Background and aims The participation in ultra-marathons and other ultra-endurance events has increased exponentially over the past decade. There is insufficient data on variation in pain mechanisms in exercise overall but especially in the ultra-endurance athlete population. To further understand peripheral and central pain sensitization we have investigated pressure pain threshold and conditioned pain modulation during three separate ultra-marathon competitions. Methods Each ultra-marathon investigated was held in the state of Florida, USA, over flat, sandy and paved surfaces under generally warm to hot, humid conditions. Pressure pain threshold was measured utilizing a Baseline © Dolorimeter. The blunt end of the dolorimeter stylus was placed onto the distal dominant arm, equidistant between the distal radius and ulna, three times in a blinded manner to insure that the testing technician did not influence the subject's responses. Conditioned pain modulation was measured immediately after the PPT measures by placing the non-dominant hand in a cool water bath maintained at 15°C. The same dolorimeter measurement was repeated two more times on the dominant arm while the non-dominant hand remained in the water. Data was analyzed with a paired t-test. Results Pressure pain threshold was significantly decreased (p<0.05) at 25, 50 and 100 miles. Conditioned pain modulation was also significantly decreased (p<0.05) at 25, 50 and 100 miles of an ultra-marathon competition. Conclusions Together these data suggest an increased peripheral and/or central pain sensitization starting at 25 miles and continuing throughout an ultra-marathon competition run in these conditions. This is the first study that provides evidence of a decreased peripheral pain threshold and decreased central pain inhibition from ultra-marathon running. Decreases in both the peripheral pain threshold and central inhibition may result from nociceptor plasticity, central sensitization or a combination of both. Implications Based on previous research that has indicated a central sensitization resulting from inflammation and the well-documented inflammatory response to the rigors of ultra-marathon competition, we suggest the decreased peripheral pain threshold and decreased descending pain inhibition results from this inflammatory response of running an ultra-marathon.
Keywords: central sensitization; conditioned pain modulation; nociceptor plasticity; pain; pressure pain threshold; ultra-endurance exercise.