# Muscle Mechanics and Arthrokinematics

 Arthrokinematics – the movement within the joints that cause movement Osteokinematics – a specific joint motion occurring at a joint Lever – a rigid bar around which a movement occurs around an axis Leverage – achieved by using levers and increases the bodies biomechanical advantage to perform work 3 Components of a lever Fulcrum – a point around which movements in the body occurForce (F) – what moves the lever around a fulcrumResistance (R) applied to the lever or what the force is moving against Mechanical advantage is – distance that the force and resistance are applied from the fulcrum of a given joint structure Force arm - the distance of the force to the fulcrum Resistance arm – the distance of the resistance to the axis MA = - The length of the force arm/The length of the resistance armMA = FA/ RA FA = Force Arm RA = Resistance Arm If FA > RA, the joint has MA If FA < RA, the joint does not have MA 1st class levers (facts) The fulcrum lies between the force and the resistanceThis is present when a segment of the body requires balance for optimal performancei.e.: teeter totters, scissors, doorknob, and steering wheel First Class levers – occurs when forces are exerted on opposite sides of each other with the axis or fulcrum in the center 2nd class levers (facts) Used when the body moves large amounts of weight by smaller amounts of forceThe resistance lies closer to the fulcrum than the forceLeast common in the bodyi.e.: wheelbarrow, nut cracker Second Class levers – occurs when a large amount of weight is supported or moved by a smaller source, this is considered a force magnifier 3rd class levers (facts) The most common lever in the bodyThe force is applied closer to the fulcrum than to the resistanceThe mechanical advantage is opposite that of the second classSmaller amounts or resistance are moved by larger amounts of forceAllows the body to move an object for a greater amount of time over a great distance Third Class levers – the effort force is central with the resistance force and the axis on either sideMost commonly seen in the bodyHas the capacity to move small weights long distancesAre considered force reducersi.e.: broom, fishing pole, tweezers, chop sticks Purpose of a pulley – to produce equilibrium between the force arm and resistance arm by increasing the number of rotation axes Length tension relationship Relates to the muscle and its ability to produce maximum contractionBased on the resting length and the number of cross links formed in the sarcomeres between actin and myosinThe optimal ________________________ is when the muscle is slightly stretched and the actin and myosin filaments slightly overlap Mechanical advantage of a muscle Need to understand the muscle and the number of joints it crossesMuscles that cross only one joint have a greater excursion than ones that cross multiple jointsIf 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 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 excursionTherefore if a muscle crosses 2 joints, it can only effectively work on one of them at a time Passive insufficiency Is related to the full stretch of the antagonist muscle when the agonist is in a state of active insufficiencyRelates to the point where the antagonist can no longer be stretchedi.e.: when the biceps (agonist) contracts to bring your hand to your mouth, your triceps (antagonist) is in a state of _____________________ Force–velocity relationship Force generated in a muscle contraction = velocity of a muscle contractionIf velocity is slowed, more cross bridges can be formed thus force increasesEccentric contractions can generate more tension (force)With concentric contractions, less tension occurs (force) Kinetics – forces acting on the body that produce stability or mobility External Forces that produce stability or mobility Gravity-constant forceWindWaterOther peopleObjects Kinetic chains - is the ability of multiple joints to move together through a full ROM Open kinetic chains – when the bony segment that is attached to the insertion of a muscle moves freely in space Closed kinetic chains – proximal bone is fixed in space and the origin moves towards the insertion 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. Internal Forces – act on the body but arise from within Internal Forces are the result of: MusclesTendonsLigamentsBones Stability of a joint is dependent on: Bony architectureLigament supportTendon and muscle tension Friction A resistive force to smooth movements Parallel Force Systems – occur when two or more parallel forces act on the same object but at some distance from each other 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 displacementThe function of the joints is obviously to provide the bones with a means of moving or being movedBut 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 Joint stability All the joints of the body do not have the same degree of strength or stabilityThe 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 Stability of a joint Is dependent on the structural support components and the convexity and concavity of a specific jointThere is a position of each joint in which stability is increased due to the ligament tension and the most bone on bone contact Closed Packed Position - when the articulating surfaces have maximum contactOccurs when there is muscle contraction around the joint causing the articulations to move closer togetherLigaments & capsules are tightDifficult to distract these joints or allow more movement in this position Open Packed Position – when the joint surfaces do not meet perfectlyLigaments and capsules are looseMore motion is possible in this positionThere is no muscle contraction around the joint to place the bony articulations close together Example of a Closed Packed Position Intrinsic Plus – MP joints of the digits in maximal flexion, IP joints in full extensionThis 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 Authorrmina71 ID8832 Card SetMuscle Mechanics and Arthrokinematics DescriptionOTA 130 Chapter 7 - Muscle Mechanics and Arthrokinematics Updated2010-03-03T00:12:55Z Show Answers