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Overview
Overview
Biomechanics: application of principles and methods of mechanics to the structure and function of the human body to produce stability & movement
Statics: branch of mechanics involving factors associated with non-moving systems
Dynamics: branch of mechanics involving two factors associated with moving systems: kinetics & kinematics
Kinetics: describes forces producing stabilization or movement in a system
Kinematics: describes motion created by forces and incorporates the factors of time, space, and mass of a moving system
Scalar: quantity describing only magnitude
Ex. Speed, length, area, volume, mass
Mass: the amount of matter in an object
Vector: describes magnitude and direction
eg. Force, velocity, acceleration
Magnitude: indicates the amount of a measurement
Direction: the direction in which magnitude is applied
Velocity: rate of change of position and direction of that change, measured by change of distance within a given time
Measured in distance/time (ft/s or miles/hr)
Dependent on musculoskeletal characteristics - limb segment movement & recruited muscles for movement
Speed is scalar, not vector; does not account for direction in addition to mag
Acceleration: rate of change of velocity (magnitude) and direction of that change
Force
Force
Force: amount & direction of push or pull applied to objects or body segments
Push - creates compression as two objects/segments are pushed together
Pull - creates traction as two objects/segments are pulled away from each other
Internal force: includes muscular contraction, ligamentous restraint, or bony support
External force: includes gravity or any externally applied resistance
Gravity: mutual attraction between earth & an object
Gravitation force: force exerted on an object/person as a result of gravity
Weight: result of a gravitation force and mass of an object and always pushes directly downward
Ground reaction: the upward force a supporting surface exerts on an object when a person pushes down on the surface
Friction: force between two surfaces that increases resistance to motion of one surface across another; impediment to motion
Linear forces: exist when two+ forces act along the same line
Ex. Two people pulling a boat with the same rope in the same direction
Parallel forces: occur in the same plane and in the same/opposite direction
Force couple: a specific configuration of parallel forces, occurring when 2+ forces act in different directions, producing either clockwise or counterclockwise rotation
Concurrent forces: occur when 2+ forces act on an object, but push or pull in different directions
Resultant force vector: represents the sum of the magnitudes and directions of each individual force vector, indicating the magnitude & direction of movement resulting from the application of all forces acting on the object
Ex. Interaction between deltoid muscle portions to abduct the shoulder
All share the distal attachment to the humerus but pull in different directions due to different proximal attachments
Force Application
Force Application
Acts on Tissues & Joints
Traction: forces cause joint distraction, in which joint surfaces pull apart from one another, placing tension on tissue that holds the joints together
Ex. Hanging from overhead bar
Compression: forces that cause joint approximation in which joint surfaces are pushed closer together
Ex. Push ups - shoulder, elbow and wrist joints are approximated
Prolonged compression = damage
Shear: forces causing a gliding motion in which joint surfaces move parallel to one another
Ex. Knee extension - femur rotates on the tibia = shear
Excessive forces = tissue strain/rupture
Like scissors that when open, move in different directions
Bending: occurs when a force is not applied at the central axis of an elongated object and the object bends, creating a concave surface on one side and convex surface on the opposite side
Compression at concave side and traction on the convex
Ex. Spinal flexion --> bending at vertebrae --> decrease in height
Torsional: where two opposing forces creating twisting within an object
Can lead to spiral fractures
Torsion = Twist
Newton's Laws
Newton's Laws
Law of Inertia (Law 1): states that an object either stays at rest or remains in motion in a constant state, unless acted upon by an external force
Law of Acceleration (Law 2): defines the relationship between force (F), mass (m), and acceleration (a)
(a) of an object is inversely related (as one increase, the other decreases) to the (m) of the object
(a) is directly proportional to the (F) applied to the object
F=m*a
Force is necessary to overcome the inertia of an object
The greater the mass, the more force is needed to act on the object
The same force applied to objects of differing mass - object with greater mass will accelerate less than the object with less mass
Law of Action-Reaction (Law 3): states that for every action, there is an equal and opposite reaction
The magnitude of the reaction is always equal to the magnitude of the action and occurs in the opposite direction
Ex. Jumping on trampoline - action = force applied to the tramp by the person landing; reaction = force the tramp exerts back on the person --> rebound and person being pushed back in the air
When the person lands on the tramp with a greater force, the rebound is greater because the reaction force is greater
Equilibrium & Stability
Equilibrium & Stability
Equilibrium: state exists when the sum of all forces acting on an object is equal to zero
Center of mass (COM): the point at which the sum of the mass of all body segments is located
Depends on the position or configuration of the different body parts
Center of gravity (COG): point at which gravity acts on the COM
Base of support (BOS): area encompassed by a body's contact with a supporting surface
Line of gravity (LOG): imaginary vertical line passed through the COG toward the center of earth
Degree of stability is determined by the interaction of BOS, COG, and mass of object/forces applied
Since the COG is within the BOS, no forces act to destabilize the body
Types of Motion
Types of Motion
Linear motion (translatory motion): occurs when all parts of an object move the same distance, in the same direction at the same time
Curvilinear motion: movement that occurs in a curved path that is not circular
Angular motion (rotary motion): movement of an object around a fixed point (axis)
All parts of the object move through the same angle in the same direction at the same time but not move the same distance
Accounts for most body movements
Movement outside the body tends to be linear
Walking is an example of simultaneous combined linear and angular motion - whole body exhibits linear motion when walking from point A to point B while hips, knees, and ankles exhibit angular motion
Levers
Levers
First-class lever: the axis is between the force and resistance = configuration: F > A > R or R> A> F
The same amount of work is required regardless of placement of a lever's axis --> when axis is closer to the resistance than to the force, RA is shorter and FA is longer
Ex. Skull on the first cervical vertebra
Axis of motion is the articulation between the skull and vertebra
The force is the anterior/posterior musculature
Resistance is the weight of the head positioned posterior/anterior, respectively, to the axis
Second-class lever: resistance is between the axis & force; configuration = A > R> F or F> R> A
Mechanical advantage is achieved by a FA that is longer than the RA, reducing the force required to lift a load
Ex. Rising on tiptoes
Axis = metatarsophalangeal (MTP) joints in the foot
Resistance = body weight pushing downward
Force = generated by contraction of ankle plantar flexor muscles
RA is only slightly shorter than the FA
Advantage - heavy loads can be lifted by a lesser force
Disadvantage - body is raised only a short distance
Third-class lever: force is between the resistance and axis; configuration: A > F > R or R> F> A
Mech. Adv. Is achieved by RA is longer than FA, increasing the distance a load is moved, requiring a greater force to move that load
Ex, moving one end of a boat away from a dock
Axis - front of the boat tied to the dock
Force - person pushing the boat
Resistance - water as the boat is pushed away from the dock
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