Pressure pain thresholds (PPT) and temporal summation (windup):
PPT and windup measurements were performed by one experienced clinician (FCV).Prior to the start of PPT and windup measurements, patients rated their pain at that moment using a paper with a numeric rating scale (NRS) with the numbers placed vertically and on which anchors were given at 0: no pain, at 1: very mild pain, at 4: moderate pain, at 8: very strong pain, and at 10: the worst imaginable pain possible. PPT was measured with an analogue Fisher algometer (FDK 40, Wagner Instruments, Greenwich). For this purpose a rubber tip of 1cm² was placed on the dorsal surface of the right hand middle finger midway between the first and the second digital joint, and at the middle of the trapezius muscle of the non-dominant shoulder. The rubber tip was placed perpendicular to the skin surface. The right middle finger was chosen irrespective of the left or right dominance, because the left hand middle finger was used for the Nexfin BP measurements. The force was gradually increased at a rate of 1 kg/s until the subject indicated that the pain level had been reached. At the two sites the procedure was performed three times with 10 sec intervals. The PPT was calculated as the mean from the last two PPT measurements.
Windup was elicited by 10 applications of the algometer at previously determined PPT value on the same locations as used for PPT measurements(35). Windup measurements started at least 3 minutes after PPT measurements. For each stimulus, pressure was increased at a rate of 1 kg/s to the previously determined PPT, where it was maintained for 1 second before being released. Stimuli were presented with an interval of 2 seconds. The same NRS paper was used as described above. Subjects were instructed to rate the pain level ranging from 0 to 10 immediately after each stimulus.