Medial gastrocnemius muscle and tendon interaction during gait in typically developing children and children with cerebral palsy

Abstract

1. Introduction Efficient gait is dependent on optimal interaction between muscles and tendons [1]. Pathological changes in the extensibility of the MG muscle fascicles, whole muscle-belly and Achilles tendon have been reported in children with spastic cerebral palsy (CP) [2]. Studying the relative length of these tissues during gait can improve our understanding of their dynamics and, inferably, the control strategies used in CP. In-vivo dynamic ultrasound imaging has been used to visualise the interaction between the MG muscle and tendon during 3D gait analysis. However, most studies combined ultrasound imaging of one variable (either fascicles or muscle-belly and tendon) with some form of musculoskeletal modelling to extrapolate the other variables, resulting in incomplete and variable findings [3–5]. 2. Research question How do MG muscle fascicles, belly, and Achilles tendon interact during gait in children with cerebral palsy (CP) and typically developing (TD) children? 3. Methods 3D gait analysis was carried out in six children with CP (11 ± 3 years, GMFCS I, uni/bilateral: 4/2) and six TD children (12 ± 4 years) as they walked at a comfortable walking speed (average CP: 0.5 m/s, TD: 1.0 m/s) on a treadmill. An ultrasound probe (Telemed SmartUS, 60 mm) was attached to the non-preferred (TD) or most-affected (CP) leg using a custom-made probe holder (Probefix Dynamic, USONO), whose position was tracked by motion analysis. Images were collected during walking first with the probe on the mid muscle-belly, imaging fascicles and secondly with the probe on the most distal muscle tendon junction (MTJ) to estimate both muscle-belly and tendon lengthchanges. Muscle-tendon unit (MTU) length-change represented combined muscle and tendon behaviour. Fascicle, MTU, muscle-belly and tendon length patterns were averaged over time-normalised gait cycles, and expressed relative to their lengths at initial contact [4]. Due to the small and heterogeneous sample, results are presented in a descriptive way. 4. Results Gait kinematic and kinetic data showed that the children with CP had mild gait deviations (1C-F). Children with CP showed reduced length-changes of all tissues compared to TD (Fig. 1A and B). In TD children, the tendon contributed more to MTU length-changes than muscle, as opposed to more equal contributions in CP. In TD children, the muscle-belly behaviour did not reflect the fascicle behaviour whereas in CP, muscle-belly and fascicle length patterns were similar. 5. Discussion Our initial findings of pathological tendon and muscle dynamics during CP gait are in line with a previous study imaging the MTJ [3], but less so with studies relying on modelling to estimate tendon length [4,5]. The similar length pattern between muscle-belly and fascicle in CP may indicate a stiff extracellular matrix. We highlight the importance of collecting experimental data from all three tissues in order to understand the pathology. This feasibility study needs to be confirmed once larger samples are collected.