Abstract
BackgroundIncreased resistance in muscles and joints is an important phenomenon in patients with cerebral palsy (CP), and is caused by a combination of neural (e.g. spasticity) and non-neural (e.g. contracture) components. The aim of this study was to simulate instrumented, clinical assessment of the hamstring muscles in CP using a conceptual model of contracture and spasticity, and to determine to what extent contracture can be explained by altered passive muscle stiffness, and spasticity by (purely) velocity-dependent stretch reflex.MethodsInstrumented hamstrings spasticity assessment was performed on 11 children with CP and 9 typically developing children. In this test, the knee was passively stretched at slow and fast speed, and knee angle, applied forces and EMG were measured. A dedicated OpenSim model was created with motion and muscles around the knee only. Contracture was modeled by optimizing the passive muscle stiffness parameters of vasti and hamstrings, based on slow stretch data. Spasticity was modeled using a velocity-dependent feedback controller, with threshold values derived from experimental data and gain values optimized for individual subjects. Forward dynamic simulations were performed to predict muscle behavior during slow and fast passive stretches.ResultsBoth slow and fast stretch data could be successfully simulated by including subject-specific levels of contracture and, for CP fast stretches, spasticity. The RMS errors of predicted knee motion in CP were 1.1 ± 0.9° for slow and 5.9 ± 2.1° for fast stretches. CP hamstrings were found to be stiffer compared with TD, and both hamstrings and vasti were more compliant than the original generic model, except for the CP hamstrings. The purely velocity-dependent spasticity model could predict response during fast passive stretch in terms of predicted knee angle, muscle activity, and fiber length and velocity. Only sustained muscle activity, independent of velocity, was not predicted by our model.ConclusionThe presented individually tunable, conceptual model for contracture and spasticity could explain most of the hamstring muscle behavior during slow and fast passive stretch. Future research should attempt to apply the model to study the effects of spasticity and contracture during dynamic tasks such as gait.Electronic supplementary materialThe online version of this article (doi:10.1186/s12984-016-0170-5) contains supplementary material, which is available to authorized users.
Highlights
Increased resistance in muscles and joints is an important phenomenon in patients with cerebral palsy (CP), and is caused by a combination of neural and non-neural components
Optimization of passive muscle properties The net knee moments as obtained from the inverse dynamic (ID) analysis revealed that the CP children had a steeper knee momentangle curve (Fig. 2a, dashed lines), and overall stiffer knee extensor muscles and knee flexor muscles
These measured net knee moment-angle curves could almost perfectly be replicated by optimizing K and S for hamstrings and vasti (Fig. 2a, solid lines)
Summary
Increased resistance in muscles and joints is an important phenomenon in patients with cerebral palsy (CP), and is caused by a combination of neural (e.g. spasticity) and non-neural (e.g. contracture) components. Cerebral palsy (CP) is the most common neurological disorder in children and is attributed to non-progressive disturbances occurring in the developing fetal or infant brain [1]. It is characterized primarily by neural deficits (caused by the brain anomalies) and secondary by muscular and bone deformities [2, 3]. Recent studies suggest that muscle stiffness in CP already appears at an early age [13] and that maladaptation to growth, rather than spasticity, plays a crucial role in developing contractures [14]
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