CHAPTER 13: THE CONDITIONS OF ROTARY MOTION KINESIOLOGY Scientific Basis of Human Motion, 12 th edition Hamilton, Weimar & Luttgens Presentation Created by TK Koesterer, Ph.D., ATC Humboldt State University Revised by Hamilton & Weimar McGraw-Hill/Irwin Copyright 2012 by The McGraw-Hill Companies, Inc. All rights reserved.
Rotary Force Eccentric Force When the direction of force is not in line with object s center of gravity, a combination of rotary and translatory motion is likely to occur. An object with a fixed axis rotates when force is applied off center. Eccentric force: a force whose direction is not in line with the center of gravity of a freely moving object or the center of rotation of an object with a fixed axis of rotation. 13-2
Examples of Eccentric Force Fig 13.1 13-3
Torque The turning effect of an eccentric force. Equals the product of the force magnitude and the length of the moment arm. Moment arm is the perpendicular distance from the line of force to the axis of rotation. Torque may be modified by changing either force or moment arm. Fig 13.2 13-4
Length of Moment Arm Perpendicular distance from the line of force to the axis of rotation. The moment arm is no longer the length of the forearm. Fig 13.3 13-5
Length of Moment Arm In the body, weight of a segment cannot be altered instantaneously. Therefore, torque of a segment due to gravitational force can be changed only by changing the length of the moment arm. d W d W Fig 13.4 13-6
Muscle Force Vectors Only the rotary component is actually a factor in torque production. The stabilizing component acts along the mechanical axis of the bone, through the axis of rotation. Thus, it is not eccentric, or off-center. 13-7
Summation of Torques The sum of two or more torques may result in no motion, linear motion, or rotary motion. Parallel eccentric forces applied in the same direction on opposite sides of the center of rotation; Ex. a balanced seesaw. If equal parallel forces are adequate to overcome the resistance, linear motion will occur; Ex. paddlers in a canoe. 13-8
Force Couple The effect of equal parallel forces acting in opposite direction. Fig 13.6 & 13.7 13-9
The Lever A rigid bar that can rotate about a fixed point (Fulcrum) when a force is applied to overcome a resistance. They are used to: Balance 2 or more forces. Favor force production. Favor speed and range of motion. Change the direction of the applied force. 13-10
External Levers Using a small force to overcome a large resistance. Ex. a crowbar Using a large ROM to overcome a small resistance. Ex. Hitting a golf ball Used to balance a force and a load. Ex. a seesaw 13-11
External Lever:
Anatomical Levers Nearly every bone is a lever. The joint is the fulcrum. Contracting muscles are the force. Do not necessarily resemble bars. Ex. skull, scapula, vertebrae The resistance point may be difficult to identify. May be difficult to determine resistance. Weight and antagonistic muscles. 13-13
Lever Arms Portion of lever between fulcrum & force point. Effort Arm (EA): Perpendicular distance between fulcrum & line of force of effort. Resistance Arm (RA): Perpendicular distance between fulcrum & line of resistance force. Fig 13.16 13-14
Classification of Levers Three points on the lever have been identified 1. Fulcrum 2. Effort force point of application 3. Resistance force point of application There are three possible arrangements of these points. This arrangement is the basis for the classification of levers. 13-15
First-Class Lever E A R E = Effort A = Axis or fulcrum R = Resistance Fig 13.12 13-16
First-Class Lever Can be used to achieve all four functions of a simple machine. Depends on relative lengths of effort arm and resistance arm: 1. Balance 2 or more forces: If effort force and resistance force are equal, effort arm and resistance arm are equal. 2. Favor force production: If effort force and resistance force are equal, effort arm is longer than the resistance arm. 3. Favor speed and range of motion: If effort force and resistance force are equal, resistance arm is longer than the effort arm. 4. Change direction of applied force: If you push down on one side of a seesaw, the other side goes up. 13-17
Second-Class Lever A R E E = Effort A = Axis or fulcrum R = Resistance Fig 13.13 13-18
Second-Class Levers Primary function is to magnify the effect of force production. The effort arm is always longer than the resistance arm. 13-19
Third-Class Lever A E R E = Effort A = Axis or fulcrum R = Resistance Fig 13.14 13-20
Third-Class Levers Primary function is to magnify speed and range of motion. Resistance arm is longer than effort arm so even though the entire lever will move through the same angular distance, the effort moves a small linear distance, while the resistance moves through a larger linear distance. 13-21