BACKGROUND:
While evidence supports efficacy of acupuncture and/or dry needling in treating musculoskeletal pain, it is unclear which needling method is most effective. This study aims to determine the effects of depth of needle penetration on muscle pain.

METHODS:
A total of 22 healthy volunteers performed repeated eccentric contractions to induce muscle soreness in their extensor digital muscle. Subjects were assigned randomly to four groups, namely control group, skin group (depth of 3mm: the extensor digital muscle), muscle group (depth of 10mm: the extensor digital muscle) and non-segmental group (depth of 10mm: the anterior tibial muscle). Pressure pain threshold and electrical pain threshold of the skin, fascia and muscle were measured at a point 20mm distal to the maximum tender point on the second day after the exercise.

RESULTS:
Pressure pain thresholds of skin group (depth of 3mm: the extensor digital muscle) and muscle group (depth of 10mm: the extensor digital muscle) were significantly higher than the control group, whereas the electrical pain threshold at fascia of muscle group (depth of 10mm: the extensor digital muscle) was a significantly higher than control group; however, there was no significant difference between the control and other groups.

CONCLUSION:
The present study shows that acupuncture stimulation of muscle increases the PPT and EPT of fascia. The depth of needle penetration is important for the relief of muscle pain.