Effects of experimental muscle pain on muscle activity & co-ordination during static & dynamic motor function

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The relation between muscle pain, muscle activity, and muscle
co-ordination is still controversial. The present human study
investigates the influence of experimental muscle pain on
resting, static, and dynamic muscle activity. In the resting
and static experiments, the electromyography (EMG) activity
and the contraction force of m. tibialis anterior were
assessed before and after injection of 0.5 ml hypertonic
saline (5%) into the same muscle. In the dynamic experiment,
injections of 0.5 ml hypertonic saline (5%) were performed
into either m. tibialis anterior (TA) or m. gastrocnemius (GA)
and the muscle activity and co-ordination were investigated
during gait on a treadmill by EMG recordings from m. TA and
m. GA.

At rest no evidence of EMG hyperactivity was found
during muscle pain. The maximal voluntary contraction (MVC)
during muscle pain was significantly lower than the control
condition (P < 0.05). During a static contraction at 80% of
the pre-pain MVC muscle pain caused a significant reduction
in endurance time (P < 0.043). During dynamic contractions,
muscle pain resulted in a significant decrease of the EMG
activity in the muscle, agonistic to the painful muscle (P <
0.05), and a significant increase of the EMG activity of the
muscle, antagonistic to the painful muscle (P < 0.05).

Muscle pain seems to cause a general protection of painful muscles
during both static and dynamic contractions. The increased
EMG activity of the muscle antagonistic to the painful muscle
is probably a functional adaptation of muscle co-ordination
in order to limit movements. Modulation of muscle activity by
muscle pain could be controlled via inhibition of muscles
agonistic to the movement and/or excitation of muscles
antagonistic to the movement.

The present results are in accordance with the pain-adaptation model
(Lund, J.P., Stohler, C.S. and Widmer, C.G. In: H. Vaeroy and H.
Merskey (Eds.), Progress in Fibromyalgia and Myofascial Pain.
Elsevier, Amsterdam, 1993, pp. 311-327.) which predicts
increased activity of antagonistic muscle and decreased
activity of agonistic muscle during experimental and clinical
muscle pain.

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