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Arthrokinematics
– the movement within the joints that cause movement
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Osteokinematics
– a specific joint motion occurring at a joint
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Lever
– a rigid bar around which a movement occurs around an axis
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Leverage
– achieved by using levers and increases the bodies biomechanical advantage to perform work
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3 Components of a lever
- Fulcrum – a point around which movements in the body occur
- Force (F) – what moves the lever around a fulcrum
- Resistance (R) applied to the lever or what the force is moving against
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Mechanical advantage is
– distance that the force and resistance are applied from the fulcrum of a given joint structure
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Force arm
- the distance of the force to the fulcrum
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Resistance arm
– the distance of the resistance to the axis
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MA =
- - The length of the force arm/The length of the resistance arm
- MA = FA/ RA
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If FA > RA,
the joint has MA
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If FA < RA,
the joint does not have MA
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1st class levers (facts)
- The fulcrum lies between the force and the resistance
- This is present when a segment of the body requires balance for optimal performance
- i.e.: teeter totters, scissors, doorknob, and steering wheel
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First Class levers
– occurs when forces are exerted on opposite sides of each other with the axis or fulcrum in the center
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2nd class levers (facts)
- Used when the body moves large amounts of weight by smaller amounts of force
- The resistance lies closer to the fulcrum than the force
- Least common in the body
- i.e.: wheelbarrow, nut cracker
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Second Class levers
– occurs when a large amount of weight is supported or moved by a smaller source, this is considered a force magnifier
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3rd class levers (facts)
- The most common lever in the body
- The force is applied closer to the fulcrum than to the resistance
- The mechanical advantage is opposite that of the second class
- Smaller amounts or resistance are moved by larger amounts of force
- Allows the body to move an object for a greater amount of time over a great distance
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Third Class levers
- – the effort force is central with the resistance force and the axis on either side
- Most commonly seen in the body
- Has the capacity to move small weights long distances
- Are considered force reducers
- i.e.: broom, fishing pole, tweezers, chop sticks
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Purpose of a pulley
– to produce equilibrium between the force arm and resistance arm by increasing the number of rotation axes
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Length tension relationship
- Relates to the muscle and its ability to produce maximum contraction
- Based on the resting length and the number of cross links formed in the sarcomeres between actin and myosin
- The optimal ________________________ is when the muscle is slightly stretched and the actin and myosin filaments slightly overlap
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Mechanical advantage of a muscle
- Need to understand the muscle and the number of joints it crosses
- Muscles that cross only one joint have a greater excursion than ones that cross multiple joints
- If a muscle crosses more than one joint, when it contracts it has to act on each joint in that segment, thus its force is spread out over those joints
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Active insufficiency
- When the actin myofilaments join together in the center of the sarcomere the contraction stops and the muscle is in a state of ____________________
- It is unable to shorten any further, force stops because of the termination of the contractile excursion
- Therefore if a muscle crosses 2 joints, it can only effectively work on one of them at a time
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Passive insufficiency
- Is related to the full stretch of the antagonist muscle when the agonist is in a state of active insufficiency
- Relates to the point where the antagonist can no longer be stretched
- i.e.: when the biceps (agonist) contracts to bring your hand to your mouth, your triceps (antagonist) is in a state of _____________________
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Force–velocity relationship
- Force generated in a muscle contraction = velocity of a muscle contraction
- If velocity is slowed, more cross bridges can be formed thus force increases
- Eccentric contractions can generate more tension (force)
- With concentric contractions, less tension occurs (force)
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Kinetics
– forces acting on the body that produce stability or mobility
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External Forces that produce stability or mobility
- Gravity-constant force
- Wind
- Water
- Other people
- Objects
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Kinetic chains
- is the ability of multiple joints to move together through a full ROM
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Open kinetic chains
– when the bony segment that is attached to the insertion of a muscle moves freely in space
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Closed kinetic chains
– proximal bone is fixed in space and the origin moves towards the insertion
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Center of Gravity
The point of the body at which the entire weight of the body is concentrated. It is where the vertical and horizontal planes meet.
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Internal Forces
– act on the body but arise from within
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Internal Forces are the result of:
- Muscles
- Tendons
- Ligaments
- Bones
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Stability of a joint is dependent on:
- Bony architecture
- Ligament support
- Tendon and muscle tension
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Friction
A resistive force to smooth movements
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Parallel Force Systems
– occur when two or more parallel forces act on the same object but at some distance from each other
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Factors affecting joint stability
The stability of a joint is a measure of:
- How difficult it is to cause disruption from its desired position or alignment, another way to describe this is a joint’s resistance to displacement
- The function of the joints is obviously to provide the bones with a means of moving or being moved
- But because such provisions bring with them a threat of instability, the joints have a secondary function for providing stability without interfering with the desired motions
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Joint stability
- All the joints of the body do not have the same degree of strength or stability
- The strength or degree of freedom follows Emerson’s law: “For everything that is given, something is taken.”
- In the shoulder, movement is gained at the expense of stability
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Stability of a joint
- Is dependent on the structural support components and the convexity and concavity of a specific joint
- There is a position of each joint in which stability is increased due to the ligament tension and the most bone on bone contact
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Closed Packed Position
- - when the articulating surfaces have maximum contact
- Occurs when there is muscle contraction around the joint causing the articulations to move closer together
- Ligaments & capsules are tight
- Difficult to distract these joints or allow more movement in this position
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Open Packed Position
- – when the joint surfaces do not meet perfectly
- Ligaments and capsules are loose
- More motion is possible in this position
- There is no muscle contraction around the joint to place the bony articulations close together
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Example of a Closed Packed Position
- Intrinsic Plus – MP joints of the digits in maximal flexion, IP joints in full extension
- This position keeps ligaments in a lengthened position during splinting or casting, this protecting the ability to move the joints when it is safe to do so
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