Applied Biomechanics M Bracher University of Southampton FHS

Applied Biomechanics M. Bracher University of Southampton FHS 2010

Aims • Discuss basic principles of biomechanics • Identify how biomechanical factors can influence movement and function • Apply biomechanical principles to the analysis of normal functional movement M. Bracher University of Southampton FHS 2010

Biomechanical Principles • Stability and Balance • Base of Support (BOS) • Centre of Gravity (COG) • Line of Gravity (LOG) • Friction M. Bracher University of Southampton FHS 2010

Stability and Balance • The stability of an object is its ability to withstand external and internal forces and remain in its current position or shape. • The balance of an object is its ability to keep its line of gravity within its base of support. This may include the ability to adjust its position or change its shape. M. Bracher University of Southampton FHS 2010

Stability. v. Instability • In order to stay still, an object/body requires a high level of stability • In order to move, an object/body needs to reduce it’s stability or become unstable M. Bracher University of Southampton FHS 2010

Key factors affecting stability • Size of the Base of Support (BOS) • Position of Centre of Gravity (COG) • Position of Line of Gravity (LOG) • Degree of friction between surfaces M. Bracher University of Southampton FHS 2010

Base of Support (BOS) • BOS = parts of an object/body in contact with the ground all the area inside the contact points • Larger BOS = greater stability • Smaller BOS = less stability M. Bracher University of Southampton FHS 2010

Base of Support in Standing M. Bracher University of Southampton FHS 2010

Large BOS Small BOS M. Bracher University of Southampton FHS 2010

Gravity • The force that attracts the body to the centre of earth • A person’s weight = their mass x gravity M. Bracher University of Southampton FHS 2010

The Centre of Gravity (COG) of an object or body is at the centre of the area of its greatest mass. This is the point at which gravity acts on an object or body and can also be identified as it’s ‘balance’ point M. Bracher University of Southampton FHS 2010

Centre of Gravity (COG) 1. COG of whole body = just in front of our second sacral vertebra when we are in the anatomical position 1. Each body segment has it’s own COG 2. The COG of the whole body moves as we change the organisation or weight of body segments M. Bracher University of Southampton FHS 2010

Centre of Gravity in Anatomical Position Centre of Gravity when standing is anterior to Sacral vertebra no. 2 M. Bracher University of Southampton FHS 2010

Centre of Gravity Centre of gravity in lying is anterior to S 2 M. Bracher University of Southampton FHS 2010

Stability and the COG 1. Lower COG = greater stability 2. When COG is more central within the object/body = greater stability 3. Higher COG = less stability M. Bracher University of Southampton FHS 2010

Changes in position of COG • As we move, our body segments are rearranged. This can change the position of the COG • The COG is able to move within or outside the physical body. • Adding weight to different parts of the body, changes the position of the COG M. Bracher University of Southampton FHS 2010

Effect of change of position on COG The COG moves up and forward as we change the position of the trunk in relation to the lower limbs Place the cursor over the photograph for further information M. Bracher University of Southampton FHS 2010

Effect of weight distribution on COG The COG moves up and forward as we add weight to the upper part of the body, (increasing it’s overall mass) Place the cursor over the photograph for further explanation M. Bracher University of Southampton FHS 2010

Line of Gravity (LOG) • LOG is an imaginary line which falls vertically from the COG to the ground. • Knowing where the LOG falls during movement, helps us identify an object’s level or state of stability M. Bracher University of Southampton FHS 2010

Line of Gravity The centre of gravity in standing The line of gravity falls vertically from the COG through the centre of the BOS therefore the person is stable M. Bracher University of Southampton FHS 2010

Stability and Instability • When the LOG falls in the middle of the BOS = maximum stability • When the LOG falls close to the edge of the BOS = minimum stability • When LOG falls outside the edge of the BOS = instability M. Bracher University of Southampton FHS 2010

The LOG viewed from the side LOG falls within the BOS but slightly to the rear of the foot. It will need to move less distance to the rear (than to the front) before the person becomes unstable, increasing the potential for a backward fall M. Bracher University of Southampton FHS 2010

Position of LOG COG The LOG falls outside the BOS therefore the person is unstable. Place the cursor over the photograph for more information M. Bracher University of Southampton FHS 2010

Friction occurs when 2 surfaces come into contact with each other Friction can either prevent movement or affect the speed of movement The degree of friction between surfaces depends on their friction co-efficients. M. Bracher University of Southampton FHS 2010

Friction Coefficients A friction co-efficient relates to the degree of texture in a material. A material with a rough surface has a high friction co-efficient and therefore will not move easily on other surfaces. A material with a smooth surface has a low friction co-efficient and therefore will move easily on other surfaces M. Bracher University of Southampton FHS 2010

Static and Dynamic Friction • Static friction opposes initial motion (eg static friction is occurring between the body and a bed when we are lying still). • Dynamic friction opposes the motion of an object that is already moving (e. g. dynamic friction occurs between a chair seat and our buttocks when we move forward in a chair). M. Bracher University of Southampton FHS 2010

Static Friction • Static friction therefore maintains an object or body in one position and prevents movement. • Static friction is important where stability or limitation of movement is needed M. Bracher University of Southampton FHS 2010

Static Friction M. Bracher University of Southampton FHS 2010

Dynamic Friction • Dynamic friction occurs when 2 surfaces move upon each other • Dynamic friction opposes motion. It therefore slows the movement down. • The effect of dynamic friction depends on; friction coefficient of each surface total surface contact pressure between surfaces M. Bracher University of Southampton FHS 2010

Reducing friction To reduce stability and facilitate movement, the effects of dynamic friction can be reduced by; • Reducing pressure between surfaces • Reducing the area of surfaces in contact • Reducing friction coefficients of opposing surfaces M. Bracher University of Southampton FHS 2010

Reducing Friction in Patient Handling High surface area contact and high friction coefficient between patient’s clothes and bed, limits movement potential Slide sheets with low friction coefficient make it easier to M. Bracher University of move the patient Southampton FHS 2010

Further Reading • Bell, F. (1998) Principles of Mechanics and Biomechanics. Cheltenham, Stanley Thornes • Levangie, P. K. and Norkin, C. C. (2005) Joint Structure and Function; A Comprehensive Analysis (4 th Edition). Philadelphia, F. A. Davis Company • Tyldesley, B and Grieve, J. I. (2002) Muscles, Nerves and Movement in Human Occupation (3 rd Edition) Oxford, Blackwell Science Ltd M. Bracher University of Southampton FHS 2010