Medial gastrocnemius and achilles tendon interplay is not optimally exploited during gait in cerebral palsy

The interplay between the medial gastrocnemius (MG) muscle and Achilles tendon (AT) is crucial for optimizing stability and propulsion. Eccentric action of the MG and reduced excursion of the AT has been found during single support at stance in spastic cerebral palsy (CP) [1]. However, more thorough understanding on the interplay of muscle and tendon during gait is needed. A direct quantification of such interplay could be a breakthrough for targeted treatment at muscle-tendon level, which are currently based on indirect measures. What is the behavior of MG-belly, MG-fascicle, and AT during gait in CP? Can their interplay explain a new equilibrium of muscle-tendon properties? Eight individuals with spastic CP (17.5±4.5 years, GMFCS I/III: 5/3) and 8 typically developing (TD) peers (17.9±4.8 years) participated. 3D gait analysis (PiG-model, Vicon, 200 Hz) was combined with a portable ultrasound (Telemed, 50 Hz) (3DGAUS) during overground walking [2]. The ultrasound probe, instrumented with four markers, was attached to either a randomly chosen (TD) or most-affected (CP) leg to image the distal MG muscle-tendon junction and MG-fascicles. 3DGAUS allowed the estimation of both MG-belly (i.e. whole muscle) and AT lengths [2], with muscle-tendon unit (MTU) length being their sum. Relative length changes were calculated from the corresponding values at initial foot contact. MTU/MG-belly (i.e. AT contribution) and MG-belly/MG-fascicle [3] were calculated every 10% of the gait cycle as ratios of their corresponding excursions within each interval. Semi-automated software was used to track the muscle-tendon junction [4] and fascicles [5]. Statistical non-parametric mapping, Mann-Whitney U-tests and Cohen’s d effect sizes were calculated to test group differences. Walking speed (m/s) was lower in CP compared to TD (1.11±0.27 vs. 1.38±0.10, p<0.05). Overall, individuals with CP showed smaller relative length changes (mm) than TD peers: for the MG-belly (2.6 vs. 7.8), AT (8.3 vs. 13.2), MG-fascicle (8.4 vs. 11.1), and MTU (10.2 vs. 17.6) during the gait cycle (Figure A-B), although without statistical differences. MTU/MG-belly ratio (Figure C) showed that the AT has a predominant role during single support in TD compared to CP. MG-belly/MG-fascicle showed an increased lengthening of MG-belly with a reduced lengthening of its fascicles in CP during single support (Figure D). Moderate-to-large effect sizes were observed for both groups.Download : Download high-res image (148KB)Download : Download full-size image Figure. Relative length changes (A-B) and muscle-tendon ratios (C-D) during gait, in TD and CP. This study deepens understanding of the tendency of reduced muscle-tendon length changes during gait in CP compared to TD [1]. In CP, the interplay between MG and AT suggests that the AT elastic mechanism at single support is not properly exploited, due to a tendency of MG-belly and fascicles to elongate during contact phase These explorative results require further evaluation with larger samples including homogenous groups of individuals with CP.