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Journal of Muscle IQ

Journal of Muscle IQ - Volume 5
February of 2025
 

RESEARCH PAPER: How Does Pain Modulate Muscle Output?

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Journal of Muscle IQ - Volume 5 - February of 2025

Author:  Christopher P. Knudsen, DPT

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Key words: Neuromodulation, Electroencephalography (EEG), Transcranial Magnetic Stimulation (TMS), TMS-evoked Potentials (TEPs), Muscle Tone, Strength, and Physical Therapy

 

Introduction

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The audience for this paper is the high-level Doctor of Physical Therapy Student.  At some point during your education you will realize you have a basic enough understanding of the Brain-Muscle connection to realize that pain is all in your head.  As a result of this conclusion you must also conclude that muscle weakness is all in your head as well.  The following paper explains why this is true.

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The basis of this statement can be summarized through the conclusions of a recent study,

 

 

Study: Alterations in cortical excitability during pain: A combined TMS-EEG Study

 

https://biorxiv.org/content/10.1101/2023.04.20.537735v2…

 

Simplified Explanation for DPT Students

 

Key Finding of the Study:

 

- Muscle output from the Motor Cortex is inhibited by pain and the worse the pain is, the greater the inhibition signals at the Motor Cortex. Background: Electroencephalography (EEG) has contributed to our understanding of the temporal sequence of pain processing. Transcranial magnetic stimulation (TMS) has been used to examine the role of inhibitory and facilitatory neural circuits in pain. When TMS is delivered over the primary motor cortex (M1), a magnetic pulse induces an electrical current in underlying cortical tissue that, if the intensity is sufficient, activates corticomotor pathways, inducing a motor evoked potential (MEP) in a target muscle. MEP responses to TMS reflect the net sum of cortical, spinal, and peripheral activity within the corticomotor pathway. This makes it unclear as to whether pain processes occur at the cortical, spinal or peripheral level.  One way of overcoming these limitations is by combining TMS and EEG to measure TMS-evoked potentials (TEPs). TEPs index cortical excitability directly from the cortex (i.e. without influence of subcortical, spinal and peripheral processes).

 

This study measured cortical output created by TMS using electroencephalography (EEG) instead of MEP.  This in more specificity which neurotransmitter circuits are being utilized during pain events.

 

TEP 45ms = GABAa

TEP 100ms = GABAb

TEP 60ms = glutamatergic 

 

The N45 Peak shows the strength of the inhibitory process of GABAa during pain stimulus.

 

Conclusions:

 

“There was strong evidence for a positive relationship such that participants who showed a larger reduction in MEP amplitude during pain reported lower pain ratings.”

 

“There was strong evidence that participants who reported higher pain ratings showed a larger increase in N45 peak amplitude during the pain block.”

 

Discussion:

 

The worse the pain is, the more the Inhibition of Motor Output from the brain due to Motor Cortex Inhibition by the GABAa neurotransmitters. The greater the reduction in MEP = the lower the pain score. What does this mean?

 

 

References 

 

1. Increased motor cortex inhibition as a marker of compensation to chronic pain in knee osteoarthritis

 

https://www.nature.com/articles/s41598-021-03281-0

 

2.The influence of sensory afferent input on local motor cortical excitatory circuitry in humans

 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4386965/

 

3. New Insight into the Time-Course of Motor and Sensory System Changes in Pain

 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4658023/https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4658023/https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4658023/

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