Accurate assessment of the talo-crural (ankle) joint angle at physical examination is important for assessing extensibility of m. triceps surae (TS) in children with spastic cerebral paresis (SCP). The main aim of this study was to quantify foot flexibility during standardized measurements of TS muscle-tendon complex extensibility (i.e. based on foot-sole rotation) in SCP children, and typical developed (TD) ones. Additionally, we aim to define a method that minimizes the confounding effects of foot flexibility on estimates of talo-crural joint angles and TS extensibility. Children, aged 6–13 years, with SCP (GMFCS I-III, n = 13) and TD children (n = 14) participated in this study. Externally applied −1 Nm, 0 Nm, 1 Nm and 4 Nm dorsal flexion foot plate moments were imposed. Resulting TS origin-insertion lengths, foot sole (φ_FoSo) rotations, and changes in talo-crural joint angle (φ_TaCr) were measured. Foot flexibility was quantified as Δ(φ_TaCr -φ_FoSo) between the 0 Nm and 4 Nm dorsal flexion conditions. In both groups, φ_FoSo rotations of approximately 20° were observed between 0 Nm and 4 Nm dorsal flexion, of which about 6° (≈30%) was related to foot flexibility. Foot flexibility correlated to φ_FoSo (r = 0.69) but not to φ_TaCr (r = 0.11). For φ_FoSo no significant differences were found between groups at 4 Nm. However, for SCP children the mean estimate of φ_TaCr was 4.3° more towards plantar flexion compared to the TD group (p < 0.05). Normalized TS lengths show a higher coefficient of correlation with φ_TaCr (r² = 0.82) than with φ_FoSo (r² = 0.60), indicating that TS lengths are better estimated by talo-cural joint angles. In both SCP and TD children aged 6–13 year, estimates of TS length and extensibility based on foot sole assessments are confounded by foot flexibility. Assessments of TS extensibility at physical examination will be more accurate when based on measurements of talo-crural joint angles.

Gait of children with spastic paresis (SP) is frequently characterized by a reduced ankle range of motion, presumably due to reduced extensibility of the triceps surae (TS) muscle. Little is known about how morphological muscle characteristics in SP children are affected. The aim of this study was to compare gastrocnemius medialis (GM) muscle geometry and extensibility in children with SP with those of typically developing (TD) children and assess how GM morphology is related to its extensibility. Thirteen children with SP, of which 10 with a diagnosis of spastic cerebral palsy and three with SP of unknown etiology (mean age 9.7 ± 2.1 years; GMFCS: I–III), and 14 TD children (mean age 9.3 ± 1.7 years) took part in this study. GM geometry was assessed using 3D ultrasound imaging at 0 and 4 Nm externally imposed dorsal flexion ankle moments. GM extensibility was defined as its absolute length change between the externally applied 0 and 4 Nm moments. Anthropometric variables and GM extensibility did not differ between the SP and TD groups. While in both groups, GM muscle volume correlated with body mass, the slope of the regression line in TD was substantially higher than that in SP (TD = 3.3 ml/kg; SP = 1.3 ml/kg, p < 0.01). In TD, GM fascicle length increased with age, lower leg length and body mass, whereas in SP children, fascicle length did not correlate with any of these variables. However, the increase in GM physiological cross-sectional area as a function of body mass did not differ between SP and TD children. Increases in lengths of tendinous structures in children with SP exceeded those observed in TD children (TD = 0.85 cm/cm; SP = 1.16 cm/cm, p < 0.01) and even exceeded lower-leg length increases. In addition, only for children with SP, body mass (r = −0.61), height (r = −0.66), muscle volume (r = − 0.66), physiological cross-sectional area (r = − 0.59), and tendon length (r = −0.68) showed a negative association with GM extensibility. Such negative associations were not found for TD children. In conclusion, physiological cross-sectional area and length of the tendinous structures are positively associated with age and negatively associated with extensibility in children with SP.

The developmental goal of 3D ultrasound imaging (3DUS) is to engineer a modality to perform 3D morphological ultrasound analysis of human muscles. 3DUS images are constructed from calibrated freehand 2D B-mode ultrasound images, which are positioned into a voxel array. Ultrasound (US) imaging allows quantification of muscle size, fascicle length, and angle of pennation. These morphological variables are important determinants of muscle force and length range of force exertion. The presented protocol describes an approach to determine volume and fascicle length of m. vastus lateralis and m. gastrocnemius medialis. 3DUS facilitates standardization using 3D anatomical references. This approach provides a fast and cost-effective approach for quantifying 3D morphology in skeletal muscles. In healthcare and sports, information on the morphometry of muscles is very valuable in diagnostics and/or follow-up evaluations after treatment or training.