Muscle function

Question 1

Define muscle agonists/antagonists/synergists

Answer 1

• agonists- “primary mover”
  
  • responsible for the initiation and execution of a specific action at a joint
  • e.g. brachialis flexes the elbow
• antagonists
  
  • oppose or reverse the action of the primary mover
  • e.g. triceps oppose the brachialis
• synergists
  
  • assist the primary mover in its action
  • e.g. biceps/brachioradialis assist brachialis

Question 2

What is the concept of reciprocal inhibition?

Answer 2

• ensures the smooth movement of a joint
• skeletal muscles typically function in pairs for maximal muscle efficiency, speed, and control
  
  • ​pair needs to coordinate their contraction to avoid muscle injury
  • requires neurological input

Question 3

What is the neurological pathway of reciprocal inhibition?

Answer 3

• in neurologically intact patients
  
  • one branch of the 1a afferent enters the spinal cord and synapses on the alpha motor neuron (causing the agonist muscle to contract)
  • the other branch of the 1a afferent synapses on the 1a inhibitory interneuron which synapses on the alpha motor neuron of the antagoinst (preventing contraction of that muscle group)
• in neurologically compromised patients
  
  • they lack reciprocal inhibition which causes spasticity (agonist and antagonist “fighting each other”)

Question 4

What are the planes of movement?

Answer 4

• frontal/coronal
  
  • divides body into anterior and posterior
  • e.g. abduction, adduction
• sagittal
  
  • divides the body into right and left
  • e.g. flexion and extension
• transverse/horizontal
  
  • divides the body into top and bottom
  • e.g. internal and external rotation

Question 5

What are the 3 types of muscle contractions?

Answer 5

• isometric
  
  • muscle length remains the same
• isotonic
  
  • concentric (muscle shortens)
  • eccentric (muscle elongates)

Question 6

Which type of muscle contraction is capable of producing the greatest amount of force?

Answer 6

**isotonic eccentric contractions;

• require less metabolic energy
• with maximal contraction they can generate much higher tension levels (up to 50% higher) when compared to concentric contractions
• important concept in the rehabilitation of tendinopathies; eccentric contraction workouts (e.g. heal lifts/dips off the stairs) generate more force and stimulate collagen growth factors

Question 7

How can you clinically evaluate muscle function?

Answer 7

• manual muscle testing (structural unit)
  
  • typically tests groups of muscles, not an isolated muscle (all agonists/antagonists)
  • tests planes of movement (helps evaluate the integrity of the nerves; the patient may move in a different plane which can help diagnose; e.g. unintential inversion during dorsiflexion)
• electromyography (EMG) and nerve conduction studies (functional unit)
  
  • detailed evaluation of muscle and nerve function/interaction

Question 8

Describe the use of EMG and nerve conduction studies clinically

Answer 8

• **Used to diagnose certain disease processes such as _nerve compression, hereditary/acquired diseases, and myopathy (_which alter the normal recorded electrical signals and recruitment patterns)
• EMG tests skeletal muscle fibers (mostly type 1, slow twitch, muscle fibers)
• Nerve conduction studies test motor, sensory, and mixed nerves (standard studies are evaluating large myelinated nerve fibers)

Question 9

What is a motor unit? How is it recruited?

Answer 9

• motor unit= the muscle fibers that are all innervated by a single axon
  
  • all the muscle fibers innervated by one motor unit are of the same muscle fiber type
• # of muscles : 1 axon= innervation ratio
  • varies widely between muscles
  • low ratio for fine motor (e.g. extraocular muscles)
  • high ratio for gross motor (e.g. soleus)

Question 10

What is MUAP?

Answer 10

**Motor unit action potential

• depolarization of all the fibers in a motor unit creates an electrical potential called the MUAP
• analysis of the MUAP (size, complexity, firing frequency) and assessment of baseline electrical signal at rest serves as the foundation for EMG
• MUAP generated by contraction of an individual muscle can be recorded by using a surface or needle electrode

Question 11

What is the Henneman size principle?

Answer 11

• to generate more force the firing rate and recruitment (smallest to largest) of more motor units must be increased
• in practice, this means that slow-twitch, low-force, fatigue-resistant muscle fibers are activated before fast-twitch, high-force, less fatigue-resistant muscle fibers
• patterns of recruitment indicate neuropathic (reduced) versus myopathic (early) conditions

Question 12

Describe the basics of a nerve conduction study

Answer 12

• nerve conduction studies are a component of EMG (they asses the integrity of the PNS)
• evaluate peripheral nerve function;
  
  • compound motor action potentials (CMAP)
  • sensory nerve action potentials (SNAP)

Question 13

How does a nerve conduction study look? There are two lines of signal; what do they represent?

Answer 13

• flexors on top (signal during flexion, silent during extension)
• extensors on bottom (signal during extension, silent during flexion)

**isometric contraction shows simultaneous signal in both lines

Question 14

What are the signs of an upper motor neuron syndrome?

Answer 14

• overactivity or “positive” signs
  
  • hyperreflexia
  • spasticity
  • due to the loss of descending inhibition
  • e.g. hemiparetic gait, clonus
• underactivity or “negative” signs
  
  • weakness
• NO inherent muscle atrophy

**MORE than just motor and sensory loss!

Question 15

What is spasticity? When does it occur?

Answer 15

• spasticity= a condition in which certain muscles are continuously contracted
  
  • velocity dependent increase in tonic stretch reflex (muscle tone)
  • hyperexcitability of the stretch reflex
  • decreased reciprocal inhibition
• **occurs commonly as a “positive sign” of upper motor neuron syndromes

Question 16

How does botulinum neurotoxin function? What are the indications for it clinically?

Answer 16

**produces denervation by pre-synaptically blocking the release of acetylcholine by cleaving the SNAP-25 protein

• Indications
  
  • spasticity/dystonia
  • migraine headaches
  • neuropathic pain
  • hyperhidrosis/sialorrhea