elbow function + muscle recruitment

Question 1

spurt and shunt at elbow joint

Answer 1

SPURT: origin far away, insertion close to joint moving
- rotational > translational = joint motion
- greater motion vector (perpendicular to bone)
- mobility role
SHUNT: origin close to joint moving, insertion far away
- translational > rotational = joint compression (stability)
- greater compressive forces (along bone)
- stability role

Question 2

SPURT and SHUNT in flexion

Answer 2

Spurt (flexion):
- biceps
- brachialis
- pronator teres
Shunt (flexion):
- brachioradialis
- common flexor origin (CFO)

Question 3

SPURT and SHUNT in extension

Answer 3

Spurt (extension):
- triceps
- anconeus
- supinator
Shunt (extension): 
- common extensor origin (CEO)

Question 4

Muscle recruitment: law of minimal muscle action + factors that affect muscle recruitment patterns

Answer 4

• Law of Minimal Muscle Action: muscles are recruited in such a way that the need for synergistic activity is minimised.

FACTORS THAT AFFECT MUSCLE RECRUITMENT PATTERNS:

• How heavy is the object? LOAD
• How quickly does my arm have to move? SPEED
• Where is the arm relative to gravity/rest of the body? JOINT POSITION
• How precise is the required action? PRECISION

Question 5

Muscle recruitment - effect of load

Answer 5

• When we flex our elbow for lifting something light, with the forearm in any position, it is possible just to use the uniarticular muscle brachialis to complete this task.

However, if you have to bend your elbow to lift a heavy object, which muscle is the best elbow flexor?

1. Biceps brachii = elbow flexion + shoulder flexion + supination
   i. Tri-articular – therefore need to consider shoulder position (may need extra scapula stability)
   ii. Supination action – if we don’t want supination, we need to incorporate a pronator
2. Brachialis = elbow flexion
   i. Uni-articular, large cross-sectional area
3. Brachioradialis = elbow flexion + forearm neutral (shunt)
   i. Brings forearm to neutral, but also stabilises elbow joint
4. Pronator Teres = elbow flexion + pronation
   i. Pronation action – if don’t want pronation need to use a supinator as a synergist

Question 6

Muscle recruitment - effect of speed

Answer 6

Increasing speed (e.g. throwing/ tennis/ rapid movements)
- Increases traction on the elbow joint (destabilises)
- Increased need to recruit shunt muscles (brachioradialis, CEO, CFO = stability)
- Rapid alternating movts require both shunt activation + some co-contraction = stability
E.G. THROWING
- accelerate over large distance (triceps - concentric)
- decelerate rapidly (brachialis, biceps, CFO, brachioradialis - eccentric)

Question 7

Muscle recruitment patterns – effect of joint position

Answer 7

Joint position (shoulder, elbow, radioulnar joints):
- Mid range more efficient for muscle contractions (can lengthen, shorten or hold relatively easily)
- Changing joint position changes gravity effects and therefore muscles used in actions (for both agonists and antagonists)
- Starting position changes muscles used – e.g. in supination/ pronation
E.G.
(1) Shoulder position
- Full elbow flexion with the shoulder held at 90° abduction in the coronal plane and in external rotation
- 0° -90° elbow flex: brachialis/ biceps concentric
- 90° -140° elbow flex: triceps eccentric
(2) Radioulnar position
- Performing full elbow flexion/ extension from anatomical position in:
- Pronation - brachialis concentrically/ eccentrically
- Supination - biceps (+brachialis) concentrically/ eccentrically

Question 8

Muscle recruitment - effect of precision

Answer 8

Precision tasks may be associated with:

• Increased concurrent activation of stabilising muscles (e.g scapula) or some co-contraction of the elbow/forearm muscles for precision tasks of the hand
• Increased movt time – Fitts Law (the longer the distance + the smaller the target’s size, the longer it takes)
• Balance b/w proximal stability function + distal dexterity in the upper limb

Question 9

Muscle recruitment patterns – principles & summary

Answer 9

• Muscles do not normally work in isolation, but generally, synergistic activity is minimised
• Our brain aims to be most efficient by selecting the best muscle for the job. So, a main agonist (prime mover) is usually the most active, and then depending on the task, a number of secondary agonists (assistant movers) recruited during activity.
• The muscle/s that will be the main agonists determined by a number of factors (in addition to muscle cross-sectional area/architecture/individual characteristics) including:
  - load
  - speed
  - joint positions
  - task demands (such as precision)