Statics and Biomechanics Steven A. Jones 12/31/2018
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. 12/31/2018
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 12/31/2018
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 12/31/2018
Skeletal Muscles Concentric: Force and contraction simultaneous (agonist) Static: Force, but no change in length Eccentric: Increased force, Increased Length Important for control Antagonist 12/31/2018
Objectives of Biomechanical Analysis Joint forces Muscle tensions 12/31/2018
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
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
The Elbow: Bones Humerus hinge joints Radius Ulna pivot joint 12/31/2018
The Elbow: Motions Pronation (palm down) Flexion Extension Supination (palm up) 12/31/2018
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 12/31/2018
Elbow: Simple Problem Statically Indeterminate. Must either (1) Simplify or (2) Have model for muscle. 12/31/2018
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
Distributed Forces A distributed load in a free body can be replaced with it a point load at the center of mass. p P 12/31/2018
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