Essential Concepts Part 2

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PTA 150: Functional Anatomy and Kinesiology:

PTA 150: Functional Anatomy and Kinesiology Essential Concepts in Kinesiology Part II

Effect of Forces on the Body:

Effect of Forces on the Body


Kinetics Effect of forces on the body Motion with regard to what causes motion. Force Push or pull that produces, arrests or modifies movement Torque Tendency of a force to produce rotation about an axis Friction Force that prevents motion


Forces Various forces can be applied to joints… Tension or traction or distraction joint surface pulled apart Compression, Approximation Joint surfaces pushed closer together Shear (parallel), glide Torsion (bending, twisting or rotary)

Effect of Forces Within Tissue:

Effect of Forces Within Tissue Healthy tissue resists deformation/changes in shape Unhealthy tissue cannot resist as well Stress Internal resistance within tissue, divided by cross-sectional area of tissue Strain Ratio of deformed length to original length

Properties of Materials/Tissue:

Properties of Materials/Tissue Stiffness Ratio of stress to strain within an elastic material Plasticity Material remains permanently deformed after the removal of a force. Elasticity Ability of a material to return to its original length after the removal of a deforming force. Viscoelasticity Material demonstrates a changing stress-strain relationship over time.

Properties of Materials/Tissue:

Properties of Materials/Tissue All normal tissues have some degree of stiffness “tightness”, abnormal stiffness Elastic energy can be useful Release of energy as tissue returns to normal length Every tissue in the body has a “breaking point” Rate of loading effects the degree of failure or point of failure “Creep” occurs when tissue is exposed to a constant load over time.

Forces Acting on the Body:

Forces Acting on the Body Internal Produced from structures within the body Can be active or passive Muscle (active) Typically the largest Connective tissues/ligaments/capsules (passive) External Produced from outside the body Gravity (body weight, free weights) Physical contact (therapist/manual resistance)

Laws of Motion:

Laws of Motion Newton’s 1 st Law An object at rest stays at rest, and an object in motion stays in motion Law of Inertia Force needed to overcome inertia and cause object to move, stop or change direction. Newton’s 2 nd Law Acceleration of an object depends upon its mass and strength of the force applied

Laws of Motion:

Laws of Motion Newton’s 3 rd Law Law of Action – Reaction For every action, there is an equal, opposite reaction

Calculating Forces:

Calculating Forces Vectors Representative of force A quantity completely specified by its magnitude and direction Represented by an arrow… Magnitude length of arrow Direction/Sense orientation of arrow Point of Application base of vector With muscle force, where muscle inserts to bone. Angle of Application angle formed between tendon of muscle and long axis of bone to which it inserts.

Calculating Forces:

Calculating Forces Angle of insertion changes as joints rotate Location of point of application depends on whether force is result of gravity or resistance Gravity acts on center of mass of body segment Point of application of resistance can occur anywhere on body Joint Reaction Force Push or pull produced by one joint surface against another


Forces Linear force Acting along the same line Parallel forces Same plane and in the same or opposite direction Concurrent forces 2 or more forces act from a common point, but pull in different directions Net effect, resultant force (somewhere in between)

Force Couple:

Force Couple 2 forces act in equal, but opposite directions Result in turning effect


Torque Torque (moment of force) Ability of force to produce rotation about an axis Rotary force Amount of force needed by a muscle to cause rotation of joint Degree of torque depends upon… amount of force distance from the axis Torque = Force x moment arm Moment arm Perpendicular distance from line of action of force to axis of rotation The greater the moment arm, the greater the torque

Muscle Torque:

Muscle Torque Torque greatest when angle of pull is 90 degrees Decreases as angle of pull decreases or increases from 90 0 Remember, moment arm is a perpendicular distance

Joint Motion:

Joint Motion Muscle’s effectiveness in producing movement is dependent upon…. moment arm size of muscle contractile strength of muscle


Stability/Equilibrium When an object is balanced…. Torques even State of equilibrium Stability dependent upon center of gravity (COG) and base of support (BOS) Gravity vertical force, directed to center of earth COG Balance point Torque on all sides equal All planes of body intersect


Stability/Equilibrium COG, just anterior S 2 (adult) Child, higher BOS Point in contact with supporting surface Line of gravity Imaginary line through COG


Stability/Equilibrium 3 States of Equilibrium Stable equilibrium To disturb, must raise COG Patient lying supine Unstable equilibrium Slight force needed to disturb Standing on one leg Neutral equilibrium COG remains the same when disturbed Ball rolling across floor


Stability/Equilibrium General Rules of Stability Lower COG = Increased stability COG must remain within BOS for stability Increased/widened BOS = Increased stability Increased mass = increased stability Increased friction = increased stability Focusing on stable object when moving, increases stability/balance

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