Lecture 12: Biomechanics of the MSK System Flashcards
Machine
a device/structure that changes the direction or magnitude of a force
Lever
long rigid segment and a point of rotation (fulcrum)
a lever changes the magnitude and direction of the forces applied to it
- forces applied = segment rotates about the fulcrum and a new force is made
Fulcrum
Point of rotation
Lever in MSK system
Bones are rigid segments
Joints are the fulcrum
Muscle Force Magnitude
Muscle force is determined by:
- amount of activation (from brain via motor nerve)
- length of the muscle (length-tension curve = best @ 2-2.5 microns)
- velocity of the muscle (via velocity-tension curve)
Muscle force direction
direction of pull of the muscle is determined by its ATTACHMENTS + WRAPPING POINT
Direction of pull
From the insertion toward the origin or the next wrapping point
Line of action
Drawn along the muscle vector direction
Determines the effect the muscle has on a jt.
Ex. if line of action is posterior to the knee along its entire length, the muscle is a flexor
Lever arm and torque
Shorter lever arm = less torque
Lever arm changes with position
Point of application
almost always its INSERTION
Determines the LEVER ARM of the muscle
Line of action and torque
If the line of action passes through the axis of rotation, there is NO perpendicular distance = NO lever arm = NO TORQUE
Perpendicular distance
Distance from point of application to axis
Wrapping points
often keep lever arm constant over the jt range
Concentric vs eccentric
Concentric contraction = lifting up a weight
Eccentric = lowering weight
Force and lever arm
Muscles usually have smaller lever arms than external forces = muscles must have larger forcemagnitudes
