Biomechanics and Its Application to Seating

Transcription

1 Biomechanics and Its Application K E L LY W A U G H, P T, M A P T, AT P S E N I O R I N S T R U C TO R / C L I N I C C O O R D I N ATO R A S S I S T I V E T E C H N O LO G Y PA R T N E R S S C H O O L O F M E D I C I N E U N I V E R S I T Y O F C O LO R A D O A N S C H U T Z M E D I C A L C A M P U S J O E L M. B A C H, P H. D. A S S O C I AT E P R O F E S S O R C E N T E R F O R B I O M E C H A N I C S A N D R E H A B I L I TAT I O N R E S E A R C H D E PA R T M E N T O F M E C H A N I C A L E N G I N E E R I N G C O LO R A D O S C H O O L O F M I N E S Contacting Us K E L LY W A U G H, P T, M A P T, AT P S E N I O R I N S T R U C TO R / C L I N I C C O O R D I N ATO R A S S I S T I V E T E C H N O LO G Y PA R T N E R S S C H O O L O F M E D I C I N E U N I V E R S I T Y O F C O LO R A D O A N S C H U T Z M E D I C A L C A M P U S E A S T 1 8 T H AV E N U E, S U I T E D E N V E R, C O LO R A D O ke l l y.w a u g u c d e n v e r. e d u w w w. a s s i s t i v e t e c h n o l o g y p a r t n e rs. o r g J O E L M. B A C H, P H. D. A S S O C I AT E P R O F E S S O R C E N T E R F O R B I O M E C H A N I C S A N D R E H A B I L I TAT I O N R E S E A R C H D E PA R T M E N T O F M E C H A N I C A L E N G I N E E R I N G C O LO R A D O S C H O O L O F M I N E S I L L I N O I S S T R E E T, W H G O L D E N, C O LO R A D O j m b a c m i n e s. e d u b i o m e c h a n i c s. m i n e s. e d u Goals Better outcomes for your clients Understanding cause and effect address the cause, not just the symptoms More efficiency for you proactive vs. reactive. Bach & Waugh,

2 Biomechanics for Seating I. REVIEW OF BASIC PRINCIPLES II. SEATING APPLICATION Basic Principles Forces Motion Equilibrium Moment Arms Center of Gravity, or Center of Mass Stability Types of Forces Unloaded shape Compression Tension Shear Bach & Waugh,

3 Types of Forces Forces can be Internal bone, ligament, tendon, muscle, etc. External support surfaces, straps & harnesses, etc. Gravity Unlike the internal and external forces, gravity is constant in magnitude and direction Body weight is the effect of the force of gravity acting on the body s mass. Pressure vs. Force In many cases, force isn t the critical variable Force divided by the area over which it is applied is pressure (or stress) In the case of bodyweight, we can t do much about the force, but we can modify the pressure. The resolution of forces In force diagrams, a force is represented by an arrow (vector) Any force can be resolved or broken down into two components (e.g. horizontal and vertical, or normal and shear) Bach & Waugh,

4 Effect of a Force A force can result in a Deformation Translation Rotation. Translational Movement = All parts of the body move exactly the same distance, in the same direction, and in the same time (Nwaobi, 1984) Rotational Movement = when a body moves along a circular path about some point in space so that all parts of the body travel through the same angle (Nwaobi, 1984) Diagrams from Nwaobi OM: Biomechanics of seating. In Trefler E, editor: Seating for children with cerebral palsy: a resource manual, University of Tennessee 1984 Joint Motion/Movement of individual body segments are Rotational Movements Motion of the limb occurs around an axis that is perpendicular to the plane. This axis is called the axis of rotation. Transverse plane motions occur about the superior-inferior axis Superior/inferior axis Medial/lateral axis Frontal plane motions occur about the anterior-posterior axis Sagittal plane motions occur about a medial-lateral axis Anterior/posterior axis Degrees of Freedom Our joints have multiple degrees of freedom (DOF) The DOF may be coupled (they aren t independent, palmar flexion results in radial deviation) There may be coupling of DOF from other joints (hip flexion pulls on the hamstrings which may cause knee flexion). Bach & Waugh,

5 Range of Motion and Limits Joints have a range of motion (ROM) for each degree of freedom The ROM may be limited Passively (internal) - bony impingement, ligament/capsule tension, muscle/tendon passive force, soft tissue impingement Passively (external) braces/orthoses, seating systems, etc. Actively (internal) muscle contraction. Moment Arms (think of a teeter-totter) The further the load acts from a joint, the greater the effect on that limb segment The further the load acts from a desired location on a body part, the greater the effect. Moment = force * distance. Moment Arms Axis of Rotation Moment arm for F2 F2 has a longer moment arm than F1, so you need less force at F2 than at F1 to open or close the door Moment arm for F1 Think of this as a top view of a heavy door Force 1 Force 2 Bach & Waugh,

6 Moment Arms All of the following result in a moment of 10 foot-pounds 1 lb at a distance of 10 feet 5 lbs at a distance of 2 feet 10 lbs at a distance of 1 foot 20 lbs at a distance of ½ of a foot 100 lbs at a distance of 1/10 of a foot So, if you need to resists hip abduction and you need 10 foot-pounds to do so, putting the blocking pad 1 foot from the hip requires ½ the force compared to if it was 6 from the hip Less force means greater comfort for your client and better durability of your hardware. Moment Arms To get the most out of the force you apply, you want it to be perpendicular to the direction of movement that you are trying to cause or restrain. Moment Arms Axis of Rotation Moment arm for F2 F2 is more effective at opening or closing the door than is F1 Why? Force 1 Force 2 Think of this as a top view of a heavy door Moment arm for F2 Moment arm for F1 Bach & Waugh,

7 Equilibrium If an object isn t changing speed, it is said to be in a state of equilibrium We tend to simplify this to bodies at rest Equilibrium tells us that all of the forces are balancing each other Example: Hold your arm out in front of you. To do so your anterior deltoid is generating a force that is creating a moment that is balancing the moment produced by gravity. Your arm is in equilibrium. Lower your arm to rest on an armrest or your thigh. The support surface creates a force that is equal and opposite to gravity. Your arm is in equilibrium. For every action (force), there is an equal and opposite reaction (force) (Newton s Third Law of Motion) Gravity pulls down on the body. That pull is resisted by the seating surfaces. The two forces are equal and opposite. A lateral knee support used to block active abduction movement experiences a force equal and opposite to that produced by the leg as it abducts. Given a state of static equilibrium, every force exerted by the person s body while sitting in a seating system is balanced by an opposite force exerted by the support surface on the person. Stability We tend to be seeking stability in most of what we do Stability = the ability of an object to maintain equilibrium or to resume its original position after displacement (American Heritage Dictionary) There are several ways to view stability Bach & Waugh,

8 Stability Positive Neutral Negative Positive ball in a bowl, the natural tendency is for the ball to return to the original position Neutral ball on a table, the natural tendency is for the ball to stop where it is released Negative ball on a dome, the natural tendency is for the ball to move away from its initial position Static vs. Dynamic Static balance on a bike while standing still Dynamic balance on a bike while rolling. Stability Postural stability describes the ability of one body segment to remain steady while an adjacent segment moves (Ward, 1994) In seating, when we stabilize a body segment, we are usually attempting to limit or prevent movement of that segment, for the purpose of improving overall postural stability or to allow improved motor control at an adjacent or more distal body segment. Center of Mass Center of Mass, COM (also referred to as Center of Gravity, CG) The point where all of the mass of an object (body, body segment) could be considered to be concentrated We can find the COM for the upper arm, the forearm, the hand and fingers, or combinations The location of the COM can vary with joint position or posture For a solid object, the COM is the point where the object would balance The COM does not have to be within the boundaries of the body. Bach & Waugh,

9 Center of gravity of upper torso in sitting How would the position of person s arms affect location of COM? Center of Mass and Base of Support A body s center of mass must be over its base of support in order to be balanced. A wider base of support provides more stability because the center of mass can move and still be within the base of support. Similarly, the lower a body s center of mass, the more stable the body is. Base of Support The footprint over which an object is stable Stand with your feet together and sway side to side, you will quickly lose your balance Stand with your feet wide apart and sway, you ll be much more stable. COM COM Base of Support Base of Support Bach & Waugh,

10 COM vs. Base of Support As long as the COM is above the base of support, the object will be stable. Stable Stable Unstable Unstable Stable Interaction of Gravity, COM, Base of Support Our bodies are not symmetric (A-P), so gravity tends to lead to movements that must be overcome When we are sitting in an upright posture, the COM of our head is anterior to the cervical spinal column. As a result, gravity produces a spinal flexion moment. To remain upright we need to contract the cervical spine extensors to create an equal extensor moment. This gets tiring. Reclining changes the line of action of the gravitational force thereby reducing or eliminating the flexion moment. If a net extension moment is created then it can be resisted by the headrest. Interaction of Gravity, COM, Base of Support Similar images as previous slide but for thoracic spine Bach & Waugh,

11 Balance and Postural Collapse in order to minimize muscular contraction and facilitate relaxation in the sitting position, the line of gravity needs to remain close to the joints which provide a biomechanically stable posture (Nwaobi, 1984) Where does the gravitational line fall in different postures? COM Relative to Joint Axis The human body can gain stability from internal or external structures Internal can be passive (bones, ligaments, capsules, etc.) or active (muscles) With the knees fully extended, the line of action of the COM is anterior to the knee axis, resulting in an extension moment. This extension is restricted by ligaments and the joint capsule. With the knees flexed even slightly, the line of action of the COM is posterior to the knee axis, resulting in a flexion moment. This flexion can only be restricted by muscular contraction. How Do We Gain Stability? The body seeks stability. Stability through muscles may not be an option, or if it is it will be fatiguing Stability through passive soft tissues (hanging on ligaments or other internal structures at end range) External support (why we are here today). Bach & Waugh,

12 How Do We Gain Stability? Axis of Rotation Axis through hip joint Axis through ischial tuberosities Biomechanics for Seating I. REVIEW OF BASIC PRINCIPLES II. SEATING APPLICATION Bach & Waugh,

13 Seating Application A. General Application Of Biomechanics Principles B. Guidelines For Addressing Difficult Postural Problems C. Posterior Pelvic Tilt and Sliding D. Forward Head Posture General Application of Biomechanics Principles Biomechanics principles come into play in these primary areas: Analyzing and predicting patterns of posture and movement Understanding Postural Collapse vs. Balance Blocking Movement as an intervention Dynamic Seating Systems and Restorative Forces Blocking Movement or Providing a Restoring Force Why? Why Block Movement? General seating strategy is to achieve balance between stability and mobility Allow and facilitate active functional movement, while providing proximal stability to support and optimize that movement May require blocking undesired movement, or stabilizing certain segments, in order to achieve more functional movement elsewhere Preventing postural deviations which interfere with health, safety, comfort or function may also require blocking some movements either active or passive movement Bach & Waugh,

14 General Application of Biomechanics Principles Blocking Movement: Equal and Opposite Forces Blocking Movement: Mechanical Advantage Blocking Movement: 3-point Control Balance vs. Postural Collapse Blocking Movement: Equal and Opposite Forces In order to most effectively block a movement, the seating support surface, or pad, should be perpendicular to the direction of movement - the opposing forces are then in opposite directions. Blocking Movement: Equal and Opposite Forces Pad is perpendicular to direction of movement, so movement is blocked. Pad is not perpendicular to direction of movement, therefore only the horizontal component is blocked, leaving the vertical component unopposed. Bach & Waugh,

15 Blocking Movement: Mechanical Advantage Apply the concept of moment arms to get the most control of rotational movements with the least pressure at pad The farther the pad is placed away from the center of rotation, the smaller the counterforce required to hold the limb, which means less pressure will be felt at the pad. Example: Blocking hip adduction pattern with medial knee support The more distal the support (farther away from hip joint which is the center of rotation), the less pressure will be felt at the pad Blocking Movement: 3 Point Control The body will move where it is mobile Importance of mat exam Blocking a movement may result in a different movement due to multiple degrees of freedom Example: Strong elbow flexion pattern If you block distally at wrist, what might happen? This leads to the concept of 3 Point Control 3 Points of Control Imagine a curved bar that we want to straighten One point of loading will just move the bar (translation) Two points of loading won t help us.bar may rotate Three points of loading allow us to correct the curve Bach & Waugh,

16 3 Points of Control To stabilize or prevent movement at a joint, both of the body segments articulating at the joint may need to be controlled. This requires at least three support pads to be applied to the body: Two pads immobilize the segments on one side, and the other pad resists movement of the center of rotation (the joint) in the opposite direction. 3 Points of Control: Blocking active hip abduction Top view of pelvis and femurs Direction of movement Active strong abduction movement Place lateral knee pad distally, perpendicular to direction of movement If hip abduction force is strong enough to overcome friction on seat, other segment will move Which direction will pelvis move? To totally immobilize, 3 points of control are needed. Illustration inspired by Cooper, DG: Biomechanics of selected postural control measures. In Proceedings, Seventh International Seating Symposium, 1991 Balance vs. Postural Collapse in order to minimize muscular contraction and facilitate relaxation in the sitting position, the line of gravity needs to remain close to the joints which provide a biomechanically stable posture (Nwaobi, 1984) Analyze the effects of gravity on the person s posture as well as movement. Use tilt, seating angles and external support surfaces NOT ONLY to change a person s orientation in space, but also to change the body s posture, so as to achieve maximum balance with minimum effort by the individual. Bach & Waugh,

17 Changing orientation in space affects posture, movement and weight distribution In seating, you can change the effect of gravity by altering the body s orientation in relation to it. The more perpendicular a body segment is to the downward line of gravity, the greater percentage of the weight it will bear Seating Application A. General Application Of Biomechanics Principles B. Guidelines For Addressing Difficult Postural Problems C. Posterior Pelvic Tilt and Sliding D. Forward Head Posture Guidelines for Addressing Difficult Postural Problems 1. Always ask WHY Remember to address the source of the problem, not just the symptom What is the cause of the postural deviation? 2. Always assess flexibility Correct the flexible components within comfort, accommodate fixed components. The body core usually takes priority for orientation when there are fixed components, to maximize function 52 Bach & Waugh,

18 Guidelines for Addressing Difficult Postural Problems 3. Simulate the basic seating equipment parameters first Determine 3 primary relative seating angles and basic dimensions, based on mat exam, then simulate. Many postural problems can be solved, or lessened just by providing the correct angles and dimensions. 53 Guidelines for Addressing Difficult Postural Problems 4. Differentiate between active movement and postural collapse If postural collapse: balance forces to decrease gravity s mechanical advantage If active movement: what is cause, or what facilitates the movement? 5. Be least restrictive - you must have a rationale for blocking movement The client or caregiver has the final say 54 Guidelines for Addressing Difficult Postural Problems 6. Set very specific postural objectives What is this person s most optimal aligned position for health, comfort, and function? Each body area Especially critical if planning to do a custom contoured seating system. 55 Bach & Waugh,

19 In general, you will be dealing with one of three conditions: 1. Posture is totally flexible Goal is neutral resting alignment, control of undesired movement patterns, and support of functional movement and health 2. Posture is partially fixed, with flexibility Goal is to maintain optimal alignment, by accommodating fixed component, and then blocking the abnormal movement or postural collapse causing the increase in deviation 3. Posture is totally fixed, with no flexibility Support the body area in the most appropriate orientation for function, comfort and health 56 Seating Application A. General Application Of Biomechanics Principles B. Guidelines For Addressing Difficult Postural Problems C. Posterior Pelvic Tilt and Sliding D. Forward Head Posture Steps towards intervention with complex postural deviations 1. Postural Analysis 2. Determine Cause of Postural Deviation - for that person 3. Set Objectives 4. Intervention Strategies based on cause and objectives 58 Bach & Waugh,

20 Steps towards intervention with complex postural deviations 1. Postural Analysis 2. Determine Cause of Postural Deviation - for that person 3. Set Objectives 4. Intervention Strategies based on cause and objectives 59 Posterior Pelvic Tilt Postural Analysis A. Pelvic Tilt Movement Rotational B. Pelvic Tilt Movement Translational C. Hip Extensor Thrust Movement D. Pelvic Tilt Movement Summary 60 Rotational Pelvic Tilt Movement - Description Center of Rotation for a freely rotating pelvis in sagittal plane is at the hip joint, or acetabulum 61 Bach & Waugh,

21 Rotational Pelvic Tilt Movement Direction of Movement The direction of movement of the different areas of the pelvis depends on its starting position. Fig. 1: Slight anterior tilt start position Fig 2: Posterior tilt start position How does this impact seating intervention? Fig.3: Severe PPT start position 62 Posterior Pelvic Tilt Postural Analysis A. Pelvic Tilt Movement Rotational B. Pelvic Tilt Movement Translational C. Hip Extensor Thrust Movement D. Pelvic Tilt Movement Summary 63 Pelvic Tilt Movement Translational Center of rotation moves Once you block one component of a movement, the movement often changes. Posterior Pelvic Tilt movement changes from being purely rotational, to a combination of rotational and translational movement, and the individual slides forward on the seat 64 Bach & Waugh,

22 Posterior Pelvic Tilt Postural Analysis A. Pelvic Tilt Movement Rotational B. Pelvic Tilt Movement Translational C. Hip Extensor Thrust Movement D. Pelvic Tilt Movement Summary 65 Hip Extensor Thrust Movement 66 Posterior Pelvic Tilt Postural Analysis A. Pelvic Tilt Movement Rotational B. Pelvic Tilt Movement Translational C. Hip Extensor Thrust Movement D. Pelvic Tilt Movement Summary 67 Bach & Waugh,

23 Summary of Pelvic Tilt Movement Possibilities The arrows represent directions of movement.all of which may result in a posterior pelvic tilt posture. You need to determine which movement predominates for successful intervention. 68 Posterior Pelvic Tilt Determine Cause Limitations in range of motion of spine, hips or knees Inaccurate seating dimensions or relative angles Seat and/or back support too soft allowing collapse into posterior tilt Body searching for stability Abnormal movement pattern with spasticity Set Adaptation Objectives based to functional the cause need of the posterior tilt, and overall goals of client 69 Posterior Pelvic Tilt Intervention Strategies A. Accommodate Range of Motion Limitations in Hips, Knees and Spine B. Adjust/Correct Primary Seating Dimensions C. Block Undesired Active Movements or Postural Collapse D. Manage severe extensor tone and spasticity E. Adapt seating environment to accommodate functional needs as necessary 70 Bach & Waugh,

24 Block Undesired Movement or Collapse Possible Directions of Movement Resulting in a Posterior Pelvic Tilt 71 Block Undesired Movement or Collapse Possible Blocking Forces Required What type of seating interventions do these arrows represent? 72 Seating Application A. General Application Of Biomechanics Principles B. Guidelines For Addressing Difficult Postural Problems C. Posterior Pelvic Tilt and Sliding D. Forward Head Posture Bach & Waugh,

25 The Forward Head Posture Direction of Movement Normal posture sagittal plane Forward head posture The Forward Head Posture Direction of Movement Where is the Center of Mass of the head? Where is the action line of gravity falling relative to the centers of rotation? Upper cervical extension centers of rotation Lower cervical flexion Normal posture sagittal plane Forward head posture The Forward Head Posture Blocking Forces Required Bach & Waugh,

26 The Forward Head Posture Intervention Strategies Use orientation in space to decrease gravitational moment encouraging collapse Gravitational bending moment is encouraging lower cervical flexion Provide support under occiput Use anterior chest or shoulder restraint to limit forward excursion The Forward Head Posture Intervention Strategies Use orientation in space to decrease gravitational moment encouraging collapse Seating Application Pelvic obliquity/scoliosis posture Bach & Waugh,

27 Blocking active movement or postural collapse into pelvic obliquity REMEMBER: You must first analyze the direction of active movement or postural collapse Is spine collapsing laterally at convexity? Is pelvis shifting laterally? To control pelvic obliquity movement or collapse you may need to: Block elevation of one side of the pelvis with an inferiorly directed force Block, or limit, depression of the other side with a superiorly directed force. Block shifting of pelvis to side with medially directed force Using 3 point control to block active movement or postural collapse into increasing scoliotic posture Also, remember to use recline or tilt when appropriate to decrease the magnitude of the gravitational moment tending to cause lateralinferior collapse These vertical lines represent the distance from application of pad counterforce to axis of rotation = lever arm Axis of rotation Bach & Waugh,

28 Determining the desired orientation in frontal plane when obliquity/scoliosis is fixed Level the pelvis or level the shoulders? Position 1 Position 2 Position 3 Think about the person s: Tolerance of pressure Potential for further collapse Functional goals Download our slides from and Its Application.pdf Bach & Waugh,