Muscle forces and levers Flashcards
What is a motor unit
A motor unit is made up of a motor neuron and the skeletal muscle fibers innervated by that motor neuron’s axonal terminals
Summarise the sliding filament theory
Muscle shortens or lengthens because the tick and thin filaments slide past each other without actually changing length
Each myosin molecule is shaped like golf clubs twisted together
Cross bridges point towards the thin filament in a spiralling arrangement - these are essential for muscle contraction
Troponin
binds to calcium, releasing tropomyosin from actin allowing myosin binding
Changes with sarcomeres with contraction
decreased I band region = z bands are pulled toward the centre of each sarcomere
h zone can disappear
Explain the sliding filament conditions during rest
cross bridges lie close to myosin
binding sites on actin closed
Explain the sliding filament conditions during excitation
nerve stimulates the muscle
action potential spreads to the T tubules causing calcium ions to be release from th sarcoplasmic reticulum
It combines with troponin molecules on actin
Hills mechanical model of muscle
Contractile components. E.g actin and myosin
Elastic components - generate passive tension lengthening,
Length tension relationship
Muscle force depends on the length of the muscle - highest force generated at 100-120% resting length
Angle of pull
Angle between muscle insertion and bone on which it inserts
Spurt muscle
Insertion is closer to the joint than the origin
Muscle force mainly acts to rotate the bone
E.g biceps brachii in elbow flexion
Shunt muscles
Origin is closer to the joint than the insertion
muscle force is directed along bone
2 joint muscles
Many skills involve movement at both joints; useful to have a muscle that contributes to both
E.g spurt muscle at elbow joint, shunt muscle at shoulder
Can 2 joint muscles produce full wage motion at both joints simultaneously?
no
What is the rectus fomoris responsible for?
Hip flexion and knee extension
muscle will act on joint where it has the largest moment arm or furthest away from the joint
Describe the relationship between velocity of muscle contraction and cross bridge
As the velocity of muscle contraction increases, the cross bridge demand increases which causes fewer cross bridges to attach to and decreased force
Describe the force velocity relationship - eccentric actions
Eccentric action created = load by antagonist muscle, gravity o another external load
Levers in the body
Bones=levers
Joints= fulcrums
What is a fulcrum ?
Point around which the lever rotates
What is an effort arm
The part of the lever than the effort force is applied to
What is a resistance arm?
The part of the lever that applies the resistance force
How do you calculate torque?
force x perpendicular distance from joint
What does it mean when the resistance torque is equal to the effort torque
isometric movement is usually taking place
describe the relationship between effort and effort arm and between resistance and resistance arm
Inverse relationship
1st class lever
Fulcrum is between muscle force and the resistance
E F R
A first class lever is designed basically to produce balanced movements when the axis is midway between the force and the resistance.
E.g a seesaw
When the acid is close to the force, the lever produces speed and range of motion
When the axis is close to the resistance, the lever produces force motion
(the force is applied where the muscle inserts in the bone)
2nd class lever
the resistance is between the fulcrum and the muscle force
F A E
Designed to produce force movements since a large resistance can be moved by a relatively small force
3rd class lever
the force is between the fulcrum and the resistance
F E R
Designed to produce speed and range of motion movements
Most levers in the body are this type
How do you calculate mechanical advantage?
Effort arm/ restate arm
What is the mechanical advantage
Efficiency in about of effort needed to overcome a particular resistance
Hill’s mechanical model of muscle
Representation of the muscle mechanical response
- Contractile components e.g actin and myosin generate active tension
- Elastic components generate passive tension
- Parallel elastic components e.g connective tissue store elastic energy when stretched, released when muscle recoils
- Series elastic components e.g tendons transmit tension produced by contractile component to attachment point of muscles
What does muscle force depend on
The length of the muscle - highest force generated at 100 - 120% of resting length
When is active tension reduced?
When muscle activated at elongated lengths (fewer cross bridges formed) and shorter lengths (filaments have exceeded ability to overlap, incomplete activation of cross bridges as fewer formed)
What contributes to total muscle tension?
Contractile and elastic components
How do tendons and other connective tissue around muscle produce a higher muscular force?
Resist lengthening and store elastic energy thus contribute passively
How do muscles create movement?
Generates tension which is transferred via tendon to bone - creates movement
Angle of pull
Angle between muscle insertion and bone on which it inserts (angle of attachment facing away from the joint)
Angle of pull decreases as the bone moves away from the anatomical position
The amount of muscular force eeed to cause joint movement is affected by the angle of pull.
Components of muscle force
Stabilising/dislocating component - acts parallel to the bone, pulling it into or away from joint
Rotary component - acts perpendicular to bone
Force velocity relationship - concentric actions
As shortening velocity increases, cycling rate of cross bridges increases so fewer attached at any one time so decreasing force
Velocity is increased as the expensive of a decrease in force
Max velocity with highest load
What happens as there is an increase in velocity of muscle contraction
Increase cross bridge demand and so fewer cross bridges to attach to and a decrease in force
Force velocity relationship - eccentric
Eccentric action created = load by antagonist muscles, gravity or another external load
Increase velocity of lengthening increases = increase force
Mechanical advantage gained by lever
Lever will either give an increase in force or speed
If effort arm is less than resistance arm, less force applied to move the resistance
Chin ups
Starting a chin up with the elbow in a position where the elbow flexor muscle group is approximately 90 degrees makes it easier because of the more advantageous angle of pull.
The effort arm is greater than the resistance arm so less force applied to move the resistance
Active and passive insufficiency
As a muscle shortens, it’s ability to exert force diminishes. When the muscle becomes shortened to the point at which it can not generate or maintain active tension, active insufficiency is reached.
If the opposing muscle becomes stretched to the point at which it can no longer lengthen and allow movement, passive insufficiency is reached.
Torque
=the turning effect of an eccentric force
The greater the distance of the force arm, the more torque produced by the force
Eccentric force
a force that is applied in a direction not in line with the centre of rotation of an object with a fixed axis
Practical application of increasing torque
purposely increase the force arm length in order to increase the torque so that we can more easily move a relatively large resistance
The resistance arm
the distance between the axis and the point of resistance application
longer the force arm, the less force required to move the lever
Mechanical advantage gained by lever
Lever with give either increase in force or speed
What does it mean if the effort arm is greater than the resistance arm
Less force applied to move the resistance
What does it mean if the effort arm is less than the resistance arm
Small movement moves lever with greater speed and range of motion
