An Exploratory Electromyography-Based Coactivation Index for the Cervical Spine

Transcription

1 738598HFSXXX / Human FactorsExploratory Electromyography-Based Coactvaton Index An Exploratory Electromyography-Based Coactvaton Index for the Cervcal Spne Peter Le, Alexander Aurand, The Oho State Unversty, Columbus, Thomas M. Best, Unversty of Mam, Coral Gables, Florda, Safdar N. Khan, Ehud Mendel, and Wllam S. Marras, The Oho State Unversty Objectve: Develop a coactvaton ndex for the neck and test ts effectveness wth complex dynamc head motons. Background: Studes descrbng coactvaton for the cervcal spne are sparse n the lterature. Of those n exstence, they were ether lmted to a pror defntons of agonst/antagonst actvty that lmted the testng to sagttal and lateral planes or conssted of sometrc exertons. Multplanar movements would allow for a more realstc understandng of naturalstc movements n the cervcal spne and propensty for neck pan. However, a gap n the lterature exsts n whch a method to descrbe coactvaton durng complex dynamc motons does not exst for the cervcal spne. Methods: An electromyography-based coactvaton ndex was developed for the cervcal spne based on prevously tested methodology used on the lumbar spne wthout a hgh-end model and tested usng a seres of dfferent postures and speeds. Results: Complex motons nvolvng twstng (.e., flexon and twstng) and hgher speed had hgher magntudes of coactvaton than unplanar motons n the sagttal or lateral plane, whch was expected. The coupled moton of flexon and twstng showed four to fve tmes hgher coactvaton than unplanar (sagttal or lateral) movements. Concluson: The coactvaton ndex developed accommodates multplanar, naturalstc movements. Testng of the ndex showed that motons requrng hgher degrees of head control had hgher effort due to coactvaton, whch was expected. Applcaton: Overall, ths coactvaton ndex may be utlzed to understand the neuromuscular effort of varous tasks n the cervcal spne. Keywords: co-contracton, neuromuscular, neck muscles, coactvaton Address correspondence to Wllam S. Marras, Spne Research Insttute, Department of Integrated Systems Engneerng, The Oho State Unversty, 1971 Nel Avenue, 210 Baker Systems Engneerng, Columbus, OH 43210, USA; e-mal: Marras.1@osu.edu. HUMAN FACTORS Vol. XX, No. X, Month XXXX, pp DOI: / Copyrght 2017, Human Factors and Ergonomcs Socety. Introducton Musculoskeletal dsorders (MSDs) of the neck-shoulder regon are one of the most frequently reported problems for the workng populaton (Larsson, Søgaard, & Rosendal, 2007; Yang et al., 2015) and present an economc burden to socety, wth medcal costs estmated at US$86 bllon a year (Martn et al., 2008). To mtgate the burden, t s mperatve to understand the etology of neck pan to enhance work (re)desgn and rehabltaton. One of the underlyng mechansms for neck pan nvolves the alteratons n neuromuscular control for head stablzaton and durng pan (Falla & Farna, 2008). Ths mechansm may be descrbed by coactvaton, or the synchronous actvaton of agonst and antagonst musculature for postural stablzaton (Lavender, Tsuang, Hafez, et al., 1992). The coordnaton between agonst and antagonst actvty results n a level of neuromuscular effort to accomplsh a task. All tasks requre some level of coactvaton. However, t s the level beyond what s necessary to accomplsh the task that ncreases the neuromuscular load. For example, patents have hgher muscular actvatons across a multple muscle system (.e., low back) when compared to asymptomatc ndvduals (Marras, Ferguson, Burr, Davs, & Gupta, 2004). To descrbe the overall actvty of the system of muscles, a coactvaton ndex s needed. Understandng coactvaton from a systems perspectve may provde nsght on the neuromuscular effort needed for the adaptaton to dfferent tasks. Gven the complexty of the cervcal spne, methods to descrbe coactvaton n the neck as a system are sparse n the lterature (Le, Best, Khan, Mendel, & Marras, 2017). Of the studes n exstence to descrbe coactvaton as an ndex, they were commonly lmted n ther utlty due to a pror defned muscular contrbuton (agonst/antagonst) (Cheng et al., 2014; Cheng, Ln,

2 2 Month XXXX - Human Factors & Wang, 2008; Cho, 2003). Based on the dependence of coactvaton patterns to stablze posture and external loads, changes n the locaton of the load may requre shfts n agonst/antagonst actvty to adjust posture relatve to the load (Lavender, Tsuang, Andersson, Hafez, & Shn, 1992). Therefore, predefnng muscle actvty as agonst or antagonst lmts the tests to sometrc testng or unplanar dynamc movements (sagttal or frontal planes). When testng under the sometrc approach, actvtes of daly lvng as well as naturalstc movements (axal twstng and other complex postures) may not be captured because dynamc muscular actvty dffers from statc. Hgher neuromuscular effort may be requred for postural control n asymmetrc postures. Currently, an ndex to quantfy asymmetrc multplanar motons for the cervcal spne does not exst. In a prevous manuscrpt, a method to descrbe coactvaton for a mult-muscle system was defned for the lumbar spne (Le, Aurand, et al., 2017). Ths nvolved the calculaton of the moments based on the actve forces defned from the musculature and allowed for the assessment of coactvaton for multplanar dynamc tasks. Snce ths approach requred the use of a bologcally asssted model, another approach was made n case a model was not accessble (Le et al., n press). Ths method was dependent on torso knematcs and normalzed electromyography (EMG) modulated by the cross-sectonal area to represent a smulated force. Although the comparablty of the EMG-based method was lower, t provded a smlar measure of coactvaton relatve to the moment-based method. In general, the EMGbased method can detect hgher coactvaton durng tasks where t was expected. These tasks nvolved hgher degrees of postural control or overall muscle actvty to generate hgher external forces (.e., precson placement and movement of a cart) throughout the seres of complex dynamc tasks. Snce the EMG-based method for the lumbar spne had a hgh fdelty when compared to the model-dependent, moment-based method, t was postulated that ths methodology could be appled to descrbe coactvaton n the cervcal spne wthout a model. The objectves of ths study were twofold: (a) Develop a coactvaton ndex for the cervcal spne usng the approach from Le and colleagues (n press) and (b) test the ndex on a seres of multplanar, complex, dynamc head-neck motons. It was hypotheszed that ths methodology would be able to dfferentate between unplanar motons and complex multplanar motons. Coactvaton Index Structure The underlyng logc of the EMG-based coactvaton ndex was based on the determnaton of smulated force components prevously descrbed by Le and colleagues (n press). Twelve muscles were ncluded n the ndex based on EMG from three blateral regons of the neck: cervcal extensors, sternocledomastod, and levator scapulae. The cervcal extensors ncluded: semspnals capts, semspnals cervcs, splenus capts/cervcs (grouped together), and the cervcal trapezus. EMG collected from the extensors were appled to the smulated force equaton for each of the cervcal extensor muscles descrbed. A cube exponental of the normalzed cross-sectonal area was utlzed to modulate the normalzed EMG to scale the effect of the smulated force (F ) (Equaton 2). Ths method provded a smlar trend as maxmal force relatve to cross-sectonal area of muscles (CSA) (Le et al., n press). CSA data were extracted from Kambayash and Rchmond (1998) wth the excepton of semspnals cervces, whch was retreved from Deng and Goldsmth (1987) (Table 1). For ths plot study, CSA data uru were fxed across all subjects. Force vectors (F ) were defned as the product of the smulated force and the unt vectors relatve to muscle lnes of acton (Table 2) and drven by the knematcs of the head usng a quaternon rotaton matrx (q Head ) (Equatons 1, 2). These lnes of acton were operatonally defned based on general anatomcal data. The rotaton/translaton of the nserton ponts (Equaton 1) was relatve to the locaton uru of C7/T1. Smulated muscle moments (m ) were the cross-product of the moment uru arm ( r ) and the assocated force vector (F ) relatve to C7/T1 (Equaton 3). The summaton uru of the smulated muscle moments (m ur uu ) resulted n the total smulated moment (M ) (Equaton 4). The dot uru product of the ndvdual muscle moments (m ur uu ) relatve to the total moment (M ) normalzed by the magntude of

3 Exploratory Electromyography-Based Coactvaton Index 3 Table 1: Muscle Cross-Sectonal Area Data Used n Cervcal Coactvaton Index CSA (mm 2 ) ur uu the total moment (M ) resulted n a scalar projecton (Proj ) defnng an ndvdual muscle s contrbuton as ether an agonst (postve) or antagonst (negatve) (Equatons 5 7). Coactvaton (CI) was then defned as the product between the balance of the antagonst/agonst systems and the normalzed magntude of the contrbuton (Equaton 8). The normalzaton was operatonally defned by the maxmum actvaton of the data set (0.68). Further detals of the logc behnd the equaton can be found n Le, Aurand, et al. (2017). uru ()= () () 1 V t q t * V t (1) Head ncsa Sternocledomastod Levator scapula Trapezus Splenus capts/cervcs Semspnals capts Semspnals cervcs Note. Cross-sectonal area (CSA) was normalzed (ncsa) relatve to largest CSA n the system. uru F t ()= uru V () t EMG t ur uu () * () t * V t EMG () ( ) max 3 (2) CSA max ( CSA ) = 112 : antagonst CI t 12 = 1 agonst ()= uru r uru m = F (3) ur uu t uru M ()= t m (4) uru = 1 uuur m M (5) Proj = uuuuuur M 0, Proj > 0 = (6) Proj, Proj 0 12 = Proj, Proj > 0 (7) = 0, Proj 0 antagonst () t 1 * 12 agonst t () = 1 12 ()+ () = antagonst t agonst t (8) Table 2: Operatonally Defned Anatomcal Geometry of Muscle Lnes of Acton n Neutral Posture Relatve to C7/T1 (mm) X O Y O Z O X I Y I Z I R sternocledomastod L sternocledomastod R levator scapula L levator scapula R semspnals capts L semspnals capts R semspnals cervcs L semspnals cervcs R splenus L splenus R trapezus L trapezus Note. O = orgn; I = nserton; X = left/rght; Y = superor/nferor; Z = anteror/posteror.

4 4 Month XXXX - Human Factors Table 3: Descrpton of the Dfferent Postures and Speeds Endured Posture Methods Speed Neutral to flexon (3 ) Slow Neutral to extenson (3 ) Preferred Flexon and extenson (3 ) R to L lateral bend (3 ) Axal twst (3 ) Flexon, hold then 3 twst whle n flexon Extenson, hold then 3 twst whle n extenson Note. R = rght; L = left. Expermental Approach A study was conducted to test the cervcal spne EMG-based coactvaton ndex through a seres of complex dynamc head motons. Subjects Twelve subjects (5 males and 7 females) were recruted for ths study (mean age = 27.8 years, SD = 6.8; mean mass = 69.6 kg, SD = 15.1; mean heght = cm, SD = 9.7). All subjects reported no medcal vsts for neck pan or surgery. Ths research compled wth the tenets of the Declaraton of Helsnk and was approved by the Insttutonal Revew Board at The Oho State Unversty. Informed consent was obtaned from each subject pror to partcpaton. Expermental Desgn Several combnatons of head posture and speed of movement were used to assess the effectveness of the cervcal spne coactvaton ndex. Independent and dependent measures. The ndependent measures ncluded seven dfferent postures wth two dfferent speeds and were collected (7 2) and repeated twce for a total of 28 dynamc trals (Table 3). Each posture and speed combnaton entaled three repeated movements wthn the same tral whle standng uprght. Speed of movement was subjectvely defned as slow (~15 /s) and normal/preferred (~50 /s). The dependent measures collected were the peak coactvaton ndex and peak head knematcs (range of moton and velocty). Apparatus EMG data were collected wth a 16-channel MA EMG system (Moton Lab Systems, Inc., Baton Rouge, LA, USA) and sampled at a rate of 1000 Hz. Sgnals were hgh-pass fltered at 30 Hz, low-pass fltered at 450 Hz, and notch fltered at 60 Hz as well as ts alases. Sgnals were rectfed and smoothed usng a zero-phase movng average flter. Knematc data were collected usng the 36 nfrared camera OptTrack Flex 41 moton capture system (NaturalPont, Corvalls, OR, USA). Procedure The subject was nformed about the detals of the experment, and after provdng consent, anthropometry was collected. Surface EMG electrodes were placed blaterally on the cervcal extensors, levator scapulae, and sternocledomastods (Sommerch, Jones, Hermans, & Moon, 2000). EMG on the cervcal extensors represented the followng muscles n the ndex: semspnals capts and cervcs, splenus capts/cervcs, and cervcal trapezus. Reflectve markers were placed on 41 landmarks for whole-body optcal moton capture. Although only the head/neck data were extracted, the 41-landmark setup was necessary to defne the dfferent segments of the body. The subjects were then asked to complete a seres of maxmum voluntary exertons (MVEs) whle standng, whch nvolved resstance durng abducton of the arms n the scapular plane and then head flexon, extenson, and lateral bends (Schuldt, 1988). Each exerton was collected twce. After MVEs were collected, the subject was nstructed on the dfferent motons nvolved and allowed ample tme to practce before the study commenced. Each movement was repeated three tmes wthn a sngle tral (.e., lateral bend nvolved three sets of rght to left motons). Statstcal Analyss General lnear models (SAS 9.2, SAS Insttute, Cary, NC, USA) were used to evaluate

5 Exploratory Electromyography-Based Coactvaton Index 5 Fgure 1. Coactvaton ndex results for the nteracton of moton and speed (p =.0136). Data are presented as mean and standard error. Note the hgher levels of coactvty for more complex motons nvolvng axal twstng/rotaton. peak coactvaton ndex relatve to the man effects and ther nteractons at α =.05. Post hoc Tukey tests were performed to assess the dfferences between condtons. Results The coactvaton ndex developed for the cervcal spne dsplayed statstcally sgnfcant results (α =.05) between the dfferent motons (p <.0001), speeds (p =.0065), and nteracton of moton and speed (p =.0136) (Fgure 1). Unplanar motons (flexon/extenson and lateral bendng) had hgher levels of coactvty relatve to the neutral posutre. As the moton became more complex through twstng/ rotaton, coactvaton became hgher than the unplanar motons, where the combnaton of flexon and twstng was the hghest. The speed was also drectly assocated wth the level of coactvaton, especally durng the axal rotaton/twstng combnatons. Normal/preferred speeds ncurred hgher coactvty than slow speeds durng movements nvolvng twstng. The covarate of gender showed that females exhbted hgher coactvaton (mean ndex =.047, SD =.039) than males (mean ndex =.028, SD =.025) across all motons (p =.0002). Most dfferences appeared durng flexon and extenson as well as flexon and twstng motons (Fgure 2a). Hgher coactvty among females was lkely due to the hgher contrbuton found n the sternocledomastod (SCM) muscles (p <.0001) (Fgure 2b). Snce knematcs are nterrelated to the levels of coactvty for postural stablzaton, t s also mportant to understand the ranges of moton experenced durng each condton. As seen n Fgure 3, the peak ranges of moton across the tasks show ncreases n lateral and axal motons wth ncreasng complexty. Durng these motons, peak coactvty tended to occur at the end ranges of moton. To understand how the coactvaton ndex was affected by the agonst/antagonst system classfcatons, two-dmensonal vsualzatons (classfed usng 3-D data) of mean agonst (green) versus antagonst (red) data across all subjects can be seen for flexon/extenson (Fgure 4), lateral bendng (Fgure 5), and axal rotaton (Fgure 6). Durng sagttal or lateral movements, the classfcatons appear to be splt perpendcular at C7/T1 relatve to the drecton of the movement. However, durng axal twstng (Fgure 6), the classfcaton s more complex as lateral and axal motons both occur wthn ths postural transton (Fgure 3). As for the magntude of ndvdual muscular contrbutons relatve to the classfcatons (represented by the sze of the crcles), hgher muscular contrbutons typcally occur wth more complex postures. More specfcally, the sternocledomastod appeared to be a strong contrbutor to the drecton of coactvty, especally durng lateral bendng and twstng. Dscusson The purpose of ths plot study was to provde an approach to assess coactvaton n the cervcal spne for complex dynamc motons. Ths objectve was acheved through the development of a method based on smulated moment contrbutons drven by muscle actvty and knematc data. The ntent was to provde a systemsperspectve descrpton of the neuromuscular effort of coactvaton for multplanar motons. The man fndngs showed that: (1) complex, multplanar motons requred hgher coactvaton; (2) hgher speed resulted n hgher coactvaton durng axal rotaton/twstng motons when compared to unplanar motons (sagttal

6 6 Month XXXX - Human Factors Fgure 2. (a) Dfferences n coactvaton between genders for the varous motons endured (p = ). Females typcally had hgher coactvty across many of the motons. (b) Ths was lkely due to the hgher sternocledomastod (SCM) contrbuton among female subjects (p <.0001). Data are presented as mean and standard error. or lateral); and (3) female subjects had hgher magntudes of coactvaton relatve to males. The results of the EMG-based, cervcal coactvaton ndex provded nsght on normal neuromuscular control among asymptomatc subjects and may be used to explore other varatons of complex tasks as well as patent populatons to assess the effectveness of rehabltatve efforts n comparson to asymptomatc populatons. The cervcal coactvaton ndex was developed based on the dea that the agonst/antagonst nature of a muscle may be determned by ts actve moment drectonalty relatve to the total actve moment (Le, Aurand, et al., 2017). Although t s understood that passve components may also play a role n drvng muscle forces, the coactvaton method descrbed s solely based on the actve contrbutons of the system of muscles, thus lmtng t to the contractle components of the musculature. Through ths methodology, t was postulated that when the drecton of the muscle moment vector was wthn 180 of the total actve moment vector, the scalar projecton would be postve, thereby deemng t an agonst (Andrews & Hay, 1983). On the other hand, f t s obtuse relatve to the drecton of the total actve moment, the projecton would be negatve, thereby deemng the muscle antagonst. Through the assessment of contrbuton from the system of antagonst muscles relatve to the agonst system and the total actvaton of the system, t can be nferred how much neuromuscular effort can be attrbuted to coactvaton. Currently, there s no gold standard to defne dynamc forces and moments n the cervcal spne as there s n the lumbar spne. Hence, a surrogate approach was sought to provde a measure of coactvty ndependent of a model. A coactvaton ndex was frst developed for the lumbar spne wthout hgh-end modelng efforts and then compared to an ndex dependent on a computatonal model to assess ts external valdty (Le et al., n press). The general approach entaled the examnaton of the dfferent modulaton factors that may be accessble wthout a model. Modulaton factors were necessary to properly scale each muscle s contrbuton. These ncluded CSA, lnes of acton, knematcs, and EMG. Overall, t was found that the normalzed CSA cubed was a reasonable alternatve to a model by weghtng the level of coactvty n the lumbar spne (Equaton 8). The fdelty was reasonable relatve to the moment-based, model-dependent method (r 2 = 0.78) whle dfferentatng between the tasks at smlar ndex magntudes. Gven the complexty n the lumbar spne as well as the cervcal spne muscles due to the varous muscle lnes of acton, t was postulated that f the methodology was applcable n the lumbar spne, t may also work for the cervcal spne. Hence, the non model based approach was appled to the cervcal spne drven by

7 Exploratory Electromyography-Based Coactvaton Index 7 Fgure 3. Peak range of moton data (across all subjects) for each moton condton n each of the three anatomcal planes (sagttal flexon/extenson, rght/left lateral bendng, and clockwse/counterclockwse axal twstng). Data are presented as mean and standard error. normalzed EMG from the cervcal extensors, sternocledomastods, levator scapulae, anatomcally defned lnes of acton relatve to C7/T1, and head knematcs. A seres of motons were tested to assess the effectveness of the cervcal coactvaton ndex to dfferentate between complex multplanar motons. Many of the fndngs were antcpated n comparson to reports n the lterature. Coactvaton s hghly dependent on posture and speed and was hghest at the end ranges of moton for each of the dfferent motons. As the moton becomes more complex (multplanar), a hgher level of neuromuscular control s necessary to stablze the head/neck. Therefore, agonst/antagonst classfcaton s mportant to understand durng these motons. As can generally be seen n Fgure 6 as compared to Fgures 4 and 5 (flexon/extenson and lateral bendng), axal rotaton/twstng nctes hgher coactvty due to the multplanar moton. Based on the assumpton that the agonst s consdered a prmary mover dependent on moment contrbutons, whch s also dependent on the level of muscle actvaton (Vasavada, L, & Delp, 1998), as expected, the cervcal extensors were deemed agonst durng peak coactvaton durng flexon and antagonst durng extenson (Fgure 4). Durng lateral bendng, antagonstc muscle contrbutons were typcally contralateral to the moton endured (Fgure 5) and agreed wth the a pror classfcaton from Cheng et al. (2008). However, the fndngs from our study of coactvty durng lateral bendng were lower than ntally antcpated. Based on the level of actvaton from the agonst and antagonst musculature durng lateral bendng found n ths study relatve to the other motons, t was nferred that t may be possble that the lower actvty may be attrbuted to lower range of moton and decreased contralateral moment-generatng capacty (Vasavada et al., 1998), thereby renderng the effort due to coactvaton to be lower. Durng axal rotaton, mean scalar projecton classfcatons (Fgure 6) agreed wth some of the agonst classfcatons from Vasavada et al. (1998) where the left splenus, rght sternocledomastod, and pslateral cervcal extensors acted as synergsts

8 8 Month XXXX - Human Factors Fgure 4. Two-dmensonal vsualzaton of antagonst (red) and agonst (green) actvty for the mean of the peak coactvaton (across all subjects) durng (a) flexon and (b) extenson. Sze of the crcle represents the mean contrbuton of the partcular muscle across all subjects. The x- and y-axes represent the locaton of each muscle s orgn relatve to C7/T1 n meters from Table 2 as X 0 and Z 0, respectvely. Although the vsualzaton s n 2-D, classfcatons were stll based on 3-D data. R/L = rght/left; SCM = sternocledomastod; Trap = cervcal trapezus; SCerv = semspnals cervcs; SCap = semnspnals capts; Spl = splenus; Lev = levator scapulae. durng left axal rotaton. These classfcatons can be affected durng rotaton because axal rotaton s typcally accompaned by lateral bendng (Fgure 3) (Ia et al., 1993; Mmura et al., 1989; Panjab, Oda, Crsco, Dvorak, & Grob, 1993). As prevously seen n the lumbar spne, asymmetrc postures have been known to ncte hgher levels of coactvty (Marras & Fgure 5. Two-dmensonal vsualzaton of antagonst (red) and agonst (green) actvty for the mean of the peak coactvaton (across all subjects) durng (a) left lateral bend and (b) rght lateral bend. Sze of the crcle represents the mean contrbuton of the partcular muscle across all subjects. The x- and y-axes represent the locaton of each muscle s orgn relatve to C7/ T1 n meters from Table 2 as X 0 and Z 0, respectvely. Although the vsualzaton s n 2-D, classfcatons were stll based on 3-D data. R/L = rght/left; SCM = sternocledomastod; Trap = cervcal trapezus; SCerv = semspnals cervcs; SCap = semnspnals capts; Spl = splenus; Lev = levator scapulae. Granata, 1995). Hence, t may be nferred that the head postures requrng axal rotaton as well as rotaton coupled wth flexon would also have hgher levels of coactvty relatve to flexon/ extenson and lateral bendng (Fgure 1). Interestngly, the coupled movement of extenson and rotaton appeared to have lower coactvty

9 Exploratory Electromyography-Based Coactvaton Index 9 Fgure 6. Two-dmensonal vsualzaton of antagonst (red) and agonst (green) actvty for the mean of the peak coactvaton (across all subjects) durng (a) counterclockwse (CCW) axal rotaton and (b) clockwse (CW) axal rotaton. Sze of the crcle represents the mean contrbuton of the partcular muscle across all subjects. The x- and y-axes represent the locaton of each muscle s orgn relatve to C7/ T1 n meters from Table 2 as X 0 and Z 0, respectvely. Although the vsualzaton s n 2-D, classfcatons were stll based on 3-D data. R/L = rght/left; SCM = sternocledomastod; Trap = cervcal trapezus; SCerv = semspnals cervcs; SCap = semnspnals capts; Spl = splenus; Lev = levator scapulae. than twstng alone. Relatve to the fndngs from Harms-Rngdahl, Ekholm, Schuldt, Nemeth, and Arborelus (1986), t was possble that extenson requred hgher tenson n the passve tssues. Ths n combnaton wth ncreased load dstrbuton to the vertebral structures due to the center of mass of the head may have reduced the actve contrbutons, thereby reducng the coactvaton ndex. Durng flexon wth axal rotaton, extensor actvty ncreased to support the forward head moment, and SCM actvty ncreased for axal rotaton. Ths synergstc combnaton ncreased both the rato of antagonst to agonst as well as total system actvaton, thereby resultng n a hgh coactvty ndex. When accountng for speed, the level of coactvty ncreased for tasks requrng axal rotaton/twstng, partcularly durng normal/preferred speed (Fgure 1). It s mportant to note that the motons endured were contnuously repeated three tmes wthn a tral at a very slow and controlled speed or comfortably at a normal speed. Fast motons were not tested to mtgate rsk of moton sckness. Ths ncrease n coactvty typcally occurred at the end ranges of moton, lkely due to deceleraton of the movement to swtch drecton. Gender dfferences were also seen between the tasks (Fgure 2a). Females tended to have hgher levels of coactvty across the dfferent tasks, and t appeared to be assocated wth hgher sternocledomastod actvatons (Fgure 2b). Ths fndng may be supported by a study by Nmbarte (2014), whch also descrbed the relatve strength dfferences n sternocledomastod actvaton between males and females. Overall, the coactvaton ndex developed for the cervcal spne could dstngush dfferent levels of coactvaton relatve to complexty n movement. To place ths study n perspectve, a seres of lmtatons must be noted. Frst, the same set of muscle lnes of acton and cross-sectonal area data were used for all the subjects. A more personalzed model would account for some of the varablty found n the data. However, ths nformaton was not avalable for our subjects. In addton, other personalzed factors such as anthropometrc dfferences were not addressed n ths study. These dfferences affect strength outputs and possbly levels of coactvty (Vasavada, Danaraj, & Segmund, 2008). Second, maxmum voluntary exertons were collected to normalze the EMG values. The problem wth the head and neck regon was that some subjects were more hestant to fully exert ther maxmum efforts for fear of njury, hence a better seres of

10 10 Month XXXX - Human Factors reference contractons may be needed to account for those ssues, partcularly for studyng a patent populaton. A lower maxmum effort may affect the ndex dependng on the set of muscles n whch the effort was lower. It was speculated that whle affectng the normalzaton of the EMG, t would ncrease the magntude of the ndex. Hence, a non-max routne may be warranted (Dufour, Marras, & Knapk, 2013). Thrd, only head motons were tested whle standng. The addton of upper extremty or lumbar-related tasks n dfferent postures and motons may affect the fndngs n the cervcal spne, especally f forces are needed to execute the task (Nmbarte, 2014; Nmbarte, Aghazadeh, Ikuma, & Harvey, 2010). The study was only lmted to the moton of the head to examne the effect of complex dynamc motons on coactvty. Prevous studes have been lmted to unplanar motons (sagttal or lateral) or sometrc exertons. Our study ncluded multplanar motons. Fourth, the system of neck muscles s hghly complex. The use of surface EMG only allowed for the collecton of superfcal musculature, possbly mssng some of the antagonst/ agonst behavor of the deeper muscles. However, a study by Bloun, Segmund, Carpenter, and Ingls (2007) showed that the neural control of agonsts between superfcal and deep musculature n the neck may share a common neural drve. Consderng the lmtatons, the study stll provdes a novel approach to understandng coactvaton n the cervcal spne durng complex dynamc tasks. Future studes would address more personalzed approaches, better reference contractons, and a wder varety of tasks. Concluson Ths study provded a descrpton of a coactvaton ndex ntended to assess the neuromuscular effort requred of the cervcal spne from a varety of head motons. Through the use of EMG, knematc data, muscular lnes of acton, and CSA data, we were able to apply an approach prevously tested n the lumbar spne (Le et al., n press) to the cervcal spne. Testng of the ndex showed that ncreased complexty n moton (partcularly motons requrng twstng) tended to have hgher coactvty due to the ncreased need for postural control. Overall, ths methodology may be appled to assess the neuromuscular efforts from complex dynamc head/neck motons for varous tasks. Key Ponts The coactvaton ndex developed for the cervcal spne allows for the assessment of multplanar, complex, dynamc head motons. Index was hghest durng condtons nvolvng axal rotaton. Index may be appled for task assessment comparsons nvolvng the cervcal spne (e.g., tasks requrng non-neutral head postons wth and wthout loads on the head). References Andrews, J. G., & Hay, J. G. (1983). Bomechancal consderatons n the modelng of muscle functon. Acta Morphologca Neerlando-Scandnavca, 21(3), Bloun, J. S., Segmund, G. P., Carpenter, M. G., & Ingls, J. T. (2007). Neural control of superfcal and deep neck muscles n humans. Journal of Neurophysology, 98(2), do: /jn Cheng, C.-H., Cheng, H.-Y. K., Chen, C. P.-C., Ln, K.-H., Lu, W.- Y., Wang, S.-F.,... Chuang, Y.-F. (2014). Altered co-contracton of cervcal muscles n young adults wth chronc neck pan durng voluntary neck motons. Journal of Physcal Therapy Scence, 26(4), do: /jpts Cheng, C.-H., Ln, K.-H., & Wang, J.-L. (2008). Co-contracton of cervcal muscles durng sagttal and coronal neck motons at dfferent movement speeds. European Journal of Appled Physology, 103(6), do: /s Cho, H. (2003). Quanttatve assessment of co-contracton n cervcal musculature. Medcal Engneerng and Physcs, 25(2), Deng, Y. C., & Goldsmth, W. (1987). Response of a human head neck upper-torso replca to dynamc loadng.2. Analytcal numercal-model. Journal of Bomechancs, 20(5), do: / (87) Dufour, J. S., Marras, W. S., & Knapk, G. G. (2013). An EMGasssted model calbraton technque that does not requre MVCs. Journal of Electromyography and Knesology, 23(3), do: /j.jelekn Falla, D., & Farna, D. (2008). Neuromuscular adaptaton n expermental and clncal neck pan. Journal of Electromyography and Knesology, 18(2), do: org/ /j.jelekn Harms-Rngdahl, K., Ekholm, J. A. N., Schuldt, K., Nemeth, G., & Arborelus, U. P. (1986). Load moments and myoelectrc actvty when the cervcal spne s held n full flexon and extenson. Ergonomcs, 29(12), do: / Ia, H., Morya, H., Goto, S., Takahash, K., Yamagata, M., & Tamak, T. (1993). Three-dmensonal moton analyss of the upper cervcal spne durng axal rotaton. Spne, 18(16), Kambayash, L. K., & Rchmond, F. J. (1998). Morphometry of human neck muscles. Spne, 23(12),

11 Exploratory Electromyography-Based Coactvaton Index 11 Larsson, B., Søgaard, K., & Rosendal, L. (2007). Work related neck-shoulder pan: A revew on magntude, rsk factors, bochemcal characterstcs, clncal pcture and preventve nterventons. Best Practce and Research: Clncal Rheumatology, 21(3), do: /j.berh Lavender, S. A., Tsuang, Y. H., Andersson, G. B. J., Hafez, A., & Shn, C. C. (1992). Trunk muscle cocontracton The effects of moment drecton and moment magntude. Journal of Orthopaedc Research, 10(5), do: /jor Lavender, S. A., Tsuang, Y. H., Hafez, A., Anderson, G. B. J., Chaffn, D. B., & Hughes, R. E. (1992). Coactvaton of the trunk muscles durng asymmetrc loadng of the torso. Human Factors, 34, do: / Le, P., Aurand, A., Dufour, J., Knapk, G. G., Best, T. M., Khan, S. N.,... Marras, W. S. (2017). Development and testng of a moment-based coactvaton ndex to assess complex dynamc tasks for the lumbar spne. Clncal Bomechancs, 46, Le, P., Aurand, A., Walter, B. A., Best, T. M., Khan, S. N., Mendel, E., & Marras, W. S. (n press). Development of a lumbar EMG-based coactvaton ndex for the assessment of complex dynamc tasks. Ergonomcs. do: Le, P., Best, T. M., Khan, S. N., Mendel, E., & Marras, W. S. (2017). A revew of methods to assess coactvaton n the spne. Journal of Electromyography and Knesology, 32, do: Marras, W. S., Ferguson, S. A., Burr, D., Davs, K. G., & Gupta, P. (2004). Spne loadng n patents wth low back pan durng asymmetrc lftng exertons. The Spne Journal, 4(1), do: Marras, W. S., & Granata, K. P. (1995). A bomechancal assessment and model of axal twstng n the thoracolumbar spne. Spne, 20(13), Martn, B. I., Deyo, R. A., Mrza, S. K., Turner, J. A., Comstock, B. A., Hollngworth, W., & Sullvan, S. D. (2008). Expendtures and health status among adults wth back and neck problems. JAMA, 299(6), do: /jama Mmura, M., Morya, H., Watanabe, T., Takahash, K., Yamagata, M., & Tamak, T. (1989). Three-dmensonal moton analyss of the cervcal spne wth specal reference to the axal rotaton. Spne, 14(11), Nmbarte, A. D. (2014). Rsk of neck musculoskeletal dsorders among males and females n lftng exertons. Internatonal Journal of Industral Ergonomcs, 44(2), do: dx.do.org/ /j.ergon Nmbarte, A. D., Aghazadeh, F., Ikuma, L. H., & Harvey, C. M. (2010). Neck dsorders among constructon workers: Understandng the physcal loads on the cervcal spne durng statc lftng tasks. Industral Health, 48(2), Panjab, M. M., Oda, T., Crsco, J. J., 3rd, Dvorak, J., & Grob, D. (1993). Posture affects moton couplng patterns of the upper cervcal spne. Journal of Orthopaedc Research, 11(4), do: /jor Schuldt, K. (1988). On neck muscle-actvty and load reducton n sttng postures An electromyographc and bomechancal study wth applcatons n ergonomcs and rehabltaton. Scandnavan Journal of Rehabltatve Medcne, 19, Sommerch, C. M., Jones, S. M. B., Hermans, V., & Moon, S. D. (2000). Use of surface electromyography to estmate neck muscle actvty. Journal of Electromyography and Knesology, 10(6), do: Vasavada, A. N., Danaraj, J., & Segmund, G. P. (2008). Head and neck anthropometry, vertebral geometry and neck strength n heght-matched men and women. Journal of Bomechancs, 41(1), do: Vasavada, A. N., L, S., & Delp, S. L. (1998). Influence of muscle morphometry and moment arms on the moment-generatng capacty of human neck muscles. Spne, 23(4), Yang, H., Haldeman, S., Nakata, A., Cho, B., Delp, L., & Baker, D. (2015). Work-related rsk factors for neck pan n the US workng populaton. Spne, 40(3), do: / brs Peter Le s a PhD canddate n the Department of Integrated Systems Engneerng and Graduate Research Assocate at The Oho State Unversty Spne Research Insttute. He receved hs MS n engneerng systems (mechancal specalty) from the Colorado School of Mnes (2007) and BS n mechancal engneerng (2006) at the Unversty of Colorado Denver. Alexander Aurand s a research engneer for The Oho State Unversty Spne Research Insttute. He receved hs MS n ndustral and systems engneerng and BS n mechancal engneerng at The Oho State Unversty. Thomas M. Best s a professor of orthopedcs and research drector for the UHealth Sports Performance and Wellness Insttute at the Unversty of Mam n Coral Gables, Florda. He s also a professor n bomedcal engneerng and knesology as well as team physcan for the Unversty of Mam Department of Intercollegate Athletcs. Safdar N. Khan s currently the Benjamn R. and Helen Slack Wltberger Endowed Char n Orthopaedc Spne Surgery, assocate professor of orthopaedcs and chef of the Spne Dvson, and assocate professor n the Department of Integrated Systems Engneerng at The Oho State Unversty Medcal Center. Hs research nterests are n the bology of spnal fuson and fracture healng, ntervertebral dsc repar, and applcaton of regeneratve molecular medcne to the treatment of spnal dsorders. Ehud Mendel currently holds the Justne Skestos Endowed Char and s the drector of the Neurosurgcal Spne Program, Spne Oncology Program, drector of Fellowshp Program, and the medcal drector of the Spne Research Insttute at The Oho State Unversty. In addton, he s a professor n the departments of neurosurgery, oncology, orthopaedcs, and ntegrated systems engneerng.

12 12 Month XXXX - Human Factors Wllam S. Marras s the executve drector and scentfc drector of the Spne Research Insttute, the executve drector of the Center for Occupatonal Health n Automotve Manufacturng (COHAM), and the executve drector of the Insttute for Ergonomcs. He s the Honda Char Professor n the department of Integrated Systems Engneerng at The Oho State Unversty and holds jont appontments n the departments of orthopaedc surgery, physcal medcne, and neurosurgery. Date receved: October 24, 2016 Date accepted: September 30, 2017