Background With growth, fascicle lengths increase through sarcomere addition to maintain force production capacity. However, children with cerebral palsy (CP) present a reduced number of in-series sarcomeres. Regarding fascicle lengths of the medial gastrocnemius (MG) in children with CP, some studies have shown smaller fascicle lengths than typically developing (TD) children, though others have shown no difference. Because fascicle length is in part responsible for contraction velocity and range of force generation, smaller fascicle lengths may imply reduced power. However, unlike TD children, in CP no fascicle length to muscle power correlation has been found, possibly due to the assumption that fascicle length represents sarcomere number. On the contrary, children with CP have shown little, over-stretched sarcomeres.

Recently, relative fascicle excursion (RFE) as a measure of sarcomere excursion has been suggested. The latter study observed children with CP have larger MG RFE than TD, suggesting larger sarcomere excursions covering the descending limb of the sarcomere force-length curve. Consequently, we consider RFE instead of fascicle length should be used to predict power generation capacity. Therefore, the aim of this study was to define a protocol to assess RFE, and compare RFE data of a sample of children with CP to that reported in the literature.

Methods Ultrasound images were taken from 22 children with spastic type CP (29 legs, 10.74±3.38 years) attending our laboratory for a gait analysis. MG architecture was assessed at rest (RJA) and maximum dorsiflexion (MDF). The probe was aligned with the fascicle plane over the mio-tendinous junction. The MG was followed with the probe from MTJ to mid-belly for fascicle length measurement. Fascicle length, absolute fascicle excursion (AFE), and RFE were measured.

Results Intra-rater fascicle length reliability was high (ICC=0.885–0.891). RFE was (mean ±SE) 41.01%±3.27%. Similarly, a RFE of (mean ±1 SD) 43,30±14,84% has been previously observed. Greater RFE reported in the literature was possibly unrelated to differences in ankle ROM as no correlations were found between RFE and ankle ROM.

Conclusions Muscle ultrasound is a readily available tool which can be used to measure spastic muscle structure to better understand function and treatment adaptations in children with CP. Through ultrasound, RFE can be accurately measured.