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Chapter 13 Equilibrium and Human Movement Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. McGraw-Hill/Irwin © 2012 The McGraw-Hill Companies, Inc. All rights reserved. Equilibrium What is torque? • the rotary effect of a force about an axis of rotation • measured as the product of force and the force’s moment arm - the shortest (perpendicular) distance between a force’s line of action and an axis of rotation Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-2 Equilibrium Where do torques occur within the human body? The product of muscle tension and muscle moment arm produces a torque at the joint crossed by the muscle. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-3 Equilibrium Skilled pitchers often maximize the length of the moment arm between the hand and total-body axis of rotation during the delivery of a pitch to maximize the effect of the torque produced by the muscles. Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-4 Biomechanical Levers • Levers: simple machines • Lever: rigid bar that turns about an axis. • Levers increase amount of torque generated by a muscle. Parts of a lever • • • • • Fulcrum Force point Resistance point Force arm (Farm) Resistance arm (Rarm) Classes of levers • Levers are classified according to the arrangement of the force point, resistance point, and fulcrum. Equilibrium F Relative locations of the applied force (F), the resistance (R), and the fulcrum or axis of rotation determine lever classifications. R First class R F Second class F R Third class Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-8 Mechanical Advantage • If the force arm > resistance arm, the lever favors force. • If the resistance arm > force arm, the lever favors speed and range of motion. • If they are equal = favors neither. Third class lever • Force point between fulcrum and resistance. • Rarm > Farm • Favors speed and ROM • Most common in human body Second class lever • Resistance point between fulcrum and force point. • Farm > Rarm • Favors force First class lever • Fulcrum between Force point and Resistance point • Length of Farm and Rarm depend on location of fulcrum. • Can favor either force or speed, depending on length of Farm and Rarm. Equilibrium What is torque? • the rotary effect of a force about an axis of rotation • measured as the product of force and the force’s moment arm - the shortest (perpendicular) distance between a force’s line of action and an axis of rotation Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-13 Center of Gravity What is the center of gravity? • the point around which a body’s weight is equally balanced in all directions • also referred to as the center of mass or mass centroid • (need not be physically located inside of a body) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-14 Center of Gravity • Point at which all body’s mass and weight is evenly distributed in all directions. • Center of gravity is dependent upon: – Body shape (anthropometrics, gender) – Body position • Base of support Stability and Balance What is stability? • resistance to disruption of equilibrium What is balance? • ability to control equilibrium Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-16 Stability and Balance What is the base of support? (area bound by the outermost regions of contact between a body and the support surface) Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-17 Stability and Balance What can increase a body’s stability? • increasing body mass • increasing friction between the body and the surfaces of contact Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-18 Stability and Balance What can increase a body’s stability? • increasing the size of the base of support in the direction of an external force Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-19 Stability and Balance What can increase a body’s stability? • horizontally positioning the center of gravity near the edge of the base of support on the side of the external force Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-20 Stability and Balance What can increase a body’s stability? • vertically positioning the center of gravity as low as possible The higher the CG, the greater the amount of torque its motion creates about the support surface. d d d Basic Biomechanics, 6th edition By Susan J. Hall, Ph.D. 13-21