Biomechanics and Kinesiology - Week 3 - Gravity (PDF)

Biomechanics and Kinesiology -1 Week 3- Gravity Sivapriya Ramakrishnan., MPT., FHEA Objectives At the end of the lecture, the student will be able to Define Gravity; Define Segmental COG; Define Equilibrium; Explain the Rules of Equilibrium; Apply the concepts to various positions in...

Biomechanics and Kinesiology -1 Week 3- Gravity Sivapriya Ramakrishnan., MPT., FHEA Objectives At the end of the lecture, the student will be able to Define Gravity; Define Segmental COG; Define Equilibrium; Explain the Rules of Equilibrium; Apply the concepts to various positions in the human body.  Introduction Gravity is an external force acting under normal circumstances which affects all the objects The first external force to be considered acting on human body Force of gravity is the pull of earth on a body or its segments and it’s a consistent one. Described as mutual attraction between the earth and an object Gravitational force is always directed towards the centre of earth which is towards the ground  Gravity- Vector Gravity being a force is a vector quantity Magnitude of gravitational force is directly proportional to the product of ✓ Point of application: COG the two masses and inversely ✓ Line of application: Centre of proportional to square of the distance mass of object between them ✓ Direction : Towards centre of the Fg=gm1m2/r² earth ✓ Magnitude: Equal to Gravitational Force  Center of Gravity Although gravity acts at all points on COG is an an object or hypothetical segment of an object,its application point at which is given as Centre of all mass would Gravity (COG) appear to be Center of Gravity is also referred as concentrated centre of Mass and is the point at which the force of gravity would appear to act.  Symmetry and COG In a symmetrical object In an asymmetrical the COG will be located object the COG will be at the geometric center located towards the of the object heavier end where all the mass is evenly distributed around that point COG in Human Body- s2 Segmental Whole When segments are combined, gravity acting on combined Each segment in segments can be represented by body is acted on by a single COG the force of gravity and has its own COG COG in Human Body- s2 When the trunk is inclined forward, location of new COG lies outside the body Location of COG varies with each of the many and varied postures the body assumes  Line of Gravity Action line and direction of the force of gravity are always vertically downward toward the centre of the earth regardless of the orientation in space of the object Gravity vector is commonly referred to as the Line of Gravity (LOG) LOG can best be visualized as a string with a weight on the end (a plumb line) with the string attached to the COG of an object Positions and COG When the posture is good the LOG passes through the mid cervical and mid lumbar vertebrae and in front of thoracic vertebrae  Base of Support Base as applied to a rigid body is the area by which it is supported and in contact with the supporting surface In lying position posterior aspect of the whole body forms the base In stride standing BOS is an area as wide as the feet and as long as the distance between their outer borders Different types of walking aids are used to increase BOS of patients  Equilibrium ✓ State in which a If the forces acting upon the body at rest tend body is at rest with to restore it to its original position after it has neither Translatory been displaced, the body is said to be in stable nor rotatory Stable equilibrium motion (static equilibrium) or in which a body is in constant motion If a body is given an initial displacement and with no the forces acting upon it increase this initial acceleration displacement ,however small the latter may be (dynamic Unstable the body is said to be in unstable equilibrium equilibrium) ✓ Equilibrium results when the forces If,in spite of displacement of a body,the height acting upon a body and position of its COG remain the same in are perfectly relation to base,t he body is said to be in balanced Neutral neutral equilibrium  Rule of Equilibrium The condition of equilibrium is most stable if, 1.The larger the BOS of an object, the greater the stability of that object 2.The closer the COG of the object is to the BOS, the more stable is the object. 3.An object cannot be stable unless its LOG falls within its BOS 4.Greater the mass of the object greater the stability 5.Greater the friction between the supporting surface and the BOS, the more stable the body will be.  Stability and COG For an object to be stable the LOG must fall with in the base of support (BOS) When LOG falls outside the BOS the object will fall(less stable) When BOS of an object is large, The LOG has more freedom to move with out passing beyond the limits of the base ExamplEs…………………………… …… When a man stands with his legs spread apart,the base is larger side to side and the trunk can move a good deal in that plane with out displacing the LOG from COG When a person grasps or leans on another object that object can become part of BOS ✓ When COG is low, movement of the object in space is less likely to cause the COG & LOG to fall outside the BOS ✓ The longer the LOG, the higher the COG, the less stable the object ✓ The shorter the LOG, the lower the COG, the more stable the object Relocation of Centre of Gravity ✓Location of COG of a body depends not only on the arrangement of segments in space but also on the distribution of mass of the object. ✓Most common way to functionally redistribute mass in the body is to add external mass ✓Every time we add an object to the body by wearing it,carrying it or using it the new COG for the combined body & the external mass will shift toward the additional weight;the shift will be proportional to the weight added When a cast is applied to the right lower limb?? 1. Results in shifting the COG down and to the right 2. Because his COG is now lower, he is theoretically more stable 3. Though he is standing only on one leg addition of crutches enlarges the base of support and thus improves the stability Holding a heavy suitcase in right hand??? Results in shift of the COG up and to the right Because the LOG would move toward the right foot the man leans to the left to compensate Man leans laterally to the left not to relocate the COG but to bring the LOG back to the middle of base of support Texbook Chapter-2 We are good to go !