Referred pain is a well-recognized phenomenon that occurs with several clinical pain conditions and is thought to reflect central sensitization. Despite its common occurrence, the relationships between primary and referred pain are not well characterized. It has been suggested that a threshold level of primary pain is necessary to induce referred pain following a brief delay (Graven-Nielsen, Scandinavian J Rheumatology 2006). To better examine these relationships, we induced multiple intensities of experimental muscle pain in the anterior tibialis muscle using an intramuscular-infusion pain model. 91 healthy adults (48 F) between the ages of 18-48 received a 5.2pH acidic phosphate buffer and an isotonic saline infusion (at 40 and 80 mL/hr) at two visits spaced approximately 1 week apart, blinded to solution (Acid40, Acid 80, Sal40, Sal80). Subjects were asked to verbally rate their pain using the Borg CR-10 at both the infusion site (primary pain) and at the ankle-foot (well-established referred pain site) every 30 seconds during the infusion. Mean primary pain ratings and incidence of referred pain for the 4 conditions were: 1.3/36.3%, 1.6/44.0%, 2.1/64.8%, and 3.0/75.8% for the Sal40, Sal80, Acid40, and Acid80, respectively. Using logistic regression with generalized estimating equations to account for repeated assessments within individuals (controlling for sex, infusion type, and infusion rate), the odds of having referred pain increased 50% for each unit increase in primary pain intensity ratings (OR 1.5). Further, sensitivity and specificity analyses indicate predictability of the incidence of referred pain is optimal when primary pain intensity is approximately 2.5 - 3.0 (0 – 10 scale). The findings substantiate the relationship between primary and referred pain, indicating a threshold and/or progressive incidence referred pain with increased primary pain intensity. Supported by NIAMS K01 AR056134. Referred pain is a well-recognized phenomenon that occurs with several clinical pain conditions and is thought to reflect central sensitization. Despite its common occurrence, the relationships between primary and referred pain are not well characterized. It has been suggested that a threshold level of primary pain is necessary to induce referred pain following a brief delay (Graven-Nielsen, Scandinavian J Rheumatology 2006). To better examine these relationships, we induced multiple intensities of experimental muscle pain in the anterior tibialis muscle using an intramuscular-infusion pain model. 91 healthy adults (48 F) between the ages of 18-48 received a 5.2pH acidic phosphate buffer and an isotonic saline infusion (at 40 and 80 mL/hr) at two visits spaced approximately 1 week apart, blinded to solution (Acid40, Acid 80, Sal40, Sal80). Subjects were asked to verbally rate their pain using the Borg CR-10 at both the infusion site (primary pain) and at the ankle-foot (well-established referred pain site) every 30 seconds during the infusion. Mean primary pain ratings and incidence of referred pain for the 4 conditions were: 1.3/36.3%, 1.6/44.0%, 2.1/64.8%, and 3.0/75.8% for the Sal40, Sal80, Acid40, and Acid80, respectively. Using logistic regression with generalized estimating equations to account for repeated assessments within individuals (controlling for sex, infusion type, and infusion rate), the odds of having referred pain increased 50% for each unit increase in primary pain intensity ratings (OR 1.5). Further, sensitivity and specificity analyses indicate predictability of the incidence of referred pain is optimal when primary pain intensity is approximately 2.5 - 3.0 (0 – 10 scale). The findings substantiate the relationship between primary and referred pain, indicating a threshold and/or progressive incidence referred pain with increased primary pain intensity. Supported by NIAMS K01 AR056134.