1. Statics and Biomechanics
Steven A. Jones
12/31/2018

2. Types of Joints Synarthrodial: e.g. skull. No motion allowed
Amphiarthrodial: e.g. vertebrae. Slight relative motions
Diarthrodial: e.g. knee, elbow. Varying degrees of motion.
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3. Components of Joints
Articular cavity: space between articulating bones
Synovial membrane: lining of the capsule that contains synovial fluid (lubricant)
Articular cartilage: cartilaginous material to absorb shock
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4. Types of Diarthrodial Joints
Gliding: e.g. vertebrae
Hinge: e.g. elbow, ankle
Pivot: e.g. proximal radioulnar
Condyloid: e.g. wrist
Saddle: carpometacarpal of thumb
Ball and socket: shoulder and hip
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5. Skeletal Muscles
Concentric: Force and contraction simultaneous (agonist)
Static: Force, but no change in length
Eccentric: Increased force, Increased Length
Important for control
Antagonist
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6. Objectives of Biomechanical Analysis
Joint forces
Muscle tensions
12/31/2018

7. Basic Assumptions for Simplified (Static) Modeling
Only one muscle group controls motion
Axes of rotation of joints are known
Locations of muscle attachments are known
Line of action of muscle tension is known
Weights and centers of gravity are known
Friction is negligible
Dynamics are not significant
Problem is 2-dimensional
No deformation of muscles, tendons, bones.
12/31/2018

8. Basic Assumptions for Simplified (Static) Modeling
Only one muscle group controls motion
Axes of rotation of joints are known
Locations of muscle attachments are known
Line of action of muscle tension is known
Weights and centers of gravity are known
Friction is negligible
Dynamics are not significant
Problem is 2-dimensional
No deformation of muscles, tendons, bones.
12/31/2018

9. The Elbow: Bones Humerus hinge joints Radius Ulna pivot joint
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10. The Elbow: Motions Pronation (palm down) Flexion Extension
Supination (palm up)
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11. Elbow Muscles Muscle Purpose Origin Insertion Biceps Flexion
Scapula, Upper Humerus
Proximal Radius
Brachialis
Mid Humerus
Upper Ulna
Brachioradialis
Lower Humerus
Upper Radius
Triceps Brachii
Extension
Pronator Teres
Pronation
(inner side)
Medial Ulna, Outer Radius
Supinator
Supination
Lower Humerus (outer side)
Upper Ulna and Outer Radius
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12. Elbow: Simple Problem
Statically Indeterminate. Must either (1) Simplify or (2) Have model for muscle.
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13. Muscle Models Muscle force is proportional to cross-sectional area
Muscle force is related to electric potential (electromyography)
Muscle force will be optimized (e.g. minimize total energy input required)
12/31/2018

14. Distributed Forces
A distributed load in a free body can be replaced with it a point load at the center of mass. p P
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15. Moment F1 F2 r F1 and F2 apply the same moment around p r’ p
Can translate F1 anywhere along its line of action. r’ p
12/31/2018