• Motor Unit - a motor neuron and muscle fibres innervate
• Motor Neuron Pool - all alpha motor neurons innervating a muscle
• Input from cortico-spinal neurons can trigger AP in axon of motor neuron if postsynaptic potential reaches threshold

Example of Neuromodulation - Persistant Inward Current

• Combined activation of ionotropic channels and release of monoanimes facilitate persistant inward currents and self-sustained firing of AP

Why Modulate the Excitability of Motor Neurons?

• State of high arousal = want motor neurons to be highly responsive
• When balancing we want less excitable motor neurons = less disruption
• muscle neurons synapse with muscle fibres at neuromuscular junction
• When an endplate potential reaches synapse then an AP may be generated in muscle fibre = contraction

Measuring Signal

1. Electromyography (EMG) - records AP of all/parts of muscle, uses 2 electrodes on skin (surface) or inserted in muscle (intramuscular)
   Limitations of surface:

• Muscles close together = record other muscles
• If muscle is deep = only records top muscles
  Strengths of intramuscular:
• Can identify single neuron

A more inhibited motor neuron pool:

• Fewer neurons likely to discharge less frequently
• muscle activation less for given amount of voluntary central drive
  A more excited motor neuron pool:
• More motor neurons likely to discharge and frequently
• Muscle activation greater for given amount of voluntary central drive

H-Reflex

• Only in limited no.muscles more when relaxed
• Influenced by presynaptic inhibition from afferent neurons

CMEPs

• Cervicomedullary motor evoked potentials - stimulates cortico-spinal neurons directly
• Possible during contractions at rest
• Painful
• Hard to evoke response in some p's

Reduced Discharge Rates Controlled by Inhibition of Motor Neurons

• Reduction in CMEPs
• Indicates inhibition of motor neuron pool
• likely driving reduction in discharge rate

Time Delay Occurs:

1. muscle excitation - the proportion of muscle fibres in a muscle that are receiving AP from motor neurons.
2. Muscle activation - the proportion of available actin and myosin cross bridges that are bound (generating tension)
   

• Tdeact = time constant describing lag between excitation of a muscle turning off and deactivation of muscle
  -Tact = time constant describing lag between excitation of a muscle and activation

Muscle Twitch vs Tetanus

• Higher rates of stimulation cause summation of force and hence a tetanus (fused force)

Muscle Architecture - fibres in series vs in parallel

• In series: length changes sum, force equal
• In parallel: length changes equal, forces sum

Muscle Architecture - resting length

• Resting fibre length influences active force production by the whole muscle
• Active and passive force-length relations vary between muscles
  Muscles with Long Resting Fibre Lengths:
• Have more sarcomeres
• maintain greater force production throughout their ROM
• Well suited to producing large length changes and mechanical work
  work = force x displacement

Muscles with long fibres and large PCSA

• Metabolically expensive
• A shorter muscle with same no. fibres in parallel (PCSA) can produce same force by activating a smaller vol of muscle = economical.

Muscle Architecture - pennation angle

• Pennation allows a greater no. fibres to be packed into a vol of muscle.
• Fibre length shortens
• Results in greater PCSA without having to increase muscle vol (+mass)

Muscle structure and FV relationships

• Shorter fibres have lower absolute max shortening speed than longer fibres.
• However, they may produce more force (greater PCSA)
  Force-length-vel and tension
• length and vel influence force
  

Structure-function relationship - proximal - to - distal leg muscle

• proximal muscles have long muscle fibres that facilitate external work production
• Distal muscles have longer compliant tendons that help improve steady running economy.
• Hip extensors have large flat tendons, making them stiff.

Length-tension relationship

• Muscle fibre length changes with joint angle
• length-tension predicts max force generated
  Muscle Moment Arms
• May change with joint angle

Torque Output and Muscle Design

• Muscle force output and moment arm relationships both contribute to resultant output by a muscle at a given joint angle.
  Muscle Moment arms - linear to ang kinematics
• change in muscle length (k) = theta x r1

Moment arm vs Muscle length

• Longer fibres required for longer moment arm
• Longer muscle fibres permit greater ROM
• Combination allows large ang displacements over which forces can be produce
  -Long fibred muscles must have a large vol to have PCSA
  Summary of Short Moment Arms
• small moment arms help muscle remain on a favourable part of the force-vel relationship, while producing fast joint rotations.
• Require more force for given torque
• Narrows force-angle relationship

Moment Arms and Effective Mechanical Advantage (EMA)

• Mechanical advantage - ratio of input to output moment arms
  EMA = External load (output force)/muscle moment arm (input force)

Bi-Articular Muscle Function

• Bi-articular muscles act across 2 joint
• This can be seen as counterintuitive for some movements
• Using bi-articular muscles, power can be transferred from large hip muscle to the knee and ankle

Elastic Power amp and stretch-shortening cycles

• Jump height = take off vel squared/2g
• Increased take off vel requires a large impulse force in a short time (force x time)

Power Amplification: where's the 'catch'?

• Some insects have a physical 'catch' mechanism - a latch, it can be used to block motion
• Frogs manipulate the muscles mechanical advantage - R/r
• High value early = tendon stretch
• Low value late = tendon recoil

What is a stretch-shortening cycle?

• Muscle actively lengthen then shortens
• eccentric-concentric sequence