Investigation of neural and biomechanical impairments leading to pathological toe and heel gaits using neuromusculoskeletal modelling

Abstract

Abstract

This study investigates the pathological toe and heel gaits seen in human locomotion using neuromusculoskeletal modelling and simulation. In particular, it aims to investigate potential cause–effect relationships between biomechanical or neural impairments and pathological gaits. Toe and heel gaits are commonly present in spinal cord injury, stroke and cerebral palsy. Toe walking is mainly attributed to spasticity and contracture at plantar flexor muscles, whereas heel walking can be attributed to muscle weakness of biomechanical or neural origin. To investigate the effect of these impairments on gait, this study focuses on the soleus and gastrocnemius muscles as they contribute to ankle plantarflexion. We built a reflex circuit model based on previous work by Geyer and Herr with additional pathways affecting the plantar flexor muscles. The SCONE software, which provides optimisation tools for 2D neuromechanical simulation of human locomotion, is used to optimise the corresponding reflex parameters and simulate healthy gait. We then modelled various bilateral plantar flexor biomechanical and neural impairments, and individually introduced them in the healthy model. We characterised the resulting simulated gaits as pathological or not by comparing ankle kinematics and ankle moment with the healthy optimised gait based on metrics used in clinical studies. Our simulations suggest that toe walking can be generated by hyperreflexia, whereas muscle and neural weaknesses partially induce heel gait. Thus, this ‘what if’ approach is deemed of great interest as it allows investigation of the effect of various impairments on gait and suggests an important contribution of active reflex mechanisms to pathological toe gait.

Key points

• Pathological toe and heel gaits are commonly present in various conditions such as spinal cord injury, stroke and cerebral palsy.

• These conditions present various neural and biomechanical impairments, but the cause–effect relationships between these impairments and pathological gaits are difficult to establish clinically.

• Based on neuromechanical simulation, this study focuses on the plantar flexor muscles and builds a new reflex circuit controller to model and evaluate the potential effect of both neural and biomechanical impairments on gait.

• Our results suggest an important contribution of active reflex mechanisms to pathological toe gait.

• This ‘what if’ based on neuromechanical modelling is thus deemed of great interest to target potential causes of pathological gait.

Abstract

Abstract figure legend Various biomechanical and neural impairments are individually modelled at the level of the plantar flexor muscles in a musculoskeletal model and a complex reflex circuit‐based gait controller. For instance, as shown on the left, the plantar flexor spindle reflex gain (KS) is increased to mimic hyperreflexia. The gait controller is then optimised for each of the impaired conditions and the resulting gaits are characterised as pathological based on ankle kinematics and ankle moment metrics used in clinical studies. Thus, this ‘what if’ approach allows the investigation of the effect of various impairments on gait presented in the table on the right.