Biomechanical models in the lower-limb exoskeletons development: a review

Abstract

Lower limb exoskeletons serve multiple purposes, like supporting and augmenting movement. Biomechanical models are practical tools to understand human movement, and motor control. This paper provides an overview of these models and a comprehensive review of the current applications of them in assistive device development. It also critically analyzes the existing literature to identify research gaps and suggest future directions. Biomechanical models can be broadly classified as conceptual and detailed models and can be used for the design, control, and assessment of exoskeletons. Also, these models can estimate unmeasurable or hard-to-measure variables, which is also useful within the aforementioned applications. We identified the validation of simulation studies and the enhancement of the accuracy and fidelity of biomechanical models as key future research areas for advancing the development of assistive devices. Additionally, we suggest using exoskeletons as a tool to validate and refine these models. We also emphasize the exploration of model-based design and control approaches for exoskeletons targeting pathological gait, and utilizing biomechanical models for diverse design objectives of exoskeletons. In addition, increasing the availability of open source resources accelerates the advancement of the exoskeleton and biomechanical models. Although biomechanical models are widely applied to improve movement assistance and rehabilitation, their full potential in developing human-compatible exoskeletons remains underexplored and requires further investigation. This review aims to reveal existing needs and cranks new perspectives for developing more effective exoskeletons based on biomechanical models.

Keywords: Assistive device; Biomechanical model; Exoskeleton; Lower-limb; Neuromuscular model.

Fig. 1

Number of papers per year focusing on applications of biomechanical models in lower limb exoskeleton development included in our review (see “Biomechanical models in developing assistive devices” section  for the search method). We covered only some part of 2023 (until end of June)

Fig. 2

Overview of various biomechanical models for gait, classified into conceptual and detailed categories along with their subcategories. Conceptual models serve as abstract representations of complex mechanics of gait. Detailed models attempt to integrate the mechanical principles of biological systems with the neural control of these systems, providing a more complete picture of the underlying interactions between the nervous and musculoskeletal systems

Fig. 3

Applications of biomechanical gait models in the development of lower-limb exoskeletons for design, control, assessment and estimation. Estimated variables (e.g., muscle activation, muscle dynamics, muscle force, joint load, metabolic cost, etc.) can be used in other mentioned applications. By leveraging biomechanical models, these applications contribute to the design and refinement of assistive devices that enhance gait in both impaired and unimpaired individuals. Predesign state refers to experimental data collection for model development and tracking simulation in the design process. We used OpenSim software to generate visualization for the musculoskeletal model [48]

Fig. 4

Using biomechanical models to design a controller for assistive devices. A Offline design of the desired trajectory. B Adaptive model-based control

Fig. 5

The number of reviewed papers that designed exoskeletons and/or controllers based on biomechanical models. Just simulation means the designed exoskeletons and controllers are tested in simulation without experimental validation. The green curve shows papers that designed an exoskeleton and/or controller in simulation and validated it in an experiment. The orange curve shows papers that used biomechanical models in a control loop without testing it in a simulation environment. The total curve shows the total number of papers which considered designing a controller based on biomechanical models for a specific degree of freedom. Each level in this graph represents five papers

Fig. 6

Distribution of user groups in lower limb exoskeleton research: biomechanical model-based design reviewed in this article (blue) vs. general exoskeleton literature review from [172, 173] (red). “Others” include multiple sclerosis, poliomyelitis, spinocerebellar degeneration patients and research papers that are not specifically categorized under the aforementioned user groups

Fig. 7

Distribution of design criteria in lower limb exoskeleton research. Biomechanical model-based design reviewed in this article (blue) vs. general exoskeleton literature review from [172, 173] (red). “Others” include cognitive effort, dependability and coordination