Objective: To investigate the analgesic effect of high frequency repetitive transcranial magnetic stimulation (rTMS) on the experimental human muscle pain and its underlying mechanism. Method: Twenty healthy Korean volunteers participated in this study. The acute muscle pain was induced by infusion of hypertonic saline (5%) into the left extensor carpi radialis longus (ECRL) muscle. During the hypertonic saline injection, 10 Hz rTMS were applied on the hot spot of left ECRL. The changes of visual analogue scale (VAS) of muscle pain and motor evoked potential (MEP) were measured from the start of saline injection to 70 minutes after the start of stimulation. At 90 minutes after the first stimulation, the subjects completed the Korean version of the McGill Pain Questionnaire (MPQ). The sham stimulation was applied with the same method as rTMS experiment. Results: In rTMS, the VAS of muscle pain was significantly decreased from 2.5 minutes and continued until 3 minutes after the last rTMS. While the amplitude of MEP was significantly increased, the latency of MEP was significantly decreased after the start of rTMS and the effect on MEP continued until 5 minutes after the last rTMS. The quality of pain experiment by rTMS and sham stimulation showed no difference in MPQ. Conclusion: The present results suggested that 10 Hz rTMS over primary motor cortex decreased the perception of muscle pain and increased the excitability of corticospinal pathway. (J Korean Acad Rehab Med 2008; 32: 558-563)

Objective
This study was performed to investigate the effect of experimental muscle pain on the stretch reflex through electrophysiologic measuring of H-reflex and ankle tendon reflex. Method: Muscle pain was produced by the continuous infusion of 5% hypertonic saline into the soleus and tibialis anterior muscles respectively in the fourteen healthy, male volunteers. Control was made with infusion of 0.9% isotonic saline. H-reflex and ankle tendon reflex were recorded at the soleus before, during and 30 minutes after infusion of saline. Results: The amplitude of ankle tendon reflex increasedsignificantly during soleus and tibialis anterior muscle pain by the infusion of hypertonic saline as compared with those of before and after injection. But the H-reflex showed no significant changes during the infusion in either muscles. Conclusion: These results have demonstrated a muscle pain increased the amplitude of the stretch reflex without a corresponding increase of the H-reflex amplitude. One explanation could be an increased dynamic sensitivity of the muscle spindles during muscle pain caused by an increased firing of the dynamic γ-motor neurons. (J Korean Acad Rehab Med 2004; 28: 240-246)

Objective
The purpose of this study was to investigate the influence of the muscle pain on the performance of motor coordination and muscular activities of upper limb according to the difficulty of activity and pain site.

Method: Experimental muscle pain was induced by the injections of 5% hypertonic saline in the upper trapezius and extensor carpi radialis longus in 20 healthy volunteers respectively. Coordinative work of upper extremity was performed through the computer simulation games which were controlled by the motion of forearm pronation and supination using the steering handle. The performance scores were measured during the tasks requiring the relatively high level (driving mode) and low level (tracking mode) of coordination. Electromyographic activities were measured from the pronator teres and biceps brachii simultaneously.

Results: The performance scores were decreased after induction of muscle pain in the high coordinative task and their decreasing rate was higher in the distal muscle pain. There were no statistical differences of electromyographic activities according to the intensity and pain site and level of coordination.

Conclusion: The upper limb coordination which required high speed and complex activities was decreased in the presence of the muscle pain. (J Korean Acad Rehab Med 2003; 27: 115-120)