Rate of Torque Development Scaling Factor Decreased following a 6-Week Unilateral Isometric Training Using Electrostimulation or Voluntary Contractions

Ci a ion: Kneže i´c, O.M.; Ša abon, N.; Ga cia-Ramos, A.; Majs o o i´c, N.; Milano i´c, S.D.; Filipo i´c, S.R.; Mi ko , D.M. Ra e o To que De elopmen Scaling Fac o Dec eased ollowing a 6-Week Unila e al Isome ic T aining Using Elec os imula ion o Volun a y Con ac ions. Appl. Sci. 2024,14, 6485. h ps://doi.o g/10.3390/ app14156485 Academic Edi o s: Rui Mendes, O lando Fe nandes and Ma ia An ónio Cas o Recei ed: 30 June 2024 Re ised: 19 July 2024 Accep ed: 23 July 2024 Published: 25 July 2024 Copy igh : © 2024 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). applied sciences A icle Ra e o To que De elopmen Scaling Fac o Dec eased ollowing a 6-Week Unila e al Isome ic T aining Using Elec os imula ion o Volun a y Con ac ions Oli e a M. Kneže i´c 1,2,* , Nejc Ša abon 3,4,5,6 , Amado Ga cia-Ramos 7,8 , Nikola Majs o o i´c 1, Sladjan D. Milano i´c 2, Saša R. Filipo i´c 2and D agan M. Mi ko 1 1Facul y o Spo and Physical Educa ion, Uni e si y o Belg ade, 11000 Belg ade, Se bia; nikola.majs o o ic@ s .bg.ac. s (N.M.); d agan.mi ko @ s .bg.ac. s (D.M.M.) 2Ins i u e o Medical Resea ch, Uni e si y o Belg ade, Na ional Ins i u e o he Republic o Se bia, 11000 Belg ade, Se bia; [email p o ec ed] (S.D.M.); [email p o ec ed] (S.R.F.) 3Facul y o Heal h Sciences, Uni e si y o P imo ska, 6000 Kope , Slo enia; [email p o ec ed].si 4And ej Ma ušiˇc Ins i u e, Uni e si y o P imo ska, 6000 Kope , Slo enia 5Human Heal h Depa men , InnoRenew CoE, 6310 Izola, Slo enia 6Labo a o y o Mo o Con ol and Mo o Beha io , S2P, Science o P ac ice, L d., 13244 Ljubljana, Slo enia 7Depa men o Physical Educa ion and Spo , Facul y o Spo Sciences, Uni e si y o G anada, 18071 G anada, Spain; amag @ug .es 8Depa men o Spo s Sciences and Physical Condi ioning, Facul y o Educa ion, Uni e sidad Ca ólica de la San ísima Concepción, Concepción 4030136, Chile *Co espondence: oli e a.kneze ic@ s .bg.ac. s Fea u ed Applica ion: Ra e o To que De elopmen Scaling Fac o (RTD-SF) is sensi i e enough o de ec changes ollowing aining p og ams based on elec os imula ion o olun a y con ac ions. Abs ac : This s udy explo ed he changes in he a e o o que de elopmen scaling ac o (RTD-SF) and maximum olun a y isome ic con ac ion (MVC) a iables ollowing six weeks o unila e al isome ic elec omyos imula ion (EMS) and olun a y (VOL) exe cises. Twen y-six physically ac i e pa icipan s we e andomly assigned o EMS (n = 13) o a VOL g oup. MVC and RTD-SF o he quad iceps emo is o bo h legs we e assessed be o e and a e aining. EMS and VOL exe cises had iden ical equency ( h ee sessions/week), in ensi y (60% MVC), olume (40 con ac ions), and wo k- o- es a io (18 min: 6.25 s o wo k/20 s o es ). The e we e no be ween-g oup di e ences o he ained leg wi h o e all inc eases in maximal o que (Tmax) o ~29% (d = 2.11–2.12), ~13% o RTDmax ( d = 0.92–1.10 ); ~23% o In e cep (d = 0.72–0.78), and educ ion in RTD-SF by ~15% ( d = 1.01–1.10 ). In he non- ained leg, signi ican mode a e change was only obse ed a e EMS o RTD-SF which dec eased by 12.5% (d = 0.76). Bo h EMS and VOL aining applied a equi alen wo kloads posi i ely impac on Tmax, RTDmax, and In e cep , bu hey nega i ely a ec he quickness wi h which muscle con ac s ac oss a wide ange o submaximal o ces. Using a mode a e aining in ensi y in egula ly physically ac i e pa icipan s could explain he absence o c oss-educa ion in he VOL g oup. Keywo ds: quad iceps; s eng h; RTD-SF; RFD-SF; EMS 1. In oduc ion When apid isome ic con ac ions o di e en submaximal in ensi ies a e pe o med, a s ong linea ela ionship is obse ed be ween peak o ce/ o que and peak a e o o ce/ o que de elopmen . The slope o his ela ionship, named he a e o o ce de- elopmen scaling ac o (RFD-SF) o a e o o que de elopmen ac o (RTD-SF), has been p oposed as a measu e o neu omuscula quickness o submaximal con ac ions [ 1 , 2 ]. Some claim ha RTD a iables may be mo e e ec i e o e alua e aining- ela ed adap a- ions, o ollow up eco e y om an inju y, o o dis inguish be ween a ious popula ions Appl. Sci. 2024,14, 6485. h ps://doi.o g/10.3390/app14156485 h ps://www.mdpi.com/jou nal/applsci Appl. Sci. 2024,14, 6485 2 o 13 han he maximal o que (Tmax) exe ed du ing maximal olun a y isome ic con ac ion (MVC) [ 3 – 6 ]. Thus i is no su p ising ha he RTD-SF has been la ely pe o med mo e equen ly and epo ed as an indica o o apid o que gene a ion [ 1 , 6 – 8 ]. Since being e i ied as eliable [ 1 , 9 – 11 ], he RTD-SF p o ocol has been used o isome ic neu omuscula assessmen o uppe and lowe limb muscle g oups [ 6 , 7 , 9 , 10 , 12 ], o explo e he e ec s o sex [ 13 ], aging [ 13 , 14 ], a igue [ 15 ], and heal h condi ions such as os eopo osis o mul iple scle osis [ 6 , 8 , 14 ], as well as in s udies explo ing in e -limb asymme ies [ 7 , 11 , 15 ]. In addi- ion, Bellumo i e al. [ 12 ] in es iga ed he e ec s o high-speed cycling on a e-dependen mobili y in ac i e olde adul s. Al hough Jacque e al. [ 16 ] ecen ly s udied he e ec o cogni i e ask-induced men al a igue on hand o ce p oduc ion capaci ies including RTD-SF, he s udy o Bellumo i e al. [ 12 ] emains he only one ha explo ed he sensi i i y o he RTD-SF p o ocol o an exe cise in e en ion. Thus, i would be o in e es o s udy how o he ypes o aining, o example, isome ic, a ec RTD-SF a iables. Knee muscle s eng h is o pa amoun impo ance o daily li e ac i i ies and a hle ic pe o mance. I has been shown ha quad iceps emo is (QF) s eng h has signi ican im- plica ions o all and inju y p e en ion as well as o desi able ou comes o ehabili a ion ollowing inju ies and su gical in e en ions [ 3 , 17 ]. Bo h olun a y isome ic con ac ion (VOL) and elec omyos imula ion (EMS) ha e been used in s eng h aining o imp o ing muscula s eng h [ 18 – 20 ] and/o igh ing de ici s in muscula ac i a ion and o imp o e s eng h (Tmax and RTD) [ 17 , 21 ]. In line wi h he speci ici y o aining e ec s, speci ic iso- me ic s eng h imp o emen s ha e been obse ed [ 22 – 24 ]. Many au ho s ha e a emp ed o de e mine whe he EMS o VOL exe cise p o ides a mo e e icien s imulus o imp o e muscle s eng h [ 22 , 24 – 26 ]. Howe e , he aining e ec s we e g ea ly dependen on ain- ing in ensi y, ype o muscle ac ion, o pa icipan s’ aining his o y. In addi ion o gains in maximum s eng h, esis ance aining can e oke signi ican inc eases in RTD pa icula ly when exe cises a e pe o med wi h in en ion o p oduce apid o que i espec i e o ac ual mo emen speed, while he simila i y be ween aining and es ing mo emen pa e ns e okes he g ea es imp o emen s [ 5 ]. Howe e , inc eases in RTD ha e also been obse ed in s udies using high load esis ance (i.e., s eng h) aining wi hou he in en ion o as mo emen speeds [ 5 , 27 , 28 ]. E en so, s udies ha in es iga ed he in luence o EMS aiming a maximum s eng h de elopmen o apid o que p oduc ion a e sca ce and less clea . In line wi h ha , he e ec s o he s eng h aining modali y on he de elopmen o RTD-SF (i.e., neu omuscula quickness) ha e no been in es iga ed so a . I he e o e emains unclea how s eng h aining ha includes EMS and VOL exe cises a ec s apid s eng h p oduc ion unde isome ic condi ions. Unde ce ain ci cums ances, neu al adap a ion o unila e al aining goes beyond he muscles di ec ly in ol ed in he exe cise and esul s in inc eased olun a y ac i a ion and s eng h gains in he con ala e al homologous muscle, i.e., non- ained limbs [ 29 ]. This c oss-educa ion phenomenon in ol es adap a ions o he cen al ne ous sys em a sup aspinal and spinal le els [ 19 , 29 – 31 ] a he han muscle o mo phological ac o s, and ha is why i is widely u ilized in spo s o clinical ehabili a ion ollowing a unila e al inju y [ 31 – 33 ]. Rega ding he e ec i eness o a ious aining modali ies o induce he c oss-educa ion e ec , G een and Gab iel [ 34 ] concluded ha in young pa icipan s he c oss-educa ion e ec (i.e., gains in he non- ained limb) was simila among isome ic, isokine ic, o isoine ial exe cises and ha EMS esul ed in subs an ially g ea e gains (27% o EMS s. 18% o o he aining modali ies). Howe e , Oakman e al. [ 26 ] and la e Zhou e al. [ 20 ] epo ed simila c oss-educa ion e ec s in he non- ained leg a e EMS and VOL aining. C oss-educa ion e ec s we e obse able e en ollowing an 8-week low-in ensi y aining in e en ion (20% MVC) [ 32 ]. None heless, i is ob ious ha bo h ypes o aining modali ies (VOL and EMS) could esul in signi ican c oss-educa ion, and his is some hing ha is independen o age [ 35 ]. Howe e , conclusions a e mos ly based on gains in Tmax while da a on changes in isome ic RTD a iables a e limi ed. Conside ing he ea lie s a ed impo ance o apid o que p oduc ion, i would be o in e es o explo e po en ial Appl. Sci. 2024,14, 6485 3 o 13 c oss-educa ion changes in he non- ained leg muscle’s capaci ies o quickly gene a e o ques o submaximal and maximal in ensi y ollowing an isome ic exe cise in e en ion. The e o e, his s udy in es iga ed he changes in RTD-SF o quad iceps emo is muscle o he ained and non- ained legs ollowing a 6-week mode a e-in ensi y unila e al isome ic aining p og am using EMS o VOL exe cises. In line wi h a p e ious s udy in es iga ing changes in RTD-SF ollowing an exe cise in e en ion [ 12 ], he planned in e en ion was expec ed o change RTD-SF along wi h Tmax and RTDmax. In addi ion, we hypo hesized ha bo h aining modali ies would esul in a c oss-educa ion e ec on he non- ained leg [20,32]. 2. Ma e ials and Me hods 2.1. S udy Design This s udy is an in e en ional clinical ial wi h ac o ial assignmen ( egis e ed wi hin ClinicalT ials.go , iden i ie : NCT04624438). Fo he sake o simplici y, a p e–pos design wi h wo andom pa allel g oups (bo h expe imen al) was used. Quad iceps emo is (QF) unc ion was assessed by MVC and RTD-SF on h ee occasions: p io o he in e en ion (p e- es ), a e 3 weeks o aining (which se ed as he adjus men o exe cise h eshold; mid- es ), and a e 6 weeks o aining (pos - es ; Figu e 1). Isome ic aining in ol ed ei he elec omyos imula ion (EMS) o he olun a y (VOL) exe cises o QF. One week p io o in e en ion, all subjec s pa icipa ed in one p ac ice session o amilia ize hemsel es wi h s imula ion pa ame e s and aining p o ocols and o de e mine he in ensi y o EMS needed o achie e 60% o MVC. P io o he p e- es , body mass and pe cen o body a we e assessed using a bioelec ic impedance me hod (In Body 720; InBody Co., L d. InBody Bldg, 625, Eonju- o, Gangnam-gu, Seoul, 06106 Ko ea) and body heigh wi h a s anda d s adiome e . A e he p e- es , he pa icipan s we e andomly assigned in o ei he an EMS o VOL g oup. Nei he pa icipan s no e alua o s we e blinded o he in e en ion o he assessmen . Appl. Sci. 2024, 14, x FOR PEER REVIEW 3 o 13 is some hing ha is independen o age [35]. Howe e , conclusions a e mos ly based on gains in Tmax while da a on changes in isome ic RTD a iables a e limi ed. Conside ing he ea lie s a ed impo ance o apid o que p oduc ion, i would be o in e es o explo e po en ial c oss-educa ion changes in he non- ained leg muscle’s capaci ies o quickly gene a e o ques o submaximal and maximal in ensi y ollowing an isome ic exe cise in e en ion. The e o e, his s udy in es iga ed he changes in RTD-SF o quad iceps emo is mus- cle o he ained and non- ained legs ollowing a 6-week mode a e-in ensi y unila e al isome ic aining p og am using EMS o VOL exe cises. In line wi h a p e ious s udy in es iga ing changes in RTD-SF ollowing an exe cise in e en ion [12], he planned in- e en ion was expec ed o change RTD-SF along wi h Tmax and RTDmax. In addi ion, we hypo hesized ha bo h aining modali ies would esul in a c oss-educa ion effec on he non- ained leg [20,32]. 2. Ma e ials and Me hods 2.1. S udy Design This s udy is an in e en ional clinical ial wi h ac o ial assignmen ( egis e ed wi hin ClinicalT ials.go , iden i ie : NCT04624438). Fo he sake o simplici y, a p e–pos design wi h wo andom pa allel g oups (bo h expe imen al) was used. Quad iceps em- o is (QF) unc ion was assessed by MVC and RTD-SF on h ee occasions: p io o he in- e en ion (p e- es ), a e 3 weeks o aining (which se ed as he adjus men o exe cise h eshold; mid- es ), and a e 6 weeks o aining (pos - es ; Figu e 1). Isome ic aining in ol ed ei he elec omyos imula ion (EMS) o he olun a y (VOL) exe cises o QF. One week p io o in e en ion, all subjec s pa icipa ed in one p ac ice session o amilia ize hemsel es wi h s imula ion pa ame e s and aining p o ocols and o de e mine he in- ensi y o EMS needed o achie e 60% o MVC. P io o he p e- es , body mass and pe - cen o body a we e assessed using a bioelec ic impedance me hod (In Body 720; InBody Co., L d. InBody Bldg, 625, Eonju- o, Gangnam-gu, Seoul, 06106 Ko ea) and body heigh wi h a s anda d s adiome e . A e he p e- es , he pa icipan s we e andomly assigned in o ei he an EMS o VOL g oup. Nei he pa icipan s no e alua o s we e blinded o he in e en ion o he assessmen . Bo h g oups o pa icipan s pe o med unila e al isome ic s eng h aining h ee imes pe week o six weeks. T aining densi y and in ensi y we e selec ed in line wi h p e ious ecommenda ions and knowing he limi a ions o EMS [24,25,36]. Pa icipan s we e ad ised o e ain om any ype o esis ance aining a ge ing he leg muscles o he du a ion o he s udy. All sessions we e pe o med a he same ime o he day o each subjec (always be ween 8 a.m. and 2 p.m.) and unde simila en i onmen al condi ions (~22 °C and ~60% humidi y). Figu e 1. An o e iew o he s udy. Bo h g oups o pa icipan s pe o med unila e al isome ic s eng h aining h ee imes pe week o six weeks. T aining densi y and in ensi y we e selec ed in line wi h p e ious ecommenda ions and knowing he limi a ions o EMS [ 24 , 25 , 36 ]. Pa icipan s we e ad ised o e ain om any ype o esis ance aining a ge ing he leg muscles o he du a ion o he s udy. All sessions we e pe o med a he same ime o he day o each subjec (always be ween 8 a.m. and 2 p.m.) and unde simila en i onmen al condi ions (~22 ◦C and ~60% humidi y). Appl. Sci. 2024,14, 6485 4 o 13 2.2. Subjec s The subjec s we e ec ui ed by wo d-o -mou h. Twen y-six physical educa ion s u- den s (14 males and 12 emales) we e andomly assigned o ei he he EMS o VOL g oup (7 males and 6 emales in each g oup; mean ± SD: age: 21 ± 1.5 yea s ( ange: 20–25 yea s); body mass: 75 ± 12 kg, body a : 18.4 ± 5.5%; body heigh : 175 ± 14 cm in EMS g oup, and mean ± SD: age: 21 ± 2.5 yea s ( ange: 20–25 yea s); body mass: 77 ± 12 kg, body a : 16.9 ± 6.1%; body heigh : 177 ± 12 cm in VOL g oup). The e we e no signi ican be ween-g oup di e ences in any o subjec s’ cha ac e is ics be o e and a e he in e en- ion (all p> 0.41). All pa icipan s we e egula ly physically ac i e h ough hei s anda d academic cu iculum ha included 6 o 8 h o physical ac i i y pe week (bo h mode a e and high in ensi y). Besides being physically ac i e, he pa icipan s we e equi ed o be ee o muscle–skele al o neu ological disease o medica ion in ake and o e ain om changing daily ou ines wi h espec o aining and sleeping. All pa icipan s ga e w i en in o med consen ha was in acco dance wi h he Decla a ion o Helsinki and app o ed by he Uni e si y Ins i u ional Re iew Boa d (02-672-2/09-04-2015). 2.3. Tes ing P ocedu es All sessions we e conduc ed on wo kdays and began wi h a s anda dized wa m-up consis ing o s a iona y cycling o 5 min and 5 min o ac i e s e ching. Following he wa m-up, he pa icipan s we e sea ed in a cus om-made chai wi h knee and hip angles ixed a 120 ◦ and 100 ◦ , espec i ely (Figu e 2, uppe le panel). The chai was adjus ed o e e y pa icipan wi h espec o hei body size. The pa icipan ’s unk, wais , and highs we e i mly s apped o he chai and he dis al pa s o hei legs we e a ixed o s ain gauge o ce ansduce s (Dongguan Sou h China Sea Elec onic Co., L d., Dongguan, China; load cell S-Type CZL302; ange 2 kN) using igid cu s. The cu s we e secu ed wi h hook and loop as ene s 2 cm abo e he malleolus la e alis. Appl. Sci. 2024, 14, x FOR PEER REVIEW 4 o 13 Figu e 1. An o e iew o he s udy. 2.2. Subjec s The subjec s we e ec ui ed by wo d-o -mou h. Twen y-six physical educa ion s u- den s (14 males and 12 emales) we e andomly assigned o ei he he EMS o VOL g oup (7 males and 6 emales in each g oup; mean ± SD: age: 21 ± 1.5 yea s ( ange: 20–25 yea s); body mass: 75 ± 12 kg, body a : 18.4 ± 5.5%; body heigh : 175 ± 14 cm in EMS g oup, and mean ± SD: age: 21 ± 2.5 yea s ( ange: 20–25 yea s); body mass: 77 ± 12 kg, body a : 16.9 ± 6.1%; body heigh : 177 ± 12 cm in VOL g oup). The e we e no signi ican be ween-g oup diffe ences in any o subjec s’ cha ac e is ics be o e and a e he in e en ion (all p > 0.41). All pa icipan s we e egula ly physically ac i e h ough hei s anda d academic cu ic- ulum ha included 6 o 8 h o physical ac i i y pe week (bo h mode a e and high in en- si y). Besides being physically ac i e, he pa icipan s we e equi ed o be ee o muscle– skele al o neu ological disease o medica ion in ake and o e ain om changing daily ou ines wi h espec o aining and sleeping. All pa icipan s ga e w i en in o med con- sen ha was in acco dance wi h he Decla a ion o Helsinki and app o ed by he Uni e - si y Ins i u ional Re iew Boa d (02-672-2/09-04-2015). 2.3. Tes ing P ocedu es All sessions we e conduc ed on wo kdays and began wi h a s anda dized wa m-up consis ing o s a iona y cycling o 5 min and 5 min o ac i e s e ching. Following he wa m-up, he pa icipan s we e sea ed in a cus om-made chai wi h knee and hip angles ixed a 120° and 100°, espec i ely (Figu e 2, uppe le panel). The chai was adjus ed o e e y pa icipan wi h espec o hei body size. The pa icipan ’s unk, wais , and highs we e i mly s apped o he chai and he dis al pa s o hei legs we e affixed o s ain gauge o ce ansduce s (Dongguan Sou h China Sea Elec onic Co., L d., Dongguan, China; load cell S-Type CZL302; ange 2 kN) using igid cuffs. The cuffs we e secu ed wi h hook and loop as ene s 2 cm abo e he malleolus la e alis. Figu e 2. Tes ing and aining se -up (uppe le panel; 1: o ce ansduce ; 2: shanks; 3: igid s aps; 4: acquisi ion and analog- o-digi al con e sion uni ; 5: moni o wi h isual eedback). No e ha only ained legs we e affixed o o ce ansduce du ing he aining sessions. Lowe le panel shows posi ioning o EMS elec odes and subjec inc easing he cu en o main ain a ge ed aining in- ensi y. Uppe and lowe igh panels depic aining p ocedu e ac oss weeks and isual eedback gi en du ing he aining sessions ( i s wo con ac ions ou o 40 a e ma ked as “1” and “2”). All es ing p ocedu es we e pe o med sepa a ely o each leg. Maximal olun a y con ac ion (MVC) was always pe o med p io o RTD-SF es ing o de e mine he g ea - es alue o o que du ing he pe o med ask [1]. Pa icipan s we e ins uc ed o ex end Figu e 2. Tes ing and aining se -up (uppe le panel; 1: o ce ansduce ; 2: shanks; 3: igid s aps; 4: acquisi ion and analog- o-digi al con e sion uni ; 5: moni o wi h isual eedback). No e ha only ained legs we e a ixed o o ce ansduce du ing he aining sessions. Lowe le panel shows posi ioning o EMS elec odes and subjec inc easing he cu en o main ain a ge ed aining in ensi y. Uppe and lowe igh panels depic aining p ocedu e ac oss weeks and isual eedback gi en du ing he aining sessions ( i s wo con ac ions ou o 40 a e ma ked as “1” and “2”). All es ing p ocedu es we e pe o med sepa a ely o each leg. Maximal olun a y con ac ion (MVC) was always pe o med p io o RTD-SF es ing o de e mine he g ea es alue o o que du ing he pe o med ask [ 1 ]. Pa icipan s we e ins uc ed o ex end he knee ‘as as and as ha d as possible’ and each maximal con ac ion was sus ained o app oxima ely 3 s [28]. Th ee MVCs we e comple ed o each leg wi h a 60 s es be ween successi e ials [ 3 , 4 ]. The MVC wi h he highes alue o o que (Tmax) o e he h ee Appl. Sci. 2024,14, 6485 5 o 13 consecu i e ials was used o u he analysis as well as o p esc ibe he subsequen submaximal as con ac ions ha we e exp essed as a pe cen age o Tmax. Following a 5 min es , he RTD-SF es ing p o ocol was conduc ed. Subjec s comple ed ~15 submaximal apid con ac ions o amilia iza ion pu poses. They we e ins uc ed o con ac hei QF muscle as quickly as possible and o elax ins an ly. Th ee se s o ≥ 20 con ac ions we e comple ed o expe imen al pu poses. In each se , he e we e 5 consecu i e con ac ions a 4 di e en in ensi ies p esen ed in ascending o de (20%, 40%, 60%, 80% o Tmax, i.e., MVC) [ 7 , 8 ]. The es ime be ween he se s was 60 s [ 1 ]. The expe imen e ga e a oice command o pe o m a new ial e e y 4–5 s. Visual eedback o o que as a a ge line was p o ided o he subjec s on a compu e moni o , bu subjec s we e ad ised no o pay a en ion o accu acy. Da a we e collec ed and sampled a 1000 Hz using he comme cially a ailable so - wa e Isome ics ( e sion 4.0.0, ‘Spo s Medical Solu ions’, Belg ade, Se bia). Signals we e il e ed wi h a low-pass (5 Hz), second-o de Bu e wo h il e . The so wa e au oma ically calcula ed he Tmax (peak alue on he o que– ime ace a e eaching he pla eau) and RTDmax (peak o i s de i a i e o he o que– ime signal o e lapping 0.1 s in e als). The peak o que and peak RTD om he isome ic o que pulses we e also au oma ically calcula ed as he maximal alue du ing he 1 s in e al o each epe i ion and as a maximum o he i s de i a i e o he o que– ime cu e, espec i ely. Each eco ding was inspec ed o ensu e ha no coun e mo emen was p esen as his would in luence RFD. Peak o que and peak RTD we e exp essed as a pe cen age o Tmax and plo ed o ob ain a eg ession line (slope and in e cep ) and R 2 alues o each pa icipan , espec i ely. The slope o he eg ession line p o ides he RTD-SF ( o mo e de ails, please see Figu e 2). The R2 ep esen s consis ency in he pe o mance apid muscula con ac ions [1,12]. 2.4. T aining P ocedu es The aining p ocedu es we e applied unila e ally o e he QF wi h lowe MVC o que a he p e- es , ega dless o leg dominance. As a no e, he e was a signi ican be ween-leg di e ence in Tmax in bo h g oups (10 ± 7%; p< 0.01 in EMS, and 12.5 ± 5%; p< 0.01 in VOL g oup). Pa icipan s’ posi ioning du ing aining sessions was iden ical o ha du ing es ing p ocedu es. The pa icipan ’s unk, wais , and highs we e i mly s apped o he chai wi h knee and hip angles ixed a 120 ◦ and 100 ◦ , espec i ely. The dis al pa o he ained leg was a ixed o a s ain gauge o ce ansduce (Figu e 2, uppe le panel). Each EMS and VOL aining session consis ed o 40 con ac ions each las ing 6.25 s and sepa a ed by 20 s in e -con ac ion in e als. The numbe o con ac ions and con ac ion- o- es a io (6.25/20 s) we e in acco dance wi h he wo k- o- es pa e n p oduced by he muscle s imula ion de ice (Figu es 1and 2—uppe and lowe igh panels). The s imula ion in ensi y was de e mined du ing he amilia iza ion when EMS was deli e ed a he indi idually maximal ole able dose and was kep close o iden ical among he session, a e aging in o al ~60% MVC pe subjec [ 24 ]. Speci ically, EMS in ensi y a ied be ween 58% and 63% ac oss sessions and subjec s, depending on hei daily pain ole ance. Due o an icipa ed posi i e exe cise e ec s on Tmax, pa icipan s we e e- es ed a e he hi d week o in e en ion (mid- es ), and hese da a se ed o adjus he exe cise h eshold (please see Figu e 2, uppe igh panel). The subjec s we e ins uc ed o s ay elaxed du ing EMS sessions, hus s imula ion was no supe imposed on o olun a y con ac ions. A po able ba e y-powe ed comme cially a ailable s imula o (Compex SP2.0, Medicompex SA, Ecublens, Swi ze land) was used in he EMS g oup. The mo o poin s we e iden i ied by s imula ing he skin su ace wi h a pen elec ode and a la ge e e ence elec ode placed o e he skin. S imula o y cu en was g adually inc eased un il a clea muscle wi ch was obse ed. The posi ions o he mo o poin s we e ma ked wi h wa e - esis an ma ke s, so he elec odes we e applied a he same si es h oughou he in e en ion pe iod. The posi i e elec odes, measu ing 25 cm 2 (5 cm × 5 cm), which had memb ane-depola izing p ope ies, we e placed as close as possible o mo o poin s o as us medialis and as us la e alis muscles. The nega i e Appl. Sci. 2024,14, 6485 6 o 13 elec ode, measu ing 50 cm 2 (10 cm × 5 cm), was placed nea he p oximal inse ion o ec us emo is muscle. The s imula o discha ged biphasic ec angula pulses las ing 400 µ s. The s imula ion equency and du y cycle we e 75 Hz and 6.25 s o s imula ion ollowed by a pause las ing 20 s (du y cycle, 24%) [ 25 , 37 ]. The selec ed s imula ion pa ame e s co espond o a maximum s eng h de elopmen p og am. The s imula ion in ensi y was moni o ed on-line and de e mined by he subjec a he s a o each EMS session acco ding o hei pain h eshold and o p oduce a o que co esponding o a leas 60% o he p e- es Tmax sco e [24]. Rega ding he VOL g oup, pa icipan s we e equi ed o each he p esc ibed o que le el only h ough olun a y ac i a ion o QF. To a ain he same numbe o con ac ions and con ac ion- o- es a io as in EMS, au oma ed audible signals we e deli e ed in acco dance wi h he con ac ion– es pa e n p oduced by he muscle s imula ion de ice. Depending on he ype o exe cise, he subjec s we e asked o inc ease he cu en (please see Figu e 2, lowe le panel) o olun a y ac i a ion in ensi y h oughou he ain- ing session o main ain a ge o que le el o ~60% MVC. The exe ed o que du ing each con ac ion was measu ed wi h he same o ce ansduce and so wa e as desc ibed o es ing p ocedu es, and eal- ime eedback along wi h he indi idual 60% MVC h eshold was p o ided on a compu e sc een. All aining sessions we e supe ised by a leas one o he esea che s who conduc ed he expe imen . 2.5. S a is ical Analyses P io o s a is ical analyses, Tmax and RTDmax da a we e no malized wi h pa ici- pan s’ indi idual body mass. No mali y o he dependen a iables was con i med using a Shapi o–Wilk es (p> 0.05). Desc ip i e s a is ics (mean and s anda d de ia ion) we e calcula ed o all a iables (Tmax, RTDmax, RTD-SF, and In e cep ). A 2 × 2 analysis o a iance (ANOVA) wi h ime (p e- es and pos - es ) as wi hin-subjec ac o and g oup (EMS and VOL) as be ween-subjec ac o was applied o each dependen a iable sepa- a ely o he ained and non- ained leg, ollowed by he Bon e oni pos hoc es . The Cohen’s e ec size (d) was used o quan i y he wi hin-g oup di e ences as d < 0.2 ( i ial o no e ec ), d = 0.2–0.5 (small), d = 0.5–0.8 (mode a e), d = 0.8–1.3 (la ge), and d > 1.3 ( e y la ge). Cohen’s d e ec size (ES) and 90% con idence in e al (CI) we e compu ed using he ha monic mean o he SD o he compa ed condi ions and an ES o 0.20 was conside ed as he minimal alue o p ac ical impo ance. When he 90% CI o e lapped subs an ial posi i e and nega i e alues, he e ec was deemed unclea ; o he wise, e ec s we e deemed clea [ 38 ]. p- alues < 0.05 we e conside ed s a is ically signi ican . The mag- ni ude compa ison analysis was pe o med using a cus om Excel sp eadshee (a ailable om: h ps://www.spo sci.o g/jou /03/wgh ials.h m, accessed on 18 Sep embe 2023), while o he s a is ical analyses we e pe o med using he so wa e package SPSS (IBM SPSS e sion 20.0, Chicago, IL, USA). 3. Resul s Consis ency in he pe o mance o apid muscula con ac ions ep esen ed by he coe icien o de e mina ion o he RTD-SF eg ession line emained high, om p e- es o pos - es (median R 2 = 0.951, ange 0.936–0.959). Figu e 3depic s he ela ionship o peak o que ela i e o Tmax and he espec i e peak RTD (%Tmax) o ained and non- ained legs in EMS and VOL g oups, espec i ely. Table 1depic s he changes in he dependen a iables om p e- es and pos - es . Rega ding he ained leg, ANOVA e ealed signi ican main e ec o ime o all dependen a iables, wi h gene ally la ge e ec sizes obse ed bo h wi hin EMS and VOL g oups (0.78–2.2). The la ges change was obse ed o Tmax (29%), ollowed by RTDmax (~13%), RTD-SF (~ − 15%), and In e cep . The main e ec o g oup ac o and in e ac ion o ac o s we e no signi ican , indica ing ha EMS and VOL exe cises induced simila changes o e a six-week pe iod, as suppo ed by he analysis o he s anda dized di e ences (Figu e 3; lowe panel), since he ES always anged om −0.20 o 0.20 (i.e., i ial di e ences). Appl. Sci. 2024,14, 6485 7 o 13 Appl. Sci. 2024, 14, x FOR PEER REVIEW 7 o 13 Figu e 3. P e- es and pos - es g oup ela ionships be ween peak o ce ela i e o Tmax and he espec i e peak RTD (%Tmax/s) o EMS and VOL, as well as o ained and non- ained leg, e- spec i ely. Table 1 depic s he changes in he dependen a iables om p e- es and pos - es . Rega ding he ained leg, ANOVA e ealed signi ican main effec o ime o all depend- en a iables, wi h gene ally la ge effec sizes obse ed bo h wi hin EMS and VOL g oups (0.78–2.2). The la ges change was obse ed o Tmax (29%), ollowed by RTDmax (~13%), RTD-SF (~−15%), and In e cep . The main effec o g oup ac o and in e ac ion o ac o s we e no signi ican , indica ing ha EMS and VOL exe cises induced simila changes o e a six-week pe iod, as suppo ed by he analysis o he s anda dized diffe ences (Figu e 3; lowe panel), since he ES always anged om −0.20 o 0.20 (i.e., i ial diffe ences). Table 1. Desc ip i e da a o dependen a iables shown as mean ± s anda d de ia ion. Mean ela- i e changes be ween p e- es and pos - es (% change) a e gi en as aw alues. Wi hin-g oup effec size is p esen ed wi h Cohen’s d. EMS VOL ANOVA P e-Tes Pos -Tes % Change Cohen’s d P e-Tes Pos -Tes % Change Cohen’s d F-Value Time F-Value G oup F-Value Time × G oup T ained leg Tmax (Nm/kg) 15.7 ± 2 20.2 ± 3.5 29.3 2.12 16.6 ± 2.2 21.4 ± 3.4 28.8 2.11 82.0 ** 0.9 0.0 RTD (Nm/kg/s) 83 ± 13 93 ± 17 12.3 0.92 85 ± 18 97 ± 15 16.6 1.10 16.4 ** 0.1 0.2 RTD-SF 6.0 ± 0.6 5.1 ± 0.7 −14.6 1.10 5.9 ± 0.8 5.0 ± 0.6 −14.9 1.01 25.1 ** 0.1 0.0 In e cep 0.9 ± 0.18 1.07 ± 0.21 22.5 0.78 0.86 ± 0.29 1.08 ± 0.17 24.9 0.72 12.5 ** 0.0 0.1 Non- ained leg Tmax (Nm/kg) 17 ± 1.9 19 ± 3.6 11.9 0.76 19.2 ± 2.8 19.4 ± 2.9 2.2 0.20 3.28 1.4 2.1 RTD (Nm/kg/s) 87 ± 16 89 ± 20 2 0.34 96 ± 18 94 ± 18 −0.6 0.01 0.04 0.9 0.4 RTD-SF 5.9 ± 0.6 5.1 ± 0.5 † −12.6 0.76 5.5 ± 0.6 5.4 ± 0.7 −0.1 0.20 6.35 * 0.0 5.5 * In e cep 0.94 ± 0.25 1.10 ± 0.17 16.6 0.60 1.02 ± 0.17 0.99 ± 0.24 −2.2 0.25 1.13 0.0 2.7 * Main effec o ac o in e ac ion signi ican a p < 0.05; ** main effec o ac o in e ac ion signi ican a p < 0.01; † pos hoc signi ican a p < 0.01. Fo he non- ained leg (Table 1), ime ac o and g oup× ime in e ac ion we e signi - ican only o RTD-SF, whe eas a subsequen pos hoc es e ealed ha signi ican change was obse ed only wi hin EMS g oup. Howe e , no e ha al hough diffe ences be ween aining modali ies eached signi icance only o RTD-SF, Figu e 4 (uppe panel) indica es ha diffe ences o RTD-SF, In e cep , and Tmax a e subs an ially g ea e o he EMS han VOL g oup (ES < −0.20). This is also suppo ed by mode a e wi hin-g oup effec sizes (0.60–0.76), whe eas Cohen’s d was i ial ac oss he VOL g oup. Figu e 3. P e- es and pos - es g oup ela ionships be ween peak o ce ela i e o Tmax and he espec i e peak RTD (%Tmax/s) o EMS and VOL, as well as o ained and non- ained leg, espec i ely. Table 1. Desc ip i e da a o dependen a iables shown as mean ± s anda d de ia ion. Mean ela i e changes be ween p e- es and pos - es (% change) a e gi en as aw alues. Wi hin-g oup e ec size is p esen ed wi h Cohen’s d. EMS VOL ANOVA P e-Tes Pos -Tes % Change Cohen’s dP e-Tes Pos -Tes % Change Cohen’s dF-Value Time F-Value G oup F-Value Time × G oup T ained leg Tmax (Nm/kg) 15.7 ±2 20.2 ±3.5 29.3 2.12 16.6 ±2.2 21.4 ±3.4 28.8 2.11 82.0 ** 0.9 0.0 RTD (Nm/kg/s) 83 ±13 93 ±17 12.3 0.92 85 ±18 97 ±15 16.6 1.10 16.4 ** 0.1 0.2 RTD-SF 6.0 ±0.6 5.1 ±0.7 −14.6 1.10 5.9 ±0.8 5.0 ±0.6 −14.9 1.01 25.1 ** 0.1 0.0 In e cep 0.9 ±0.18 1.07 ±0.21 22.5 0.78 0.86 ±0.29 1.08 ±0.17 24.9 0.72 12.5 ** 0.0 0.1 Non- ained leg Tmax (Nm/kg) 17 ±1.9 19 ±3.6 11.9 0.76 19.2 ±2.8 19.4 ±2.9 2.2 0.20 3.28 1.4 2.1 RTD (Nm/kg/s) 87 ±16 89 ±20 2 0.34 96 ±18 94 ±18 −0.6 0.01 0.04 0.9 0.4 RTD-SF 5.9 ±0.6 5.1 ±0.5 †−12.6 0.76 5.5 ±0.6 5.4 ±0.7 −0.1 0.20 6.35 * 0.0 5.5 * In e cep 0.94 ±0.25 1.10 ±0.17 16.6 0.60 1.02 ±0.17 0.99 ±0.24 −2.2 0.25 1.13 0.0 2.7 * Main e ec o ac o in e ac ion signi ican a p< 0.05; ** main e ec o ac o in e ac ion signi ican a p< 0.01; †pos hoc signi ican a p< 0.01. Fo he non- ained leg (Table 1), ime ac o and g oup × ime in e ac ion we e signi i- can only o RTD-SF, whe eas a subsequen pos hoc es e ealed ha signi ican change was obse ed only wi hin EMS g oup. Howe e , no e ha al hough di e ences be ween aining modali ies eached signi icance only o RTD-SF, Figu e 4(uppe panel) indica es ha di e ences o RTD-SF, In e cep , and Tmax a e subs an ially g ea e o he EMS han VOL g oup (ES < − 0.20). This is also suppo ed by mode a e wi hin-g oup e ec sizes (0.60–0.76), whe eas Cohen’s d was i ial ac oss he VOL g oup. Appl. Sci. 2024,14, 6485 8 o 13 Appl. Sci. 2024, 14, x FOR PEER REVIEW 8 o 13 Figu e 4. S anda dized mean diffe ences (90% con idence in e als) o all dependen a iables be- ween he EMS and VOL g oup (ES = EMS mean - VOL mean/SD bo h). The shaded a ea ep esen s a i ial diffe ence (ES om -0.20 o 0.20). Tmax—maximum o que; RTDmax—maximum a e o o que de elopmen ; RTD-SF— a e o o que de elopmen scaling ac o . 4. Discussion The aim o his s udy was o in es iga e he changes in he quad iceps emo is muscle a e o o que de elopmen scaling ac o (RTD-SF) ollowing unila e al isome ic ain- ing. The aining in e en ion included ei he elec os imula ion o olun a y exe cises (EMS and VOL, espec i ely). The main indings ega ding ou i s hypo hesis a e as ol- lows: (1) bo h in e en ions induced simila dec emen s in RTD-SF (app oxima ely 15%); (2) In e cep was inc eased ollowing bo h aining modali ies (22 and 17% o EMS and VOL, espec i ely) bu due o he high a iabili y he change was signi ican only ollow- ing EMS; (3) bo h in e en ions inc eased Tmax by ~30%; and (4) compa able inc emen s, al hough o lowe magni ude (14%), we e obse ed o RTDmax. Ou second hypo hesis was no con i med because he c oss-educa ion effec o he non- ained leg was obse ed only o he EMS g oup. As was hypo hesized, he applied aining in e en ions had a signi ican effec on he RTD-SF. In ac , o ou su p ise, bo h EMS and VOL induced a dec ease in RTD-SF, which could be pa ly explained by a highe ela i e inc ease in Tmax han in RTDmax. Al hough p e ious s udies e ealed ha he RTD-SF p o ocol could be used o disc imi- na e be ween pa icipan s o diffe en ages [14], sexes [13], a igue o physical ac i i y le els [11,15], o in e -limb asymme ies [7,39], cu en ly no published da a a e a ailable ega ding how s eng h aining in e en ions affec he RTD-SF. The s udy ha in es i- ga ed changes in RTD-SF ollowing exe cise in e en ion was conduc ed on ac i e olde adul s and i in ol ed a 6-week high-in ensi y in e al cycling in e en ion [12]. Hence, Figu e 4. S anda dized mean di e ences (90% con idence in e als) o all dependen a iables be ween he EMS and VOL g oup (ES = EMS mean - VOL mean/SD bo h). The shaded a ea ep esen s a i ial di e ence (ES om -0.20 o 0.20). Tmax—maximum o que; RTDmax—maximum a e o o que de elopmen ; RTD-SF— a e o o que de elopmen scaling ac o . 4. Discussion The aim o his s udy was o in es iga e he changes in he quad iceps emo is muscle a e o o que de elopmen scaling ac o (RTD-SF) ollowing unila e al isome ic ain- ing. The aining in e en ion included ei he elec os imula ion o olun a y exe cises (EMS and VOL, espec i ely). The main indings ega ding ou i s hypo hesis a e as ollows: (1) bo h in e en ions induced simila dec emen s in RTD-SF (app oxima ely 15%); (2) In e cep was inc eased ollowing bo h aining modali ies (22 and 17% o EMS and VOL, espec i ely) bu due o he high a iabili y he change was signi ican only ollowing EMS; (3) bo h in e en ions inc eased Tmax by ~30%; and (4) compa able inc emen s, al hough o lowe magni ude (14%), we e obse ed o RTDmax. Ou second hypo hesis was no con i med because he c oss-educa ion e ec o he non- ained leg was obse ed only o he EMS g oup. As was hypo hesized, he applied aining in e en ions had a signi ican e ec on he RTD-SF. In ac , o ou su p ise, bo h EMS and VOL induced a dec ease in RTD-SF, which could be pa ly explained by a highe ela i e inc ease in Tmax han in RTDmax. Al hough p e ious s udies e ealed ha he RTD-SF p o ocol could be used o disc imina e be ween pa icipan s o di e en ages [ 14 ], sexes [ 13 ], a igue o physical ac i i y le els [ 11 , 15 ], o in e -limb asymme ies [ 7 , 39 ], cu en ly no published da a a e a ailable ega ding how s eng h aining in e en ions a ec he RTD-SF. The s udy ha in es iga ed changes in RTD-SF ollowing exe cise in e en ion was conduc ed on ac i e olde adul s and i in ol ed a 6-week high-in ensi y in e al cycling in e en ion [ 12 ]. Hence, he au ho s epo ed a 34% imp o emen in RTD-SF (i.e., a highe slope) o QF in he exe cise g oup Appl. Sci. 2024,14, 6485 9 o 13 bu no in con ols. This is no su p ising since obus imp o emen s in explosi e capaci ies a e expec ed ollowing bo h high- and low-speed dynamic exe cises and ballis ic isome ic exe cises pe o med agains lowe load (e.g., <60% o maximum o que) and wi h he in en o apid o que p oduc ion [ 5 ]. In he cu en s udy, i seems ha apid o que gene a ion was una ec ed by aining in e en ion o in ensi ies < 50% MVC (Figu e 2). Howe e , a isible dec ease in apid p oduc ion can be obse ed o highe in ensi ies (50–80% MVC) ha we suspec esul ed in lowe RTD-SF when compa ed o p e- es alues. Conside ing ha isome ic exe cises used in his s udy we e no as ballis ic as in he RFD-SF assessmen p o ocol, he applied aining modali ies had a signi ican nega i e impac on neu omuscula quickness assessed h ough RTD-SF. This inding could be o impo ance o esea che s in es iga ing muscle unc ion and o coaches wo king wi h speed- ained a hle es who may conside using mode a e-in ensi y isome ic con ac ions in hei aining o ehabili a ion ou ines as well. In e cep is he a iable ha has mos ly s ayed un epo ed o no discussed as i was conside ed o p o ide no addi ional in o ma ion [ 1 , 2 , 6 , 12 ]. A s udy ha in es iga ed how cogni i e a igue a ec s RFD-SF p esen ed a ely epo ed esul s on In e cep ha emained una ec ed [ 16 ]. Kozinc e al. [ 2 ] specula ed ha a highe In e cep oge he wi h he same RFD-SF could indica e supe io quickness ac oss he ange o con ac ion in ensi ies, which is only pa ly he case in ou s udy since In e cep inc eased while he slope, i.e., RFD-SF, declined. Ne e heless, mo e s udies a e needed o answe he ques ion on he physiological and po en ially clinical ele ance o he In e cep . Rega ding maximal s eng h and RTD gains, i should be no ed ha he le el o he elec ically e oked o que achie ed in his s udy anged be ween 58% and 63% MVC ( he a e age in ensi y ac oss pa icipan s was 60% MVC). The in ensi y was selec ed ollowing p e ious ecommenda ions and knowing he limi a ions o EMS [ 24 , 25 , 36 ]. Speci ically, o p omo e s eng h gains, EMS was p e iously mos commonly applied a in ensi ies o ~60% MVC [ 24 ]. To ensu e a compa able aining olume be ween he wo aining modali ies we ma ched he con ac ion in ensi y and du a ion o he EMS and VOL g oups. Al hough highe olun a y exe cise loads a e ecommended o p omo e muscle hype ophy and s eng h gains [ 40 ], signi ican s eng h gains could be achie ed e en when using medium loads (50–60% MVC) [ 25 , 41 – 44 ]. In ac , Sze o e al. [ 44 ] demons a ed a 31% inc ease in MVC ollowing isome ic VOL aining a 50% MVC, while Filipo ic e al. [ 24 ] epo ed gains in maximum s eng h o 32.6 ± 17.6% in ained subjec s and o 32 ± 15.6% in eli e a hle es ollowing isome ic EMS. None heless, he esul s ob ained in he cu en s udy a e in line wi h hose epo ed p e iously [ 24 , 25 , 42 – 44 ] and show no subs an ial di e ence in MVC o que gains be ween VOL and EMS aining modali ies. While MVC Tmax was among he mos in es iga ed a iables in EMS s udies [23,24] , changes in RTDmax ha e been sca cely epo ed. Fo example, Speiche e al. [ 45 ] demon- s a ed signi ican inc eases o up o 16% in he QF muscle du ing he ea ly ime sec ions o RTD (100–200 ms om he con ac ion onse ) bu no in maximal RTD a e dynamic whole-body combina ion EMS aining [ 24 ]. Tha is no su p ising since i is well known ha pe o ming isome ic exe cises in a ballis ic manne can maximize he imp o emen o RTD [ 40 ]. Ne e heless, as shown in he cu en s udy, an inc ease in RTD max (15% o bo h aining modali ies) is wa an ed e en when isome ic exe cises a e pe o med on a single muscle g oup and in a non-ballis ic manne . Ou second hypo hesis and ou inal indings conce n he c oss-educa ion o he non- ained leg. A signi ican change in RTD-SF o he non- ained leg was ob ained only ollowing EMS aining, sugges ing po en ial cen al adap a ions. Mode a e, non- signi ican change was also obse ed o he In e cep . Al hough he c oss-educa ion e ec is well desc ibed in he li e a u e [ 18 , 19 , 26 , 32 ], his is he i s s udy o in es iga e his phenomenon conce ning RTD-SF and s eng h aining. In he a o emen ioned s udy o Bellumo i e al. [ 12 ], he au ho s epo ed ha hei lowe limb speed-cycling aining p og am had a posi i e c oss-educa ion e ec on RFD-SF o non- ained uppe limbs. Those who compa ed he e ec s o olun a y and s imula ed isome ic exe cises on he