3. Basic concepts of MSK - Muscles

Question 1

Why do muscles produce force?

Answer 1

Provide stability - eg posture

Propel body segments - Propulsion is achieved through the controlled movements of bones at joints

Question 2

3 Key principles of muscle contraction

Answer 2

1. Muscles can only PULL (CANNOT PUSH) + act on joints they (or their tendons cross) —-> antagonistic pairs
2. 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
3. The action of a muscle is a function of the starting position of the joint.

Question 3

What is the origin of a muscle?

Answer 3

The origin is the stationary anchor point and is usually sited proximally in the limb

Question 4

What is the insertion of a muscle?

Answer 4

The insertion is the mobile attachment point and is usually sited distally in the limb.

Question 5

How is muscle contraction symmetrical?

Answer 5

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

Question 6

How can the origin and insertion of muscles be reversed?

Answer 6

If the insertion point becomes fixed and the usual origin becomes mobile

Question 7

Which two major structures coordinate muscle movement?

Answer 7

Brain and spinal cord

Question 8

Do muscles work in isolation?

Answer 8

no - Muscles work together, and almost never in isolation.

Question 9

What is a muscle agonist? Antagonist?

Answer 9

Prime muscle/s responsible for particular movement

Muscles that oppose this movement.

Question 10

What are synergists?

Answer 10

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.

Question 11

What is the function of neutralisers?

Answer 11

Prevent the unwanted actions that an agonist can perform

Question 12

What is the function of fixators/stabilisers?

Answer 12

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)

Question 13

What are the three type of muscle contraction?

Answer 13

Concentric
Eccentric
Isometric

Question 14

What is a concentric contraction? Give an example

Answer 14

Muscle pulls while shortening

e.g. biceps curl

Question 15

What is eccentric contraction? Give an example

Answer 15

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

Question 16

What is Isometric contraction?

Answer 16

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

Question 17

What does the amount of force a muscle is able to produce during an isometric contraction depend on?

Answer 17

The length of the muscle during the contraction

Each muscle has an optimum length at which the maximum isometric force can be produced

Question 18

What are the 3 main divisions of skeletal muscle fibres?

Answer 18

Parallel: subdivided into strap, fusiform and fan-shaped (triangular or convergent).
Pennate: subdivided into unipennate, bipennate and multipennate
Circular

Question 19

Describe strap muscles

Answer 19

shaped like a strap or belt with the fibres running longitudinally, parallel to the direction of contraction.

Question 20

Describe Fusiform muscles

Answer 20

are cylindrical and are wider in the centre, tapering off at the ends.

Question 21

Fan-shaped/triangular/convergent

Answer 21

Have fibres that converge at one end

Question 22

Parallel muscles

Answer 22

Fibres run parallel to force generating axis

Question 23

Pennate muscles

Answer 23

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

Question 24

Unipennate muscle

Answer 24

All fascicles are on the same side of the tendon

Question 25

Bipennate muscle

Answer 25

fascicles on both sides of a central tendon

Question 26

Mulitpennate muscle

Answer 26

where a central tendon branches into several tendons from which fascicles originate.

Question 27

Which generates a greater force : pennate or parallel muscles of the same length and why?

Answer 27

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.

Question 28

Circular muscle

Answer 28

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.

Question 29

Which muscles can shorten the most?

Answer 29

Parallel

Question 30

Which muscles can contract most powerfully?

Answer 30

Pennate

Question 31

What is Compartmentalisation within limbs?

Answer 31

Muscles are contained within fascial compartments.

The muscles within a compartment usually share a common innervation and action

Question 32

What is a fascial compartment?

Answer 32

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.

Question 33

What questions should you ask yourself to predict muscle action?

Answer 33

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?