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”
• 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.
• What determines which one is working is
the purpose of movement, acceleration
(speeding-up) or deceleration (slowingdown).
• Examples
Steps to determine contraction type
1. Identify the joint movement
2. Identify the agonist (concentric contractor) and
antagonist (eccentric contractor) for the joint
movement
3. 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
Elbow and Radioulnar Joint
Movements
• elbow - flexion and extension
• Radioulnar (forearm) - pronation and
supination
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
Brachialis
O: Distal ½ anterior shaft of humerus
I: Coronoid process of ulna
A: True flexion of the elbow
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
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
Anconeus
O: Posterior surface of lateral condyle of
humerus
I: Posterior surface of olecranon process
and proximal ¼ of ulna
A: extension of elbow
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
Pronator quadratus
O: Distal fourth anterior side of ulna
I: Distal fourth anterior side of radius
A: Pronation of forearm
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
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
Radial Collateral Ligament
Ulnar Collateral Ligament
Annular Ligament
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
• 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.
• 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.
• 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.
θ
C
A
θ
B
• Furthermore, the following equations are derived from
the Pythagorean theorem:
sin θ = opposite / hypotenuse
cos θ = adjacent / hypotenuse
tan θ = opposite / adjacent
θ
C
A
θ
B
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?
Known: angle of pull = 45 degrees
Muscle force (resultant) = 20 lbs
Unknown: rotary (y) force
dislocating (x) force
Rotary force (y) calculation:
sin θ = opposite / hypotenuse
sin 45 = rotary (y) / 20 lbs
sin 45 x 20 lbs = rotary (y)
rotary (y) = 14.14 lbs
Dislocating force calculation:
cos θ = adjacent / hypotenuse
cos 45 = dislocating (x) / 20 lbs
cos 45 x 20 lbs = dislocating (x)
dislocating = 14.14 lbs
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
399.88 lbs = (muscle force)2
√ 399.88 = muscle force
20 lbs = muscle force