Posture Biomechanics 2 Kinetics and Kinematics

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Biomechanics of the posture : 2 kinetics & kinematics :

Biomechanics of the posture : 2 kinetics & kinematics Dr. Dibyendunarayan Bid [PT] The Sarvajanik College of Physiotherapy, Rampura, Surat

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The muscle strategies described in response to perturbations are examples of the active internal forces employed to counteract the external forces that affect the equilibrium and stability of the body in the erect standing posture. The following section examines the effects of both external and internal forces on the body in the standing posture. 7/2/2012 2 dnbid71@gmail.com

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The external forces that will be considered are inertia, gravity, and ground reaction forces (GRFs). The internal forces are produced by muscle activity and passive tension in ligaments, tendons, joint capsules, and other soft tissue structures. 7/2/2012 3 dnbid71@gmail.com

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The sum of all of the external and internal forces and torques acting on the body and its segments must be equal to zero for the body to be in equilibrium. Stability is maintained by keeping the body’s CoM over the BoS and the head in a position that permits gaze to be appropriately oriented. 7/2/2012 4 dnbid71@gmail.com

Inertial and Gravitational Forces:

Inertial and Gravitational Forces In the erect standing posture, little or no acceleration of the body occurs, except that the body undergoes a constant swaying motion called postural sway or sway envelope. The extent of the sway envelope for a normal individual standing with about 4 inches between the feet can be as large as 12 in the sagittal plane and 16 in the frontal plane. 7/2/2012 5 dnbid71@gmail.com

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The inertial forces that may result from this swaying motion usually are not considered in the analysis of forces for static postures. However, in a postural study using laser technology, Aramaki and colleagues investigated angular displacements, angular velocity and acceleration around the hip and ankle joints. 7/2/2012 6 dnbid71@gmail.com

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Naturally, inertial forces must be considered in postural analysis of all dynamic postures such as walking, running, and jogging in which the forces needed to produce acceleration or a change in the direction of motion are important for understanding the demands on the body. 7/2/2012 7 dnbid71@gmail.com

Ground Reaction Forces :

Ground Reaction Forces Whenever the body contacts the ground, the ground pushes back on the body. This force is known as the GRF, and the vector representing it is known as the ground reaction force vector (GRFV). The GRF is a composite (or resultant) force that represents the magnitude and direction of loading applied to one or both feet. The GRF is typically described as having three components: a vertical component force (along the y-axis), and two force components directed horizontally. 7/2/2012 8 dnbid71@gmail.com

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One of the two horizontal forces is in a medial-lateral direction (along the x-axis) , whereas the other horizontal force is in an anterior-posterior direction (along the z-axis) on the ground. The composite or resultant GRFV is equal in magnitude but opposite in direction to the gravitational force in the erect static standing posture. 7/2/2012 9 dnbid71@gmail.com

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The GRFV indicates the magnitude and direction of loading applied to the foot. The point of application of the GRFV is at the body’s CoP, which is located in the foot in unilateral stance and between the feet in bilateral standing postures. If a person were doing a handstand, the CoP would be located between the hands. 7/2/2012 10 dnbid71@gmail.com

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The CoP, like the CoG , is the theoretical point where the force is considered to act, although the body surface that is in contact with the ground may have forces acting over a large portion of its surface area. The path of the CoP that defines the extent of the sway envelope can be determined by plotting the CoP at regular intervals when a person is standing on a force plate system (Fig. 13-5). 7/2/2012 11 dnbid71@gmail.com

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Much of the research on postural control uses the pattern of displacements of the CoP to evaluate the effects of attentional demands and perturbations on standing posture. 7/2/2012 dnbid71@gmail.com 12

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The GRFV and the LoG have coincident action lines in the static erect posture. The LoG represents the force of gravity-on-person and is generally equal in magnitude to and in the same direction as the force of person-on-ground. 7/2/2012 13 dnbid71@gmail.com

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The GRF is a more common name for the force of ground-on-person. In equilibrium during static stance, we would expect the force of gravity-on-person (represented by the LoG) to be equal in magnitude and opposite in direction to the GRF represented by the GRFV. 7/2/2012 dnbid71@gmail.com 14

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In many dynamic postures, the intersection of the LoG with the supporting surface may not coincide with the point of application of the GRFV. The horizontal distance from the point on the supporting surface where the LoG intersects the ground and the CoP (where the GRFV acts) indicates the magnitude of the external moment that must be opposed to maintain a posture and keep the person from falling. 7/2/2012 15 dnbid71@gmail.com

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The technology required to obtain GRFs, the CoP, and muscle activity may not be available to the average evaluator of human function. Therefore, in the following sections, a simplified method of analyzing posture will be presented with the use of diagrams and with the combined action of the LoG and the GRFV as a reference. 7/2/2012 16 dnbid71@gmail.com

Coincident Action Lines:

Coincident Action Lines In an ideal erect posture, body segments are aligned so that the torques and stresses on body segments are minimized and standing can be maintained with a minimal amount of energy expenditure. The coincident action lines formed by the GRFV and the LoG serve as a reference for the analysis of the effects of these forces on body segments (Fig. 13-6). 7/2/2012 17 dnbid71@gmail.com

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When the LoG and the GRFV coincide, as they do in static posture, it is possible to assess the effects at each joint by using one or the other. However, the reader should be aware that the horizontal forces are not being considered separately. We will use the LoG in the remainder of this chapter. The location of the LoG shifts continually (as does the CoP) because of the postural sway. 7/2/2012 18 dnbid71@gmail.com

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As a result of the continuous motion of the LoG, the moments acting around the joints are continually changing. Receptors in and around the joints of lower body segments and on the soles of the feet detect these changes and relay this information to the CNS. 7/2/2012 19 dnbid71@gmail.com

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Sagittal Plane:

Sagittal Plane The effect of external forces on body segments in the sagittal plane during standing is determined by the location of the LoG in relation to the axis of motion of body segments. When the LoG passes directly through a joint axis, no external gravitational torque is created around that joint. 7/2/2012 22 dnbid71@gmail.com

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However, if the LoG passes at a distance from the axis, an external gravitational moment is created. This moment will cause rotation of the superimposed body segments around that joint axis unless it is opposed by a counterbalancing internal moment (an isometric muscle contraction). 7/2/2012 dnbid71@gmail.com 23

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The magnitude of the gravitational moment of force increases as the distance between the LoG and the joint axis increases. The direction of the external gravitational moment of force depends on the location of the LoG in relation to a particular joint axis. If the LoG is located anterior to a particular joint axis, the gravitational moment will tend to cause anterior motion of the proximal segment of the body supported by that joint. 7/2/2012 24 dnbid71@gmail.com

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If the LoG is posterior to the joint axis, the moment will tend to cause motion of the proximal segment in a posterior direction . In a postural analysis, external gravitational torques producing sagittal plane motion of the proximal joint segment are referred to as either flexion or extension moments. 7/2/2012 25 dnbid71@gmail.com

Example 13-2:

Example 13-2 If the LoG passes anterior to the ankle joint axis, the external gravitational moment will tend to rotate the tibia (proximal segment) in an anterior direction (Fig. 13-7). Anterior motion of the tibia on the fixed foot will result in dorsiflexion of the ankle. 7/2/2012 26 dnbid71@gmail.com

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Therefore, the moment of force is called a dorsiflexion moment. An internal plantarflexion moment of equal magnitude will be necessary to oppose the external dorsiflexion moment and establish equilibrium. 7/2/2012 dnbid71@gmail.com 27

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Example 13-3:

Example 13-3 If the LoG passes anterior to the axis of rotation of the knee joint, the gravitational moment will tend to rotate the femur (proximal segment) in an anterior direction (Fig. 13-8). An anterior movement of the femur will cause extension of the knee. 7/2/2012 29 dnbid71@gmail.com

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Therefore, the moment of force is called an extension moment. An internal flexion moment of equal magnitude will be necessary to balance the external extension moment. 7/2/2012 dnbid71@gmail.com 30

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Optimal Posture:

Optimal Posture Because the force of gravity is constantly acting on the body, an ideal standing posture would be one in which the body segments were aligned vertically and the LoG passed through all joint axes. Normal body structure makes such an ideal posture impossible to achieve, but it is possible to attain a posture that is close to the ideal. 7/2/2012 32 dnbid71@gmail.com

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In an optimal standing posture, the LoG is close to, but not through, most joint axes. Therefore, the external gravitational moments are relatively small and can be balanced by internal moments generated by passive capsular and ligamentous tension, passive muscle tension (stiffness), and a small but continuous amount of muscle activity. 7/2/2012 33 dnbid71@gmail.com

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Slight deviations from the optimal posture are to be expected in a normal population because of the many individual variations found in body structure. However, deviations from an optimal standing posture that are large enough to cause excessive strain in passive structures and to require high levels of muscle activity need to be identified, and remedial action must be taken. 7/2/2012 34 dnbid71@gmail.com

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If faulty postures are habitual and assumed continually on a daily basis, the body will not recognize these faulty postures as abnormal, and over time, structural adaptations such as ligamentous and muscle shortening or lengthening will occur. 7/2/2012 35 dnbid71@gmail.com

Analysis of Standing Posture:

Analysis of Standing Posture Observational analysis of posture in the sagittal plane involves locating body segments in relation to the LoG. A plumb line, or line with a weight on one end, dropped from the ceiling and passing through the external auditory meatus of the ear may be used to rep-resent the LoG. Evaluators of posture should be able to determine whether a body segment or joint deviates widely from the normal optimal postural alignment by using their observational skills. 7/2/2012 36 dnbid71@gmail.com

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A skilled observational analysis can yield basic information about an individual’s posture that can be used either for developing a treatment regimen for the correction of poor posture or to decide whether a more sophisticated analysis such as radiography is warranted. 7/2/2012 37 dnbid71@gmail.com

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End of part - 2 7/2/2012 39 dnbid71@gmail.com