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

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.