1. Agonist and Antagonist Relationship
Agonist – is a muscle described as being primarily responsible for a specific joint movement while contracting
Antagonist – is a muscle that counteracts or opposes the contraction of another muscle
Simply, these are relative terms describing “opposites”

2. If an agonist muscle is considered a concentric contractor for a movement then the antagonist muscle is the eccentric contractor for the same movement.
Generally, concentric and eccentric contractions do not occur at the same time for a given movement.

3. What determines which one is working is the purpose of movement, acceleration (speeding-up) or deceleration (slowing-down).
Examples

4. Steps to determine contraction type
Identify the joint movement
Identify the agonist (concentric contractor) and antagonist (eccentric contractor) for the joint movement
Determine if the movement is speeding-up (accelerating) or slowing-down (decelerating)
If speeding-up then agonist working concentrically
If slowing-down then antagonist working eccentrically

5. Elbow and Radioulnar Joint Movements
elbow - flexion and extension
Radioulnar (forearm) - pronation and supination

6. Biceps Brachii* O: Long Head – supraglenoid tubercle above the superior lip of glenoid fossa Short Head – coracoid process and upper lip of glenoid fossa I: Tuberosity of radius and bicipital aponeurosis A: Flexion of elbow, supination of forearm (radioulnar), weak flexion of shoulder, and weak abduction of shoulder

7. Brachialis O: Distal ½ anterior shaft of humerus I: Coronoid process of ulna A: True flexion of the elbow

8. Brachioradialis O: Distal 2/3 of lateral condyloid (supracondyloid) ridge of humerus I: Lateral surface distal end of radius at the styloid process A: Flexion of elbow, pronation from supinated position to neutral (thumb up), supination from pronated position to neutral

9. Triceps brachii O: Long head – infraglenoid tubercle below inferior lip of glenoid fossa of scapula Lateral head – upper ½ posterior surface of humerus Medial head – distal 2/3 of posterior surface of humerus I: Olecranon process of ulna A: All heads: extension of elbow Long head: extension, adduction, and horizontal abduction of shoulder

10. Anconeus O: Posterior surface of lateral condyle of humerus I: Posterior surface of olecranon process and proximal ¼ of ulna A: extension of elbow

11. Pronator teres O: Distal part of medial condyloid ridge of humerus and medial side of proximal ulna I: Middle third of lateral surface of radius A: Pronation of forearm (radioulnar) and weak flexion of elbow

12. Pronator quadratus O: Distal fourth anterior side of ulna I: Distal fourth anterior side of radius A: Pronation of forearm

13. Supinator O: Lateral epicondyle of humerus and neighboring posterior part of ulna I: Lateral surface of proximal radius just below the head A: Supination of forearm

14. Ligaments of the Elbow
Radial collateral ligament – provides lateral stability of the elbow; resists lateral displacement of elbow
Ulnar collateral ligament – provides medial stability of the elbow; resists medial displacement of elbow
Annular ligament – stabilizes the head of radius to the ulna and allows smooth articulation with the ulna

15. Radial Collateral Ligament

16. Ulnar Collateral Ligament

17. Annular Ligament

19. Introduction to Linear Kinetics
Linear Kinetics – the study of linear forces associated with motion (ex. force, momentum, inertia).
Linear Kinematics – the study of linear motion.
Force = mass x acceleration
Force → Acceleration → Velocity → Displacement

20. Vector – is a quantity that has both magnitude (how much) and direction.
Used as a measuring tool for linear variables which have both magnitude and direction.
Illustrated by an arrow where the tip represents direction and the length representing magnitude.

21. Muscle Force – can be measured with vectors, since muscle force pulls on bone in a linear fashion.
Vector composition – is a process of determining a single vector (usually called resultant) from two or more vectors.

22. Vectors can typically be analyzed as having horizontal (x) and vertical (y) components.
In this case, these perpendicular component vectors can be used to form a right triangle.
A common trigonometric principle used is the Pythagorean theorem, where A2 + B2 = C2. C θ A θ B

23. sin θ = opposite / hypotenuse cos θ = adjacent / hypotenuse
Furthermore, the following equations are derived from the Pythagorean theorem:
sin θ = opposite / hypotenuse
cos θ = adjacent / hypotenuse
tan θ = opposite / adjacent C θ A θ B

24. Sample Problem
If the muscle force generated by the biceps brachii is 20 lbs, how much rotary (y) force is generated by the muscle? How much dislocating (x) force?

25. Known: angle of pull = 45 degrees
Muscle force (resultant) = 20 lbs
Unknown: rotary (y) force
dislocating (x) force

26. Rotary force (y) calculation:
sin θ = opposite / hypotenuse
sin 45 = rotary (y) / 20 lbs
sin 45 x 20 lbs = rotary (y)
rotary (y) = lbs

27. Dislocating force calculation:
cos θ = adjacent / hypotenuse
cos 45 = dislocating (x) / 20 lbs
cos 45 x 20 lbs = dislocating (x)
dislocating = lbs

28. Pythagorean Check A2 + B2 = C2
(rotary force)2 + (dislocating force)2 = (muscle force)2
(14.14 lbs)2 + (14.14 lbs)2 = (muscle force)2
lbs = (muscle force)2
√ = muscle force
20 lbs = muscle force