Interactive Benefits from Switching Electrical to Magnetic
Muscle Stimulation
Yudai Tanaka
University of Chicago
yudaitanaka@uchicago.edu

Akifumi Takahashi
University of Chicago
akifumi@uchicgao.edu

Pedro Lopes
University of Chicago
pedrolopes@uchicago.edu

Figure 1. We examine the benefits of switching from electrical (EMS) to magnetic muscle stimulation (MMS). While much ink has
been spilled about the advantages of EMS, not much work has investigated circumventing its key limitations: (a) electrical impulses
cause an uncomfortable “tingling” sensation; EMS relies on pre-gelled electrodes, which require direct contact with the user’s skin,and dry up quickly. To tackle these limitations, we study (b) force-feedback based on magnetic muscle stimulation, which we found
to reduce uncomfortable tingling and enable stimulation over the clothes (VR scene replicated from [42] to contrast EMS and MMS).
ABSTRACT
improves user experience, including for VR feedback,Electrical muscle stimulation (EMS) became a popularmethod for force-feedback without mechanical-actuators.
While much has been written about the advantages of EMS,
not much work has investigated circumventing its key
limitations: (1) as impulses traverse the skin, they cause an
uncomfortable “tingling”; (2) impulses are delivered via
gelled-electrodes, which not only require direct skin contact(must be worn under clothes); but, also (3) dry up after a few
hours. To tackle these, we explore switching from electricalto magnetic muscle stimulation (MMS), via electromagnetic
fields generated by coils. The first advantage is that MMS
coils do not require direct skin contact and can actuate up to
5 cm away (Study#1)—this enables applications not possible
with EMS, such as stimulation over the clothes and without
ever replacing electrodes. Second, and more important,
MMS results in ~50 % less discomfort caused by tingling
than EMS (Study#2). We found that reducing this tingling
discomfort has two downstream effects for interactive
systems: (1) participants rated MMS force-feedback as more
realistic than that of EMS (Study#3); and (2) participants
could more accurately perceive the pose actuated by theinteractive system (Study#4). Finally, we demonstrated
applications where our proposed switch from EMS to MMS
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gaming, and pose-control.
Keywords

Magnetic Stimulation, Muscles, Force Feedback, Haptics.
INTRODUCTION

Stimulating a user’s muscles using electrical currents has
become a popular method for achieving force-feedback ininteractive experiences without requiring mechanicalactuators. Over the last decade, this technique, known aselectrical muscle stimulation (EMS), has revitalized the useof force-feedback in new contexts due to its portability.
However, while much has been written about the advantagesof EMS as an interface capable of actuating the user's body,not much rigorous work has proposed technical alternatives
that address most of its key limitations. While some work has
been done on improving the accuracy of movements inducedby means of EMS recently [24, 25, 54, 68], other key factorsthat drastically affect the user’s experience have been left
unturned. We identify outstanding and unsolved limitationsof EMS and trace their root cause to the EMS’ need forelectrodes. We summarize the three most important of thefive issues we analyzed: (1) EMS induces anuncomfortable tingling sensation: in conventional EMS,
before an electrical impulse can stimulate the muscle fibers(which contract to generate the desired effect), it conducts
via an electrode and traverses skin receptors, generating an
unwanted and uncomfortable tactile sensation often

described as “tingling” [29, 55, 58], or “buzzing” [37]; (2)EMS must be worn under clothes: users need to attach theinterface on exposed skin or under their clothes (i.e., this is
why most EMS interfaces depict users with rolled-up sleeves[9, 41, 53] or even short pants [23, 57])—this prevents thistype of actuation technique to be used in more walk-up
scenarios, in which there is no time to attach interfaces underclothes. (3) EMS electrodes dry up: conventional EMS
utilizes pre-gelled electrodes (already meant to minimize thetingling), but these electrodes dry up over time and typicallyare only good for a few hours.
To tackle these and other limitations, we explore switchingfrom electrical to magnetic muscle stimulation (MMS),
which uses electromagnetic fields generated by coils insteadof electrical impulses via electrodes (Figure 1). The first key
advantage is that MMS coils do not require direct contact
with the user’s skin and, unlike the electrodes of EMS, canactuate a muscle and provide force-feedback up to 5 cm away
(as validated in our Study#1). This is transformative as itenables applications not possible with EMS, such as muscle
stimulation over clothes or through a furniture piece, orapplications that actuate users in a walk-up setting without
needing to replace sticky-pad electrodes.
Second, and more importantly, we found that force-feebackusing MMS exhibits ~50% less skin-tingling sensation thanthat of EMS (as validated in our Study#2). Critically, we
found that reducing this uncomfortable tingling has two key
downstream effects for interactive systems: (1) users wereable to more accurately perceive the pose of their body when
actuated by the interactive system (as validated in our
Study#3); and, (2) users rated MMS force-feedback as more
realistic than that of EMS (as validated in our Study#4).
In light of these benefits, we present the first set of interactive
MMS applications, exemplifying cases previously difficult
to realize via EMS. We hope these examples will inspire
researchers to extend MMS to new interactive domains.
Finally, it is important to note that we do not consider MMS
as a way to completely replace existing EMS applications;rather, we view it as a new option for force-feedback
stimulation that enables muscle actuation in interactive
contexts where EMS tends to fail, such as walk-up-use
interfaces (e.g., game arcades, museums, public kiosks, etc.).
BACKGROUND & RELATED WORK

Our work is built on electrical muscle stimulation (EMS) andmagnetic stimulation. First, we analyze EMS’ limitations.
Then, we discuss the principles of magnetic stimulation thatoffer a promise towards addressing EMS’ shortcomings.
Electrical Muscle Stimulation

While force-feedback is useful in many interactive
experiences by directly guiding the user’s body or adding
haptic realism in an immersive experience, such devices
typically require mechanical actuators that are heavy, thus
encumbering the user’s movement. Electrical muscle
stimulation (EMS) is a more recent alternative to this long-standing challenge of actuating limbs. EMS uses electrodes
attached to the skin near or atop a muscle. Passing a current
through those electrodes causes the muscle fibers to contract
and, in turn, actuate the user’s muscle [37]. While EMS
originated in the field of medical rehabilitation [75], manysee it as a promising interface to miniaturize force-feedback
today [43]. As such, EMS has been adopted in a variety offorce-feedback applications: adding realism to immersive
experiences [24, 38, 42]; guiding the user’s limb movement[40, 57, 69, 70]; conveying information via proprioceptive
sensations for eyes-free interactions [41, 53]; or evenenhancing the user’s abilities [29, 30, 62].
Our Analysis of the Limitations of EMS

While EMS has been used as an alternative to mechanicalforce-feedback devices for its attractive form factor,
researchers have rarely investigated its limitations, with mostof the work focusing only on improving the lack of accuracy[24, 25, 54, 68] or agency [29], but not improving the lack of
comfort, its reliance on electrodes, etc. In this section, we
shed light on five additional limitations of EMS.
1. EMS induces an uncomfortable tingling sensation. In
conventional EMS, electrical impulses must travel throughelectrodes attached to the user's skin to reach deep into theirmuscles. This presents a significant limitation because,
before the electrical impulse can stimulate muscle fibers(which ultimately contract to generate the desired effect), it
inevitably traverses skin receptors [28], producing an
unwanted and uncomfortable tactile sensation often
described as "tingling" [29, 43, 55, 58], or “buzzing” [37].
This has been known since the early days of EMS as aninterface: “Triggering of [skin] receptors might also havecaused the feelings of pain or buzzing (…) sometimes
noticed by the users” as put by Kruijff et al. [37] in their first
seminal study on the interactive use of EMS. The authors
also found that "two users reacted negatively (...) and had themost problems with user comfort" [37]. It is known that thisis not just a limitation of EMS but of most electrical-basedstimulation techniques, in the seminal work of Francini et al.
in 1979, they describe that as the intensity of skin-based
electrical stimulation increases, it “evokes three different
sensations with consecutive sensory thresholds: tactile
sensation, tingling, and pain” [14, 72]. As noted by Kruijff etal.’s participants, we argue this is a major limitation to users’
experiences that warrants novel solutions. As we will findlater, via our Study#2, switching from electrical to magnetic
stimulation dramatically reduces tingling by 50 %.
2. EMS demands exposed skin. EMS requires that theelectrodes are attached directly to the user’s skin. Thisimplies that users need to expose their skin to the interface
or wear the interface under their own clothes—this is why
most EMS interfaces depict their users with rolled-up sleeves[9, 41, 53] or even short pants [23, 57]. This needs to directcontact with the user’s skin prevents this type of actuation
technique to be used in more walk-up interfaces [63], inwhich there is no time to attach interfaces under clothes.

3. EMS electrodes dry up. Moreover, these electrodes areusually pre-gelled to ensure the comfort of the stimulation as
their impedance matches that of the user’s skin (i.e., they are
already designed to minimize the tingling). As Tachi,
Kajimoto, and Kanno put it, “[the gel is a] means foralleviating a sensation experienced by the wearer as a resultof the stimulation (…) The conductive gel layer has aresistance value equivalent to that of the [outermost layer] of
the skin” [67]. Unfortunately, like with any hydrogel, thesepre-gelled electrodes dry up over time and are typically only
good for a few hours [3]. As Tamaki et al. reported in theirseminal paper, “dry (…) pads cause pain when the contact
area is small” [69]. Alternatively, it is also possible to deliver
the stimulation via dry electrodes [58], or even integratethem into garments to improve wearability [34, 52].
However, dry electrodes are not optimized to match the skin
impedance [59] and will elicit more uncomfortable tingling
or even pain [69]. As such, most interactive EMS is limited
to gel-based electrodes.

electrodes as they dry up and lose conductivity—this, again,
limits the applicability of EMS. Also, in such installations,audiences might be informed via disclaimers if the electrodes
are used across multiple users—this, again, illustrates alimitation with the electrode’s need for direct contact(hygiene in public use). Finally, because of the adhesive
nature of electrodes, these need to be detached with aconsiderable amount of force, typically a technician or, in thecase of the aforementioned artwork, a motorized system oflevers—this further emphasizes how the reliance onelectrodes prevents EMS from being more widely applied tocontexts, such as art installations, products, or public spacesthat might require “walk-up use” interactions.

4. Electrodes are adhesives = no quick detachment. Thesegel-based electrodes are essentially adhesives, making it
difficult to detach them; we preliminary measured that a
fresh electrode from Syrtenty required 4.2 N to detach fromthe skin. This makes it challenging to envision EMS in awalk-up-use interface, in which the user rapidly experiences
force-feedback and leaves after (e.g., EMS in a public space).
5. Electrodes are often limited to a per-user basis. Lastly,
as the user’s dead skin cells and other organisms adhere to
the electrodes upon the detachment, re-using the same
electrodes across users raises hygienic concerns. Thus, most
EMS demonstrations are limited to a per-user basis. While
EMS installations for the public have been performed atconferences and museums, they typically require newelectrodes per user, or a disclaimer about the sharedelectrode hygiene (e.g., the EMS artwork ad infinitum [2]).
Showcasing EMS’ limitations via case studies. Let us
exemplify these limitations in interactive systems that use
EMS. For instance, Lopes et al’s “VR wall” (Figure 2a), theuser feels the desired force but, unfortunately, isaccompanied by an uncomfortable tingling that distracts
them from the experience, as one of their participants put it:“EMS tingles and hence reveals the source of theforce” [42]—this depicts how our first limitation (tingling)decreases the effectiveness of this work (reduced realism).
Moreover, the users of these “VR walls” wore tank-top shirts
allowing experimenters to reach the skin on the shoulder
muscles and attach the electrodes—this depicts our second
limitation (no use over clothes). Extending EMS to walk-up
scenarios reveals even more challenges: public EMS
installations such as the artwork “ad infinitum” (Figure 2b),require that an audience member first rolls-up their sleeves,otherwise electrodes will not attach to their muscles—this,again, depicts how EMS limitations prevent walk-up use.
Moreover, EMS demonstrations and art installations requiremonitoring from a technician, who regularly replaces

Figure 2. Two use-cases to exemplify how the electrodes in EMS
bring five additional limitations. (Figures from [42] and [2]).
Magnetic Stimulation

Similar to electrical stimulation, magnetic stimulation is atechnique that originated from medicine/neuroscience [18].
Unlike its electrical counterpart, magnetic stimulators do notdirectly provide an electrical current to the user’s body;instead, a magnetic stimulator uses a coil to produce achanging magnetic field. Because this magnetic field isoscillating rapidly, it creates an electrical current (known aseddy currents). The difference is that this current originatesin a spatial area around the coil by a magnetic field, not byelectrodes that produce high current density at their surface.
As such, this current appears inside the target body region
and does not require conduction from the source to the target
by means of cables or electrodes—it is magnetic [21].
Use in brain stimulation. The primary use of magnetic
stimulation is for the most popular technique for noninvasive brain stimulation as it directly stimulates neurons
under the skull [21]. First confirmed by Barker et al. in 1985
[4] and known as transcranial magnetic stimulation (TMS).
It uses coils placed atop the patient’s skull to stimulate thebrain neurons without needing to operate (or open-up partsof) the skull. It is frequently used as a tool to map brain
functions to cortical areas, e.g., motor control [74], vision[6], language processing [13], working memory [50], andmore. TMS has also been used as therapy for improvingmotor function in Parkinson’s patients [39], reducing
epileptic seizures [15], and as a depression treatment [19].
Use in rehabilitation. While magnetic stimulation finds its
primary use as TMS, stimulating muscles via coils atop—
known for repetitive peripheral magnetic stimulation(rPMS) [61]—has also been applied for muscle
rehabilitation [5, 49]. Szecsi et al. reported that rPMS
induced less pain compared to EMS during leg rehabilitationof patients with paresis [66]—a muscular weakness caused
typically by nerve damage or neurological conditions. Abe et
al. also observed the pain reduction in the wrist extensor
muscle of healthy subjects [1]. In light of this, neuroscientists
have pursued this stimulation for various clinicalapplications, including leg rehabilitation [65], shoulder
rehabilitation [16], finger rehabilitation [64], or evenswallowing rehabilitation for post-stroke patients [47, 48].
Building on prior efforts in rehabilitation, our work
demonstrates the potential of MMS in interactive systems.
We analyze what new types of interactions are possible when
we remove EMS electrodes, as well as characterize thereduction of tingling sensation for healthy participants (notexplored in prior work). We found that this reduction in
tingling has two downstream implications for interactive
systems: (1) clearer proprioceptive (i.e., pose) sensation; and(2) haptic realism of force-feedback compared to EMS.
Use in interactive systems. Although magnetic stimulation
has gained recognition in rehabilitation, its presence inhuman-computer interaction remains limited. To ourknowledge, the sole instance of this is by Kim et al., whoused magnetic fields to stimulate hand nerves for tactile
sensations [32, 33]—not muscles. Our work explores thebenefits that interactive systems can reap by switching fromelectrical to magnetic muscle stimulation.
CONTRIBUTION, BENEFITS, AND LIMITATIONS

despite wearing the coil’s full weight, participants in ourstudy preferred MMS to EMS for the realism of VR forcefeedback. Fourth, the size and power consumption of thestimulator do not rival the small form factor of EMS (i.e., weused a stationary medical-grade magnetic stimulator,
powered from AC outlet, not battery). Later, we discuss theselimitations in detail and suggest potential directions for each.
Finally, we reflect on the nature of our proposal for using
magnetic muscle stimulation as force-feedback. This
technique is not new—it originates from neuroscience as amethod to stimulate the brain. However, the value of our
proposal lies in its translation as a tool for force-feeback,coupled with our investigation of the benefits it brings tohuman computer interaction. In fact, similar translational
efforts have previously advanced our field. For instance, thePossessedHand [69] translated a 50-year-old medical tool(i.e., electrical muscle stimulation) into our field, leading toa growth in force-feedback research. Likewise, our efforts
are the first to translate magnetic muscle stimulation from the
medical realm into new knowledge for interactive systems.
OVERVIEW OF STUDIESIn the subsequent sections, we present a series of four user
studies designed to explore the interactive advantages
resulting from the transition from EMS to MMS. The
following is an overview of the critical insights gleaned fromthese investigations: in Study#1 we found that MMS
stimulated participants' muscles up to 5 cm away; in Study#2
we found that MMS reduced the tingling sensation by about
50% compared to EMS; in Study#3 we found that MMS
rendered more realistic force-feedback in VR than EMS;
and, in Study#4 we found that participants could better
recognize the pose output of the MMS actuation than of
EMS. All four studies were approved by our Institution
Review Board (IRB21-0055).

Our key contribution is that we propose that many interactive
systems could benefit from a switch from electrical tomagnetic muscle stimulation (MMS). We found, by meansof four user studies, four benefits of MMS over EMS: (1)
MMS does not require electrodes or physical contact with theskin and can stimulate over the user's clothing—eliminating
the issues with electrodes such as their adhesiveness andneed for replacement; (2) MMS reduces uncomfortabletingling sensations by 50%; (3) MMS provides higher
perceived realism of force-feedback in VR; and (4) MMS
enables the user to more clearly perceive poses that wereactuated by the stimulation.
MMS is not without limitations, which we believe areimportant to discuss in light of this being the first explorationof MMS in interactive systems. First, as with any electricalstimulation, MMS requires calibration (e.g., intensity andplacement, just as with EMS). Second, the coil generates aloud sound, while EMS is soundless. Third, a coil is heavier
than electrodes (1.1 kg), which leads us to mostly
recommend MMS for stationary applications as depicted inmost of our examples; however, it is worth noting that

Figure 3. Overview of our study setups. In Study#1, we had an
additional spacer to adjust the coil distance (e.g., 20 mm), andthe EMS electrodes were not present as this study focused solely
on MMS. In all EMS conditions, we used 1.5-inch pre-gelled
electrodes and biphasic stimulation, a method commonly
employed in previous work (e.g., [42]), to mitigate skin tingling.
USER STUDY #1: HOW FAR MMS CAN STIMULATE?

Our first study characterized the relationship between theforce generated by MMS and the coil’s distance to the skin.

Participants. We recruited ten participants from ourinstitution (3 identified as female, 6 male, 1 as non-binary;
23.8±2.3 years old; all right-handed). None had any form of
motor impairments. Each study session took ~10 minutes.
Apparatus. Participants sat with their dominant arm resting
on a tabletop (Figure 3a). This provided a similar setup toprior work that characterized forces generated by EMS [44].
We used a medically compliant magnetic stimulator(Magstim Super Rapid2) with a butterfly coil (Magstim
D702) connected to it. We used five different spacers (20, 30,
40, 50, and 60 mm) to control the distance. The coil wasplaced above the participant’s forearm muscle (flexor carpi
radialis) over the spacer with the support of an armrest. The
participants wore earplugs. We also attached a 3 mm acrylic
plate to the MMS coil to approximate the thickness ofclothes. We used a 5-kg load cell with a HX711 amplifiersampled by an Arduino to measure the force.
Procedure. We calibrated the MMS so that the participants’
wrist flexor was actuated. Then, we set the stimulation to a
one-second duration, 1200 mT intensity (recommended
maximum intensity for this stimulator [45]), at 20 Hz (asin [64]), while ensuring that the stimulation was pain-free forparticipants at the 20 mm distance. Each participant
completed 10 trials (2 repetitions for 5 distances), resulting
in a total stimulation duration of ~10 sec. Trials began at the
20 mm condition, incrementally increased by 10 mm, up to
60 mm; then, decreased step-by-step back to 20 mm. In eachtrial, we applied magnetic muscle stimulation and logged the
maximum force value measured during stimulation.
Results. As shown in Figure 4, the amount of generated force
via MMS measured the averages of 2507 g (M=24.6 N;
SD=13.1) at 20 mm, 1514 g (M=14.8 N; SD=10.6) at 30mm, 588 g (M=5.8 N; SD=7.4) at 40 mm, 92 g (M=0.9 N;
SD=1.8) at 50 mm, and 4 g (M=0.04 N; SD=0.08) at 60 mm.
As expected the decay of a magnetic field is sharp, yet we
observed that MMS still produced ~1N of force at 50 mm,
which can trigger hand movement [69]. As such, we deemed
this as the maximum distance for force-feedback generation.

Figure 4. The force-distance relationship found in Study 1.
USER STUDY #2: REDUCING TINGLING VIA MMS

Our second user study characterized the comparison of MMS
and EMS regarding the skin-tingling sensation. Although
prior research in the rehabilitation field has shown that MMS
generates less pain in the leg [66] or wrist extensor [1]muscles, our study is the first in focusing to evaluate thetingling sensation still arises during pain-free operations—the most common condition for such interactive systems.
Our hypothesis was that MMS would induce less tingling
than EMS. As such, our study design is based on these prior
works [1, 66].
Participants. We recruited the same ten participants from
Study#1. Each study session took about 30 minutes and theparticipants received $10 USD as compensation.
Conditions. Participants experienced muscle actuation of
their wrists in two conditions: MMS or EMS. We chose tostimulate the wrist flexor muscle since it is one of the mostpopular targets for EMS, which enables us to understand how
MMS might facilitate grasping/touching-type of interactions.
Apparatus. We used the same apparatus as in our previous
study (Figure 3a) with the addition of a medically-compliant
EMS stimulator (HASOMED Rehamove3). For the MMS
condition, the coil was kept 3 mm away from the skin via thesame spacer from Study#1, which replicates the thickness ofclothing.
Calibration. We first calibrated the maximum intensities of
MMS and EMS that ensured pain-free operation while
logging their resulting forces in the same way as Study#1.
For each, starting from the minimum, we applied one secondof stimulation and increased its intensity by the unit amount.
Both stimulations were at 30 Hz—considered to be a lower
bound for EMS to reliably contract muscles [12]. We madethe increase upon the participant’s agreement so that they
were pain-free; otherwise, we defined the intensity before theincrease to be an upper bound. Once we reached the upperbound, we logged the stimulation intensity and the generated
force at that level. After calibrating all participants, we found
an average intensity of 526 mT (SD=148) for MMS, and
9.5 mA (SD=2.6) for EMS.
MMS-EMS equalization on the resulting force. As MMS
and EMS operate on different stimulation modalities (i.e.,
direct current and electromagnetism), the MMS stimulator
controls the stimulation intensity via the strength of magnetic
fields (with a unit of 12 mT), while the EMS stimulator
controls the current amount (with a unit of 1mA). As such,after the calibration, we had to equalize the MMS intensities
to those of EMS on the amounts of resulting force in order tocompare them for the tingling sensations. The earlier
calibration informed which of the two had the larger upperbound force. Since the force range based on the smaller upperbound is always included in that of the larger one, we used
25%, 50%, and 75% of the smaller upper bound as target
force levels. Then, for each stimulation technique, wecalibrated its intensity so that the output force was closest to
the target force amounts using the staircase method.
Procedure. In each trial, participants rated and compared the
MMS and EMS stimuli at the calibrated intensities. Aftereach stimulation, the participant rated their perceived amountof tingling on a 7-point Likert scale (1: least, 7: most). Then,only after feeling both stimuli, they rated the tingling amountfor each using a visual-analog scale, i.e., which stimulation
tingled more and by how much. We repeated this process for
six trials (three force levels ´ two repetitions). Note that thepresentation orders of the stimuli and the force levels wererandomized.
Results. Firstly, through our calibration, we found, at theirmaximum intensities, MMS produced more force(M=23.9 N; SD=12.1) than EMS (M=13.5 N; SD=12.3),
which was confirmed using a paired t-test (p<0.01). More
importantly, the participants rated MMS’ tingling less thanthat of EMS as depicted in Figure 5a. We conducted a twoway repeated measures ANOVA and found the main effects
for stimulation techniques (F (1, 9)=14,71, p=0.004,
hp2=0.62), force levels (F (2, 18)=9.42, p=0.002, hp2=0.51),
and interaction (F (2, 18)=8.30, p=0.003, hp2=0.48). As wefound interaction, we performed a post-hoc test with
Bonferroni corrections on the main effect of stimulation
techniques and found significant differences for the simple
main effect in all force levels, which showed that MMS wasless tingling than EMS: 25% (p<0.05), 50% (p<0.05); and75% (p<0.05). To analyze visual-analog scale results, wefollowed prior work [51] and calculated the quotients of
MMS’ and EMS’ visual-analog scale scores, i.e., the portionof EMS tingling that corresponds to that of MMS:
M=58.4 % (SD=46.7) at 25 %; 44.4 % (SD=43.4) at 50 %;and 50.5 % (SD=55.8) at 75 % (Figure 5b). These resultsoverall suggest that MMS reduced the tingling sensation by
about 50% compared to EMS.

Figure 5. The Likert-scale rating (a) and the ratio (mms/ems) of
the perceived amounts of skin-tingling sensations (b). For both
tingling rating and tingling ratio, less depicts a superior result.
USER STUDY #3: MMS VS. EMS ON REALISM

The study took ~30 min, including ~15 mins inside of the VRexperiences. Participants received $10 USD compensation.
Tasks. Participants performed two VR tasks (depicted laterin Figure 11) using their dominant arm: box-task: pushing
three cargo boxes aside; lever-task: turning on an electricalgenerator by pulling a lever three times. During the box-task,
we stimulated the bicep muscle to simulate the resistive force
experienced when pushing a heavy object (as in [42]). In thelever-task, we stimulated the participant's tricep muscle toemulate the resistance of the lever (as in [42]).
Apparatus. Participants stood in a 1.5 ´ 1.5 m open space,wearing a Meta Quest 2 headset (Figure 3b). Participants
wore either an MMS coil or EMS electrodes connected totheir respective stimulators. While the EMS electrodes wereattached directly to the skin after rolling up participants'
sleeves, the MMS coil was placed over clothing and secured
using velcro straps. Participants also wore headphones toprovide sound effects, such as the box dragging on the flooror the mechanical detents of the lever, without the use of
earplugs or white noise to mask any acoustic noise from the
MMS.
Procedure. Prior to actual trials, we calibrated MMS andEMS so that both actuated the participant's bicep and tricep
muscles by 30 degrees over one second of stimulation(Figure 3b), with the zero-degree angle defined where theparticipant's forearm was parallel to the ground and the upperarm was perpendicular to the ground. After the calibration,
each participant performed four trials (2 conditions ´ 2tasks), with the presentation order of the simulation
techniques counter-balanced. After each trial, participants
were asked to rate haptic realism using a 7-point Likert scale.
After completing both tasks in one condition, participantsalso rated the overall haptic realism of that condition. The
procedure was then repeated for the remaining stimulation
condition. At the end of the study session, we collected
preferred conditions and open-ended feedback by asking
participants about their experiences.
Results. The participants rated MMS’ force-feedback more
realistic than that of EMS (i.e., box, lever, and overall) as
depicted in Figure 6. We conducted a two-way repeatedmeasures ANOVA and found the main effect for stimulation
condition (F(1, 11)=13.5, p=0.004, hp2=0.55). We did not
find main effects for tasks (F(2, 22)=3.5, p=0.08,
Greenhouse Geisser corrected for sphericity, hp2=0.24) nor
interaction (F(2, 22)=0.24, p=0.78, hp2=0.02).

Our third user study explored how MMS’ reduction in
tingling might lead to an increase in realism during forcefeedback with tasks inspired by [42]. Our hypothesis wasthat a VR application making use of MMS-based forcefeedback would be perceived as more realistic than the same
application using EMS-based force-feedback.
Participants. We recruited 12 participants (5 identified as
female, 7 as male; 21.7 ±2.1 years old, all right-handed), of
which three were recruited from the pool of our first study.

Figure 6. Participants’ VR realism rating for MMS and EMS.

Qualitative feedback. All 12 participants preferred MMS to
EMS. Eleven out of the 12 participants attributed theirpreference to MMS from having “less tingling”, “numbing”,
or “sharp”, “strong skin sensations” or “jerky vibrations”.
For example, P2 stated “[MMS] was very comfortable. There
was no tingling” (P2), while P6 stated “[MMS’s] tingling is
not as intense as electrodes”. P9 stated “EMS felt a little bit
jerky vibrating” (P9). P12 stated “[MMS’s] pulse feels likesmooth” (P12). Additionally, four participants reported thatthe tingling sensation induced by EMS decreased the realismof the experience, with P3 stating, “[EMS's] sharp, painfulstimulation aspects hold me away from the realism”. The
only participant who did not directly mention a difference intingling still stated “EMS makes a more unwanted movement
like a vibration, MMS was much more clean[er]” (P8).
USER STUDY #4: CLARITY OF THE ACTUATED POSE

In our fourth user study, we continued examining how the
MMS’ reduced tingling might provide additional interactive
benefits. In particular, we examined how it might let
participants better recognize their body’s pose when actuatedby MMS compared to EMS. This leads to a benefit inapplications where the user’s body poses act as values output
by interactive systems, e.g., gestural IO or proprioceptive
interactions [41]—a user can perceive the output moreaccurately. To this end, we evaluated how well participants
could recreate their hand pose after having been stimulated
by MMS or EMS—using the study design from [41]. Our
hypothesis was that the participants would have smaller
errors between the actuated and recreated poses in MMS.
Participants. We recruited 12 participants (2 identified as
female, 9 as male, 1 as non-binary; 25.0±3.0 years old; allright-handed), of which five were recruited from our priorstudies (two from Study#1 and three from Study#3). The
study took ~30 min and participants received $10 USD.
Task. We used the EMS-pose task design from [41]: westimulated the participant's wrist extensor using MMS or
EMS. After the stimulation, we asked the participants tovoluntarily recreate the pose induced by the stimulation. We
chose this muscle as it has minimal risk of the fingers
touching each other or the palm upon an actuation, which
would introduce tactile sense as a confounding factor in task
performance. We then evaluated the error between theactuated and recreated pose for each stimulation technique.
Apparatus. As depicted in Figure 3c, participants sat at adesk with their dominant hands resting on a desktop. We
used the same stimulators as Study#1-3. An MMS coil and
EMS electrodes were placed atop the wrist extensor muscle(extensor carpi radialis) with the support of an armrest.
Throughout the trials, we blindfolded the participants inorder to let them recognize the pose, solely relying on theirproprioceptive sense, (same as [41]). The participants did notwear earplugs nor hear white noise since shutting off theacoustic noise of MMS could bias task performance in favorof MMS. Note that the noise of MMS is independent of thewrist pose so it was not effective as a cue for this task. For

optical tracking, we used an Optitrack Flex 3 system, with a
total of six markers attached to each of the fingernails andthe back of the hand.
Procedure. Prior to the trials, we calibrated MMS and EMS
so as to actuate the participant’s wrist by 45 degrees(measured by the tracking system), following the same
calibration protocol from Study#3. Each participant
performed six trials (2 stimulation conditions ´ 3 repetitions)with the presentation order of the simulation techniques
randomized. In each trial, after a random waiting period, theparticipant received either MMS or EMS, which was
automatically shut off once their wrist angle had reached 45
degrees. We instructed the participant to memorize theirhand pose right before it turned off. Then, participants waited
five seconds and, at an audible tone, they recreated thepreviously sensed pose. To confirm their pose was finalized
they pressed a key on a keyboard with their non-dominanthand. We calculated the total error between the marker
positions recorded at the moment stimulation stopped andthose recorded when the participant confirmed their pose.
Results. As shown in Figure 7, we found that participants
had smaller total errors in recreating the pose rendered by
MMS (M=18.8 cm; SD=6.2) than EMS (M=25.9 cm;
SD=12.5) as confirmed by a paired t-test (p<0.01). We also
observed a smaller mean error for the back of the hand for
MMS (M=1.8 cm; SD=1.3) than EMS (M=2.3 cm; SD=1.5).

Figure 7. Total error in pose recreation with MMS and EMS.
APPLICATIONS

Now that we have confirmed the benefits obtained bytransitioning from EMS to MMS, we showcase how thesebenefits improve EMS-based applications, or even enableapplications that were previously challenging with EMS.
A Pose-Based I/O Interface (e.g., a video scrubber)

We demonstrate how MMS enables a new rendition of the
I/O pose-control previously shown with EMS (e.g., [41]).
The key is that, as found in our Study#4, MMS likely enables
users to more clearly recognize the output of the interactive
application as it actuates their pose. Figure 8 depicts how theuser scrubs through a video they are watching using posecontrol: (a,b) the user sitting on a couch rests their arm on thearmrest that has an MMS coil inside; (c) the user activatesthe system by performing a pinching gesture; (d) our system
immediately takes over and renders the current playback
position of the video by actuating the user’s wrist; and (e)now, by moving the wrist voluntarily left/right, the user
scrubs the video (e.g., rewinds by moving their wrist to theleft). When finished, the user makes the pinch gesture again
to exit the interaction. Note that we purposefully drew anoutline of the arm on the surface of the armrest to let the user
better understand which area allows for stimulation (Figure8a). While this is obviously optional and not always needed(e.g., our next applications do not use this method), it also
lets users better align themselves on demand with the coil’s
location.

bus starts/stops/shakes. (c, d) Proof-of-concept prototype that
activates MMS based on IMU sensor data inside the handrail.
Stimulating the User’s Back in a Driving Simulator

While EMS has been explored for force-feedback, very few
systems provide force to a user’s back [31]. This is likely due
to the difficulty in attaching electrodes to the back. Figure 10
depicts a driving simulator where MMS enables forcefeedback applied to the user’s back without the need for any
electrodes, instead, the coils are on the chair. As the user
experiences inertia while the car turns left/right, in response,the corresponding MMS coil outputs stimuli to actuate theuser’s back muscles (latissimus dorsi) even through thefabric of the chair. Actuating this dorsal muscle causes theuser’s body to tilt. We use a second coil (Magstim AirFilm)and multiplex the connection to a single magnetic stimulator.

Figure 8. Our video-scrubbing tool based on MMS can extend
EMS-based I/O interaction by improving the clarity of its
output and installing the system into the furniture piece.
Envisioning Muscle Actuation in Public Spaces

This application envisions how MMS might enableapplications previously challenging with EMS, namely
deployment of muscle actuation in a publicly available
interface (i.e., walk-up-use, quick interactions, many users).
Figure 9 envisions the use of MMS-based force-feedback ina bus: (a) the passenger places their hand on a handrail but is
not firmly holding it. However, (b) when the bus starts
driving, an MMS coil is actuated, causing the passenger toinvoluntarily grab the handle, reminding them of this safety
behavior. (c) Depicts how this finger flexion is achieved
using a back-of-hand stimulation [68] since the coil is placedin the wall, parallel to the back of the hand muscles. (d) To
detect acceleration and trigger the coil, our proof-of-concept
implementation uses an IMU sensor.

Figure 10. Muscle actuation through the fabric of the chair. As
the car turns (a) left or (b) right, users feel force-feedback.
MMS as Wearable Force-Feedback

While we demonstrated stationary coils embedded in objects,
it is also possible to have users wear coils over their clothing.
We show this by replicating the EMS VR force-feedback
application of [42], but without the need for the tank-top
since MMS stimulates over clothing—this is the application
used in our Study#3 and depicted in Figure 11: (a) whenusers push the cargo box they feel a resistive force; then,when users pull the lever to turn on the electrical generator,
they feel resistance. While we recommend that most MMS
applications use stationary coils, it is worth noting thatparticipants from Study#3 all preferred this condition to
EMS even after 15 minutes of wearing the heavy coil.

Figure 9. (a, b) Envisionment of MMS in a bus handle where its
handle automatically actuates the passenger’s grab when the
Figure 11. MMS for VR feedback (box replicated from [42]).

SOFTWARE IMPLEMENTATIONIn our user studies and applications, we implemented thecontrol of a medical-grade magnetic stimulator (Magstim
Super Rapid2) using the MagPy Python Toolbox [46]. Our
middleware routed serial communication to the stimulator
and received commands from our Unity applications via
Open Sound Control (OSC). Regarding tracking, we
employed Meta Quest's hand/pose tracking for the VR andvideo-scrubbing applications. For the video-scrubbing
application, the headset was mounted on a tripod locatedbehind the couch (inspired by [22]). We made all the source
codes and materials publicly available1.
DISCUSSION ON LIMITATIONS & GUIDELINESIn this section, we note the limitations of current MMS
stimulation common to most magnetic stimulators, in which
we elaborate on potential directions to address them. Basedon this, we further discuss guidelines for interactive MMS.
Current Hardware Limitations

Acoustic noise. Rapidly oscillating magnetic fields create
coil vibrations due to magnetostriction, producing pressure
waves that result in a clicking sound. In prior work, thisacoustic noise was measured at ~65 dB (similar to theloudness of a human conversation) at the lower bound of
stimulation intensity (~370 mT) that actuates the fingers [17,36]. We also measured the noise using a decibel meter(AS824), positioned 50 cm away from the coil, while
increasing the intensity to 540 mT (the average upper-boundintensity observed in Study#2’s calibration). We observed apeak amplitude of 80.7 dB, which is similar to that ofpropeller-based haptic devices (e.g., 83 dB [27]).
Consequently, it is advisable to reduce the level of perceived
noise either by utilizing sound effects to mask the noise (asin our VR applications), or by implementing MMS inenvironments that are already noisy. Ultimately, addressing
noise is required to enable MMS in applications where users
are engaged in sound-sensitive activities (e.g., listening tomusic, etc.). Peterchev et al. demonstrated a coil prototype
for brain stimulation that reduced noise by 19 dB, showingthat engineering the coil casing can decrease noise byshifting its dominant frequency outside the human audible
range [56]—as these authors state, further optimization of
coils and pulse shapes could lead to greater noise reduction.
Coil form-factor. To produce magnetic fields, MMS coils
are significantly larger than electrodes. The coil used in ourwork (Magstim D702) measures 18×11×2 cm and weighs
1.95 kg with its 2 m cable. When worn by a user with thecable connected to the stimulator, its effective weight is
1.1 kg. While participants in our Study#3 preferred MMS to
EMS even while wearing the coil, we believe that a 1.1 kg
weight may negatively impact experiences in prolonged use
and do not recommend it for most deployments of interactive
MMS. Improving the coil form factor is paramount to enable
more wearability. One approach is tweaking coil geometries.
1

https://lab.plopes.org/#MMS

The butterfly-shaped coil we used consists of two loops,
whereas a single-loop coil can reduce the size by about half[11]. One could even use a smaller coil designed for smaller
mammals [71]. Although these alternative coil designs
compromise the depth of the stimulation [11], they might still
effectively reach most surface muscles.
Stimulator form-factor & power. MMS stimulators aremuch larger than EMS stimulators. For instance, thestimulator that energizes our coil (Magstim Super Rapid2)
measures 46×38×31 cm and is typically used for stationary
applications. This is mostly because MMS stimulators arecomprised of an array of high-voltage capacitors and a power
supply. To produce these magnetic fields, the capacitor
arrays consume 4600 W at peak stimulation—approximately
the energy required to drive two hairdryers. This is in stark
contrast to EMS’ low power consumption. As such, wecurrently recommend MMS as a tethered device. However,advances in engineering are likely to change this. Sauvage etal. designed a compact magnetic brain stimulator measuring
11×27×29 cm and weighing 9 kg, which operates on a small
24 V DC power supply [7]. This is capable of generating
magnetic fields of 700 mT at 1 Hz. Since MMS typicallyrequires less than 540 mT, it appears feasible to miniaturize
the hardware and enable it to operate on a 24 V LiPo battery.
Coil-alignment. Much as with EMS, MMS requires theactuator to be aligned with the target muscle. Given the
aforementioned size limitations with current MMS
stimulators, we recommend placing the coil in strategic
locations where the user’s body will align naturally (e.g.,
embedding the coil in the seat of a chair or in the handle of adoor). Obviously, when the coil is installed on an object, thetarget muscle needs to be within a certain area around thecenter of the coil for the stimulation to be effective. Based on
our Study#1 results, we recommend the maximum distances
between the coil and target muscles as follows: 3 cm for
VR/gaming to render 10N~; and 4~5 cm for applications
requiring less force (e.g., pose). This means that users cannot
move away from the coil during the stimulation, a restriction
similar to other walk-up-use haptics (e.g., seat vibrations[26]). Figure 12 illustrates how deviations in alignment
influence the resulting muscle contraction, using the wrist
flexor muscle as an example. In this example, the arm can bepositioned 2 cm in either direction of the center of the coilwhile still producing the intended wrist flexion. Deviations
larger than that start affecting the actuation. Note that, in thisobservation, the coil output was fixed at 540 mT (the averageintensity observed in Study#2’s calibration)—increasing thecoil output to its maximum is likely to widen the area more.
As demonstrated in our pose-based I/O application, one way
to assist the alignment is to visually represent the coil’s
position. All in all, further research is needed for a more casespecific discussion with respect to different muscles or othertypes of instrumented surfaces.

Figure 12. An example and measurement of how deviation incoil alignment could impact the resulting wrist movements.

Moreover, while we recommend that most interactive
deployments of MMS should make use of stationary coils,
we also acknowledge that other options are possible, withdifferent tradeoffs and complexity, such as: attaching the coilto the user’s body (e.g., as in our VR study by means of
Velcro straps); or even, automatically aligning the coil withthe target muscle by motorizing the coil. Figure 13 depicts
two examples in which the MMS coil is mechanically moved
into position: (a) depicts the simpler instance in which an XY gantry is used to move the coil—this is useful inapplications on surfaces, such as actuating the user’s finger
on a tabletop; (b) depicts a more involved mechanicalapproach, in which a robotic arm moves the coil—this isuseful in applications where users have some degree ofmobility (e.g., VR). While we still tend to think that MMS
lends itself best for stationary applications without the needfor any mechanical actuators, we denote that the addition ofa mechanical actuator does not compromise the benefits of
MMS. First, the mechanical actuator only needs to havesufficient force to move the coil (~2 kgf). This is lighter than
the force typically required for a mechanical actuator tomove a human body [73] (an average human arm without
exerting any voluntary force is still ~4.5 kgf [8]). As such theactuator needed for MMS alignment would be smaller thanan active force-feedback motor. More importantly,
mechanically moving the MMS coil still reaps more benefitsof MMS, i.e., no direct contact or attachment is needed withthe user’s body. Instead, if one would use a gantry or robotic
arm to actuate the user’s body directly (as with mechanicalforce-feedback devices), one would require to attach orconnect the user’s body to the mechanical actuator.

Figure 13. An alternative to strategic coil placement is to
motorize the coil, e.g., using (a) X-Y gantry or (b) robotic arm.
Safety Guidelines

Establishing safety guidelines for interactive MMS is crucial
for wider deployment. A large portion of safety guidelines of
EMS [35] may apply to MMS as both pass electrical currents
to muscles (e.g., they both require calibration, etc).
Moreover, due to its use of an electromagnetic field, MMS
also shares some of the safety guidelines of fMRI [10] andTMS [60], i.e., remove any ferromagnetic materials from thestimulation range (e.g., implants, or metallic accessories
such as jewelry) and maintain some distance from electronic
devices (e.g., smart watches) to prevent unintended
interactions. Moreover, as an interactive system, it isadvisable to ensure that users are in control of the MMS
stimulation and that the stimulation stops whenever it isagainst the users’ intention. Such a function can be
implemented by an emergency stop switch [20], or byincorporating motion sensing measures [40, 41].
CONCLUSION

We explored how switching from electrical (EMS) tomagnetic muscle stimulation (MMS) brings interactive
benefits and addresses the key limitations of EMS. Unlike its
electrical counterpart, MMS offers novel advantages: it does
not require direct skin contact and causes approximately 50%
less discomfort due to tingling sensations. Our studies haveshown that MMS can actuate muscles up to 5 cm away,
enabling applications that are not possible with EMS, suchas over-the-clothes stimulation and without ever replenishingelectrodes. Additionally, we found that participants rated
MMS force-feedback as more realistic than EMS and could
more accurately perceive actuated poses. Finally, weshowcased a range of applications unique to MMS (e.g.,
walk-up-use interfaces, such as force-feedback on a publicly
used bus) and outlined a roadmap of future challenges toexpand its applicability in interactive contexts.
In sum, by significantly reducing the tingling discomfort andremoving electrodes from the skin, we believe MMS has thepotential to improve many interactive force-feedback
experiences previously realized by means of EMS. As such,
we humbly believe that our work will inspire researchers toconsider MMS as a new tool for force-feedback.
ACKNOWLEDGMENTS

We would like to express our sincere gratitude to Prof.
Howard Nusbaum and John Veillette for their generous
support and training with magnetic stimulation. We would
also like to extend our thanks to our colleague Alan Shen for
his assistance in audiovisual production, and to Prof. Sarah
Sebo, Tewodros Ayalew, and our colleague Noor Amin for
their assistance. Moreover, we are grateful for theconstructive feedback from the reviewers.
This work was supported by NSF grant 2047189 and2024923. Any opinions, findings, conclusions, orrecommendations expressed in this material are those of theauthors and do not necessarily reflect the views of any
funding agencies.
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