Calf muscle passive stiffness and function during gait in children with cerebral palsy

Abstract

Objective: The calf muscle of individuals with cerebral palsy (CP) are smaller, weaker, more resistant to stretch, have increased antagonist co-contraction and longer Achilles tendon. These neuromuscular adaptations reduce muscular force generation and could contribute to altered gait kinematics. This study examines the relationship between ankle joint, medial gastrocnemius muscle fascicle and muscle-tendon unit (MGfas, MGmtu) passive mechanical properties and MGfas and MGmtu function during walking in children with CP. Design: Cross-sectional study. Method: Fourteen children with CP, mean age 9 years 10 months (SD 2y 9mo), including 9 males, 8 hemiplegia, 6 diplegia, GMFCS I=10, II=4, participated in the study. Passive ankle torque-angle and torque-MGfas length curves were obtained using a custom-built ankle dynamometer at three knee angles: knee extended, mid-range, flexed. Ankle torque, range and MGfas ultrasound were measured simultaneously during full-range passive ankle rotation. MGfas slack length was defined as the initial increase in fascicle length with dorsiflexion and ankle slack angle as the angle at MGfas slack length. Ankle and MGfas stiffness were calculated as the slope of the associated torque curves. Participants performed 10 barefoot walking trials while 3D-kinematics, kinetics and MGfas ultrasound were simultaneously acquired. MGfas were analysed using a semi-automated tracking algorithm and MGmtu were determined using OpenSim. Length change in MGmtu and MGfas during mid- and terminal-stance were calculated. Passive ankle and MGfas outcomes between knee angles (independent t-test) and passive and gait outcomes (Pearson correlation) were analysed (p<0.05). Results: The three test knee angles were significantly different (p<0.01). Ankle and MGfas slack angle and stiffness approached significance at the three knee positions. In midstance gait the MGmtu and MGfas lengthened 5.5 (3.8) mm and 1.6 (1.9) mm respectively. Ankle stiffness at knee midrange was significantly correlated with MGmtu lengthening during mid-stance gait (r=-0.71, F=11.96, p<0.01). Conclusion: Children with CP whom had greater passive ankle stiffness in the mid-range knee position also had less MGmtu lengthening during mid-stance walking. This finding is clinically intuitive as ankle stiffness is predominantly determined by the properties of the calf muscle-tendon unit and dependent on knee position emphasizing the impact of the two-joint calf muscle and the plantar flexor/knee extension couple in children with CP. Furthermore, during mid-stance walking, the contractile component of the calf muscle-tendon unit, MGfasc, lengthens (contracts eccentrically) highlighting the inability of the muscle to support body weight and the reliance on passive properties of the MGmtu to absorb and return energy during gait in children with CP.L6148361512017-03-21 <br />