The Area of Pressure-Induced Referred Pain Is Dependent on the Intensity of the Suprathreshold Stimulus: An Explorative Study

Abstract

Objective: To investigate the pain referral area (number of pixels) and extent (vector length) as elicited from increasing intensities of pressure-induced pain at the shoulder. Design: Cross-sectional design. Setting: Clinical laboratory setting. Participants: Twenty-two healthy men and women participated in two experimental sessions. Methods: Delayed onset of muscle soreness (DOMS) was induced in the dominant shoulder and assessed 24 hours later. Participants rated the level of DOMS on a 6-point Likert scale. Four different intensities (pressure pain threshold [PPT]+20%, PPT+30%, PPT+40%, and PPT+50%) were applied to the infraspinatus in a randomized, balanced fashion for 60 seconds from low to high intensity or vice versa. The resulting location, area, and extent of referred pain as drawn by the participants on a digital body chart were extracted and expressed in pixels. The extent of pain was defined as the vector length extending from the ipsilateral earlobe to the most distal location of the pain. Results: The referred pain area from PPT+20% was smaller than PPT+30%, PPT+40%, and PPT+50%. The extent of referred pain did not differ between the pressure pain intensities. Conclusions: Pressure intensity at PPT+30%, but no more, produces the greatest referred pain area as compared with the traditional pressure intensity of PPT+20%. Thus, the intensity of PPT+30% may be ideal for exploring the mechanisms of referred pain. The extent of the pain represents an independent expression of the intensity of the provoking stimulus and may be more closely related to the location of the stimulus.