Stretching and relaxing the plantar fascia may change plantar fascia thickness but not pressure pain thresholds: a cross-sectional study of patients with plantar fasciopathy

Background: Despite the established relevance of ultrasonography and assessment of pressure pain thresholds in patients with plantar fasciopathy, patient and probe positioning has been mostly ignored and are not necessarily reported in research. The primary aim of this study was to compare plantar fascia thickness in stretched and relaxed positions in patients with plantar fasciopathy. The secondary aim was to compare plantar heel pressure pain thresholds in these positions. Methods: In this cross-sectional study, we measured the plantar fascia thickness with ultrasonography, and localised pressure pain thresholds using pressure algometry of 20 patients with plantar fasciopathy. These were assessed bilaterally, with the plantar fascia in both a stretched and relaxed position. In the stretched position, toes were maximally dorsiflexed, while in the relaxed position participants' feet were hanging freely over the end of the table. Results: The plantar fascia of the most symptomatic foot was significantly thicker when stretched compared with the relaxed position (sagittal: mean difference 0.2 mm, 95%CI: 0.1-0.4, P = 0.013; frontal: mean difference - 0.27, 95%CI: - 0.49 to - 0.06, P = 0.014). The plantar fascia was significantly thinner in the frontal plane compared with the sagittal plane in both positions (stretched: mean difference - 0.2 mm, 95%CI: - 0.42 to - 0.03, P = 0.025; relaxed: mean difference - 0.3 mm, 95%CI:-0.49 to - 0.08, P = 0.008). There was no difference between pressure pain thresholds in stretched or relaxed positions in either foot (P > 0.4). Conclusions: The plantar fascia was significantly thicker in a stretched compared with a relaxed position and in the sagittal compared with the frontal plane, but differences were smaller than the standard deviation. Pressure pain thresholds were not different between the positions. These results highlight the importance of how ultrasonography is performed and reported in research to allow for replication.