1. Induction of experimental pain
acute pain
STERILE, CONTRACTION
During each experimental session (~ 1 hour), acute pain was induced by intramuscular injections (0.2 ml) of sterile hypertonic saline (58.5 mg/ml) into the most prominent point of the right side masseter muscle (assessed during contraction) by a non-blinded examiner (LA–for all men; and LL–for all women). Before injection, the skin was cleaned with a swab containing isopropyl alcohol (70%). To ensure intramuscular injection, a cannula of 19*0.4 mm was used and inserted perpendicular to the skin-surface covering the masseter muscle to a depth of 15 mm, as previously described by Christidis After the first 30 minutes of the experiment, including injection and assessments, one random piece of chocolate (3.6 g) was provided to the participant and 5 minutes later a second injection of hypertonic saline was given in the exact same manner as the first injection. All assessments were repeated in the same manner, as shown in 