The relationship between triceps surae muscle–tendon morphology and shear modulus across passive ankle range of motion in cerebral palsy

Alterations in skeletal muscle morphology and composition are critical factors in cerebral palsy (CP), including changes in passive stiffness and in belly and fascicle lengths. In this study, we quantified the relative contributions of muscle and tendon to passive stiffness across the ankle range of motion in individuals with CP and typically developing (TD) peers. We also investigated morphological factors underlying increased muscle stiffness. Twelve individuals with CP and 12 age-matched TD peers were recruited. 3D freehand ultrasonography was used to image the medial and lateral gastrocnemius, soleus, and Achilles tendon at three angles across the passive range of motion. From these datasets, muscle belly and fascicle lengths were estimated. Shear wave elastography assessed tissue passive stiffness. The shear modulus at the neutral ankle angle was significantly (p < 0.0038) higher in CP (26.8 kPa for the medial and 20.2 kPa for lateral gastrocnemius) than in TD (19.7 and 14.1 kPa, respectively). When relating shear modulus to muscle belly strain, a significantly steeper slope in CP (3.31 kPa) than in TD (1.00 kPa) (p = 0.001) was found. In the CP group, the slope of muscle belly strain differed significantly from that of fascicle strain, whereas no such difference was observed in the TD group. Our results confirm an increase in passive muscle stiffness in individuals with CP, which remains consistent across the joint range. This elevated stiffness seems primarily associated with whole muscle belly strain, suggesting that changes in the extracellular matrix, rather than fascicle elasticity, may be the main contributor.