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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 FROM AUTHOR]

Objectives: To clarify a characteristic of dynamic stability during gait in idiopathic normal pressure hydrocephalus (iNPH) and Parkinson's disease (PD), and to explore the association between dynamic stability and disease severity in each disease. Materials and Methods: The 5‐m gait of 36 iNPH (precerebrospinal fluid drainage), 20 PD (medicated state), and 25 healthy controls (HC) were evaluated using three‐dimensional motion analysis. Ambulatory dynamic stability was defined as the ability to maintain the extrapolated center of mass within the base of support at heel contact, with the distance between the two referred to as the margin of stability (MOS). Results: Anteroposterior direction (AP) MOS was significantly larger in the iNPH and PD groups than in the HC group; no significant difference was found between the iNPH and PD groups. Mediolateral direction (ML) MOS was significantly larger in the iNPH and PD groups than in the HC group and significantly larger in the iNPH group than in the PD group. In the iNPH group, the disease severity was positively correlated with only ML MOS. In the PD group, the disease severity was positively correlated with the AP MOS and ML MOS. Conclusions: Dynamic stability in iNPH increases in AP and ML, and it may be associated with not only iNPH‐associated gait disturbance but also with a voluntarily cautious gait strategy. Dynamic stability in PD only increased in AP, and this may be associated with PD symptoms. These findings will help physicians understand the difference in pathological gait including dynamic stability between patients with iNPH and PD. [ABSTRACT FROM AUTHOR]

New Findings: What is the topic of this review?This narrative review explores past and recent findings on the mechanical determinants of energy cost during human locomotion, obtained by using a mechanical approach based on König's theorem (Fenn's approach).What advances does it highlight?Developments in analytical methods and their applications allow a better understanding of the mechanical–bioenergetic interaction. Recent advances include the determination of 'frictional' internal work; the association between tendon work and apparent efficiency; a better understanding of the role of energy recovery and internal work in pathological gait (amputees, stroke and obesity); and a comprehensive analysis of human locomotion in (simulated) low gravity conditions. During locomotion, muscles use metabolic energy to produce mechanical work (in a more or less efficient way), and energetics and mechanics can be considered as two sides of the same coin, the latter being investigated to understand the former. A mechanical approach based on König's theorem (Fenn's approach) has proved to be a useful tool to elucidate the determinants of the energy cost of locomotion (e.g., the pendulum‐like model of walking and the bouncing model of running) and has resulted in many advances in this field. During the past 60 years, this approach has been refined and applied to explore the determinants of energy cost and efficiency in a variety of conditions (e.g., low gravity, unsteady speed). This narrative review aims to summarize current knowledge of the role that mechanical work has played in our understanding of energy cost to date, and to underline how recent developments in analytical methods and their applications in specific locomotion modalities (on a gradient, at low gravity and in unsteady conditions) and in pathological gaits (asymmetric gait pathologies, obese subjects and in the elderly) could continue to push this understanding further. The recent in vivo quantification of new aspects that should be included in the assessment of mechanical work (e.g., frictional internal work and elastic contribution) deserves future research that would improve our knowledge of the mechanical–bioenergetic interaction during human locomotion, as well as in sport science and space exploration. [ABSTRACT FROM AUTHOR]