Muscle Activating Force Detection Using Surface Electromyography

Transcription

1 Muscle force, F v (v m ) (Fracton of maxmum sometrc force) Muscle force, F l (l m ) (Fracton of maxmum sometrc force) Muscle Actvatng Force Detecton Usng Surface Electromyography Saran KEERATIHATTAYAKORN Canddate for the Degree of Doctor of Phlosophy Supervsor: Prof. Shgeru TADANO Dvson of Human Mechancal and Systems and Desgn 1. Introducton Human body movement s generated by muscle forces. Daly actvtes such as graspng an object, usng a hand tool and walkng requre cooperatve of muscles n upper and lower extremtes. From a bomechancal standpont, there s a relaton between the nternal muscle force that produces jont moments and the external force that s human body mparts on a work object. In order to gan more nsght nto muscle functon and mechancs, measurng of muscle force durng contracton s necessary. Drect measurement of muscle force s mpractcal and nvasve. Electromyography (EMG) s a boelectrcal sgnal that usually used to observe muscle actvty durng contracton. Usng surface EMG sgnal whch detected from skn surface s one way to measure muscle actvty n a non-nvasve manner. The am of ths research s to develop methods to estmate muscle forces usng surface EMG sgnal. 2. EMG-drven Model for Estmatng Muscle Force In order to determne the muscle forces, many methods based on mathematcal models were developed. EMG sgnal was well known to be related to muscle force generaton and thus, EMG was ntroduced nto the model to estmate the muscle force. The advantage of the EMGdrven model s that the processed EMG sgnal reflects the actvaton of each muscle crossng the jont, thus facltatng the accurate estmaton of the ndvdual muscle force. Hll s muscle model s used to descrbe the mechansm of muscle durng contracton. In conceptual, Hll muscle model s that the contractle propertes of muscle can be represented by relatonshp between tenson, length and velocty of the muscle fber controlled by muscle actvaton[1]. Muscle model conssts of passve element (PE) and the contractle element (CE) as shown n Fg The force produced by CE can be expressed by F CE l F v F t F F (2.1) 0 l m v m EMG where F 0 s the maxmum sometrc force possble by muscle model, F l (l m ) s the fracton of the maxmum sometrc force the muscle can produce at current length, F v (v m ) s the fracton of the maxmum sometrc force whch the muscle can produce at current velocty and F EMG (t) s the current actve state of muscle. Surface electrode can be used to measure muscle actvaton and muscle length can be estmated va the relatonshp between elbow angle and muscle length usng musculoskeletal model. a) b) c) F MT Tendon w Passve force Muscle fber length (% of optmum length, L FOPT ) 1.6 Actve force at 100 % actvaton 1.4 Actve force at 50 % actvaton 1.2 n curvature of 1 hyperbola defned by 0.8 Hll s equaton Lengthenng Actve force Shortenng Muscle fber Velocty (% of maxmum velocty, v 0 ) Tendon Actve force at 100 % actvaton Actve force at 50 % actvaton Fgure 2.1 Hll type muscle model a) The muscle model s composed of a contractle element (CE) n seres wth a tendon and parallel to a passve element (PE). b) The force-length relatonshp of the CE element. Values are normalzed by maxmum sometrc force (F 0 ) and optmum muscle length (L FOPT ). w s the wdth of the parabola n the force-length curve. c) The force-velocty relatonshp of the CE. Values are normalzed by F 0 and maxmum shortenng velocty (v 0 ). 2.1 Experment procedure The subjects performed a seres of elbow and knee flexon /extenson wth predetermnated movement frequences range from 0.2 Hz to 1.2 Hz. Jont flexon angles were measured by usng an electrogonometer. The external load was measured by the load cell that was fxed to the ground (Fg. 2.2). Pars of bpolar surface electrode were attached to the major muscles of elbow and knee jonts. Frstly, the raw EMG were hgh-pass fltered usng Butterworth flter (20 Hz) to remove movement artfact, then full wave rectfed and fltered usng Butterworth low-pass flter (3 Hz) then normalzed by MVC to obtan the fltered, rectfed and normalzed EMG. Angular acceleraton profle was obtaned by numercal dfferentaton of the poston jont angle obtaned from electrogonometer.

2 a) Load cell T Surface electrode Flexon angle θ Electrogonometer b) Fgure 2.3 Muscle forces durng elbow flexon/extenson, bceps (blue sold lne) and trceps (red sold lne) at varous movement frequences ( Hz). Fgure 2.2 Schematc drawng of the expermental setup a) elbow jont b) knee jont 2.2 Optmzaton process From the muscle model (Fg. 1). The specfc set of muscle parameters for each movement frequency was determned usng an optmzaton method. We have assumed that the external force appled to the subject s wrst estmated by the model should match those measured from the load cell. A smulated annealng algorthm was used to tune the model parameters by mnmzng the objectve functon, (J) gven by: 2 J (2.2) n 1 T m T c where n s the number of samples durng the entre movement n each tral, T m s the measured external force from the load cell and T c s the predcted external force calculated from the model. 2.3 Result and dscusson In ths study, an EMG-drven model was developed and used to estmate muscle force durng elbow and knee flexon/extenson movement. The optmzaton process used data from varous movement frequences. The change n muscle force contrbuton can be observed nonnvasvely. Durng elbow flexon/extenson movement, both bceps and trceps generate force to move forearm as shown n Fg Level and tmng of muscle force generaton change wth movement speed. As movement speed ncreases more force s requred to generate faster movement. Durng flexon (0-50% of flexon/extenson movement), bceps works as agonst muscle and trceps works as antagonst muscle. Whle durng extenson (50- Fgure 2.4 Muscle forces durng knee flexon/extenson, rectus femors (RF), vastus laterals (VL), bceps femors (BF) and gastrocnemus (GaS) durng knee extenson/flexon at varous movement frequences ( Hz). 100% of flexon/extenson) trceps work as agonst and the bceps works as antagonst muscle. The results seem consstent wth the anatomcal nformaton. The cooperatve actvty of bceps and trceps was presented. The pattern seems smlar n every movement speed. The maxmum bceps force occur at about 20% of flexon/extenson cycle whch requred to move forearm and the maxmum trceps force occur later at about 40% of flexon/extenson cycle to break the forearm movement. Indvdual muscle forces were estmated durng knee flexon/moton at varous movng speeds (Fg. 2.4). The muscle contrbutons changed durng the moton. The muscle force of the vastus laterals (VL) showed a mnmal change n muscle force when the movement speed ncreased. In contrast, the rectus

3 femors (RF) and bceps femors (BF) generated more force when the movement speed ncreased. Durng slow movement (e.g., 0.2 and 0.4 Hz), the VL was the man muscle that contrbuted to generatng knee extenson. For faster movements, the RF was the man muscle that accelerated the body segment to acheve the desred speed. The ncreased co-contracton of the RF and BF was observed at hgher speeds (e.g., 0.8 and 1.0 Hz). The peak force of the VL occurred at about 60% of the extenson/flexon tme and stayed the same for all speeds. The gastrocnemus provded only a small contrbuton durng knee flexon/extenson movements. In summary, a practcable muscle force estmaton technque called EMG-drven model was developed and mplemented to estmate muscle force durng elbow and knee jont movement. EMG sgnal from the muscles were measured by pars of bpolar electrode and used to estmate muscle force. The results show the effect of dynamc moton on the EMG sgnals. Increasng n muscle force or movement speed affects the ampltude of EMG sgnal. It seems that EMG-drven model s practcal n estmate muscle force n major jont where surface EMG sgnals from muscles are detectable, lke those of the upper arm and legs. However, n the human forearm, there are many muscles that are used to control the complex movement of fngers and hand. These muscles are relatvely small and dffcult to detect by conventonal surface electrode. The problem of detectng unwanted sgnals or cross-talk s the man problem n assessng muscle actvty n forearm regon. 3. Muscle Stress Dstrbuton n the Forearm Usng EMG-CT Method In a prevous study, an electromyography computed tomography (EMG-CT) method was developed to measure the dstrbuton of muscle actvty wthn the forearm[2]. The muscle actvtes n the whole cross secton were calculated from EMG sgnals obtaned from 40 pars of bpolar surface electrodes, whch were bound around the forearm. An EMG conducton model was formulated for reverse-estmaton of muscle actvtes from the measured EMG sgnals. EMG-CT provdes a new way to vsualze muscle actvty dstrbuton n a nonnvasve manner. The level of muscle actvty represent the effort of muscle durng the contracton. The muscle actvty dstrbuton dentfed by EMG CT has hgh potental to estmate muscle stress n the forearm muscles, defned as force generated n the muscles per unt crosssectonal area of the forearm. Determnng stress dstrbuton n the forearm s of great mportance because t could help to understand how nternal load s shared wthn the forearm muscles and to provde more nsght nto muscle mechancs. The am of ths study was to develop a method to estmate stress dstrbuton wthn the forearm muscle durng grppng usng muscle actvty obtaned by means of EMG-CT. A forearm model was constructed from subject s forearm geometry. In order to estmate stress dstrbuton, a mathematcal model relatng to muscle actvty and force was formulated. The stress dstrbuton wthn the forearm durng grppng was estmated and vrtualzed usng tomographc mage. In ths study, muscle stress generated n the forearm durng hand grppng was performed as a tral. Hand grppng requres forces exerted by the fngers and the thumb, whch are manly controlled by the muscles of the forearm; thus, the grppng abltes of the hand are drectly related to the strength of the forearm muscles. Durng performng a hand grp as shown n Fg. 3.1, maxmum grppng force F was defned as the force requred by the subject s hand to keep a hand grp at full grp poston, as descrbed by the followng equaton: x2 F w f dx (3.1) x1 where w f s the force dstrbuted along the handle of the hand grp durng full grp poston. x 1 and x 2 are the dstances from the center of rotaton of the hand grp to the respectve ends of the handle. 3.1 Forearm model constructon To construct a forearm model of each subject, an outlne of the forearm cross-sectonal area was obtaned accordng to the subject s forearm geometry. The subject s arm was scanned wth a 3D scanner. Durng measurements, the subject sat on a char wth hs arm extended away from the torso n supne poston, the same poston as n the grppng tral. An examner held the 3D scanner and moved t around the subject to scan the whole arm. The scan data ponts of subject s arm obtaned from the 3D scanner numbered approxmately 130,000 ponts at a resoluton of a pont-to-pont space of approxmately 0.6 mm. The subject s forearm geometry was reconstructed from scan data ponts (Fg. 3.2a). The forearm axs was drawn by connectng ponts between two anatomcal landmarks: the lateral epcondyle of the humerus and the radal stylod. The ponts wthn ± 2-mm nterval from EMG CT analyzed secton whch perpendculars to the forearm axs were projected to a plane. An outlne of the forearm crosssectonal area was thus created by connectng the ponts usng a smple convex polygonal approxmaton method (Fg. 3.2b). Fgure 3.1 A hand grp used n the experment. The subject exerted a unformly dstrbuted force w f along the handle to keep the hand grp n the full grp poston.

4 bpolar electrodes j, l 0 s the unt length (1 mm), and b(d j ) s the power exponent of the attenuaton under the dstance d j. In ths study, d j was 15 and 45 mm. V 0 and b were 162 mv s/ma dpole and at d j = 15 mm and 115 mv s/ma dpole and at d j = 45 mm. The actvty of each muscle element m was calculated usng a sequental quadratc programmng method to mnmze the objectve functon (OF), whch was defned as the sum of the power of the dfferences between the measured EMG V Mj and the calculated EMG V j, as shown by the followng equaton: j 2 OF V Mj V j (3.3) Fgure 3.2 (a) Subject s forearm geometry based on 3D scan data. (b) Forearm cross-sectonal area outlne extracted from the EMG-analyzed secton. (c) Forearm EMG conducton model. The muscle regon wthn the forearm outlne was dvded nto small elements usng Vorono tessellaton. Surface electrodes were placed around the outlne of the forearm. The conducton dstance was the dstance between the muscle element and the surface electrode. An EMG conducton model was constructed usng the outlne of the forearm cross-sectonal area. Muscle element nodes were dstrbuted across the entre crosssectonal wth an element sze of 1 mm at the surface and 5 mm for the nsde regon. The area of each muscle element was dvded usng Vorono tessellaton. The surface electrodes were placed around the outlne of the forearm model. The conducton dstance of each muscle element was the dstance between the muscle element and the surface electrode (Fg. 3.2c). 3.2 Muscle actvty calculaton The attenuaton of the acton potental depends on tssue conductvty and the dstance between a muscle element and the surface electrode. In ths study, we consdered that each muscle element was actvated ndependently. Thus, the statstcal summaton of power of the EMG sgnals was possble [2]. The mean square value of EMG from all muscle elements detected by bpolar electrodes j, V j can be smply expressed by the followng equaton: 2bd j l j V j V0 d j m (3.2) l0 where V 0 (d j ) (mv s/(ma dpole)) s a transformaton coeffcent that depends on the dstance between the par of bpolar electrodes d j, m (ma dpole/s) s the muscle actvty of the element, l j (mm) s the conducton dstance between the muscle element and a par of 3.3 Stress calculaton When external forces are appled to the fngers, the muscles of the forearm generate reacton forces to mantan a statc equlbrum. Total muscle actvty Σm s defned as the sum of muscle actvty wthn the forearm cross-sectonal area and reflects the amount of force generated by the muscles durng contracton. A lnear relatonshp between F and Σm was assumed n ths study. The muscle actvty-force coeffcent α (N s/ma dpole) was defned as shown by the followng equaton: F (3.4) m Here, the muscle force generated by each element can be calculated from the stress wthn each element multpled by ts area, as shown by the followng equaton: f a (3.5) where f (N) s the force generated by muscle element, σ (MPa) s the stress n muscle element, and a (mm 2 ) s the area of a muscle element. Thus, F s equal to the sum of the force generated by all elements wthn the forearm and can be expressed by the followng equaton: F f a m (3.6) The stress of each element can be calculated from the followng equaton: m (3.7) a Thus, the stress dstrbuton n the forearm muscles can be calculated usng muscle actvty measured by EMG CT and Eq. (3.7). 3.4 Expermental setup Three male subjects partcpated n ths study. The subjects sat on a char wth ther domnant arm placed on a horzontal table. The upper arm was at approxmately 0 of abducton, the elbow jont was flexed at approxmately 90, and the wrst was placed n supne poston. EMG sgnals from the forearm were recorded

5 usng an EMG band [2], consstng of 20 electrode plates. The mddle pont of the EMG band was postoned at the mddle pont of the forearm lengthwse, between the lateral epcondyle of the humerus and the radal stylod. Before attachment of the EMG band, the subject s forearm skn was cleaned wth an alcohol swab. Fat and skn thckness were measured usng a sknfold calper. For each grppng tral, the subject held a hand grp between the mddle phalanges of fngers and palm and then squeezed the handles to full grp poston and held for 5 s, three trals per load wth 5 s of relaxaton between trals. EMG sgnals were recorded durng each grppng tral and relaxaton usng a custom program (LabVIEW 8.5, Natonal Instruments, TX, USA). 3.5 Results and dscusson Fgure 3.3 shows the relatonshp between F and the total muscle actvty wthn the forearm of all subjects. Total muscle actvtes were plotted aganst grppng force, and the relatonshp was descrbed by a lnear regresson (R 2 = 0.97 ± 0.04). The mean of the total muscle actvty of all subjects ncreased from 1626 ± 344 to 3878 ± 170 ma dpole/s when the grppng force ncreased from 77 to 242 N. α of subjects A, B, and C were 0.054, 0.060, and N s/ma dpole, respectvely. Fgure 3.4 shows tomographc mages representng stress dstrbuton wthn the forearm muscles durng grppng n all subjects. Patterns of stress dstrbuton could be observed under all nvestgated condtons. The results showed a change n level and area for all condtons, and an ncrease n stress concomtant wth load ncrease was observed. A magnetc resonance mage (MRI) of the forearm cross-sectonal area of each subject s shown n the top row for anatomcal comparson (Fg. 3.4a). The actvated area and the maxmum value of muscle stress ncreased wth grppng force n all subjects. Under the 77 and 124 N load condtons, muscles n ulnar regon and radal dorsal regon were actve n all subjects. Under the 242 N load condton, muscles n the palmar regon were actve n subjects A and B. The average maxmum stresses n all subjects under 77, 124, and 242 N load condtons were 0.08 ± 0.01, 0.11 ± 0.01, and 0.18 ± 0.02 MPa, respectvely. In subject A, the maxmum stress was found n the ulnar regon under all load condtons, whereas n subject B and C, the maxmum stress was found n the radal dorsal regon under all load condtons. In ths study, we nvestgated the relatonshp between external force and muscle actvty durng grppng. The muscle force requred to mantan a statc equlbrum ncreased wth the grppng force. A lnear model appears to offer a good approxmaton of the relatonshp between muscle actvty and force under sometrc condtons. Under 77 and 124 N grppng load condtons (Fg. 3.4b and c), the stress s concentrated n two muscle groups: on the ulnar sde, whch may nclude the flexor dgtorum superfcals (FDS) and the flexor dgtorum profundus (FDP), and on the radal dorsal sde, whch may nclude the extensor dgtorum communs. Subject A appeared to use more muscles durng grppng, resultng n less stress concentraton than the other subjects. Under Fgure 3.3 Relatonshp between grppng force F and total muscle actvty Σm wthn the forearm durng grppng. Fgure 3.4 (a) Magnetc resonance mage of forearm crosssectonal area. Stress dstrbuton generated n the forearm muscle durng grppng (b) 77 N, (c) 124 N, and (d) 242 N. the 242 N load condton (Fg. 3.4d), stress was dstrbuted across the whole area. Addtonal stress concentraton was found n subjects A and B on the palmar sde whch may nclude the flexor carp radals. The basc grppng functon nvolves fnger flexon generated by muscle forces from the fnger flexor muscles n the forearm. Ths mechansm s shown by the fndng that both the FDS and FDP generate stress durng. It s also consstent wth results of prevous studes that used wre electrodes to study fnger moton, fndng that the FDP s

6 responsble for synchronous flexon of fnger jonts. The maxmum stress was found n the extensor regon of subject B and C under the 242 N load condton (Fg. 3.4d). It appears that extensors play an mportant role under hgh grppng load. Ths study has shown hgh potental for estmatng stress dstrbuton generated n the forearm muscles usng an EMG CT band and a 3D scanner. The use of a new EMG conducton model devsed from subjects forearm geometry and a model for calculatng stress from muscle actvty represent an mprovement n the EMG CT method and make t more clncally applcable. Ths mprovement allows vsualzng stress wthn the forearm muscles, a capablty that may advance the development of dagnostc tools. References [1] Zajac, F. E., Muscle and tendon: propertes, models, scalng, and applcaton to bomechancs and motor control, Crtcal revews n bomedcal engneerng, Vol.17, No.4 (1989), pp [2] Nakajma, Y., Keerathattayakorn, S., Yoshnar, S. and Tadano, S., An EMG-CT method usng multple surface electrodes n the forearm, Journal of electromyography and knesology, Vol.24, No.6 (2014), pp Summary In ths study, methods to estmate muscle force from surface EMG sgnal were developed. Muscle forces generated durng upper and lower lmb movement were estmated usng the developed methods. EMG-drven model was developed and used to estmate muscle forces n elbow and knee jont durng flexon/extenson moton. Body knematc data and EMG sgnal were measured and used to estmate ndvdual muscle force. The results show that there s the relatonshp between movement speed and EMG sgnal. The EMG-drven model usng surface EMG s very useful n estmatng muscle force n elbow or knee jont whch s moved by large muscles. However, the lmtatons of conventonal surface EMG sgnal prevent the possblty of usng ths system to estmate force n human forearm whch many small muscles are deeply resde wthn. A novel method called EMG-CT was developed to estmate muscle actvtes wthn the forearm. Surface EMG sgnals wthn the forearm regon were detected by multple surface electrodes. EMG conducton model was formulated for reverse-estmaton of muscle actvtes usng detected surface EMG sgnals. The ndvdual muscle actvtes n the deep regon were estmated and present n EMG tomographc mage. A method for estmatng muscle force n the forearm regon based on EMG-CT was developed. The study has shown hgh potental for estmatng stress dstrbuton generated n the forearm muscles usng an EMG band and a handy 3D scanner. The use of a new EMG conducton model devsed from subjects forearm geometry and a model for calculatng stress from muscle actvty represent an mprovement n the EMG CT method and make t more clncally applcable. Ths mprovement allows vsualzng stress wthn the forearm muscles, a capablty that may advance the development of dagnostc tools.