Biostomp: A Biocontrol System for EmbodiedPerformance Using Mechanomyography

Çagrı Erdem∗

Istanbul Technical Universitycagrierdemo@gmail.com

Anıl Çamcı†

University of Illinois at Chicagoacamci@uic.edu

Angus G. Forbes‡

University of Illinois at Chicagoaforbes@uic.edu

ABSTRACTBiostomp is a new musical interface that relies on the usemechanomyography (MMG) as a biocontrol mechanism inlive performance situations. Designed in the form of a stompbox, Biostomp translates a performer’s muscle movementsinto control signals. A custom MMG sensor captures theacoustic output of muscle tissue oscillations resulting fromcontractions. An analog circuit amplifies and filters thesesignals, and a micro-controller translates the processed sig-nals into pulses. These pulses are used to activate a step-per motor mechanism, which is designed to be mounted onparameter knobs on effects pedals. The primary goal in de-signing Biostomp is to offer a robust, inexpensive, and easy-to-operate platform for integrating biological signals intoboth traditional and contemporary music performance prac-tices without requiring an intermediary computer software.In this paper, we discuss the design, implementation andevaluation of Biostomp. Following an overview of relatedwork on the use of biological signals in artistic projects, weoffer a discussion of our approach to conceptualizing andfabricating a biocontrol mechanism as a new musical inter-face. We then discuss the results of an evaluation studyconducted with 21 professional musicians. A video abstractfor Biostomp can be viewed at vimeo.com/biostomp/video.

Author KeywordsBiocontrol systems, mechanomyography, stomp box, em-bodied performance, user evaluation

ACM ClassificationH.5.5 [Information Interfaces and Presentation] Sound andMusic Computing, H.5.2 [Information Interfaces and Pre-sentation] User Interfaces

1. INTRODUCTIONBiomonitoring systems offer a vast range of applications forobserving various functions of the human body. While mostof these applications pertain to medical research, the inter-pretation of signals internal to the human body has also

∗http://cagrierdem.net†http://anilcamci.com‡http://creativecoding.evl.uic.edu

Licensed under a Creative Commons Attribution4.0 International License (CC BY 4.0). Copyrightremains with the author(s).

NIME’17, May 15-19, 2017, Aalborg University, Copenhagen, Denmark..

been used in arts as a control mechanism. In this paper, wediscuss the design and implementation of one such appli-cation, named Biostomp, where we utilize mechanomyogra-phy to augment the control space of a musical performer bytranslating the oscillations from naturally-occurring mus-cle activity during a performance into control signals thatmanipulate various effects that are commonly used by mu-sicians.

A mechanomyogram is a low-frequency mechano-acousticsignal generated by contractions in muscle fibres [6, 18]. Themonitoring of mechanomyograms, known as mechanomyo-graphy (MMG), is most commonly used in medical appli-cations such as prosthetic control, and the study of mus-cle disorders and fatigue. With Biostomp, we exploit themechanical nature of MMG as a means to extract the un-mediated byproduct of muscle activities inherent to a mu-sical performance. We have adopted a stomp box model inour design to alleviate the complexity of a computer-basedsetup, which is often found in biocontrol systems. Our sys-tem is designed as a plug-and-play tool that does not requireextra configuration or calibration beyond what is commonto stomp box effects pedals.

Biostomp comprises of a custom MMG sensor that is at-tached to the performer with an armband. The signals fromthe sensor are passed to the stomp box, which houses acircuitry to amplify, filter, and interpret the MMG signals.The processed signals are then used to activate a motor thatcontrols parameter knobs on effects pedals, such as those fordelaying and overdriving an instrument signal. We have de-signed an adjustable motor mounting system that can easilybe attached to knobs on most effects pedals.

Biostomp augments the musician’s control over existingparameters of a performance setup in various scenarios. Inits primary use, the system listens to the biosignals thatoccur during a musician’s regular performance, and mapsthese to the control of parameters on effects pedals. Butthe musician can also engage in an extended performancewhere they actively control these parameters with consciousmuscle movements beyond what their usual interaction withtheir instruments would mandate. In addition to such pri-mary uses of the system, Biostomp also provides a separateaudio output for the processed biosignals, which facilitatesthe use of the system as a stand-alone instrument.

We conducted a user evaluation study to explore howaugmenting the control space of a performer by interpretingMMG signals affects the performance, to what extent theperformer feels in control of the resulting changes, and towhat extent these changes are considered to be desirable ina musical context. In doing so, we aimed to evaluate theoptimal applications of the proposed system, and delineatea road-map for the future development of our platform.

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2. RELATED WORKScientific investigation of signals internal to the human bodydates back to the physician Luigi Galvani’s discovery of ani-mal electricity in 1791; biomonitoring mechanisms has sincebeen a prominent component of medical research [12]. Thestethoscope, for instance, is a well-known application in thisfield.

The use of biosignals in arts, however, has a relativelyrecent history. In one of the first works to utilize such sig-nals, the composer Alvin Lucier used EEG sensors placedon his scalp to pickup alpha waves that the brain generatesunder a focused state. Premiered in 1965, his work Musicfor Solo Performer relied on this technique for the controlof actuators attached to instruments [8].

In 1980, the artist Stelarc introduced his work Third Hand,which used EMG signals captured from his abdominal andleg muscles to control a mechanical hand attached to hisright hand. This work was initially intended as a semi-permanent prosthesis but was re-purposed as a performancedevice due to the discomfort caused by the apparatus. Ac-cording to the artist, the artwork has“contributed to cyborgdiscourses on the body”.1

In the early 1990s, the artist Atau Tanaka drew attentionto the use of biosignal interfaces in performance art. Hewas the first artist to be comissioned to work with BioMuse,which is a multi-sensor biocontrol system first developed byR. Benjamin Knapp and Hugh Lusted in 1988. This sys-tem was able to monitor and transmit electroencephalogram(EEG), electrooculography (EOG) and electromyography(EMG) signals. A year after the introduction of BioMuse,Tanaka co-founded the Sensorband, a trio of musicians whouse bioelectric sensors, with Zbigniew Karkowski and Ed-win van der Heide [2]. The artist describes his work in thismedium as a “corporeal activation of sound” that can notonly decode experimental performance, but also offer “anentry point to possible intimate spaces created by digitalinteraction” [15]. BioMuse project was later developed intoa brand that consisted of various consumer-oriented biocon-trol devices.

The artist Marco Donnarumma coined the term“biophys-ical music” [4] in 2011, following the development of hisXth Sense device, which is an MMG-based sensor systemthat captures biological sounds. Xth sense comprises of anMMG sensor armband and a hardware amplification unit;the hardware is coupled with a custom software that en-ables mapping, filtering and sound-processing. The artistuses the device in his performance art pieces, where hisbody becomes the primary sound source.

In 2014, Thelmic Labs released a consumer version oftheir armband EMG sensor MYO, which is a general-purposegesture controller. Accompanied by a variety of softwareproducts, MYO is marketed as a cross-platform biocontrolinterface. It relies on the use of pre-defined arm and handgestures to activate certain software commands. For in-stance, in the MYO Control App, making a fist opens themenu, and spreading fingers launches an application.

3. BIOSTOMPBiostomp aims to integrate biocontrol into musical perfor-mance. Initially developed for guitarists, Biostomp can beuseful for any performer who utilizes effects pedals in theirmusic. Additionally, the audio output capabilities of oursystem enables its use as a standalone instrument, in whichthe muscle sounds themselves act as a sound source.

Stomp boxes often comprise a toggle switch for turningthe effect on or off, permanently mapped parameter con-

1http://stelarc.org/?catID=20265

trol knobs, an audio input, and an audio output for theprocessed signal. Our primary design consideration withBiostomp was to capture the simplicity of this model andachieve an unmediated connection between biosignals andthe parameters these can be used to control. Most biosignalsystems used in artistic projects communicate with a com-puter software that generates or manipulates sounds. Insuch cases, the computer can be considered as the sound-producing instrument. The computer can also be used as anintermediary when using biosignals to control external me-chanical sound sources. While this approach offers precisemonitoring and intricate mapping of biosignals, putting anextra layer between the performer and the instrument canbe undesirable in live performance situations.

Biostomp is a self-contained system that does not requirea software interface or external calibration beyond what ispresented on its physical interface. The system thereforeacts as an analogous mediator that is capable of mechani-cally manipulating hardware signal processing devices, suchas effects pedals, with biosignals generated during a musicalperformance.

Biostomp relies on MMG acquisition for translating mus-cle activity into control signals. MMG is most commonlycompared to electromyogram (EMG), which represents theelectrical potential of muscle activity [12]. The hardwareand signal processing components of an EMG acquisitionsystem is often technically complicated and expensive tomanufacture [1, 6]. Furthermore, EMG sensors have a noisyresponse under low muscle activity [19], and are sensitive toenvironmental factors, such as power line interference [7],electro-magnetic pollution [16], and perspiration [11]. Ad-ditionally, EMG signals detected by surface electrodes takeplace prior to muscle activation in the sensorimotor sys-tem flow [5] whereas mechanical vibrations picked up by anMMG sensor are generated upon the force production. Thisimplies that control signals acquired with an EMG sensorprecede the actual muscle activity. While this makes EMGparticularly useful in certain applications, such as prostheticlimb systems, it masks the effect of, for example, physicalmuscle fatigue, which can be utilized as a meaningful com-ponent of a musical performance. Finally, when comparedto EMG signals, MMG signals are less sensitive to place-ment of the sensor over the muscle [17]; this allows MMGsensors to be placed more freely over certain muscle regionsand makes them less susceptible to shifts in placement dur-ing monitoring. Fig. 1 shows the components and operationscheme of Biostomp.

Performer Biostomp Effects pedal

Sensor armband for

MMG acquisition

Stepper motor for turning parameter

knobs

Signal amplification, filtering, and interpretation

Processed muscle sound

Instrument

Processed instrument

sound

Figure 1: Biostomp components and operation scheme

3.1 Sensor DesignAn MMG sensor, an air-chamber that houses this sensor,seen in Fig. 2, and an armband that holds these componentsin place, seen in Fig. 3, are the principal components of

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Figure 2: 3D model of the air-chamber housed in the arm-band

Biostomp’s MMG extraction mechanism.The frequency of core muscle vibrations range from sin-

gle digit values to approximately 45 Hz [10] with that ofsuperficial muscle contractions go up to 100 Hz [3]. Whileindustrial measurement microphones, such as the MicrotechGefell’s MK250, can offer a frequency response of 3.5 Hzto 20 KHz, this is a considerably wider range than thatis necessary for our system. Instead, we used a KingstateKECG2742PBL-A electret condenser microphone, which issuggested as being ideal for MMG applications in a com-parative study conducted by Donnarumma et al. [5].

Previous research describes ideal specifications for MMGsensor and air-chamber designs: Watakabe et al. [18] testedseveral dimensions for cylindrical air-chambers and recom-mended a diameter of at least 10mm and a height of 15mm,pointing out the correlation between slight changes in thisdimension and frequency response. Silva and Chau [14] pro-posed the use of a microphone-accelerometer composite sen-sor with a cylindrical air chamber of 1.3cm in diameter and0.2cm in height, sealed with a silicone membrane, whichprovides the highest signal-to-noise ratio. Posatsky [13] in-vestigated multiple designs with both cylindrical and coni-cal air chambers and found that cones offer a higher averagegain of 6.79 +/-1.06 dB/Hz in low frequencies, in compari-son to cylindrical air-chambers.

To achieve flexibility in where the sensor can be placedon the body, instead of a small silicon-based enclosure, weadopted the air-chamber design by the researcher MartinMa, who proposed the use of wider dimensions for monitor-ing large muscle regions rather than individual muscles [20].We modelled this proposal in a 3D design environment (see

Figure 3: The armband that houses the sensor

Fig. 2) and used oil-based plastic acrylonitrile-butadienestyrene (ABS) as our base material. This is a tough-yet-lightweight material appropriate for molding robust objectsfor everyday use.

3.2 Circuit DesignThe electret microphone is powered via a 12V regulatedcircuit; its output is amplified with a 2-gain-staged high-quality audio preamp, designed by Rod Elliot.2 Note thatin our system, the R7 resistor is modified with a 4.7 kOhmresistor to achieve a total gain of 12.9 db.

Based on the frequency range to be monitored with theMMG sensor, a 5th-order low-pass Butterworth filter in aSallen Key topology with a cut-off frequency at 100 Hz isused. From the filters we experimented with, the cut-offslope and the pass-band characteristics of this design pro-vided us with an optimal noise elimination. OPA27 opera-tional amplifiers were preferred due to their low-noise andhigh-precision stability characteristics.

The filtered signal is passed to a Msgeq7 equalizer IC,which is a CMOS chip that provides a DC representation ofthe amplitudes of 7 fixed frequency bands. In our case, weonly use the first band, which has a center frequency of 63Hz. We also implemented a signal path for the AC signalto allow for the direct monitoring of the audio signal.

Figure 4: Biostomp enclosure

Upon detection of a consistent hum in our first prototype,we switched from a plastic enclosure to a metal one, as seenin Fig. 4. The conductive enclosure works as a local earthand offers shielding from electromagnetic interference [9].A sensitivity knob attached to a 100-kOhm potentiometerenables the control of the preamp’s gain. A 3PDT foot-switch is used for switching between motor control or audiooutput. When the audio output is not used, this foot-switcheffectively turns off Biostomp.

3.3 Motor SystemEnvelope following of the band-limited 20Hz-to-63Hz signalis sent to one of the analog inputs on the micro-controller,in this case an Arduino Nano. Further smoothing, scal-ing and mapping is applied to the digital signal. An A4988driver board is used before the motor for pulse transmis-sion, current adjustments, and over-heating protection. A2http://sound.whsites.net/project88.htm

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NEMA 14-size bipolar stepper motor is attached to a cus-tom, adjustable motor mounting system, as seen in Fig. 5,which can be attached to stomp boxes with different knobconfigurations.

Figure 5: Adjustable motor mounting

4. USER EVALUATIONA user evaluation was conducted to explore the responsive-ness of Biostomp, and its ease of use for musicians who mayor may not be experienced in incorporating digital controlinterfaces into their performances.

4.1 Preliminary StudyTo evaluate our first prototype of Biostomp, we conducted apreliminary study with 11 guitar players between the agesof 25 and 45. Two primary issues that surfaced in thisstudy were the lack of ease in mounting the sensors, andthe discomfort caused by the armband during performance.Furthermore, the participants reported an inconsistent re-lationship between their activities and the audible outcomeof the device’s interpretation of these activities. Such feed-back prompted us to revise the wearable components of oursystem to improve comfort during performance. Further-more, we revised the grounding and amplification schemesin our circuitry, which helped eliminate some of the noisethat impacted the sense of correlation between the bodilymovements and the changes in audio effects negatively.

4.2 MethodAfter revising our system based on the results of the prelim-inary study, we conducted a primary study with 21 profes-sional musicians, which included 13 guitar players, 1 trum-pet player, 1 violinist, 1 keyboard player, 3 percussionists, 1drummer, and 1 custom electronic instrument player. 11 ofthe participants reported limited experience with new mu-sical interfaces, with 4 participants reporting no experiencewhatsoever. The remainder of the participants reported ex-perience with a range of interfaces, such as Arudunio-basedsensor systems, MIDI keyboard and controllers, tablets,computer software, and natural interaction devices like theMicrosoft Kinect and the Leap Motion. The styles of musicperformed by the participants included classical, folk, jazz,free improvisation, rock, noise, and ambient.

The study took 30 minutes on average. After a brief ver-bal description of the system, the users were given a periodof time to warm up. The sensitivity of the monitoring wasthen adjusted for the individual performer. This was fol-lowed by a performance, which was recorded audio-visually

and lasted 15 minutes on average. Each performer usedthe system to control the delay time parameter on a delaypedal, and the drive parameter on a fuzz/distortion pedal.

The participants were than asked to fill out a 15-questionsurvey, which consisted of 10 linear scale questions askingusers to rate different aspects of their experience using Bios-tomp, as well as 4 open-ended writing prompts, and 1 mul-tiple choice question asking users which of the effects (i.e.delay or drive) was preferred.

4.3 Results & DiscussionFig. 6 shows the distribution of responses to the 10 linearscale questions about various aspects of their experienceperforming with Biostomp.

While it was left up to the performers to decide whetherthey would let the system accompany their regular perfor-mance, or if they would extend their playing with deliberatemuscle movements, users reported the armband gave theman awareness of the system that they wanted to “play into”regardless of their approach. While most users reportedswitching back and forth between regular and extended per-formances, one user described that, in addition to his nor-mal guitar playing, he contracted his biceps to the rhythmof the tune; this is an interesting in-between approach thatmaintains a regular performance but augments it with extraembodied controls. This can be resembled to a performer’stapping of a foot to the rhythm of his or her playing; such anactivity is often a natural bodily expression that can serveas a means of timekeeping. While such expressions wouldnot effect the sound output directly, in this case, Biostomptranslates these expressions into variations in sound, offer-ing a new degree of freedom.

The users reported that they felt a significant potential forimproving their control over the system with more practice,implying that the system can be capable of affording varyingdegrees of virtuosity. In accordance with this, most usersindicated that their ability to predict the system’s effecthas noticeably improved over time. One user stated thatonce they figured out the limits of the parametric changesenabled by the system, their control over it significantly im-proved. These results imply an ability to gain expertise withBiostomp, which can contribute to its long-term integrationinto performance practices.

Out of the two effects the participants performed with,there was a preference towards the control of the delay pa-rameter (62%) than that of the drive parameter (38%). De-spite this result, multiple users reported feeling more in con-trol of the drive parameter. Other users expressed that thesense of contracting a muscle was more analogous with theaudible effect of the drive pedal rather than that of the de-lay pedal. This indicates an embodied relationship betweenthe variations in physical effort and sound dynamics, whichis a rather intuitive relationship that is often observed withacoustic instruments. When asked about which effects pa-rameters they thought would be suitable for controlling withBiostomp besides the ones they performed with during thestudy, users listed size parameter on reverberation effects,cut-off frequency on EQ and WAH pedals, depth and timeparameters on flanger and phaser pedals, and pitch valueon pitch-shift pedals.

While most of the participants described their movementsand the resulting changes in effects to be coherent, theyhave also suggested that personalizing the experience byusing different effects pedals, and attaching the sensor toother limbs could help improve the sense of coherence. Oneof the users expressed that one of the most engaging as-pects of their experience with the system was figuring outits effect on their playing, and added that it wouldn’t be

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as expressive if it was entirely predictable. Another partici-pant noted that the unexpected outcomes have surprisinglyaffected their performance in a positive way. This is appar-ent in Fig. 6 (b) and (c), which imply that while most usersacknowledged a sense of control over the system, the coher-ence between their performance and the resulting changes ineffects were rated higher with a similar Likert distribution.This could be explained through the unmediated connectionthe system creates between muscle movements and param-eter changes: although the users might not be conscious oftheir muscle activity when playing their instrument, suchactivity is nevertheless a part of their performance; Bios-tomp merely maps this to an existing parameter within theircontrol space. By nature of its design, the system is immuneto misinterpretations, discontinuities or data loss.

The users reported the armband having a physical effecton the performance to varying degrees. While some usersdescribed that the system did not cause any additional phys-ical strain during their performance, others indicated thatit could get uncomfortable beyond the 15-20 minutes pe-riod they spent with the system during the study. Oneuser noted that the awareness of the device forced them toperform extra movements that might have contributed totheir sense of post-performance fatigue. In line with this,another user stated that becoming more experienced withthe system would presumably help them be more deliber-ate with it and therefore reduce the risk of fatigue. Anotheruser suggested that it might be comfortable to play with asmaller armband; the current design of the armband hasbeen resembled to a blood pressure cuff.

Despite the significant variation of musical styles playedby the participants, nearly all of them expressed that play-ing with Biostomp would be compatible with their style.While psychedelic rock, free improvisation and experimen-tal music were the most commonly listed genres consideredsuitable for performing with this system, the list includedmore mainstream genres as well. Some users noted that thedevice would be particularly useful in performances wherebodily movements are well-articulated, or more physicallyexpressive. One user mentioned that they would want tocompose new pieces with this system in mind, while an-other felt the device “expanded the playground for the freeimproviser.”

When asked about possible improvements to the system,the participants requested better response times especiallywith the release stage of the effect; a user noted that itoccasionally became hard to decay the effect before a newactivation was triggered. Some users indicated that it couldbe interesting if multiple limbs were monitored to controlmultiple parameters at once. Given the audio output ca-pability of the system, one user suggested that Biostompcould be integrated into a modular synthesizer performanceas a CV source. One of the users suggested that the sys-tem could be used in an art-form that is more corporeallyexpressive, such as theatre or performance art.

5. CONCLUSIONSThis paper introduces a new interface for augmenting mu-sical performance with biosignals through the use of a self-contained stomp box system that translates muscle activityinto changes in parameters on effects pedals. A user studyof 21 musicians provides initial validation of the success ofBiostomp in terms of ease of use, controllability, expressive-ness, and fatigue.

Feedback from our preliminary study guided the funda-mental design of our system. The improved comfort andmore accurate processing hardware in the new design en-

(a) Do you have prior expe-rience with new musical in-terfaces (e.g. sensor systems,digital control devices)? 0: Ihave no experience with suchinterfaces. 10: I have consid-erable experience with suchinterfaces.

(b) To what extent did youfeel in control of the changesin sound effects as you per-formed with this device? 0:I felt it was totally out of mycontrol. 10: I felt it was com-pletely under my control.

(c) Regardless of the extentof your sense of control overchanges in sound effects, howcoherent do you think thesechanges were with your per-formance? 0: I did not find itcoherent at all. 10: I found itvery coherent with my play-ing.

(d) Did you feel you had tocontract your muscles delib-erately to activate the device,and to what extent? 0: Thedevice acted upon my naturalarm movements during per-formance. 10: I controlled thedevice entirely with deliber-ate muscle contractions.

(e) Regarding the two scenar-ios presented in the previousquestion, which one do youthink is the ideal use of thisdevice? 0: When it changesthe effects as a result of myregular performance withoutdeliberate contractions. 10:When it responds my delib-erately executed muscle con-tractions.

(f) To what extent did the de-vice affect your performancephysically? 0: It did not af-fect my playing physically atall. 10: It significantly af-fected my playing physically.

(g) Have you noticed a vari-ation in your sense of controlover the changes in sound ef-fects? 0: There was no no-ticeable difference in my senseof control. 10: I felt my senseof control improved over time.

(h) Have you noticed a vari-ation in your ability to pre-dict the device’s effect onsound processing during thecourse of your performance?0: The device acted unpre-dictably throughout my per-formance. 10: It became eas-ier to predict the effect ofthe device as the performanceprogressed.

(i) What is the effect ofperforming with this deviceon your overall fatigue af-ter the performance? 0:This device had no effect onmy post-performance fatigue.10: This device increased mypost-performance fatigue.

(j) How compatible do youthink this device is to yourplaying style? 0: The func-tion of this device is not com-patible with my playing styleat all. 10: The functionof this device is considerablycompatible with my playingstyle.

Figure 6: The distribution of user responses to our survey,following the performance of multiple musical styles withBiostomp.

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sured a minimum threshold of musical responsiveness forinstrumentalists with a wide range of stylistic backgrounds.In our user study, we observed an ease of adaptation duringeach participant’s experimentation with the system. Someusers especially indicated that they appreciated the inti-macy that the familiar “effects pedal” presentation of Bios-tomp provided. We observed that users who were intro-duced to the original sound of the control signal were moreeasily able to incorporate Biostomp into their performances.Users were intrigued by the increased range of expressivityour system enabled, and we will continue to explore andevaluate the use of Biostomp to control a wider range ofmusical effects.

We believe that the overall comfort of the armband willneed to be improved for the next iteration of the hardware.In addition to the foot-switch that shifts the signal frommotor control to audio output, and the input gain knob, weplan to add extra control knobs (e.g. for adjusting rotationdecay time) and switches (e.g. for switching motor rotationpolarity) based on the feedback we received from musicians.We also see a great potential in adding a DC output withinsignal Eurorack standards for the integration of Biostompwith modular synthesizers.

We hope that the results discussed here will not only helpus improve our own design, but also aid the design of newbiosignal interfaces, particularly taking into considerationsome of the issues highlighted in this study, such as theeffects of mapping strategies on the embodied relationshipbetween the performer and the sound, the benefits of ad-hering to a performer’s existing interaction paradigm, andthe need for a range of expressiveness within which the per-formers can improve their control over the system. Overall,we believe that the integration of biocontrol into music per-formance opens up a lot of possibilities for both traditionaland contemporary performance practices, and that Bios-tomp offers a notable contribution in this area.

6. ACKNOWLEDGMENTSWe would like to thank Alp Tugan for helping with 3Dmodelling, Musa Yılmaz for helping with manufacturing thearmband, and Mehmet Omur for video editing.

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