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.

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Comprehensive instrumented muscle and joint assessments should be considered when prescribing Botulinum NeuroToxin-A (BoNT-A) treatment in spastic paresis. In a child with spastic paresis, comprehensive evaluation following treatment with BoNT-A, serial casting, and physiotherapy showed that short-term improvements in gait occurred without changes in muscle morphology. Rather, foot flexibility increased.

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Typically, Spastic Paresis (SCP) causes morphological changes of m. gastrocnemius medialis (GM) that may change its mechanical characteristics. An enhanced resistance to dorsal flexion may in part be explained by such changes.@en

Spasticity, i.e. exaggerated velocity-dependent stretch reflex activity, is one of the key impairments in neurological diseases, but its effect on gait is unclear [1]. To quantify spastic reflexes during gait, ankle rotations have been applied by mechanical devices [2,3], but these are cumbersome and can affect gait. Recently, it has been shown that treadmill accelerations can be used to evoke stretch reflexes during gait in the calf muscles of healthy adults [4]. However, the applicability of this approach in children and in patients with neurological diseases has yet to be demonstrated.@en

Spasticity as part of a central neurological disorder is characterized by a ‘velocity dependent hyperactive stretch-reflex’ [1]. Secondary, morphological adaptations of the muscle-tendon complex reduce the passive joint angle-moment relationship (i.e. passive ROM) [2]. Potentially, joint hyper-resistance, as a result of either the neurological disorder, muscle morphology or both, can be clinically assessed [3]. Botulinum Toxin-A (BoNT-A), in combination with casting and physiotherapy are regularly used as conservative treatment in children with a spastic paresis to improve gait. While in some studies improvements resulting from this approach are reported, large treatment response variability persists [4]. Heterogeneity in treatment effectiveness may be due to a clinical focus at the joint impairment level rather than on the contributing mechanisms of joint hyper-resistance. In recent years great advances have been made in standardized, objective assessments of stretch reflexed induced joint hyper resistance [5]. 3D ultrasound (3DUS), allows morphometry of the muscle-tendon complex in children with spastic paresis [6]. The combination of instrumented assessments of neurological, muscle morphology and gait characteristics following treatment has not been carried out.@en