Introduction
Chronic pain patients are often treated equally not taking into consideration, which of the central pain mechanisms are affected. The treatment can be individualized by distinguishing between the mechanisms. Temporal summation (TS) and conditioned pain modulation (CPM) are affected in many chronic pain conditions, but the interaction between the mechanisms is sparsely assessed. The aim of the study was to examine this interaction in healthy volunteers before and during delayed onset muscle soreness (DOMS).

Methods
Twenty healthy subjects (10 males and 10 females, mean age 28,9±8,52) participated in four experimental sessions. DOMS was randomly induced in quadriceps femoris and triceps surae muscles. The subjects were assessed using randomized quantitative sensory testing (QST) paradigms with handheld pressure algometry and computer controlled cuff algometry.

Results
DOMS was detected by assessment with handheld algometer but there was no significant reduction in pain thresholds when using the cuff-system. Visual analogue scale (VAS) scores were increased during TS before and during DOMS. Pressure pain thresholds were reduced during CPM. In quadriceps femoris there was a significant difference in VAS scores for TS before (p<0,01) and during CPM (p<0,05) both before and during DOMS. In the triceps surae there was a significant difference in the VAS scores for TS during CPM (p<0,001) during DOMS, but not before (p<0,146).

Conclusion
TS seems to be inhibited by CPM in the healthy pain system, but both facilitated and inhibited during DOMS. The pain response of temporal summation during DOMS was intensified indicating that central mechanisms may play a role in pain facilitation. These findings suggest that the interactions of CPM and TS shifted during DOMS. Further clinical studies on the interaction between TS and CPM are needed.

Keywords: Temporal summation, conditioned pain modulation, delayed onset muscle soreness, quantitative sensory testing.