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Shear wave velocity of the right VL was measured by an ultrasound SWE apparatus (Aixplorer version 12, Supersonic Imagine, Aix-en-Provence, France) with a linear array probe (SL10-2), as previously reported (7 (link)). Briefly, the ultrasound probe was placed at 50% of the thigh length and aligned in the plane of the VL fascicles so that several fascicles were identified across the B-mode image, at the following three knee joint angles: (i) the knee fully extended in the supine position; (ii) the knee flexed at 90° in the seated position; and (iii) the knee passively (supported by the examiner) fully flexed in the seated position. The SWE measurements were performed in this order. The participants were instructed to relax completely throughout the measurements. In each position, three measurements were performed (i.e., three images were acquired). The SWE data were analyzed using the software included with the ultrasound apparatus to calculate SWV over the region of interest, which was as large as possible, in VL without aponeurosis or subcutaneous adipose tissue. It was confirmed that no pixel in the region of interest reached the saturation limit of SWV (16.3 m/s). For each position, the average of three measurements was used for further analyses. All measurements and analyses of the SWE data were performed by an examiner with more than 5 years of experience.

The Young’s modulus of the AT was quantified using an ultrasound shear wave elastography system (SuperSonic Imagine, Aix-en Provence, France) with a 40 mm linear array transducer (2–10 MHz, SL10-2). The settings were set as follows (Zhou et al., 2019a ,b (link); Liu et al., 2020 ): the initial condition was standard musculoskeletal mode. The opacity was 85%. The elastic modulus of the AT was 0–800 kPa, and the preset of B-scan ultrasound was adjusted to a depth of 0–2.5 cm. In the SWE examination, the Q-box diameter of AT was defined as the thickness of the AT (Liu et al., 2020 ). The color scale used in the Young’s modulus (in kPa) showed the lowest values in blue and the highest values in red.

All cUS were performed using the CE-marked Aixplorer ultrasound system (Supersonic Imagine, Aix-en-Provence, France) with a 6 MHz central frequency linear probe (SL10-2 Supersonic Imagine, 192 elements, pitch 0.2 mm). Both conventional B-mode cUS and SWE imaging sequences complies with international standard in terms of power and acoustic emission for pediatric transfontanellar ultrasound system applications (Food and Drug Administration (FDA) Track 3: (MI < 1.9, ISPTA < 720 mW/cm² and ISPPA < 190 W/cm²). B-mode acquisitions were performed using sagittal and coronal planes, and Shear Wave Elastrography (SWE) mode were acquired using sagittal planes. The same three operators performed the examinations. The operators were radiology technologist specialized in paediatric ultrasound acquisition for more than 7 years. For preterm neonates, examinations were performed at day of life 3 (DOL 3), 8 (DOL 8), 21 (DOL 21) and at term equivalent age (TEA, 40 ± 1 weeks of postmenstrual age) in accordance with our standardized practice. For term neonates, there was only one acquisition at DOL 3.

Participants were placed in the prone position as mentioned above, and a 40 mm linear array transducer (SL10-2, Supersonic Imagine, France) was used to measure the sagittal plantar fascia thickness (PFT). The transducer was placed over the surface closest to the medial foot, and then the PFT was measured at the anterior margin of the calcaneus and stored for offline analyses (Figure 2; Gamba et al., 2018 (link); Gatz et al., 2020 (link); Petrofsky et al., 2020 (link)).

The TLF shear modulus of the ultrasound examinations was performed using an Aixplorer ultrasound device(Aixplorer Supersonic Imagine, France) with a 40 mm linear array sensor(SL10-2, Supersonic Imagine, France), using the instrument’s default standard musculoskeletal settings, with the selection of SWE mode (enhanced mode, 85% opacity).The range of measurable was adjusted from 0 to 300 kPa.

Hamstrings shear modulus was assessed using two similar ultrasound scanners (Aixplorer, v11; Supersonic Imagine, Aix-en-Provence, France; Aixplorer, v12; Supersonic Imagine, Aix-en-Provence, France) in shear wave elastography (SWE) mode (musculoskeletal preset, penetrate mode, smoothing level 5, opacity 100%, scale: 0–800 kPa for active (i.e., during contraction), coupled with a linear transducer array (SL10-2, 2–10 MHz. Super Linear, Vermon, Tours, France). The SWE procedures were detailed in the previous study with a similar test protocol (Pimenta et al., 2023b (link)). The transducer was placed to align with the muscle fascicles orientation, and to perform minimal pressure during the measurements. To maximize the window of opportunity of the effects in both tasks, both examiners collected data simultaneously in the pairs BFlh + SM and ST + BFsh. The utilization of casts and the measurement with two examiners was previously validated (Pimenta et al., 2023d (link)).

An ultrasound shear wave elastography system (Aixplorer Supersonic Imagine, France) with a 40 mm linear array transducer (SL10-2, Supersonic Imagine, France) was used. The settings of the SWE system were set as follows: the instrument was set in the musculoskeletal mode. The frequency was 2~10 MHz. The SWE Opt was the penetration mode. The opacity was 85%. The elastic modulus range of the gastrocnemius was 0~200 kPa, while the elastic modulus range of the Achilles tendon was 0~800 kPa. The color scale used in the shear modulus (in kPa) showed the lowest values in blue to the highest values in red. The depth of the B-scan was 3.0 cm [30 (link)]. For the Achilles tendon, the size of the regions of interest (ROI) had to be set to 25∗12 mm and the Q-Box™ diameter was defined by the thickness of the tendon, which was the distance between the superior and inferior borders of the Achilles tendon [23 (link)]. For the MG and LG, the size of the ROI had to be set to 10∗10 mm and the diameter of the Q-Box™ is 5∗5 mm [31 (link)]. The transducer was positioned along the longitudinal axis of the AT, MG, and LG.