Optimization of Pulse Duration and Pulse Charge During Transcutaneous Electrical Nerve Stimulation

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ptimizatin f Pulse Duratin and Pulse Charge During Transcutaneus Electrical Nerve Stimulatin The main effects and interactin f puise duratin and puise charge n sensry, mtr and painful stimulatin were

1 ptimizatin f Pulse Duratin and Pulse Charge During Transcutaneus Electrical Nerve Stimulatin The main effects and interactin f puise duratin and puise charge n sensry, mtr and painful stimulatin were examined n six male subjects. Surface electrdes were placed ver the triceps brachii muscle. Pulse duratin was varied between 5 and 0 jus. Peak current, muscle trque and fur excitatry respnses were determined. Sequential rder f sensry, mtr and painful stimulatin was evidenced. Selective excitatin f these different physilgical respnses was easier and required less charge as pulse duratin became shrter. The greatest nn-pamful trque was reached at ps pulse duratin. The mst suitable range fr mtr stimulatin was 20 t 200 ]is. Fr painful stimulatin, a 5 t 10 \xs duratin was favured. A range f 20 t ps was recmmended fr sensry stimulatin. GAD ALN Gad Aln, Ph.D., R.P.T., is an Assistant Prfessr in the Department f Physical Therapy, Schl f Medicine, University f Maryland, Baltimre, U.S.A.: he was the Deputy Directr f the Lewis Natinal Prsthetic Institute f Israel during the cnductin f this study. JNATHAN ALLiN Jnathan Allin is an Electrical Engineer and a Ph.D. candidate at the Technin-israel Institute f Technlgy, Department f Electrical Engineering, Haifa, Israel GIDEN F INBAR Giden Inbar is an Assciate Prfessr f Electrical Engineering, Technin-lsraei Institute f Technlgy, Department f Electrical Engineering, Haifa, Israel, Many years f practical, experimental and theretical testing f Transcutaneus Electrical Nerve Stimulatin (T.E.N.S ) have yielded an enrmus amunt f useful data (Bentn et al 1981) Prgress has been characterized by much mre rapid develpment f clinical applicatin than in determining ptimal parameters required fr such applicatin. Selecting criteria assciated with patient safety has als been neglected t sme degree (Cper et al 1977, Seligman 1982). The advancement f electrphysilgy, particularly with reference t excitatin f peripheral nerves, has revealed many phenmena which affect bth the physilgical and chnical respnses t the applicatin f T.E.N S. (Burtn and Maurer 1974). Amng the mre reprducible physilgical respnses is the ability t excite sensry, mtr and pain-cnducting fibres. Islated excitatin f each f these three different nerve grups may be easier t btain with shrt duratin pulses, as can be deduced frm the Strength Duratin (S-D) curves reprted by Li and Bak (1976) and Hwsn (1978) Thus, ne can classify the main physilgical respnses f the excitatry system as sensry, mtr and painful stimulatins. Each can be used t achieve a desired clinical result. In the area f physical medicine and rehabilitatin, these clinical respnses include pain mdulatin, edema reductin, muscle re-educatin, eliminatin f prtective muscle spasm, nrmalizatin f muscle tne fllwing spasticity, maintenance r imprvement f jint mbility and hastening tissue healing. Selecting an ptimal treatment is therefre a cmplex task. ptimizatin f the electrical parameters, physilgical respnses, treatment techniques and chnical respnses in different physical impairments must all be evaluated fr their main effects as well as their interactins. Deciding n suitable electrical parameters t achieve a physilgical respnse has been the fcus f a number f investigatins. In particular, the ptimal pulse duratin fr an ultimate mtr respnse was studied by several researchers. Crag et al (1974) culd nt draw definite cnclusins as t the best pulse duratin fr eliciting excitatin frm the finger flexrs. Peckham et al(1975) preferred jus fr the same grup f muscles, whereas Bwman (1980) fund a 300 l*s pulse duratin ptimal fr stimulating the Quadriceps Femris muscle. Different muscle grups were recently tested by Mren-Aranda and Seireg (1981) fr their mtr, nn-painful respnse. Favurable results ccurred with a 10,000 Hz pulse frequency which crrespnds t a 50 /*s duratin per phase. ptimizatin f pulse duratin fr sensry stimulatin was investigated by Hwsn (1978). Very shrt pulses, between 10 and 50 JUS seemed preferable because f a better ability t differentiate the sensry frm the mtr and painful stimulatin. Cnversely, fr a painful respnse, the ptimal pulse duratin was shwn by Ntermans (1966) t be 5 ms. Mst f these representative studies did nt include data n pulse charge which is nw regarded as an imprtant factr in terms f the safety f stimulatin (Seligman 1982, Crag et al 1972, Linzer and Lng 1976). Frm The Australian Jurnal f Physitherapy Vl 29, N 6, December,

2 available data, a cnclusin can be reached that the shrter the pulse, the lwer the charge necessary t btain threshld stimulatin f all types f excitatry respnses (Crag et al 1972 Gracanin and Trnkczy 1975, Dley etal 1978). Hwever, whether minimal pulse charge results in ptimal excitatry respnses and prvides the best sensry, mtr r painful stimulatin is an imprtant questin t which n answer culd be fund in the literature. The purpse f the present study was t examine the main and interactin effects f pulse duratin and pulse charge n sensry, mtr and painful stimulatin. Particular attentin was paid t pulse charge density, its magnitude during the attainment f the varius excitatry respnses and its relevance t the safety f stimulatin, ther electrical parameters such as uniphasic versus biphasic, pulse shape, pulse frequency and pulse tram alng with their effects n the physilgical respnses were beynd the scpe f the present study, system t minimize mvement between the latter and the limb. The stimulatr was a labratry prttype cnstant current generatr with a pulse duratin adjustable frm 0.5 t 0 us. Pulse shape was a mnphasic square wave with a maximum intensity f 500 ma. Pulse rate was fixed at 20 pulses per secnd Stimulatin prtcl included measuring S-D curves fr each f fur physilgical respnses: 1) threshld f sensry stimulatin; 2) threshld f mtr stimulatin; 3) threshld f painful stimulatin, which represented the strngest muscle cntractin withut pain; and 4) maximal tlerance level f painful stimulatin. Pulse duratin was varied at lgarithmic intervals frm 5 jus t 0 ^s with the intensity being adjusted t achieve each f the fur physilgical respnses. Pulse charge (Q) was calculated as a prduct f current intensity and pulse duratin. Trque measurements were recrded frm the strain-gauge utput Methd f the six healthy male subjects wh vlunteered t be tested, three had experienced electrical stimulatin and three had nt The target regin was the drsal side f the left arm. Stimulating electrdes were secured ver the mtr pints f the medial and lateral heads f the Triceps Brachn muscle. The electrdes were made f carbn impregnated spnge and their 2x2 cm size presented an area f 4cm 2 A subject was seated with his shulder at 90 abductin, his elbw at 90 flexin, and his frearm in mid-psitin between supinatin and prnatin. A specially cnstructed arm rest allwed fr this psitin and included a strain-gauge system t measure the extensin trque prduced by the Triceps during mtr stimulatin. The arm and frearm were fastened firmly t the mechanical Q. E C 200 N Pulse Duratin (^s) Figure 1: Strength Duratin Curves f the fur majr excitatry respnses (A) sensry threshld, (B) mtr threshld, (C) pain threshld, (D) maximal tlerable pain respnse 196 The Australian Jurnal f Physitherapy Vi 29, N 6, December, 1983

3 Results A mean peak current necessary t achieve the afrementined physilgical respnses at each f eight pulse duratins is illustrated in Figure 1 As can be seen, there was an insufficient current utput t reach a maximal pain sensatin at 5 and 10 micrsecnds An imprtant bservatin is that at each pulse duratin there was a sequential rder f threshld excitatin starting with sensry, fllwed by mtr and ending with stimulatin f the pain-cnducting fibres. In additin t the threshld intensities f each f the three grups f nerves, the fur curves present three imprtant intensity intervals which represent minimum t maximum ranges f stimulatin intensity fr each f the three types f excitatry respnses, As evidenced, the largest interval ccurs between the mtr and C 0) CD Pulse Duratin (jus) pain respnses. The range f tlerable pain respnses is smewhat smaller and the smallest range is that between sensry and mtr stimulatin. Intervals at each pulse duratin can be calculated as the abslute difference f intensity r as percent change Abslute and percent differences between mtr and sensry threshlds and between pain and mtr threshlds are graphed in Figure 2 and Figure 3, respectively. Whereas it is very clear that abslute differences mcrease as the pulse duratin becmes shrter, the percent changes d nt seem t fllw a specific pattern. Figure 3 may ffer a trend, but statistical testing f its existence may nt be warranted due t the small sample size. Figure 4 depicts that eliciting a painful respnse may require increasing intensity nearly 800 percent beynd sensry stimulatin At 5-10JUS, a relatively small increase in the current Figure 2: Abslute and percent differences between mtr versus sensry threshlds 120 Q> n S D i_ S 0) -Q needed fr sensry stimulatin may evke a painful respnse. Lnger pulse duratins exhibit a much greater increase f intensity befre painful sensatin is reached. These bservatins suggest that a pulse duratin f 20 ^s r lnger shuld be selected when sensry stimulatin alne is being sught. Pulse charge as a functin f pulse duratin is pltted in Figure 5 fr each f the fur physilgical threshlds. A sharp increase f charge was required t achieve each f the physilgical excitatins abve 200 us. n the ther hand, it is clear that the shrter the pulse, the less charge was required t btain the fur physilgical respnses. Charge density, calculated as charge per cm 2, is recrded in Table 1. Mean values fr the six subjects ranged frm a minimum f 0.12 ^ic/cm 2 t 8.45 juc/cm 2 and were well within safety bundaries. Figure 6 cmbines the charge f the maximal nn-painful elbw extensin trque and the Charge Trque Rati (CTR) bth as functins f pulse duratin As seen, the ptimal pulse duratin necessary t generate the largest trque was ^s. But, relative t the amunt f charge, the 20 ^s pulse duratin required the least charge per unit trque. In general, the middle interval (between 20 and 200 ^s) appears preferable fr mtr respnse generatin t the interval between 5 and 20 pts r the interval between 200 and 0 ys. Discussin The S-D curves btained in the present investigatin reprduced the basic relatinships between pulse duratin and current intensity fund in the literature (Li and Bak 1976, Crag et al 1974, Mren-Aranda et al 1981). The inability t stimulate pain-cnducting fibres at 5 and 10 ^s pulse duratin is prbably due t an insufficient current surce. At these duratins, the maximum f 500 ma and the electrde area f 4 cm 2 culd The Australian Jurnal f Physitherapy Vl 29, N 6, December,

4 S # relative Pulse Duratin (^s) _ E CD 80 t 0) 60 Figure 3: Abslute and percent diffences between pain versus mtr threshlds nly result in a maximal pulse charge spectively These charges were suffidensity f 0.62 /ic/cm 2 and 1.2 jic/cm 2 cient t elicit maximal painful respnse fr 5 and 10 ^s pulse duratins re- frm nly tw f the six subjects C -Q The hypthesis f a sequential rder f stimulatin where sensry fibers are excited befre mtr fibres and the latter befre pain-cnducting fibres is strngly supprted by the present data Sensry and mtr fibres are the largest in diameter and their cnductin velcity the fastest (Bentn ef al 1981, Li and Bak 1976) They require the least amunt f electrical charge t becme deplarized. Thus, at any given pulse duratin, bth sensry and mtr fibres are mst likely t be the first t respnd t the electrical stimulatin. f the tw, sensry respnse precedes because these fibres are likely t be clser t the stimulating electrde when surface stimulatin is applied. Likewise, excitatin f mtr fibres shuld precede the pain-cnducting fibres as lng as bth types are at clse prximity under the electrde. Althugh the latter fibres are usually mre superficial than the frmer, their smaller diameter and much higher resistance t current flw makes their excitatin characteristics inferir t mtr fibres. Selective excitatin f the different physilgical respnses was shwn t be easier as pulse duratin became shrter. Hwsn (1978) reprted similar results. Hwever, finding that the percent change is nt necessarily the greatest at the shrtest pulse duratin cntradicts Hwsn's data. Table 1: Charge density (/ic/cm 2 ) 5iiS 10n$ 20\JLS 50\xs IJJLS 200\xs 500ixs WpiS Sensry stimulatin Minimal mtr stimulatin Maximal mtr stimulatin Maximal painful stimulatin QM Insufficient charge fr these shrt pulse duratin The Australian Jurnal f Physitherapy Vl 29, N 6, December, 1983

5 800 r c D CD Q ~r L J Pulse Duratin (^s) Figure 4: Percent differences between pain versus sensry threshlds Amng the reasns fr this discrepancy, ne may cnsider methdlgical differences such as pulse characteristics, electrde size and electrde psitin all f which may affect the results (Linzer and Lng 1976, Wlf et al 1978). This infrmatin was nt disclsed by Hwsn, whse data was btained frm a single subject and shuld therefre be cnsidered with sme reservatin. The present results imply that the ability t selectively stimulate the different nerve types at shrt pulse duratin may nt be a physilgical phenmenn but rather a technical prblem. If ne desires an accurate selectivity at a lnger pulse duratin, the generatr shuld have a vernier cntrl s that smaller, mre sensitive adjustments f intensity culd be made. ptimizatin f pulse parameters t achieve maximal physilgical respnses may be based n several assumptins. ne is that pulse charge density must nt exceed 20- /ic/cm 2 s that the stimulatin wuld be safe (Sehgman 1982). Withm the limits f the present study, the maximal and minimal charge densities were 8.45 ^C/cm 2 and 0.12 uc/cm 2 fr the mst painful and least sensry stimulatin, respectively. Thus, the entire range f physilgical respnses was achieved well within the limits set fr nnharmful stimulatin A secnd questin f interest is whether pulse charge shuld preferably be kept at minimum fr each f the fur majr excitatry respnses s that the stimulatin wuld be mst efficient. The tendency f the electrical charge t decline with the shrtening f pulse duratin was evident fr the sensry, mtr and painful stimulatin. ne wuld then like t hypthesize that 5-10 pis pulse duratin wuld be ptimal fr all the excitatry respnses. The hypthesis is mst prbably false, at least in regard t mtr stimulatin. Gracanin and Trnkczy (1975) fund als that pulse charge declined as pulse duratin was shrtened, but illustrated that at less than \xs the charge rse again. In their study, current intensity was increased until a predetermined, cnstant muscle trque was generated even if the stimulatin became painful. In the present investigatin, muscle trque was nt cnstant Rather, it was measured at the pint f beginning f pain s that muscle trque was maximal but withut pain. Direct cmparisn f the tw investigatins can be made by lking at the Charge Trque Rati (CTR). Evidently, the amunt f charge per unit trque als increased at 5-10 j-ts pulse duratin, thus shwing agreement with Gracanin and Trnkczy findings. Examinatin f the trque-pulse duratin relatinship, irrespective f charge suggests that the ptimal value fr elbw extensrs shuld be arund us pulse duratin Peckham et al (1975) preferred the same value fr the finger flexrs. Examining a similar questin n the Quadriceps Femris muscle led Bwman (1980) t prefer a 300 fis pulse duratin. This seems t be slightly ut f the ptimal range reprted herein. It is pssible that varius muscle grups have different ptimal pulse duratin and the difference in the sexes f the subjects in the tw studies may have als cntributed t the varying results. Finally, the present study included a wide range f pulse duratins, whereas Bwman and his assciates cmpared nly 50 and 300 jus and may have missed ptimizatin at r 200 jus. The Australian Jurnal f Physitherapy Vl 29, N 6, December,

6 i i cd D ZJ CL Pulse Duratin (pts) Figure 5: Puise charge and pulse duratin relatinships fr the fur majr excitatry respnses (A) sensry threshld; (B) mtr threshld, (C) pain threshld, (D) maximal tlerable pain respnse The freging results and discussin may lead t the general cnclusin that a range f 20 t 200 ^s pulse duratin is mst suitable fr mtr stimulatin. In this range, the CTR is lw while the maximal mtr respnse withut pain is still nly mderately different frm maximum. Abve 200 ^s a significantly greater pulse charge is required and a similar increase in CTR ccurs. This indicates that the range is less suitable fr mtr stimulatin. The 5-10 us range is als undesirable due t the increase f the CTR which is evidenced. Painful stimulatin is sught fr sme clinical applicatins such as acupuncture pints in chrnic lw back pain (Fx and Melzack 1976, Ma et al 1980). Within the range studied, 5 us gave painful stimulatin with minimum pulse charge and thus increased the subject's safety. Furthermre, at this pulse width pain respnse was achieved with the least amunt f muscle trque. Since mtr respnse is nt required during trigger-pint stimulatin, it can be minimized by using very shrt pulses. During the experiment, all six subjects reprted that the quality f pain perceptin was distinctly different between the very shrt pulses and the very lng pulses. The frmer was a sharp, pain prick while the latter was a burning pain sensatin. These statements agree with thse reprted by the subjects in Ntermans' study (Ntermans 1966), At present, the effect f different types f painful stimulatin n clinical results is nt knwn. Thus, within the limits f the present data and discussin, it may be prpsed that the shrtest pulse duratin shuld be favred, prvided that sufficient peak current and charge density accmpany such pulse duratin. Sensry stimulatin is widely and successfully used t achieve pam mdulatin in a variety f medical cnditins. Tw examples are the wrks f Hymes et al (1974) and Slmn et al (1980). Linzer and Lng (1976) suggested 50- us as suitable fr sensry stimulatin, but did nt test shrter than 50 us pulses. Present results demnstrated that sensry respnses were btained thrughut the studied range. Cnsidering nly the minimal charge criterin, the 5-20 ^s is the best range fr pulse duratin. Hwever, if ne wishes t achieve sensry stimulatin withut mtr r pain respnses, then the ratis f mtr ver sensry and painful ver sensry charges shuld be maximized. N bvius maximum was apparent fr the frmer, whereas the latter is greatly imprved abve 20 us. Since 200 The Australian Jurnal f Physitherapy Vl 29, N 6, December, 1983

7 16 h 14 h Pulse Duratin (^s) threshld charge fr sensry stimulatin is nt greatly different between 20 and f*s, such a range may be recmmended fr sensry respnse. References Benin LA, Baker LL, Bwman BR and Water RL (1981), Functinal/Electrical Stimulatin A Practical Clinical Guide, 2nd Editin, Ranch Ls Amigs Rehabilitatin Engineering Center, Ranch Ls Amigs Hspital, Dwney, Califrnia, 1-30 Bwman BR (1980), Effectiveness f surface stimulatin, Annual Reprts f Prgress, Ranch Ls Amigs Rehabilitatin Engineering Center, Dwney, Califrnia, Burtn C and Maurer DD (1974), Pain suppressin by transcutaneus electric stimulatin, IEEE Transactins f Bimedwal Engineering, BME 21, Cper IS, Amin I and Uptn, A (1977), Safety and efficacy f chrnic stimulatin, Neursurgery, 1, Crag PE, Peckham PH, Mrtimer JT and Van Der Meulen JP (1974), The chice f pulse duratin fr chrnic electrical stimulatin via surface, nerve and intramuscular electrdes, Annals f Bimedical Engineering, 2, Crag PE, Peckham PH, Van Der Meulen JP and Mrtimer JT (J972), Minimizatin f charge transfer and energy dissipatin during electrical stimulatin f muscle and nerve, paper delivered at the Third Annual Cnference f the Bimedical Engineering Sciety, Baltimre, Maryland 0 Figure 6: Elbw extensin trque {right rdmate) and the charge trque rati {left rdinate) as a functin f pulse duratin E D Diey DM, Sharkey J, Keller W and Kasprak M (1978), Treatment f demyelmating and degenerative diseases by electr-stimulatin f the spinal crd, Medical Prgress and Technlgy 6, 1-14 Fx EJ and Mefzack R (1976), Transcutaneus electrical stimulatin and acupuncture Cmparisn f treatment fr lw-back pain, Pain, 2, Gracanin F and Trnkczy A (1975), ptimal stimulus parameters fr minimum pain in the chrnic stimulatin f innervated muscle, Archives f Physical Medicine and Rehabilitatin 56, Hwsn DC (1978), Peripheral neural excitability, Physical Therapy, 58, Hymes AC, Ynehir EG et al (1974), Electrical surface stimulatin fr treatment and preventin f ileus and atelectasis, Surgical Frum, 25, Li CL and Bak A (1976), Excitability characteristics f A- and C-fibres in peripheral nerve, Experimental Neurlgy, 50, Linzer M and Lng DM (1976), Transcutaneus neural stimulatin fr relief f pain, IEEE Transactins f Bimedical Engineering, BME 23, Ma W, Ghia JN et al (1980), High versus lw intensity acupuncture analgesia fr treatment f chrnic pain Effects f platelet sertnin, Paw, 8, Mren-Aranda J and Seireg A (1981), Electrical parameters fr ver-the-skin muscle stimulatin, Jurnal f Bimechanics, 14, Mren-Aranda J and Seireg A (1981), Investigatin fr ver-the-skin electrical stimulatin parameters fr different nrmal muscles and subjects, Jurnal f Bimechanics, 14, Ntermans SLH (1966), Measurement f the pain threshld determined by electrical stimulatin and its clinical applicatin, Neurlgy, 16, Peckham PH, Mrtimer JT and Marslais EB (1975), Alteratin in the frce and fatigabihty f skeletal muscle in quadraplegic human fllwing exercise induced by chrnic electrical stimulatin, Clinical rthpaedics 114, Sehgman LJ (1982), Physilgical stimulatrs Frm electric fish t prgrammable implants, IEEE Transactins f Bimedical Engineering, BME 29, Slmn RA, Viernstein MC and Lng DM (1980), Reductin f pstperative pain and narctic use by transcutaneus electrical nerve stimulatin, Surgery, 87, Wlf SL, Gersh MR and Kutner M (1978), Relatinship f selected clinical variables t current delivered transcutaneus electrical nerve stimulatin, Physical Therapy 58, The Australian Jurnal f Physitherapy Vl 29, N 6, December,

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