Muscle Mechanics and Arthrokinematics

– the movement within the joints that cause movement

– a specific joint motion occurring at a joint

– a rigid bar around which a movement occurs around an axis

– achieved by using levers and increases the bodies biomechanical advantage to perform work

• 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

– distance that the force and resistance are applied from the fulcrum of a given joint structure

- the distance of the force to the fulcrum

– the distance of the resistance to the axis

• - The length of the force arm/The length of the resistance arm
• MA = FA/ RA

Force Arm

Resistance Arm

the joint has MA

the joint does not have MA

• 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

– occurs when forces are exerted on opposite sides of each other with the axis or fulcrum in the center

• 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

– occurs when a large amount of weight is supported or moved by a smaller source, this is considered a force magnifier

• 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

• – 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

– to produce equilibrium between the force arm and resistance arm by increasing the number of rotation axes

• 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

• 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

• 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

• 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 _____________________

• 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)

– forces acting on the body that produce stability or mobility

• Gravity-constant force
• Wind
• Water
• Other people
• Objects

- is the ability of multiple joints to move together through a full ROM

– when the bony segment that is attached to the insertion of a muscle moves freely in space

– proximal bone is fixed in space and the origin moves towards the insertion

The point of the body at which the entire weight of the body is concentrated. It is where the vertical and horizontal planes meet.

– act on the body but arise from within

• Muscles
• Tendons
• Ligaments
• Bones

• Bony architecture
• Ligament support
• Tendon and muscle tension

A resistive force to smooth movements

– occur when two or more parallel forces act on the same object but at some distance from each other

• 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

• 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

• 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

• - 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

• – 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

• 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