HESS 510 Chapter 3 Basic Biomechanical Factors Concepts

HESS 510 Chapter 3 Basic Biomechanical Factors & Concepts PPT Series 3 A

Biomechanics Mechanics - Study of physical actions of forces. These forces may be both Static and Dynamic. Statics - Study of systems that are in a constant state of motion, whether at rest with no motion or moving at a constant velocity without acceleration. • Statics involves all forces acting on the body being in balance, which results in the body being in equilibrium Dynamics - Study of systems in motion with acceleration. • A system in acceleration is unbalanced due to unequal forces acting on the body Biomechanics - Study of the mechanics as it relates to the functional and anatomical analysis of biological systems

Biomechanics Terminology • Kinematics - Description of motion and includes consideration of time, displacement, velocity, acceleration, and space factors of a system's motion • Kinetics - Study of forces associated with the motion of a body • Mechanical advantage – is the advantage of using a machine to transmit the application of force and can be calculated by load (resistance) divided by effort (force). • Ideally, using a relatively small force (or effort) to move a much greater resistance. Smaller force application to move a larger resistance = greater mechanical advantage.

Types of Machines Found in the Body Musculoskeletal system may be thought of as a series of simple machines • Machines - Used to increase mechanical advantage, and have four distinct mechanisms as they can: • balance multiple forces • enhance the force applied to reduce the total force needed to overcome a resistance • enhance range of motion and speed of movement so that resistance may be moved farther or faster than the applied force • alter the resulting direction of the applied force Musculoskeletal system contains three types of machines in producing movement • Levers (most common) • Wheel/axles • Pulleys

Machines: Levers Musculoskeletal movement is facilitated through a system of levers • Lever - A rigid bar that turns about an axis of rotation, or fulcrum • Axis (fulcrum)- Point of rotation about which the lever moves • Levers rotate about the axis(fulcrum) as a result of force (effort, E) being applied to cause its movement against a resistance (sometimes referred to as load or weight) In the Musculoskeletal System • Bones represent the bars • Joints are the axes • Muscles contract to apply force

Machines: Levers Resistance can vary and bones alone or the weight of the body segment may be the only resistance applied All lever systems have three possible arrangements that determine the type and application of the lever. • Axis (A)/fulcrum - The point of rotation • Point (F) of force application (usually muscle insertion) - Effort • Point (R) of resistance application (center of gravity of lever) or location of an external resistance

Machines: Levers • Axis (A)/fulcrum - The point of rotation • Point (F) of force application (usually muscle insertion) Effort • Point (R) of resistance application (center of gravity of lever) or location of an external resistance • First class lever - Axis (A) is somewhere between force (F) and resistance (R) • Second class lever Resistance (R) is somewhere between axis (A) and force (F) • Third class lever Force (F) is somewhere between axis (A) and resistance (R)

Machines: Levers The mechanical advantage of levers may be determined using the following equations: Mechanical advantage (MA) = Resistance Force or Mechanical advantage = Length of force arm (distance from axis to point of force application) Length of resistance arm(distance from axis to point of resistance application) Ideally, using a relatively small force to move a much greater resistance. Smaller force application to move a larger resistance = greater mechanical advantage.

Machines: Levers - First-Class Levers Produce balanced movements when the axis is midway between the force and resistance (Example - Seesaws) • MA is equal to 1 Produce speed and range of motion when the axis is close to the force (Example - Triceps in elbow extension) • MA is lesser than 1

Machines: Levers - First-Class Levers Produce force motion when the axis is close to the resistance (Example Crowbar) • MA is greater than 1 Agonist and antagonist muscle groups contract simultaneously on either side of a joint axis • Agonist produces the force whereas the antagonist supplies the resistance

Machines: Levers - First-Class Levers • Example - Triceps applies the force to olecranon (F) in extending the nonsupported forearm (R) at the elbow (A) Force is applied where muscle inserts in bone, not in the belly of the muscle • Example 1 - In elbow extension with the shoulder fully flexed and the arm beside the ear, the triceps applies the force to the olecranon of the ulna behind the axis of the elbow joint • As the applied force exceeds the amount of forearm resistance, the elbow extends • Example 2 - Placing the hand on the floor, as in performing a push -up to push the body away from the floor • The same muscle action at this joint now changes the lever to a second-class lever because the axis is at the hand the resistance is the body weight at the elbow joint

Machines: Levers - Second-Class Levers Produce force movements, and a large resistance can be moved by a relatively small force MA is greater than 1 • Wheelbarrow • Nutcracker • Raising the body on the toes • Example - Plantar flexion of the foot to raise the body up on the toes where ball (A) of the foot serves as the axis as the ankle plantar flexors apply force to the calcaneus (F) to lift the resistance of the body at the tibiofibular articulation (R) with the foot There are relatively few second class levers in the human body

Machines: Levers - Third-Class Levers Produce speed and range of motion movements and are the most common in the human body MA is less than 1 Require a great deal of force to move even a small resistance • Paddling a boat • Shoveling - Application of lifting force to a shovel handle with the lower hand while the upper hand on the shovel handle serves as axis of rotation

Machines: Levers - Third. Class Levers • Example - Biceps brachii • Using the elbow joint (A) as the axis, the biceps brachii applies force at its insertion on the radial tuberosity (F) to rotate the forearm up, with its center of gravity (R) serving as the point of resistance application • Biceps brachii supinates the forearm (applying the rotational force of a first class lever as in a wheel and axle to the radius) as it flexes, so its third class leverage applies to flexion only • Example - Brachialis - True third class leverage • Pulls on the ulna just below elbow • Pull is direct and true since the ulna cannot rotate

Machines: Levers - Torque (moment of force) - The turning effect of an eccentric force. Eccentric force - Force applied “off center”, or in a direction not aligned with the center of rotation of an object with a fixed axis. In objects without a fixed axis, it is an applied force that is not in line with the object's center of gravity. For rotation to occur, an eccentric force must be applied The amount of torque is determined by multiplying the amount of force (force magnitude) by the force arm Force arm - Perpendicular distance between the location of force application and axis • Axis is known as moment arm or torque arm • Shortest distance from the axis of rotation to the line of action of the force • The greater the distance of force arm, the more torque produced by the force

Machines: Levers - Torque Resistance arm - Distance between the axis and the point of resistance application There is an inverse relationship between length of the two (force, resistance) lever arms • The longer the force arm, the less force required to move the lever if the resistance and resistance arm remain constant • Shortening the resistance arm allows a greater resistance to be moved if force and force arm remain constant • If either of the resistance (resistance, resistance arm)components increases, there must be an increase in one or both of force components (force, force arm).

Machines: Levers - Torque First-class levers • In figure A, if the force arm and resistance arm are equal in length, a force equal to the resistance is required to balance it • In figure B, as the force arm becomes longer, a decreasing amount of force is required to move a relatively larger resistance • In figure C, as the force arm becomes shorter, an increasing amount of force is required to move a relatively smaller resistance, but the speed and range of motion that the resistance can be moved are increased

Machines: Levers - Torque Second-class levers (MA > 1) • In figure A, placing the resistance halfway between the axis and the point of force application provides a MA of 2 • In figure B, moving the resistance closer to the axis increases the MA but decreases the distance that the resistance is moved • In figure C, the closer the resistance is positioned to the point of force application, the less the MA but the greater the distance it is moved

Machines: Levers - Torque Third-class levers (MA < 1) • In figure A, a force greater than the resistance, regardless of the point of force application, is required due to the resistance arm always being longer • In figure B, moving the point of force application closer to the axis increases the range of motion and speed but requires more force • In figure C, moving the point of force application closer to the resistance decreases the force needed but also decreases the speed and range of motion

Machines: Levers - Torque Human leverage system is built for speed and range of motion at the expense of force, however, short force arms and long resistance arms require great muscular strength to produce movement • Examples - Biceps and triceps attachments: (A) Biceps force arm is 1 to 2 inches; and (B)Triceps force arm is less than 1 inch. Human leverage for sport skills requires several levers. Throwing a ball involves levers at the shoulder, elbow and wrist joints

Machines: Levers - Torque The longer the lever, the more effective it is in imparting velocity • A tennis player can hit a tennis ball harder with a straight-arm drive than with a bent elbow because the lever (including the racket) is longer and moves faster • Long levers produce more linear force and thus better performance in some sports such as baseball, hockey, golf, field hockey, etc.

Machines: Wheels and Axles Used primarily to enhance range of motion and speed of movement in the musculoskeletal system and function essentially as a form of a first class lever When either the wheel or axle turns, the other must turn and both complete one turn (rotation) at the same time. • Center of the wheel and the axle both correspond to the fulcrum • Both the radius of the wheel and the radius of the axle correspond to the force arms

Machines: Wheels and Axles If the wheel radius is greater than the radius of the axle, then, due to the longer force arm, the wheel has a mechanical advantage over the axle • A relatively smaller force may be applied to the wheel to move a relatively greater resistance applied to the axle • If the radius of the wheel is five times the radius of the axle, then the wheel has a five to one mechanical advantage over the axle • Mechanical advantage of a wheel and axle can be calculated by considering the radius of the wheel over the axle Mechanical advantage = radius of the wheel radius of the axle

Machines: Wheels and Axles If application of force is reversed and applied to the axle, then the mechanical advantage results from the wheel’s turning a greater distance at greater speed • If the radius of the wheel is five times the radius of the axle, the outside of the wheel will turn at a speed five times that of the axle • The distance that the outside of the wheel turns will be five times that of the outside of the axle • Mechanical advantage for this example can be calculated by considering the radius of the wheel over the axle Mechanical radius of the axle advantage = radius of the wheel

Machines: Wheels and Axles Example - Muscles apply force to the axle to result in greater range of motion and speed, which can be seen in the upper extremity in the case of internal rotators attaching to the humerus • Humerus acts as the axle • Hand wrist are located at the outside of the wheel when the elbow is flexed approximately 90 degrees • With minimal humerus rotation, the hand wrist travel a great distance • Using the wheel and axle allows us to significantly increase the speed at which we can throw objects

Machines: Pulleys A) Single pulleys function to change the direction of the force application • Mechanical advantage is equal to one B) Pulleys may be combined to form compound pulleys to increase the mechanical advantage • Each additional rope increases the mechanical advantage by one Example - Lateral malleolus acting as a pulley around which the tendon of peroneus longus runs • As the peroneus longus contracts, it pulls toward its belly (toward the knee) • Using the lateral malleolus as a pulley, force is transmitted to the plantar aspect of the foot resulting in plantar flexion and eversion of the foot End of PPT Series 3 A