The influence of experimental muscle pain on the human soleus stretch reflex during sitting and walking.
OBJECTIVE
The stretch reflex is functionally important during human locomotion. Muscle pain has been found to increase the stretch reflex amplitude during sitting, possibly due to an altered fusimotor drive. To further study the importance of altered fusimotor activity due to muscle pain we investigated the combined effect of muscle pain and motor task on the soleus stretch reflex.


METHODS
Stretch reflexes were elicited before, during and after experimentally induced muscle pain in soleus (i.m. infusion of 6% saline) in 3 experiments: (1) in the relaxed soleus muscle and before, during and after an isometric ramp contraction (500 ms, 0-10 Nm), (2) at 3 different time periods during walking, and (3) at matched pain intensity and soleus activity during sitting and walking.


RESULTS
Infusion of hypertonic saline into the soleus muscle caused a significant facilitated stretch reflex in the relaxed muscle (P<0.01), but not during walking or during sitting and walking at matched soleus EMG and matched pain levels. The infusion of isotonic saline (non-painful) did not cause any changes (P = 0.75).


CONCLUSIONS
The main findings of the present study were that experimental muscle pain facilitated the stretch reflex during pain in the relaxed muscle, but caused no changes in stretch reflex amplitude during sitting and walking at higher "functional" background EMG levels.
Matre
D A
DA
Center for Sensory-Motor Interaction, Aalborg University, Denmark. dagfinn.matre@stami.no
Sinkjaer
T
T
Knardahl
S
S
Andersen
J B
JB
Arendt-Nielsen
L
L
D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D004576:Electromyography; D005260:Female; D006801:Humans; D008297:Male; D009132:Muscles / Q000503:physiopathology*; D010146:Pain / Q000503:physiopathology*; D010147:Pain Measurement; D011187:Posture / Q000502:physiology*; D012026:Reflex, Stretch / Q000502:physiology*; D016138:Walking / Q000502:physiology*