P.468 - Are muscle volume and echo-intensity related to rate of force development in children with Duchenne muscular dystrophy?

Abstract

Muscle weakness in children with Duchenne muscular dystrophy (DMD) is one of the main symptoms and is mainly caused by alterations in muscle morphology. Several studies have used muscle imaging, such as ultrasonography (US), to quantify these alterations and their association with muscle weakness. An increase in echo-intensity (EI) was found to negatively correlate with the outcomes of maximal voluntary isometric contractions (MVICs). However, to what extend muscle volume and EI are related to MVIC-outcomes and rate of force development (RFD) has not yet been established. Maximal force (Nm/kg), RFD (N/s), muscle volume (ml/kg) and EI of the knee-extensors and knee flexors were collected in 10 DMD children (median age (25–75%): 8.15 (6.77 – 9.99)). Muscle volume was acquired with a 3D freehand US technique and extracted using MeVisLab and EI was calculated according to the method described by Shkylar et al (2014). Spearmans' rank correlation coefficient was used to evaluate to what extend MVIC-outcomes and RFD-values were correlated to muscle volume and EI, and the classification of Altman was used to interpret these correlations. A decrease in RFD of the knee-flexors was related to an increase in muscle volume of that same muscle group (r = -0.43). Further, an increase in EI in the knee-flexors was associated with a decrease in knee-flexor strength (r = -0.44). A decrease in muscle strength and RFD in the knee-flexors could be attributed to an increase in EI and muscle volume respectively, most likely caused by an increment of fatty and fibrous tissue. No relationship between MVIC-outcomes and RFD-values, and muscle volume and EI was found for the knee-extensors. Because changes in muscle structure in children with DMD are known to be age- and muscle specific, the study group and number of analysed muscles should be enhanced in future studies.