mechanisms that underlie movement changes with pain Flashcards
Musculoskeletal pain
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
Mechanisms that underlie
movement changes with pain
Motor unit discharge rate is lower during force matched contractions with pain
pain adaptation theory cycle 1
Pain»_space; Group III & IV afferents > generalised inhibition of agonist > decrease muscles activity > decreased amplitude and velocity of movement
reduced risk of further injury
pain adaptation theory cycle 2
Pain»_space; Group III & IV afferents >
decreased amplitude and velocity of movement > excitation of antagonist > increase muscles activity >
reduced risk of further injury
How is force maintained if motor unit discharge rate is lower during pain?
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
How do the changes in motor unit recruitment
during pain influence surface EMG measures?
Both increased and decreased SMU activity with pain SEMG = summation of all SMU activity
Pain adaptations may be overlooked using SEMG
surface EMG
no change in surface EMG with pain
Take home message
- 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
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Pain
What drives altered motoneurone discharge during pain?
- Input to motoneurone pool from nociceptive stimulation
- Changes to descending drive proposed but not tested e.g. pain inhibitory system, motor cortex
Test one without the other
Does motoneurone discharge change during anticipation of pain when there is no nociceptive afferent stimulation?
Gait, reaction time, postural responses
Is motoneuron discharge altered during anticipation of pain (no direct nociceptive stimulation)?
- less pain perception when anticipated
Yes….. Higher centers likely involved with altered discharge during pain (not simply spinal)
Why is motor unit discharge altered during pain? Is there a purpose or is it an epiphenomenon?
Hypothesis 1
Larger units: protective & faster force production
- greater nociceptive input to smaller neurons
Why is motor unit discharge altered during pain? Is there a purpose or is it an epiphenomenon?
Hypothesis 2
Alter load within painful part: Protective
changes in biomechanics with pain
Elastography
detects stress with pain
Is recruitment of new motor units during pain associated with a change in force direction?
No pain - smaller Absolute angle
Pain - greater Absolute angle
Are the motor adaptions to acute pain task dependent?
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)
Are the motor adaptions to acute pain task dependent?
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.
Summary
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
Summary 2
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
