REVIE W Open Access
Compliant lower limb exoskeletons: a
comprehensive review on mechanical
design principles
Maria del Carmen Sanchez-Villamañan, Jose Gonzalez-Vargas, Diego Torricelli
*
, Juan C. Moreno and Jose L. Pons
Abstract
Exoskeleton technology has made significant advances during the last decade, resulting in a considerable variety of
solutions for gait assist ance and rehabilitation. The mechanical design of these devices is a crucial aspect that
affects the efficiency and effectiveness of their interaction with the user. Recent developments have pointed
towards compliant mechanisms and structures, due to their promising potential in terms of adaptability, safety,
efficiency, and comfort. However, there still remain challenges to be solved before compliant lower limb
exoskeletons can be deployed in real scenarios. In this review, we analysed 52 lower limb wearable exoskeletons,
focusing on three main aspects of compliance: actuation, structure, and interface attachment components. We
highlighted the drawbacks and advantages of the different solutions, and suggested a number of promising
research lines. We also created and made available a set of data sheets that contain the technical characteristics of
the reviewed devices, with the aim of providing researchers and end-users with an updated overview on the
existing solutions.
Keywords: Assistance, Compliant actuation, Mechanical compliance, Mechanical design, Lower limb exoskeleton,
Rehabilitation
Background
Robotic wearable exoskeletons
1
have potential impact in
several application domains, like industry [1], space [2]
and healthcare [3]. In the healthcare sector, this technol-
ogy is expected to contribute by reducing the clinical
costs associated with the assistance and rehabilitation of
people with neurological and age-related disorders [3–
6]. Research in this area is clearly shifting toward the in-
clusion of compliant elements (i.e. actuators, structure
2
,
etc.) as a way to overcome the main drawbacks of rigid
exoskeletons, in terms of adaptability, comfort, safety
and efficiency [7 ].
Currently, there is a large variety of designs of lower
limb compliant exoskeletons aimed at gait rehabilitation
or assistance. However, there is a lack of detailed infor-
mation about the mechanical components of these de-
vices, which has been largely overlooked by previous
reviews (e.g. [7–9]). These variety and lack of
information makes it difficult for developers to identify
which design choices are most important for a specific
application, user’s need or pathology. For this reason, we
aimed to bring together available literature into a com-
prehensive review focused on existing lower limb wear-
able exoskeletons that contain compliant elem ents in
their design.
In this work, we refer to ‘compliant exoskeleton’ as a
system that includes compliant properties derived from
non-rigid actuation system and/or structure. Our review
focused on three particular aspects: the actuation tech-
nology, the structure of the exoskeleton and the inter-
face attachment components
3
.
We have gathered the mechanical and actuation char-
acteristics of 52 devices into standardized data sheets
(available at Additional file 1), to facilitate the process of
comparison of the different solutions under a unified
and homogeneous perspective. We consider that such a
comprehensive summary will be vital to res earchers and
developers in search for an updated design reference.
© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons. org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver
(http://creativecommons.org/publicdomain/zero/1.0/ ) applies to the data made available in this article, unless otherwise stated.
* Correspondence: diego.torricelli@csic.es
Neural Rehabilitation Group, Cajal Institute, Spanish Nati onal Research
Council (CSIC), Avda Doctor Arce, 37, E-28002 Madrid, Spain
Sanchez-Villamañan et al. Journal of NeuroEngineering and Rehabilitation
(2019) 16:55
https://doi.org/10.1186/s12984-019-0517-9
