Mirror Therapy Reduces Pain and Inhibits Reduction of Corticomotor Excitability in Experimental Skeletal Muscle Pain

Skeletal muscle pain can reduce corticomotor excitability (CE). We examined the effects of mirror therapy (MT), which can increase CE, on skeletal muscle pain. Fifteen healthy adults were injected with hypertonic saline (5.8% NaCl, 0.2 mL) into the first dorsal interosseous muscle (FDI) of the right hand to induce an experimental skeletal muscle pain condition and then divided into pain + MT and pain groups. The pain + MT group observed their left index finger abducting and adducting for 4 min after injection, creating the illusion that they were performing the movement with their right index finger. However, the pain group remained at rest. CE was assessed by measuring motor-evoked potentials (MEPs) of the right FDI through transcranial magnetic stimulation. Pain was assessed using a numerical rating scale (NRS). MEP amplitudes were significantly higher in the pain + MT group; the same trend was observed after MT. NRS scores were significantly lower in the pain + MT group from 60 to 390 s after injection; the time to pain disappearance was also shorter. MT could decrease CE reduction and alleviate skeletal muscle pain. MT may be effective in preventing plastic changes in the neural system associated with skeletal muscle pain and providing early pain relief.