## 2 Methods
### Plantar Tissue Mechanical Property  
An elastographic ultrasound (Aloka Pro Sound Alpha 7, Hitachi Healthcare Americas, Twinsburg, OH, USA) with a linear array transducer (UST-5412, 5-13 MHz, Hitachi Healthcare Americas) was used to measure plantar tissue mechanical property. Elastographic images were obtained by applying cyclic compressions on the first metatarsal head [21; 22]. The measurements included both B-mode and elastographic images. Elastographic images represent the tissue strain ratio [23]. The tissue strain ratios of plantar foot were then used to quantify plantar stiffness. The principle of strain elastography is based on the elastic modulus (E), defined as a ratio of stress to strain. Under similar stresses, strain in harder tissue is lower than strain in softer tissue [24]. Thus, a comparison between strains of the different soft tissues can be used to assess elasticity changes. The strain ratio represents the relative difference in tissue stiffness [25].  
The stiffness of the plantar foot was calculated based on the strain ratio of the elastographic images [21; 22]. First, a rectangular area was drawn on the B-mode image to define the soft tissue  
Figure 1: B-mode and elastographic ultrasound images of the plantar tissue of the first metatarsal head. (**A**) The ultrasound probe is placed over the first metatarsal head. Five measurements are taken for each condition. (**B**) The rectangular area is placed on the B-mode ultrasound image to define the region of interest (ROI) of the soft tissue over the first metatarsal head. (**C**) The same rectangular area is superimposed on the corresponding elastographic ultrasound image to acquire strain ratios of the ROI. (**D**) The elastographic images of five measurements are then used to calculate the plantar tissue stiffness of the first metatarsal head.  
region between the skin and bone at the first metatarsal head (Figure 1B). Then, the corresponding elastographic image was superimposed on the B-mode image to define the soft tissue over the first metatarsal head area (Figure 1C). The 5 ultrasound scans were then used to represent soft tissue under the first metatarsal head (Figure 1D). The pixel value of the elastographic image (Figure 2A) was converted to 100 levels (Figure 2B), representing soft (level 1) to hard (level 100) [21; 22]. The pixel value of the elastographic image is usually shown in the RGB format for visual inspections by clinicians. In an elastographic image obtained by elastographic ultrasound, the strain ratio is a numeric scale ranging from level 1 (red color indicating soft) to level 100 (blue color indicating hard). In this study, RGB values of elastographic images are defined as (1,0,0) as red, (0,1,0) as green and (0,0,1) as blue [26]. The color map level was converted to use blue as soft (level 1) and red as hard (level 100) (Figure 2B). The mean strain ratio of the plantar soft tissue was calculated within this rectangular area [27]. Third, the stiffness of the metatarsal head, from the skin to the bone (Q-All), was evenly divided into four layers, including the superficial layer (from the skin surface, Q1), sub-superficial layer (Q2), sub-deep layer (Q3) and deep layer (Q4) (Figure 2B).