched.

5. The prime mover and its antagonist are usually on opposite sides of the joint they act on.
The agonist and its antagonist can change roles.
Ex. The triceps is the agonist for elbow extension and the biceps is the antagonist for elbow extension.

6. Synergists Synergist = “together” “work”
Synergists are muscles that help the prime mover.
They can:
1. Add extra force for the movement

7. Synergist roles continued
2. Reduce undesirable movements that can result from the prime mover acting.
Ex. Finger flexors cross the wrist, but you can make a fist without flexing the wrist due to synergists that stop that action.

8. Fixators
A fixator is a specific type of synergist that immobilizes a bone or the muscle’s origin so that the prime mover has a stable base on which to act.
Ex. Fixator muscles run from the axial skeleton to the scapula to immobilize it (since it is not really attached to the axial skeleton otherwise).

9. Naming skeletal muscles
Names reflect several aspects of the muscle:
1. Location – May include the location of the body where the muscle is found.
Ex. Temporalis is located on the temporal bone.
2. Muscle shape –
Ex. Deltoid = triangular (The delta symbol is a triangle.)

10. 3. Muscle size
Ex. Maximus = biggest
Minimus = smallest
Longus = long
Brevis = short

11. 4. Direction of muscle fibers:
A. Rectus = straight (parallel to an imaginary axis).
Ex. Rectus abdominus – parallel to a vertical line at the midline of the abdomen.
B. Transverse – (at a right angle to the imaginary line)
C. Oblique – At an odd angle to the imaginary line.

12. 5. Number of origins
A. Biceps – 2 origins
B. Triceps – 3 origins
C. Quadriceps – 4 origins

13. 6. Location of attachments
Origin is named first and insertion is named second.
Ex. Sternocleidomastoid – has an origin at the sternum and one at the clavicle. Inserts at the mastoid process of the temporal bone.

14. 7. Action – what the muscle does.
A. Flexor – causes flexion
B. Extensor – causes extension
C. Adductor – causes adduction
D. Supinator – causes supination