Kinesiology Lever System Lecture Notes PDF

Kinesiology 5th lecture lever system Dr. Ahmed Aboulfotouh  lever system - Lever is a simple machine consisting of a rigid rod suspended across a pivot point. - A lever is any rigid segment that rotates around a fulcrum. - A lever system exists whenever t...

Kinesiology 5th lecture lever system Dr. Ahmed Aboulfotouh  lever system - Lever is a simple machine consisting of a rigid rod suspended across a pivot point. - A lever is any rigid segment that rotates around a fulcrum. - A lever system exists whenever two forces are applied to a lever in a way that produces opposing torques. - One function of a lever is to convert a linear force into a rotary torque. A lever is any rigid segment that rotates around a fulcrum.  Components of lever system - In a lever system, the force that is producing the resultant torque (the force acting in the direction of rotation) is called the Effort force (EF). - Because the other force must be creating an opposing torque, it is known as the resistance force (RF). Another way to think of effort and resistance forces acting on a lever is that the effort force is always the winner in the torque game, and the resistance force is always the loser in producing rotation of the segment. - The moment arm for the effort force is referred to as the effort arm (EA), whereas the moment arm for the resistance force is referred to as the resistance arm (RA).  Musculoskeletal Levers - In order to apply concepts of levers to a bony segment, the following equivalents are considered : 1- Bone is the rigid bar 2- Joint axis is the fulcrum 3- Muscles produce the effort 4- Resistance is represented by the weight of the segment plus any weight attached to the segment 5- Effort arm : is the moment arm of the muscle 6- Resistance arm ; is the moment arm of the resistance.  Classifications of lever According to the position of the fulcrum, effort and resistance the lever is classified into Three classes , First , Second and Third classes.  Three classes of levers - The most dominant forces involved with musculoskeletal levers are those produced by muscle, gravity, and physical contact within the environment. - The pivot point, or fulcrum, is located at the joint. As with the seesaw, the internal and external torques within the musculoskeletal system may be equal, such as during an isometric activation; or, more often, when one of the two opposing torques dominates, resulting in movement at the joint. Levers are classified as either first, second, or third class.  First-Class Lever - A first-class lever is a lever system in which the axis lies somewhere between the point of application of the effort force and the point of application of the resistance force. First-Class Lever  Second-Class Lever Second-class lever is a lever system in which the resistance force has a point of application between the axis and the point of application of the effort force, which always results in the effort arm being larger than the resistance arm. A second-class lever always has two features. First, its axis of rotation is located at one end of a bone. Second, the muscle, or internal force, possesses greater leverage than the external force. Second-Class Lever Second-Class Lever  Third-Class Lever A third-class lever is a lever system in which the effort force has a point of application between the axis and the point of application of the resistance force, which always results in the resistance arm being larger than the effort arm. The Third-class lever is the most common lever used by the musculoskeletal system. Levers in musculoskeletal system B A C - An example of a first-class lever in the human body is the head-and-neck extensor muscles that control the posture of the head in the sagittal plane. Second-class levers are very rare in the musculoskeletal system. The classic example is the calf muscles producing the torque needed to stand on tiptoes. The elbow flexor muscles use a third-class lever to produce the flexion torque required to support a weight in the hand.  Mechanical Advantage The Mechanical advantage (MA) of any machine is a measure of the mechanical efficiency of this machine that means the ability of the machine to magnify force or to increase the out put in relation to the input. in the lever system it is the relative effectiveness of the effort force in comparison with the resistance force. (MA) of a musculoskeletal lever can be defined as the ratio of the internal moment arm to the external moment arm. 𝑒𝑓𝑓𝑜𝑟𝑡 𝑎𝑟𝑚 MA = 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑎𝑟𝑚  Mechanical advantage and class of lever Depending on the location of the axis of rotation, and so according to the class of lever The first-class lever can have an MA equal to, less than, or greater than 1. Second-class levers always have an MA greater than 1. Third-class levers always have an MA less than 1.  Mathematic expression of MA MF IMA = EF EMA (Eq. 1.1) Where: MF = Muscle force EF = External force IMA = Internal moment arm EMA = External moment arm IMA/ EMA = EF/MF (Eq. 1.2)  In some first-class levers, IMA/EMA = 1; the torque equations balanced only when MF = EF. In some first-class and all second-class levers, IMA/EMA > 1; the torque equation is balanced only when MF is less than EF. In some first-class and all third-class levers, IMA/EMA < 1; the torque equation is balanced only when MF is greater than EF. MA can also be expressed by the ratio of external force to muscle force (EF/MF). Although this is correct.  For example, The majority of muscles throughout the musculoskeletal system function with an MA of much less than 1. Consider, for example, the biceps at the elbow, the quadriceps at the knee, and the supraspinatus and deltoid at the shoulder. Each of these muscles attaches to bone relatively close to the joint’s axis of rotation. The external forces that oppose the action of the muscles typically exert their influence considerably distal to the joint, such as at the hand or the foot. Problem solving  Supraspinatus and deltoid - External weight of 35.6 N (8 lb) is held in the hand. - The muscles have an internal moment arm of 2.5 cm - The center of mass of the external weight has an external moment arm of 50 cm (about 20 inches). Can you calculate the force of supraspinatus that is required to balance this position against the load?  Solution MF IMA = EF EMA MF x 2.5 = 35.6 x 50 MF= 712 N MA= IMA/ EMA = 2.5/50 = 0.05 0.05 = 1/20 1/20 MA requires that the muscle would have to produce 712 N (160 lb) of force, or 20 times the weight of the external load!  Trade-Off between Force and Distance - Most muscles are obligated to produce a force much greater than the resistance applied by the external load. - At first thought, this design may appear biomechanically flawed. - The design is absolutely necessary, however, when considering the many functional movements that require large displacement and velocity of the more distal points of the extremities. - Consider the small mechanical advantage of 1/20 described earlier for the supraspinatus and deltoid muscles. - This MA implies that the muscle must produce a force 20 times greater than the weight of the external load.  Trade-Off between Force and Distance - Because work is the product of force and distance, it can be performed through either a relatively large force exerted over a short distance or a small force exerted over a large distance. What must also be considered, however, is that the muscles need to contract only 5% (1/20) the distance that the center of mass of the load would be raised by the abduction action. - A very short contraction distance (excursion) of the muscles produces a much larger vertical displacement of the load. - When considering the element of time in this example, the muscles produce a relatively large force at a relatively slow contraction velocity. The mechanical benefit, however, is that a relatively light external load is lifted at a much faster velocity. Obtaining a high linear velocity of the distal end of the extremities is a necessity for generating large contact forces against the environment. These high forces can be used to rapidly accelerate objects held in the hand, such as a tennis racket, or to accelerate the limbs purely as an expression of art. Regardless of the nature of the movement, muscle-and-joint systems that operate with an MA of less than 1 must pay a force “penalty” by generating relative large internal forces, even for seemingly low load activities.  Trade-Offs of Mechanical Advantage and velocity - However, as the muscle pulls its point of application (on the proximal forearm-hand segment) through a very small arc, the distal portion of the segment is displaced through a much greater arc. - Although the magnitude of force needed to create the rotation is large in comparison with the magnitude of the resistance force, the result is that linear muscle displacement and velocity are small compared to the linear displacement and velocity of the segment’s more distal components. - Because one of the goals of human function is to maximize angular displacement of a distal segment through space while minimizing muscle length and muscle velocity changes, third-class lever systems are very common.  Pulley - Pulley is a simple machine that performs a mechanical job - The function of any pulley is to redirect a force to make a task easier. - Pulleys change the direction without changing the magnitude of the applied force.  Anatomic pulley - Frequently, the fibers of a muscle or a muscle’s tendon wrap around a bone or are deflected by a bony prominence. - When the direction of pull of a muscle is altered, the bone or bony prominence causing the deflection forms an anatomic pulley.  Examples for anatomic pulley - Patella under the quadriceps tendon ( the largest sesamoid bone in the body) - Head of humerus under the deltoid - Lateral malleolus for the tendon of peroneus longus - Pisiform bone for the tendon of flexor carpi ulnaris. - Condyles of interphalangeal joint for long flexor tendons of the hand Head of humerus under the Dletoid muscle in shoulder patella as a pulley for the quadriceps tendon  Effects of anatomic Pulleys on action lines, and moment arms - The function of any pulley is to redirect a force to make a task easier. The “task” in human movement is to rotate a body segment. - Anatomic pulleys (in the majority of instances) make this task easier by deflecting the action line of the muscle farther from the joint axis, thus increasing the moment arm of the muscle force. - By increasing the moment arm of a muscle force, a force of the same magnitude will produce greater torque.  Functions of patella as a pulley for the quadriceps tendon 1- Move the muscle away from the joint 2- Increase the moment arm of muscle 3- Increase the quadriceps torque. 4- Increase the angle of pull of muscle = change the direction of pull 5- increase the mechanical advantage and efficiency of quadriceps ASSIGNMENT Calculate the muscle force and mechanical advantage ASSIGNMENT Calculate the muscle force and mechanical advantage ASSIGNMENT Calculate the muscle force and mechanical advantage