Effect of pain location on spatial reorganisation of muscle activity.
BACKGROUND
We aimed to determine whether the changes in muscle activity (in terms of both gross electromyography (EMG) and motor unit (MU) discharge characteristics) observed during pain are spatially organized with respect to pain location within a muscle which is the main contributor of the task.


METHODS
Surface and fine-wire EMG was recorded during matched low-force isometric plantarflexion from soleus (from four quadrants with fine-wire EMG and from the medial/lateral sides with surface EMG), both gastrocnemii heads, peroneus longus, and tibialis anterior. Four conditions were tested: two control conditions that each preceded contractions with pain induced in either the lateral (Pain(L)) or medial (Pain(M)) side of soleus.


RESULTS
Neither the presence (p = 0.28) nor location (p = 0.19) of pain significantly altered gross muscle activity of any location (lateral/medial side of soleus, gastrocnemii, peroneus longus and tibialis anterior). Group data from 196 MUs show redistribution of MU activity throughout the four quadrants of soleus, irrespective of pain location. The significant decrease of MU discharge rate during pain (p < 0.0001; Pain(L): 7.3 ± 0.9-6.9 ± 1.1 Hz, Pain(M): 7.0 ± 1.1 to 6.6 ± 1.1 Hz) was similar for all quadrants of the soleus (p = 0.43), regardless of the pain location (p = 0.98). There was large inter-participant variation in respect to the characteristics of the altered MU discharge with pain.


CONCLUSIONS
Results from both surface and fine-wire EMG recordings do not support the hypothesis that muscle activity is reorganized in a simple systematic manner with respect to pain location.
The University of Queensland, NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane, Australia; University of Nantes, Laboratory "Motricité, Interactions, Performance" (EA 4334), F-44000 Nantes, France. Electronic address: f.hug@uq.edu.au.
François
F
Hug
Paul W
PW
Hodges
Kylie J
KJ
Tucker
D000328:Adult; D000818:Animals; D004576:Electromyography; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D008297:Male; D051379:Mice; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D063806:Myalgia / Q000139:chemically induced / Q000503:physiopathology*; D011187:Posture / Q000502:physiology; D012462:Saline Solution, Hypertonic; D013647:Task Performance and Analysis
D012462:Saline Solution, Hypertonic
Discharge rate; Electromyography; Hypertonic saline; Motor unit recruitment; Triceps surae