Kinesiology 3rd Lecture - Force Analysis PDF

Kinesiology 3 rd lecture Force Analysis Dr. Ahmed Aboulfotouh  Force analysis Forces can be analyzed in different manners depending on the context of the analysis. Several forces can be combined into a single resultant force, represented by a single vector. Adding...

Kinesiology 3 rd lecture Force Analysis Dr. Ahmed Aboulfotouh  Force analysis Forces can be analyzed in different manners depending on the context of the analysis. Several forces can be combined into a single resultant force, represented by a single vector. Adding forces together uses processes called vector composition. Alternatively, a single force may be resolved or “decomposed” into two or more forces, the combination of which has the exact effect of the original force. This process of decomposing a single force into its components is termed vector resolution.  Methods of Force Analysis Composition and resolution of forces can be accomplished using: - Graphic methods of analysis, - Mathematic methods including the simple addition and subtraction of vectors. - Right angle trigonometry.  Forces composition The process of composition of forces is used to determine whether a net unbalanced force (or forces) exists on a segment, because this will determine whether the segment is at rest or in motion. Furthermore, the direction/orientation and location of the net unbalanced force or forces determine the type and direction of motion of the segment. The process of composition depends on the relationship of the forces to each other: that is, whether the forces are in a linear, concurrent, or parallel force system.  Composition of Forces Two or more forces are simply combined as a single resultant. force  Resolution of Forces The rectangular components of the force are shown at right angles to each other and are referred to as the X and Y components. The X component is set to be parallel to the long axis of the segment. (non rotatory component) the Y component represents the component of force that acts perpendicularly to the long axis of the segment. (rotatory component)  X- Y+ Y- X+ If the force is vertical or horizontal as the Y & X coordinate system , there is no need for force resolution  Resolution of Forces f f f fy fx  2 1 3  3 Fx Fy Fy Fy F Fx 1 3  Muscle force resolution In case of muscle force resolution, the Y component represents the component of the muscle force that acts perpendicularly to the long axis of the segment. (rotatory component). While the x component of muscle represents the component of the muscle force that acts parallel to a bony segment. Because of the internal moment arm associated with this Y component, one effect of MY is to cause a rotation (i.e., produce a torque).  Fy Fx  Y Muscle Force Component Acts perpendicular to a bony segment. Often indicated as MY, depending on the choice of the reference system. Can cause translation of the bone and/or torque if moment arm >0. In a simple hinge joint model, MY creates a shear force between the articulating surfaces. (In reality, MY can create shear, compressive, and distractive forces depending on the anatomic complexity of the joint surfaces.)  X Muscle Force Component Acts parallel to a bony segment. Often indicated as MX, depending on the choice of the reference system. Can cause translation of the bone. Often does not cause a torque because the chosen reference system reduces the moment arm to zero. In a simple hinge joint model, MX creates a compression or distraction force between the articulating surfaces. How to calculate X &Y components? Cosine θ= Fx/F Sine θ= Fy/F Fx= (F) cosine θ Fy= (F) sine θ  Musculoskeletal Torques Forces exerted on the body can have two outcomes. First, forces can potentially translate a body segment. Second, the forces, if applied at some distance perpendicular to the axis of rotation, can also produce a potential rotation of the joint. The perpendicular distance between the axis of rotation of the joint and the force is called a moment arm. The product of a force and its moment arm produces a torque or a moment. Torque of the muscle = muscle force x moment arm  Types of torque The torque may be : Internal Torque : It is the torque produced by the muscle force. or External Torque : It is the torque produced by the external force.  Methods for determining internal torque Internal torque is the torque produced by the muscle force. Two methods for determining internal torque : Method 1 calculates the internal torque as the product of MY and its internal moment arm (IMAMY ).  Methods for determining internal torque, Method 2 uses the entire muscle force (M) and therefore does not require this variable to be resolved into its rectangular components. In this method, internal torque is calculated as the product of the muscle force (the whole force, not a component) and IMAM (i.e., the internal moment arm that extends perpendicularly between the axis of rotation and the line of action of M). Methods 1 and 2 yield the same internal torque because they both satisfy the definition of a torque (i.e., the product of a force and its associated moment arm). The associated force and moment arm for any given torque must intersect each other at a 90-degree angle. Manually applying external torques during exercise and strength testing Assignment  The previous figure showing a change in knee joint angle affects the magnitude of the components of the external forces generated by the leg- and-foot segment weight (S) and exercise weight (W) , print this figure and complete the following : 1- determine the x & y components on the diagram for the S and W forces at each of the three angles of knee flexion. 2- If the S = 43 N and W = 67 N , calculate the X and Y components of S and W forces at each angle of knee flexion.