Forearm heat pain does not inhibit electrically induced tibialis anterior muscle pain

Abstract

Abstract Objectives Women report more musculoskeletal pain than men. A dysfunctional pain inhibitory system has been launched as a contributing factor for these gender differences. This study used a diffuse noxious inhibitory controls (DNIC) paradigm and asked the following questions: (1) is electrically induced muscle pain inhibited by a painful heat stimulus to the forearm, and (2) does women show signs of reduced inhibition compared to men?. Methods Forty healthy subjects (20 females; 18–45 years) participated in a cross-over design with painful (45–49 °C) or non-painful (35 °C) conditioning heat stimuli (in balanced order) to the contralateral forearm. The subjects received 10 painful electrical stimuli in the tibialis anterior muscle before, during and after conditioning and rated each electrical stimulus on a 0–10cm visual analogue scale (VAS). There was 30 min between experiments. All VAS scores were normalized to scores before conditioning (100%) and analyzed by RM-ANOVA. Females participated during the ovulatory phase (days 12–14). Results There was a main effect of conditioning. VAS scores during conditioning were reduced to 90 ± 24% (mean ± S.D.) with respect to before conditioning (p = 0.02). There was no difference between painful and non-painful conditioning (p = 0.31). Neither was there any difference between genders (p = 0.28); mean male VAS scores were 91 ± 21% and mean female scores were 89 ± 27% during (vs. before) conditioning. Conclusion Electrically induced muscle pain was not inhibited by a painful heat stimulus to the forearm. The reduction in muscle pain may be results of habituation. The lack of a DNIC effect may have been caused by the relatively small skin area receiving the conditioning stimulus, and excludes a conclusion on gender effects.