Week 10 (MC) - MU recruitment & Muscle Force

Question 1

Why does versatility and regulation of motor recruitment matter?

Answer 1

You want your body to work in the most energy efficient way to reduce fatigue states

• The force generated by muscles is limited in magnitude and time (fatigue)
• In response we must adopt and control strategies within energy cost constraints

Question 2

Define Motor unit and its properties

Answer 2

A motor unit is a single motor neuron and all the muscle fibers it innervates. It is the functional unit for muscle contraction

Typical muscle is controlled by approx 100 MU

Question 3

At what level is neural control operated?

In a muscle

Answer 3

Motor unit level NOT muscle fibre

Question 4

What are the 3 muscle fibre types

Answer 4

Type I: Slow twitch
Type IIa: Fast twitch fatigue resistant fibres
Type IIb: Fast Twitch Fatigable FIbres

Question 5

What are the properties of the Type I: Slow-Twitch Fibres

Answer 5

aka - Slow-Oxidative (SO)
Relies on oxidative metabolism
Oxidative metabolism = Glucose & oxygen used almost indefinitely to regenerate adenosine triphosphate (ATP) that fuels the contractile apparatus

Question 6

What are the properties of
Type IIa: Fast-Twitch Fatigue-Resistant Fibres

Answer 6

aka Fast-Oxidative-Glycolytic (FOG)
Combines relatively fast twitch dynamics & contractile velocity with enough aerobic capacity to resist fatigue for several minutes.

Question 7

What are the properties of Type IIb: Fast-Twitch Fatigable Fibres

Answer 7

aka Fast-Glycolytic (FG)
Uses anaerobic metabolism.
Has relatively large stores of glycogen that provide energy to phosphorylate adenosine diphosphate (ADP) rapidly as glucose is converted into lactic acid.
However, rapid depletion of glycogen stores limits these fibres to brief bursts of force, after which they take hours to recover fully

Question 8

Difference between small and large motor units?

Answer 8

Smaller Motor Units
- Produce little force - have smaller & fewer muscles fibres, & fibres produce less force per unit area
- Contract relatively slowly & are resistant to fatigue (i.e., Type I).
- Use less energy at all speeds of contraction

Larger Motor Units
- Produce larger forces recruit more & bigger muscles fibres, & fibres produce greater force per unit area
- Contract relatively fast & become fatigued rapidly (i.e., Type IIb).
- Use more energy at all speeds of contraction

Question 9

What is Hennemans Motor Unit Size Principle?

Answer 9

Motor units are recruited from smallest to largest, reflecting low-high in force requirements for movement taks.

Will first recruit slow twitch fibres as they generally innervate smaller pools of fibres
then goes to fatigue resistable then to fatigable fibres (strongest)

Question 10

What is the relationship with energy cost of a muscular contraction and unit force?

Answer 10

Motor unit force increases non-linearly with level of motor unit recruitment. Fewer of the larger motor units produce higher forces

Muscle energy cost increases non-linearly with level of muscle contraction (recruitment)

Question 11

What 2 purposes does Hennemens size principle serve?

Answer 11

1. Minimises fatigue development (conservative)
   - Allows most fatigue resistant muscle fibres to be used, holding more fatiguable fibres in reserve until needed
2. Proportional control
   - Muscle force occur increases (jumps) in accordance with no. of activated units. Relates to motor unit size & composition of fibre types.
   Force control achieved by number & type of active motor units; &, frequency of motor unit firing (rate of innervation)

Question 12

What is the relationship with proportional control and accruacy/precision?

Answer 12

At higher muscular force outputs there is a decrease in the ability to be precise / difficulty to maintain control

Question 13

What is the relationship between accuracy, skilled performers and muscular output?

Answer 13

Movement accuracy reduces as force or MU activation increases
Better accuracy is attainable at LOWER levels of MU or force

Superior Accuracy attained using lower muscular output is what is expected of skill performers
- with practice, the amount of force produced in an accurate manner would be higher

Question 14

What is the key part of skill learning in relation to accuracy and energy

Answer 14

A key part of skill learning is to reduce energy cost & develop accuracy, achieved via re-organising efferent neural signals & motor unit recruitment.