Shear wave elastography of the plantar fascia: A cross-sectional observational study in asymptomatic runners and sedentary controls.

The plantar fascia (PF) is exposed to repetitive loading in runners; it remains unclear whether chronic training in young, asymptomatic athletes leads to changes in PF thickness or stiffness. To compare PF thickness and elasticity between asymptomatic elite runners and sedentary controls using B-mode ultrasound and shear-wave elastography (SWE), and to test the hypothesis that prolonged mechanical loading induces adaptive changes in PF stiffness. In this cross-sectional study, 56 participants were included: 38 elite runners (12 short-distance, 11 middle-distance, 15 long-distance) and 18 sedentary controls. Bilateral B-mode ultrasound and longitudinal 2-dimensional SWE virtual touch tissue imaging quantification were performed at proximal (≈1 cm from the calcaneal insertion) and distal (≈6 cm) PF sites using an Acuson S2000 system (Siemens, Munich, Germany) with a 4 to 9 MHz linear array transducer. Primary outcomes were PF thickness (mm) and shear-wave velocity (m/s). Gender distribution did not differ between groups (χ2, P = .748). Age differed across subgroups (analysis of variance, P = .011), and body mass index was lower in middle- and long-distance runners than in sedentary controls (P = .042). Thickness: No differences were observed between runners and sedentary controls (all P > .10); among runner subgroups, a difference was detected only at the left-proximal site (P = .043), with the highest values in short-distance runners. Proximal thickness exceeded distal thickness (P < .001). SWE: No differences were found between-group at any site (all analysis of variance, P > .45); effect sizes were small (Hedges, g  -0.07 to -0.29), and post hoc power to detect such small effects was low (~6%-16%). Proximal SWE was higher than distal SWE (P < .001); at the distal site, left-sided values were higher than right-sided values (Wilcoxon, P = .032). Among sedentary participants, females had higher proximal SWE than males (P ≈ 0.014-0.053), whereas no gender effect was observed in runners. Reliability: Intraclass correlation coefficient was 0.965 to 0.979 for distal SWE (excellent) and 0.755 for right-proximal SWE (good); thickness intraclass correlation coefficients ranged from 0.765 to 0.814. In young, asymptomatic elite runners, chronic training is not associated with a measurable change in PF thickness or stiffness compared with sedentary peers. Differences are predominantly site-dependent (proximal > distal), and a gender effect was observed only among sedentary individuals. Adaptation of the PF to mechanical loading may occur without macroscopic hypertrophy or increased stiffness.