mechanisms that underlie movement changes with pain

Question 1

Musculoskeletal pain

Answer 1

1/7Australians expected to be living with chronic pain
- Contribute to long term persistence or reoccurrence of pain e.g altered activation patterns linked to more rapid progression KOA

Question 2

Mechanisms that underlie

movement changes with pain

Answer 2

Motor unit discharge rate is lower during force matched contractions with pain

Question 3

pain adaptation theory cycle 1

Answer 3

Pain&raquo_space; Group III & IV afferents > generalised inhibition of agonist > decrease muscles activity > decreased amplitude and velocity of movement
reduced risk of further injury

Question 4

pain adaptation theory cycle 2

Answer 4

Pain&raquo_space; Group III & IV afferents >

decreased amplitude and velocity of movement > excitation of antagonist > increase muscles activity >
reduced risk of further injury

Question 5

How is force maintained if motor unit discharge rate is lower during pain?

Answer 5

Recruitment of new motor units during pain
different recruitment of SMU for actions with and without pain
Group decrease in discharge rate confirmed previous work
- POST PAIN DATA ~50% units remained changed

Question 6

How do the changes in motor unit recruitment

during pain influence surface EMG measures?

Answer 6

Both increased and decreased SMU activity with pain SEMG = summation of all SMU activity
Pain adaptations may be overlooked using SEMG

Question 7

surface EMG

Answer 7

no change in surface EMG with pain

Question 8

Take home message

Answer 8

• Not a simple generalised inhibition during pain
• There are research implications
• SEMG not sensitive to altered motor unit recruitment during pain Care when interpreting EMG in pain studies
  20
  0
  20 10 0
  100 ms
  Pain

Question 9

What drives altered motoneurone discharge during pain?

Answer 9

• Input to motoneurone pool from nociceptive stimulation

- Changes to descending drive proposed but not tested e.g. pain inhibitory system, motor cortex

Question 10

Test one without the other

Does motoneurone discharge change during anticipation of pain when there is no nociceptive afferent stimulation?

Answer 10

Gait, reaction time, postural responses

Question 11

Is motoneuron discharge altered during anticipation of pain (no direct nociceptive stimulation)?

Answer 11

• less pain perception when anticipated

Yes….. Higher centers likely involved with altered discharge during pain (not simply spinal)

Question 12

Why is motor unit discharge altered during pain? Is there a purpose or is it an epiphenomenon?
Hypothesis 1

Answer 12

Larger units: protective & faster force production

- greater nociceptive input to smaller neurons

Question 13

Why is motor unit discharge altered during pain? Is there a purpose or is it an epiphenomenon?
Hypothesis 2

Answer 13

Alter load within painful part: Protective

changes in biomechanics with pain

Question 14

Elastography

Answer 14

detects stress with pain

Question 15

Is recruitment of new motor units during pain associated with a change in force direction?

Answer 15

No pain - smaller Absolute angle

Pain - greater Absolute angle

Question 16

Are the motor adaptions to acute pain task dependent?

Answer 16

clear alternative, during a bilateral task

• ~30% plantarflexion force transferred from the painful leg to the non painful leg
• The decrease in force was not well correlated with a decrease in soleus (painful muscle SEMG)

Question 17

Are the motor adaptions to acute pain task dependent?

Answer 17

Cycling at 130w and 80 rpm
Before and when pain induced into VL and VM of one limb Pedal power measured by instrumented pedals
Pedal power and EMG decreased in the painful limb
Although there are numerous ways to compensate (multi-jointed task), only between leg compensations were observed.

Question 18

Summary

Answer 18

In contrast to a generalised inhibition during pain:

• Redistribution of activity within and between muscles during acute pain and anticipation of pain
• Does not necessarily resolve after pain has ceased
• Redistribution not observed using SEMG, and changes in SEMG do not necessarily reflect changes in force

Question 19

Summary 2

Answer 19

Associated with a “absolute” change in force and stress (we do change, but not all in the same way)
Systematic decrease in stress/force when a clear option to compensate is available
The pain adaptation needs to be considered with direct reference to constraints related to the task