3. Basic concepts of MSK - Muscles Flashcards
Why do muscles produce force?
Provide stability - eg posture
Propel body segments - Propulsion is achieved through the controlled movements of bones at joints
3 Key principles of muscle contraction
- Muscles can only PULL (CANNOT PUSH) + act on joints they (or their tendons cross) —-> antagonistic pairs
- The action of a muscle on a joint is a function of the orientation of its fibres and the relation of those fibres to the joint
- The action of a muscle is a function of the starting position of the joint.
What is the origin of a muscle?
The origin is the stationary anchor point and is usually sited proximally in the limb
What is the insertion of a muscle?
The insertion is the mobile attachment point and is usually sited distally in the limb.
How is muscle contraction symmetrical?
An equal force is exerted on the origin and the insertion
It is the stabilisation of the origin (e.g. by contraction of other muscles) that leads to the insertion becoming the only mobile attachment point
How can the origin and insertion of muscles be reversed?
If the insertion point becomes fixed and the usual origin becomes mobile
Which two major structures coordinate muscle movement?
Brain and spinal cord
Do muscles work in isolation?
no - Muscles work together, and almost never in isolation.
What is a muscle agonist? Antagonist?
Prime muscle/s responsible for particular movement
Muscles that oppose this movement.
What are synergists?
Muscles that act to assist the prime mover /agonist.
Acting alone they cannot perform the movement of the agonist, but their angle of pull assists the agonist in achieving its action.
What is the function of neutralisers?
Prevent the unwanted actions that an agonist can perform
What is the function of fixators/stabilisers?
act to hold a body part immobile whilst another body part is moving. In most activities, proximal joints are stabilised whilst distal joints move.
Eg stabilisation of shoulder during elbow flexion. The fixators active in elbow flexion are the muscles that stabilise the position of the scapula (e.g. rhomboids) and those that stabilise the shoulder joint (rotator cuff muscles)
What are the three type of muscle contraction?
Concentric
Eccentric
Isometric
What is a concentric contraction? Give an example
Muscle pulls while shortening
e.g. biceps curl
What is eccentric contraction? Give an example
Muscle lengthens as it contracts
e.g. knee extensors walking downhill, lowering the dumbbell down in a bicep curl exercise. The fibres within the biceps branchii muscle are contracting to reduce the rate at which the dumbbell is lowered, but the bicep muscle is lengthening passively
What is Isometric contraction?
No change in length of contracting muscle eg carrying an object; the weight of the object is pulling your arm downwards, but your muscles
are contracting to hold the object at the same level
What does the amount of force a muscle is able to produce during an isometric contraction depend on?
The length of the muscle during the contraction
Each muscle has an optimum length at which the maximum isometric force can be produced
What are the 3 main divisions of skeletal muscle fibres?
Parallel: subdivided into strap, fusiform and fan-shaped (triangular or convergent).
Pennate: subdivided into unipennate, bipennate and multipennate
Circular
Describe strap muscles
shaped like a strap or belt with the fibres running longitudinally, parallel to the direction of contraction.
Describe Fusiform muscles
are cylindrical and are wider in the centre, tapering off at the ends.
Fan-shaped/triangular/convergent
Have fibres that converge at one end
Parallel muscles
Fibres run parallel to force generating axis
Pennate muscles
Have one or more aponeuroses (thin fibrous sheet of tissue forming a tendon) running through muscle body from tendon. The fascicles of muscle fibres attach to these aponeuroses at an angle (the ‘pennation angle’) to the direction of movement. These muscles are further classified into unipennate, bipennate, multipennate
Unipennate muscle
All fascicles are on the same side of the tendon
Bipennate muscle
fascicles on both sides of a central tendon
Mulitpennate muscle
where a central tendon branches into several tendons from which fascicles originate.
Which generates a greater force : pennate or parallel muscles of the same length and why?
A pennate muscle can generate a greater force than a parallel muscle of the same length as it has a greater number of muscle fibres. However, the amount of shortening achieved is less than for a parallel muscle, so there is a trade-off between force generation and movement.
Circular muscle
where the fibres form concentric rings around a sphincter or opening. Such muscles attach to skin, ligaments and fascia of other muscles rather than to bone. Examples include orbicularis oculi around the eye and orbicularis oris around the mouth.
Which muscles can shorten the most?
Parallel
Which muscles can contract most powerfully?
Pennate
What is Compartmentalisation within limbs?
Muscles are contained within fascial compartments.
The muscles within a compartment usually share a common innervation and action
What is a fascial compartment?
region of the limb that contains muscles, nerves and blood vessels, surrounded by deep fascia.
Each compartment usually has a separate nerve and blood supply to its neighbours. The muscles in a compartment will often be supplied by the same nerve.
What questions should you ask yourself to predict muscle action?
Where does it attach (origin and insertion)?
How many joints does it cross?
How is it related to the joint(s) i.e. crossing anterior, posterior etc?
Which direction do the fibres run in?
