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**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”

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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.

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

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**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

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**Elbow and Radioulnar Joint Movements**

elbow - flexion and extension Radioulnar (forearm) - pronation and supination

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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

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

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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

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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

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

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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

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

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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

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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

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**Radial Collateral Ligament**

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**Ulnar Collateral Ligament**

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Annular Ligament

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**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

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**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.

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**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.

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**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

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**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

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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?

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**Known: angle of pull = 45 degrees**

Muscle force (resultant) = 20 lbs Unknown: rotary (y) force dislocating (x) force

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**Rotary force (y) calculation:**

sin θ = opposite / hypotenuse sin 45 = rotary (y) / 20 lbs sin 45 x 20 lbs = rotary (y) rotary (y) = lbs

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**Dislocating force calculation:**

cos θ = adjacent / hypotenuse cos 45 = dislocating (x) / 20 lbs cos 45 x 20 lbs = dislocating (x) dislocating = lbs

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**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

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