Pressure-Pain Thresholds of the Plantar Foot reflect Relative Tissue Thickness and are Systematically Higher in Active Runners than Non-Runners

Claire Game¹Tom Walsh²Nathan Stevenson³Werner Klingler^4,5Scott Wearing^4*

• ¹Podiatry Professionals, Canberra, Australia
• ²Princess Alexandra Hospital, Woolloongabba, Australia
• ³QIMR Berghofer Medical Research Institute, Herston, Australia
• ⁴Technical University of Munich, Munich, Germany
• ⁵Universitatsklinikum Ulm, Ulm, Germany

Background: This study evaluated topographical pressure-pain sensitivity maps of the plantar foot in competitive distance runners and non-runners, and explored the relationship between pressure-pain thresholds (PPTs) and skin and subcutaneous tissue morphology. Methods: Mechanical PPTs were measured in 23 competitive distance runners (age, 39.7±12.0 years) and an equivalent number of healthy non-runners (age, 36.6±10.1 years). PPTs were determined, bilaterally, at the plantar calcaneal area (PCA), Abductor Hallucis muscle belly (ABH), plantar metatarsal area of the first (1MH), third (3MH), and fifth (5MH) metatarsal heads, Abductor Digiti Minimi muscle belly (ADM), and the Abductor Pollicis Brevis muscle belly (THE) of the corresponding hand. Skin and subcutaneous tissue thickness at each site was measured using B-mode ultrasound. Between-group differences in PPT and tissue thickness were assessed using three-way repeated-measures ANOVA. Relationships between PPTs and tissue thickness were explored using nonlinear regression, with tissue thickness as the independent variable. Akiake's Information Criterion was used to assess logit and polynomial fits (linear, quadratic and cubic). Results: Mean PPT values in runners were 24% higher than those of non-runners across all sites (F=4.6, P=.038). Pain sensitivity varied across the plantar surface of the foot in both groups (F=82.5, P<.001). PPTs at the PCA were higher (range, 18.6–31.7 kPa) and the ABH lower (range, -31.7−-6.2 kPa) than those at all other foot sites (P<.05). Similarly, mean PPT measured at the THE was significantly lower than all foot sites (range, -36.9 − -5.1 kPa). Runners presented with significantly thinner tissues than non-runners (F=14.1, P=.016) at the PCA (-1.5mm [-2.8,-0.2], P<.05), 1MH (-1.0mm [-2.0,-0.1], P<.05), and ADM (-1.4mm [-2.6,-0.2], P<.05). The relationship between PPT and tissue thickness was best described by a logit function in runners and non-runners (R2, 88-95%). Normalization of PPT values to the THE, mitigated the bias in PPTs between groups without altering the shape of the logit function. Conclusion: Endurance runners present with lower sensitivity to mechanical pain than non-runners, despite thinner plantar tissues. Topographical variation in PPTs across the foot sole can be effectively modeled as a function of relative tissue thickness. The hypoalgesic bias in runners likely reflects differences in centrally-mediated pathways.