TYPE Systematic Review

PUBLISHED 10 January 2023

DOI 10.3389/fbioe.2022.1044275

Modelling the interaction between

OPEN ACCESS wearable assistive devices and
EDITED BY
Ajay Seth,
Delft University of Technology,
digital human models—A
Netherlands

REVIEWED BY
systematic review
Massimo Sartori,
University of Twente, Netherlands
Thomas K. Uchida,
David Scherb*, Sandro Wartzack and Jörg Miehling
University of Ottawa, Canada
Friedrich-Alexander-Universität Erlangen-Nürnberg, Engineering Design, Erlangen, Germany
*CORRESPONDENCE
David Scherb,
scherb@mfk.fau.de

SPECIALTY SECTION
Exoskeletons, orthoses, exosuits, assisting robots and such devices referred to as
This article was submitted
to Biomechanics, wearable assistive devices are devices designed to augment or protect the human
a section of the journal body by applying and transmitting force. Due to the problems concerning cost-
Frontiers in Bioengineering and and time-consuming user tests, in addition to the possibility to test different
Biotechnology
configurations of a device, the avoidance of a prototype and many more
RECEIVED 14 September 2022
advantages, digital human models become more and more popular for
ACCEPTED 21 December 2022
PUBLISHED 10 January 2023 evaluating the effects of wearable assistive devices on humans. The key
indicator for the efficiency of assistance is the interface between device and
CITATION
Scherb D, Wartzack S and Miehling J human, consisting mainly of the soft biological tissue. However, the soft
(2023), Modelling the interaction between biological tissue is mostly missing in digital human models due to their rigid
wearable assistive devices and digital
body dynamics. Therefore, this systematic review aims to identify interaction
human models—A systematic review.
Front. Bioeng. Biotechnol. 10:1044275. modelling approaches between wearable assistive devices and digital human
doi: 10.3389/fbioe.2022.1044275 models and especially to study how the soft biological tissue is considered in the
COPYRIGHT simulation. The review revealed four interaction modelling approaches, which
© 2023 Scherb, Wartzack and Miehling. differ in their accuracy to recreate the occurring interactions in reality.
This is an open-access article distributed
under the terms of the Creative Commons
Furthermore, within these approaches there are some incorporating the
Attribution License (CC BY). The use, appearing relative motion between device and human body due to the soft
distribution or reproduction in other biological tissue in the simulation. The influence of the soft biological tissue
forums is permitted, provided the original
author(s) and the copyright owner(s) are
on the force transmission due to energy absorption on the other side is not
credited and that the original publication in considered in any publication yet. Therefore, the development of an approach to
this journal is cited, in accordance with integrate the viscoelastic behaviour of soft biological tissue in the digital human
accepted academic practice. No use,
distribution or reproduction is permitted
models could improve the design of the wearable assistive devices and thus
which does not comply with these terms. increase its efficiency and efficacy.

KEYWORDS

digital human model, musculoskeletal modelling, multi-body dynamic, interaction

modelling, systematic review, soft tissue, wearable assistive device

1 Introduction
Exoskeletons, orthoses, exosuits, assisting robots and such devices are special products
designed to support the human body in a specific way. These devices augment or protect the
human body by applying force to it and are additionally wearable. Attempting to identify a
generic term for these types of devices, in this publication the expression “wearable assistive
devices” (WADs) following Asbeck et al. (2014) is used. In other publications, these product
types are referred to as “physical assistive devices” (Mombaur and Ho Hoang, 2017), “wearable
power assistive devices” (Imamura et al., 2011), “support systems/devices” (Miehling et al.,
2018), “physical support systems” (Argubi-Wollesen and Weidner, 2018), “assistive devices”
(Sartori et al., 2012) or just “exoskeletons” (Collins et al., 2015). Regarding their intended use,

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WADs can be classified into three groups (Young and Ferris, 2017). elimination of the need to create prototypes in early design stages, the
The first group are the devices that augment the performance of possibility to test different configurations of the device, with the effects
healthy subjects. First, they increase the humans’ strength and of these variations being identified easily and more quickly, the early
endurance and minimize the risk of injury (Tröster et al., 2020; acquisition of more technical knowledge, a better performance and
Fritzsche et al., 2022), making them suitable in a working quality in the final product and, most importantly, a reduction in
environment, for e.g., lifting objects (Millard et al., 2017) and design time and cost (Agarwal et al., 2013; Ferrati et al., 2013; Zhou
working over-head (Molz et al., 2022). Second, the human effort and Li, 2016; Dembia et al., 2017). However, all insights during the
for accomplishing activitites should be reduced. A common example execution of user tests cannot be completely substituted by
for this application is the metabolic energy decrease for different musculoskeletal simulation studies, which makes the simulation a
activities (Collins et al., 2015; Dembia et al., 2017; Miehling et al., good complementary tool to experimental testing (Manns et al., 2017).
2018). The second group of classification for WADs aims to support In order to build up a complete human-WAD model, a lot of
humans with an enduring disability. Motor or neurological disorders elements have to be considered for modelling. The main goal is always
like stroke, spinal cord injury or cerebral palsy can lead to difficulties the recreation of natural circumstances in the virtual representation to
for affected subjects to execute movements. WADs aid people to account for the correctness of simulation results and transferability to
perform movements they are unable to do on their own, like walking reality (Ferrati et al., 2013). Thus, elements like the device itself with its
(Afschrift et al., 2014; Michaud et al., 2019; Yamamoto et al., 2019) or dimensions and material properties resulting in parameters like mass
arm movements (Rahman et al., 2007). The third category are devices or inertia (Ferrati et al., 2013), the combination of single parts of the
for therapeutic rehabilitation, which accounts for a specific crossover WAD, e.g., via joints (Millard et al., 2017), functional parameters of
with the second group. These devices assist disabled humans after a the devices [e.g., orthosis stiffness (Yamamoto et al., 2019)],
disease until the previous performance of the body is restored (Shi, controllers for regulation of the device (Durandau et al., 2019; Yin
2018; Akbas and Sulzer, 2019; Zhou et al., 2020). Therefore, the et al., 2019), actuation power and timing (Fournier et al., 2018) and
difference to the second group is represented by the temporary use many more elements, depending on the specific WAD itself, have to be
of the products. included into the virtual environment. A key indicator for the efficient
WADs need to provide sufficient support to the human body to be interaction between the human body and the device is the interface
useful. By performing user tests, the efficacy of the devices can be (Sánchez-Villamañán et al., 2019). The interface consists of the
evaluated and improved. Popular parameters investigated are the attachment types of the product (mostly straps or cuffs) and
oxygen consumption or heart rate of the subjects to observe the mainly of the biological soft tissue (skin, fat, muscles, etc.), which
physical load on the body (Danielsson and Sunnerhagen, 2004; covers the movement-executing structure of the human body, the
Fritzsche et al., 2021) and user (dis-)comfort to analyze product bones, which the WAD actually targets (Yandell et al., 2017; Young
acceptance and product safety (Mills et al., 2012; Lucas-Cuevas and Ferris, 2017). The biological soft tissue determines human
et al., 2014; Linnenberg and Weidner, 2022). Thus, user tests are comfort and possible occurring injuries, like scratches or bleedings
beneficial to improve the design of the WAD and make it more (Young and Ferris, 2017). Ultimately, two main factors concerning the
suitable to users’ requirements. However, there are some problems biological soft tissue influence the resulting efficiency and effectivity of
with user tests. The steps of building a prototype, testing it on different the transmitted assistance by the device (Sánchez-Villamañán et al.,
users, gathering user feedback, adapting the design of the device, 2019). The first one is the occurring relative motion between the
building a new prototype and thus starting the cycle from the human body and the device, resulting in the so-called misalignment
beginning results in a very cost- and time-intensive process to (Figure 1). Due to the attachment of the WAD on the skin of the
design a final product (Agarwal et al., 2013; Ferrati et al., 2013; human, a distance (x) between the joint centers of both collaborators is
Fritzsche et al., 2021). Due to the interaction of WADs with the present in reality (Figure 1A). Consequently, the movement of the
human body, it is also suitable to investigate the effects on the human limb results in a shift Δ L and a rotation φ of the device
biomechanics of the users (Yamamoto et al., 2019). The (Figure 1B), which affects the efficacy of the support and can cause
biomechanical parameters, however, are mostly either hard to discomfort and pain for the users due to compressed and sheared skin
measure, e.g., using electromyography (EMG) to investigate muscle (Schiele and van der Helm, 2006; Zanotto et al., 2015; van Dijk et al.,
activations (Fritzsche et al., 2022; Molz et al., 2022) or in some cases 2017). Furthermore, the identification of the subject’s joint axes to
even impossible to determine, e.g., joint reaction forces (Neptune et al., perfectly align the WADs joint axes is very challenging,
2000; Zhou, 2020) during a study. Additionally, the use of WADs in consequently resulting in the misalignment (Näf et al., 2018;
user tests can be limited due to ethical and legal restrictions (Neptune Mallat et al., 2019). The second factor influencing assistance of
et al., 2000; Fritzsche et al., 2022). Patient safety is the most critical a WAD is the tendency of the biological soft tissue to act as an
aspect to consider, restricting the freedom of testing different energy sink (Young and Ferris, 2017). Yandell et al. (2017)
properties of the device and requiring justification of the benefits demonstrated in their study that about 25% of the power
of the test type. In order to address these challenges, the trend of using provided by their device was lost during transmission. The
digital human models (DHMs), and more precisely musculoskeletal remaining 75% indeed augmented the human body, but part of
human models (MHMs) to evaluate WADs has emerged in recent it was absorbed during loading and had therefore a delayed
years. Especially MHMs provide the advantages of investigating the contribution to the assistance. Other contributions even report
effects on the human body itself, in particular on parameters of the a power loss of 50% of the provided energy (Asbeck et al., 2015).
musculoskskeletal human system like muscle activations and joint Thus, the compliance and viscoelastic behaviour of the biological
reaction forces, and evaluating the interactions between the human soft tissue can have a considerable influence, resulting in a desired
and the device design (Fournier et al., 2018) without compromising support for the human body (Pons, 2010; Rossi et al., 2011;
the health of the user. Further advantages of this tool include the Quinlivan et al., 2016). Asbeck et al. (2014) are even indicating

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FIGURE 1
Misalignment shown on a simple degree of freedom joint attached with a WAD based on (Schiele and van der Helm, 2006); (A) initial aligment of human
limb and WAD limb, (B) Occurring relative motion between human limb and WAD limb due to movement of the single degree of freedom joint of the human
limb.

human-machine interface compliance as a major roadblock to synonyms and, second, WADs and other names of this product type
designing exoskeletons. (as already mentioned in the introduction) and names of single
Considering the rising use of DHMs for designing WADs, the products like exoskeleton or orthosis. The used search string was:
biological soft tissue is missing in DHMs due to the underlying rigid [(“musculoske* model*”) OR (“musculoske* simul*”) OR (“digital*
body dynamics. This raises the question whether this important and human model*”) OR (“human model*”) OR (“biomech* model*”) OR
non-negligible influence of the compliant biological soft tissue is taken (“biomech* simulat*”) OR (“computat* model*”) OR (“human
into account. A missing consideration could lead to results that deviate neurom* model*”)] AND [(exoskelet*) OR (“assisti* device*”) OR
from reality and, accordingly, to different or incorrect implications for (orthos?s) OR (orthotic*) OR (exosuit*) OR (“assist* robot*”) OR
the design and application of the device and, consequently, to negative (“assist* robot* device*”) OR (“support* device*”)]. The specific
effects for the user (Schiele and van der Helm, 2006; Zanotto et al., search strings for each database can be found in the Supplementary
2015). There should be different possibilities or approaches to model Material (“Full search strings”). This generic string was chosen to find
the interaction between human body and device, in order to ensure a publications using multibody simulations of WADs and to identify the
valid transfer of the obtained information to reality. Thus, we want to modelled interaction or interface between the model and the device.
investigate and answer the following research questions in this This strategy was applied to increase the probability of finding papers
contribution: describing the modelled interaction or interface of digital human
RQ1: How is the interaction between wearable assistive devices models and WADs, since “(interaction OR interface) modelling” is not
and human represented by existing models in the digital simulation explicitly mentioned in many papers. For the applied search string, all
environment? paper types published in English language were included in the
RQ2: How is the influence of the biological soft tissue at the systematic review. In “Web of Science” the literature search was
interface between human and wearable assistive devices (regarding the conducted in all fields, whereas in “Scopus” the search was
occurring relative motion and alteration of power support) considered conducted on title, Abstract and Keywords. The date of the
in these interaction models? literature research was 18 March 2022. After finishing the analysis,
Our aim is to show different possibilities and approaches for an additional check of the databases was conducted on 15 August
modelling the interaction between DHMs and WADs, which was not 2022 to identify published material in the meantime.
done yet to our knowledge and to analyze these interaction modelling
approaches in terms of their consideration of real-world interface
behaviour. Furthermore, we wanted to investigate the consideration of 2.2 Literature scanning process
the biological soft tissue in the digital human simulation of WADs,
which is in our opinion a key factor for a sufficient transfer of gained The found literature of every database was extracted and saved in
simulation results to the design of a physical device. Microsoft Excel (Microsoft, 2016). Then, a self-written macro was
applied to identify and remove all duplicates. With the resulting
publications, the real screening process began. First, the titles and
2 Methods abstracts of the identified publications were screened. In this step,
mainly papers not using DHMs (e.g., finite-element models) and
2.1 Search strategy and study selection papers analyzing devices not embedded to the definition of WADs
from the introduction (e.g., wheelchairs or hearing aids) were
To answer the aforementioned research questions, a systematic excluded. In the next steps, the remaining papers were screened by
review of the literature was conducted. The electronic databases reviewing the material and methods and full paper. The material and
“Scopus” and “Web of Science” were searched. The basic search methods part was especially screened because the type of modelled
string resulted in a combination of, first, digital human models and interaction or interface created is described in this part. During these

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FIGURE 2
PRISMA flow diagram of study selection and screening process.

two review loops, publications were excluded if they, first, were not 779 publications for screening. Papers were excluded during the
dedicated to the interaction of DHMs and WADs, but rather depict the screening process based on the aforementioned exclusion criteria.
interface between the device and a computer to regulate the device During screening of title and abstract 420 records and after
(Fleischer and Hommel, 2008). Secondly, papers were excluded that screening of material and methods 191 records were excluded.
used DHMs as a pre-investigating tool for the design of the WADs. For The full text of the remaining publications was checked for
example, if investigations concerning the biomechanical changes of eligibility of these records resulting in an exclusion of another
humans with a disease (e.g., after stroke or with crouch gait) were 47 records. By the backward search in the reference lists of the
conducted to gain knowledge for a later use or design of WADs (Knarr remaining records, three papers were added to the literature
et al., 2013; Steele et al., 2013; Waterval et al., 2021) or if DHMs were review, resulting in 125 included publications that were further
analysed to identify intervals for muscle activities or joint torques the analyzed.
device should support or replace (Durandau et al., 2019). The last
exclusion criterion was the use of DHMs to evaluate effects of physical,
manufactured WADs on a diseased patient (Ewing et al., 2016; Choi 3.2 Interaction modelling approaches for
et al., 2017; Yamamoto et al., 2019). In this scenario, a patient is mostly DHMs and WADs
equipped with a WAD, is recorded in a motion laboratory and the
DHM is used to identify biomechanical data of the recorded data, like A detailed list of the included papers and assigned classification for
joint angles or torques (Yamamoto et al., 2019).Basically, papers were these publications is provided in the Supplementary Material (“Paper
included in the literature review that simulated the effect of WADs on classification”). The reviewed literature reveals four different
DHMs and therefore had to couple the two collaborators in the approaches for simultaneously coupling a WAD with a DHM. The
simulation. Additionally, the reference lists of the included approaches are arranged according to their accuracy of reproducing
publications were screened to identify important literature that the interaction in the real world. In order to illustrate the used
were missed during the mentioned screening process and also interaction modelling approaches, the example of an ankle-foot
included in the literature review. The full text of all included orthosis (AFO) is used (Figure 3).
papers was then analyzed for eligibility and to identify and classify The first group is characterized by the missing virtual
approaches for interaction/interface modelling between DHMs and representation of the WAD in the simulation environment, which
WADs and to examine them regarding their strengths and limitations. means that only the effect of the device is considered. This effect can
either be a provided torque (Cholewicki, 2004; Farris et al., 2014;
Karavas et al., 2015; Jackson et al., 2017) or an applied force (Sawicki
3 Results and Khan, 2016; Inose et al., 2017; Kim et al., 2017; Yang et al., 2019).
Furthermore, the efficacy of this assistance is always provided ideally
3.1 Filtering via search strategy (Uchida et al., 2016; Dembia et al., 2017; Franks et al., 2020). Hence, a
torque is provided exactly for the coordinate axis to be supported
Figure 2 presents the study selection and screening process and (Font-Llagunes et al., 2011; Afschrift et al., 2014) or the force is applied
the final outcome of included papers in the literature review. After to one steady, non-changing point in perfect effective line for the
searching all databases, 1,033 records were identified. 254 of these assistance (Ueda et al., 2007; Chen et al., 2019). In this group the
records could be removed due to duplications, resulting in abstraction of the WAD’s impact as an added virtual muscle in the

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FIGURE 3
Identified approaches for modelling the interaction between WADs and DHMs arranged according to their reproducibility of the real interface behaviour,
represented on the example of an ankle-foot-orthosis (AFO). To symbolise the possible motions at the interface of WAD and AFO figures of mechanical
support structures are used.

DHM accounting for the applied force (Ganesan and Gupta, 2021; device and DHM as soft, which allows small relative motions
Harbauer et al., 2022) is also classified. (Zhou et al., 2017; Fritzsche et al., 2022). Due to motion of the
In the second group of interaction modelling approaches a virtual device to the model, the force being applied is also not steady and
representation of the WAD is now considered in the simulation. The does not operate idealized, which represents the real occurrence
interaction of the WAD and the DHM is realized by a rigid connection during performing with a WAD.
(Imamura et al., 2011; Ferrati et al., 2013; Lancini et al., 2016) resulting The fourth classification of interaction modelling approaches
in no possible shift or rotation between the two partners. This between WADs and DHMs highlights no difference to group three
connection is done by fixing one point of the device to one point in terms of the connection between the two partners. However, an
of the model’s corresponding bone (e.g., in the MHM simulation important parameter for the design of the device (Silva et al., 2010;
software OpenSim (Delp et al., 2007; Seth et al., 2011; Seth et al., 2018) Serrancoli et al., 2019) is considered, which is the occurring interaction
this is done by a weldJoint (Yamamoto et al., 2021) or via a constraint force (Fi in Figure 4) at the interface. Due to the applied force on the
(Ferrati et al., 2013)) and thereby matching the device to the human’s human body by the WADs, the biological soft tissue is on the one hand
anatomy (see Figure 3.2) (Ferrati et al., 2013; Michaud et al., 2019; pressured (pressure force) and on the other hand sheared due to the
Yamamoto et al., 2021). Kruif et al. (2017) even align the WAD to the occuring relative motion (friction force) (Pons, 2010; Silva et al., 2010).
model’s bones, in other words assumes the device to be integrated into Both of these forces combine to the interaction force at the interface of
the human kinematical system. In this classification the effect of the the device and human. The resulting pressure of the interaction over
inserted WAD is either determined by the set stiffness of the device the contact area is a key factor for user comfort, safety and possible
resulting in a force being applied to the fixation points of device and occurring injuries (Pons, 2010; Fournier et al., 2018; Serrancoli et al.,
model (Ferrati et al., 2013; Tröster et al., 2020; Yamamoto et al., 2021) 2019; Zhang et al., 2021). Therefore, the publications classified in
or by applying an external force or torque equally to group 1 (Kruif group 4 provide modelling approaches to simulatively estimate the
et al., 2017; Yin et al., 2019; Gordon et al., 2022). interaction forces and pressures and to evaluate by that the (dis-)
The identified third group for interaction modelling approaches comfort of the device. The identified approaches can be divided in two
constitutes one basic change compared to the previous one. The virtual different types, which are both based on using contact models for
representation of the WAD is also present in the simulation determining the interaction force. The first type is based on the
environment, but the connection with the model is supplemented definition of two points, one for the body and one for the WAD,
by additional degrees of freedom (Figure 3.3) (To et al., 2005; Zhou who are coincident in their initial position (Popovic, 1990; Fournier
et al., 2017; Panero et al., 2020). Thereby, translation and rotation of et al., 2018; Serrancoli et al., 2019; Zhou, 2020). Due to possible relative
the device to the human model is possible, the execution depends on motion between WAD and DHM, the two points will shift from each
the studied use case (Arch et al., 2016; Moosavian et al., 2018; Fritzsche other. By applying a tri-directional spring-damper force element
et al., 2022). The connection is done via kinematic constraints (To between the two anchor points (Lee et al., 2018; Luo et al., 2018;
et al., 2005; Panero et al., 2019; Liu et al., 2021) or defined joints (e.g., in Serrancoli et al., 2019), the occurring interaction force can then be
OpenSim via FreeJoint or CustomJoint) (Zhu et al., 2013; Zhou et al., calculated due to the equation shown in Figure 4A (Zhou, 2020). With
2015; Arch et al., 2016). Furthermore, the AnyBody modelling given stiffness constants kx, ky, kz and damping constants cx, cy, cz
software (Damsgaard et al., 2006; AnyBody Technology, 2022) depending on the resistance in every spatial direction the interaction
provides a possibility to set the fixed connection between the force in each direction (Fx, Fy, Fz) can be calculated based on the

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FIGURE 4
Identified approaches for simulating the occurring interaction force at the interface between DHM und WAD based on the contact models (A) based on
the deviation of the two contact nodes and the calculation of the force via a spring-damper system (B) based on the integration of the device node in the
cylindrical area around the model node and generation of the force; blue indicates the node of the DHM, grey indicates the node of the WAD.

this type of interaction force calculation is based on a rigid-body

method.

4 Discussion
The aim of this systematic review was to identify approaches for
modelling the interaction between a WAD and a DHM. The identified
approaches should be analysed regarding their consideration of the
real-world interface behaviour. Additionally, we wanted to study how
this interaction modelling approaches respect the influence of the
biological soft tissue at the interface regarding the occurring relative
motion between device and human and the effect on the force
transmission due to the compliance of biological soft tissue.
Arranging the publications included in this literature review by the
respective year they were published also shows the increasing use of
DHMs for the investigation of the effects of WADs on humans over
FIGURE 5 the last years (Figure 5). Furthermore, this highlights the trend and
Number of publications arranged according to the year of relevance of this topic for the future design of WADs based more on
publication of the included papers to the literature review, the dotted line the findings of virtual results and also the aid that is provided by
is the trend of publications; 2022* indicates that literature is only
considered that has been published to the date of literature search. DHMs to improve the performance and quality of the devices.

4.1 Interaction modelling approaches

distance of the two points (x, y, z) according to the initial assembly (x0, between WADs and DHMS
dy dz
y0, z0) and the derivative of the distances (dxdt , dt , dt ) resulting in the
interaction force (Fi). This calculation type accordingly incorporates In this chapter, RQ1 is answered. The systematic review revealed
the viscoelastic behaviour of the soft biological tissue and utilizes the four approaches for modelling the interaction between a WAD and a
modelling via spring and damper. The second type is also based on the DHM, which are arranged in ascending order regarding their ability to
definition of one node for each collaborator (Cho et al., 2012; Chander recreate the occurring interactions in reality. The first level depicts an
and Cavatorta, 2020; Zhang et al., 2021; Chander et al., 2022). The approach, where only the effect of the device, i.e., force or torque,
nodes are in this case not coincident with each other in the initial without any power loss is applied to the model and therefore respected
situation (Figure 4B). The node of the DHM is assigned as the base in the simulation. This approach appears as a pretty simplified
object and the node of the device as target object (Silva et al., 2010; Cho representation. The force or torque is assumed to operate always in
et al., 2012; Zhang et al., 2021). Then, for the base object a cylindrical an idealized way and even the device itself is not present in the
shape with pre-given dimensions (height h and radius r) is defined simulation environment, which also accounts for no possible
(Cho et al., 2012; Zhang et al., 2021). Contact is assumed, when the simulative interaction between the WAD and the model. However,
node of the device is moved inside the cylinder around the base object, the use of this approach is suitable and beneficial in the early phases of
generating the occurring of an interaction force (Fi). This force is the product design process, like the concept phase, to see and evaluate
realized by the virtual integration of an artificial muscle, e.g., an effects easily and fast (Miehling et al., 2018; Franks et al., 2020; Gneiting
actuator in OpenSim (Chander and Cavatorta, 2020). Thus, et al., 2022). Especially, when the design of the product is not known yet

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or the treatment of musculoskeletal diseases or their underlying causes considered. Thus, for all solutions a difference in the occurring relative
is studied, the approximation to the effect of the device is very useful motion from the simulation to the reality is pretty likely, but was not
(van der Spek et al., 2003; Bae and Tomizuka, 2011; Bianco et al., 2022). validated in the investigated publications. The identified fourth level
The second approach shows the integration of the WAD in the introduces and investigates the occurring interaction forces at the
simulation environment and the coupling of the device with the interface between WAD and model. By modelling the interaction
DHM. By adding the device to the human model, mass, center of forces, a virtual user (dis-)comfort assessment is possible. With this
mass and inertia is considered (Lancini et al., 2016; Kruif et al., 2017) gained results, the design of the devices can be improved and optimized
and accordingly influences the simulation results. The problem of this to users’ requirements (Fournier et al., 2018; Serrancoli et al., 2019),
interaction modelling approach is that device and model are fixed with which further underlines the targeted design of WADs in the virtual
each other. Thus, no slipping or rotation can occur between the environment. However, in some publications only the normal pressure
collaborators as in reality. Considering the realisation of rigid and not the friction or shear forces is investigated, which has an huge
fixation of both co-workers, researchers have to be aware of influence on the humans’ comfort (Boutwell et al., 2012; Fournier et al.,
preserving correct kinematic chains. For example, trying to model 2018; Zhang et al., 2021). Furthermore, practically no paper did a
the shown example of the orthosis (Figure 3- interaction type 2) in validation of the simulated interaction force against the real one (Silva
OpenSim by applying a weldJoint between both orthosis parts and the et al., 2010; Cho et al., 2012; Zhou, 2020). Serrancoli et al. (2019) were
corresponding bone (shank and foot) and a rotational joint between the only ones to compare their predicted forces against experimentally
both orthosis parts results in a non-viable kinematic chain. A solution measured ones, but only considered a 2D-movement and did not
could be the use of a constraint as replacement for one weldJoint as done measure the shear forces, explaining the divergence in force values.
by Kruif et al. (2017) or by combining both parts with other modelling Zhang et al. (2021) also point out that rather the trend of the calculated
solutions than a joint (Yamamoto et al., 2021). The suitable workaround interaction forces should be analysed than the magnitude. Comparing
depends essentially on the use case and WAD that should be modelled. the two different contact models for determining the interaction force,
Furthermore, since the connection is rigid, the assistance of the device is the approach with the force calculation via a spring-damper system
still idealized and without the consideration of torque loss as in level one seems to be the simpler realisation (in terms of modeling/
approaches. As a difficult classification, there are some special implementation) (Zhou, 2020) by just defining the stiffness and
approaches combining the mass, inertia and center of mass of the damping constant values in each direction accounting for a linear
WAD with the corresponding bone of the model (Schemschat et al., viscoelastic behaviour. However, a constant value for stiffness and
2016; Manns et al., 2017; Lerner et al., 2019; Nguyen et al., 2019). Here, a damping constant appears to be questionable due to the resulting, non-
virtual prototype is not present in the simulation, normally accounting linear deformation of the soft biological tissue in reality. The determination
for a classification to group one. However, due to the consideration of of these constant values is also quite a challenge. Furthermore, the
the devices’ influence on the dynamic simulation results, the interaction force is always calculated, also when the two partners
publications are classified to level two. Publications classified in level deviate from each other, which means that the WAD is moving away
three incorporate the advantages of group two and additionally provide from the body and thus does not produce an interaction force in reality.
the possibility of relative motions between the device and model. The The second approach with the induction of a force when the device node is
result is that the assistance is not idealized anymore, but shifts from the in the cylindrical shape around the DHM node is a more modelling-heavy
optimal effective line, which accounts for a loss of the transmitted task having to define the nodes of the collaborators, the size and range of
torque by device. This behaviour represents the actions also occurring in the cylindrical shape around the model node and the definition of the
reality due to misalignment. The relative motion is mainly realized by artificial muscle. The advantages are the improved definition of the
allowing additional degrees of freedom between WAD and DHM, again occurring interaction forces (Fournier et al., 2018) and the improved
having to be aware of viable kinematic chains. In the most publications representation of a planar influence on the interaction force (Silva et al.,
either one (To et al., 2005; Arch et al., 2016) or two degrees of freedom 2010; Cho et al., 2012). On the other hand, the contact force depends on the
(Panero et al., 2020; Liu et al., 2021) per fixation point are modelled at chosen optimum value of the artificial muscle to activate it. Furthermore,
most. Due to the kinematic redundancy (Yang et al., 2007) this the appearing friction force is also modelled depending on the normal force
limitation of degrees of freedom is necessary to allow the model to via a friction coefficient assuming a linear, constant dependency between
solve the present problem. The possible degrees of freedom are therefore the parameters (Christensen et al., 2021). A comparison of the two
reduced to the main directions depending on the specific use case. The calculation types has not been conducted so far. The four interaction
solution of “soft constraints” implemented in AnyBody depicts another modelling types are arranged according to their ability to recreate the
approach to respect the possible relative motions. By setting a constraint occurring interactions in reality. This ranking, however, is only valid
to soft, the constraint does not have to be fulfilled, but should be fulfilled in situations, when the included model of the WAD to the DHM is also
as well as possible due to an optimization algorithm (Christensen et al., represented correctly and in good quality. A poor representation or poorly
2021). During the implementation, the definition of single degrees of adjusted parameters do not necessarily improve the insights one might
freedom in one connection is also possible (like soft for two translations gain from the simulation, but rather lead to false implications. More
and the third translation is fixed) (Damsgaard et al., 2006). The force detailed models are not necessarily equivalent to better models. However,
from the WAD is then transmitted via the resulting, shifting position of the ambition should be directed to realising more complex models,
the interface. Furthermore, the calculation and determination of the i.e., higher rankings in this classification, in order to account for the
occurring relative motion can also be done in an extra model or tool transferability of the results to reality but users have to be aware that more
prior to the DHM (Molz et al., 2022) accounting for a special use case of uncertainties and accordingly errors can be established. Therefore, a
this level three classification. The force application on the human model validation of adjusted settings and modifications with the results from
is then similar to level one, with the difference that the possible non- real user tests is essential to ensure a high quality of the executed modelling
ideal effect due to the calculated varying point of force application is (Serrancoli et al., 2019; Franks et al., 2020).

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4.2 Consideration of soft biological tissue’s be referred to as “musculoskeletal models,” “human models,”
influence at the interface “computational models” and so on. This non-uniform
nomenclature makes it hard to find a search string including all
In the following section, RQ2 is discussed. The influence of the relevant publications for interaction modelling of WAD and DHM.
soft biological tissue for limiting the efficiency of the WADs’ This problem was assumed to be handled by the development of a
assistance to the human body is based on its compliant behaviour. rather generic and comprehensive search string capturing a high
Two main factors/causes for influencing the efficiency resulting number of publications and thus, a time-consuming screening
from that compliance were identified, the occurring relative process. However, the literature search does not ensure, that all
motion between device and human—referred to as relevant literature was discovered, since only the databases
misalignment—and the influence on the power transmission “Scopus” and “Web of Science” were used. To face this limitation
due to energy absorption. The literature review and the on the one hand a further search with the search string in “Google
identified interaction modelling approaches show that there are Scholar” was conducted and the first 100 most relevant found
several publications (the ones classified to the previously literature was studied and on the other hand a backwards search in
mentioned group three and four) incorporating the the reference lists of the identified publications was done to reveal
misalignment in their simulation of WADs with DHM. As missing relevant publications. Furthermore, a certain bias of the
already mentioned, this implementation is sometimes limited authors in the understanding and interpretation of the described
and requires further research, but reached a pretty remarkable approaches in the literature cannot be eliminated.
advancement in past years. Concerning the energy absorption
behaviour of the biological soft tissue and thus affected efficacy of
the WAD’s assistance, there are no existing approaches to 5 Conclusion and outlook
implement this effect in DHMs. Gordon et al. (2018) are the
only ones attempting to integrate the influence in their study. As a conclusion, in this literature review the approaches for
Designing an active pelvis orthosis with a musculoskeletal human modelling the interaction/interface between WADs and DHMs
model—orthosis fixed to pelvis bone and external torque applied were investigated. The analyses revealed four different
(interaction modelling approach level 2)—the power loss between approaches that were introduced and investigated according to
the generated torque and the experienced torque is simulated. For their reproduction of the real-life behaviour resulting in a listing
the generated torque a certain curve was given. The given torque of these approaches with increasing accuracy. With a special
was divided in loading and unloading phase of the human. During focus, the consideration of the soft biological tissue’s influence in
loading a percentage of the generated power is absorbed and the modelling approaches was studied. The results show that the
during unloading a percentage of this absorbed power is returned occurring relative motion between the device and
(Yandell et al., 2017; Young and Ferris, 2017; Sánchez- human—misalignment—in reality is already integrated in the
Villamañán et al., 2019) resulting in a change of the interaction modelling approaches in some way. The influence on
experienced torque curve compared to the generated torque the power transmission due to the non-linear viscoelastic
curve. The values of the percentages absorbed and returned behaviour of the soft biological tissue on the other hand is not
were taken from Yandell et al. (2017). The result was that considered in the modelling approaches so far. Due to the
compared to the ideal support of the orthosis qualitative increasing use of DHMs for investigations of the effects of
changes in the metabolic energy consumption for different WADs on the human body (compare increasing number of
walking conditions and that in these walking conditions literature in past years in Figure 5), the improvement of the
(especially for slow walking) strong differences in the energy interaction modelling determining the efficiency of the WAD
consumption of single leg muscles, like tibialis posterior or biceps assistance towards the reproduction of the real-life behaviour
femoris, were observable. However, Gordon et al. (2018) made should be the main vision. By reaching this, the need for time-
some assumptions like the simple interaction modelling approach and cost-consuming user tests will decrease and the quality of the
or the pure transfer of the data from Yandell et al. (2017), who devices can be improved. There are many possible ways to
determined the absorption and return percentage values on the continue existing research, like the comparison between
lower leg. Nevertheless, the results show that a consideration of virtual and real relative motion, the validation of simulated
the soft biological tissue’s influence on the power transmission of and real interaction forces and comparing both identified
WADs in DHMs could have a high benefit and could lead to methods (rigid-body vs. viscoelastic) for simulating the
further improvements for the virtual design of WADs. interaction forces. A promising direction, in our opinion, is
the development of an approach to consider the soft biological
tissue’s influence on the power transmission in the DHM
4.3 Limitations of the conducted systematic simulation since this is neglected so far. There are mutliple
literature review ways to realize such an approach, e.g., by the use of spring-
damper-systems (Sánchez-Villamañán et al., 2019) or by
There are also some limitations in our literature review. There is combining the tools to compute the interaction force with
no common term for what we introduced as “wearable assistive finite element models (Périé et al., 2004; Cheung and Zhang,
devices.” As already mentioned in the introduction, there are many 2008). With such a possibility, the device could be better designed
different designations for these types of devices in publications or even according to the natural circumstances and thereby, the
just the name of the device itself is mentioned (like exoskeleton or efficiency and efficacy of the WADs could be significantly
orthosis). The same applies for the “digital human models,” which can increased.

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Scherb et al. 10.3389/fbioe.2022.1044275

Data availability statement Conflict of interest

The original contributions presented in the study are included in The authors declare that the research was conducted in the
the article/Supplementary Material, further inquiries can be directed absence of any commercial or financial relationships that could be
to the corresponding author. construed as a potential conflict of interest.

Author contributions Publisher’s note

DS: review design, systematic review, data analysis and
All claims expressed in this article are solely those of the
manuscript preparation, SW: funding acquisition, supervision
authors and do not necessarily represent those of their affiliated
and manuscript review, JM: funding acquisition, supervision,
organizations, or those of the publisher, the editors and the
project administration, discussion and manuscript review. All
reviewers. Any product that may be evaluated in this article,
authors agreed to be accountable for all aspects of the work.
or claim that may be made by its manufacturer, is not guaranteed
or endorsed by the publisher.
Funding
This work was (partly) supported by the Deutsche Supplementary material
Forschungsgemeinschaft (DFG, German Research Foundation) under
Grant WA 2913/43-1 and MI 2608/2-1. This work was (partly) supported The Supplementary Material for this article can be found online at:
by the Deutsche Forschungsgemeinschaft (DFG, German Research https://www.frontiersin.org/articles/10.3389/fbioe.2022.1044275/
Foundation) under Grant SFB 1483–Project-ID 442419336. full#supplementary-material

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