Does Higher Intensity Increase the Rate of Responders to Endurance Training When Total Energy Expenditure Remains Constant? : A Randomized Controlled Trial

Reu e e al. Spo s Medicine - Open (2023) 9:35 h ps://doi.o g/10.1186/s40798-023-00579-3 ORIGINAL RESEARCH ARTICLE Open Access © The Au ho (s) 2023. Open Access This a icle is licensed unde a C ea i e Commons A ibu ion 4.0 In e na ional License, which pe mi s use, sha ing, adap a ion, dis ibu ion and ep oduc ion in any medium o o ma , as long as you gi e app op ia e c edi o he o iginal au ho (s) and he sou ce, p o ide a link o he C ea i e Commons licence, and indica e i changes we e made. The images o o he hi d pa y ma e ial in his a icle a e included in he a icle’s C ea i e Commons licence, unless indica ed o he wise in a c edi line o he ma e ial. I ma e ial is no included in he a icle’s C ea i e Commons licence and you in ended use is no pe mi ed by s a u o y egula ion o exceeds he pe mi ed use, you will need o ob ain pe mission di ec ly om he copy igh holde . To iew a copy o his licence, isi h p:// c ea i eco mmons. o g/ licen ses/ by/4. 0/. Spo s Medicine - Open Does Highe In ensi y Inc ease heRa e o Responde s oEndu ance T aining When To al Ene gy Expendi u e Remains Cons an ? ARandomized Con olled T ial Ma cel Reu e 1,2* , F iede ike Rosenbe ge 2,3, And eas Ba z1,2, And eas Venho s 1, Lau a Blanz1,2, Anne Hecks eden4 and Tim Meye 1 Abs ac Backg ound S anda dized aining p esc ip ions o en esul in la ge a ia ion in aining esponse wi h a subs an- ial numbe o indi iduals ha show li le o no esponse a all. The p esen s udy examined whe he he esponse in ma ke s o ca dio espi a o y i ness (CRF) o mode a e in ensi y endu ance aining can be ele a ed by an inc ease in aining in ensi y. Me hods Thi y-one heal hy, un ained pa icipan s (46 ± 8 yea s, BMI 25.4 ± 3.3 kg m−2 and ˙ V O2max 34 ± 4 mL min−1 kg−1) ained o 10 weeks wi h mode a e in ensi y (3 day week−1 o 50 min pe session a 55% HR ese e). He ea e , he alloca ion in o wo g oups was pe o med by s a i ied andomiza ion o age, gende and VO2max esponse. CON (con inuous mode a e in ensi y) ained o ano he 16 weeks a mode a e in ensi y, INC (inc eased in ensi y) ained ene gy-equi alen o 8 weeks a 70% HR ese e and hen pe o med high-in ensi y in e al aining (4 × 4) o ano he 8 weeks. Responde s we e iden i ied as pa icipan s wi h VO2max inc ease abo e he echni- cal measu emen e o . Resul s The e was a signi ican di e ence in ˙ V O2max esponse be ween INC (3.4 ± 2.7 mL kg−1 min−1) and CON (0.4 ± 2.9 mL kg−1 min−1) a e 26 weeks o aining (P = 0.020). A e 10 weeks o mode a e aining, in o al 16 o 31 pa icipan s we e classi ied as VO2max esponde s (52%). A e ano he 16 weeks con inuous mode a e in ensi y ain- ing, no u he inc ease o esponde s was obse ed in CON. In con as , he ene gy equi alen aining wi h inc eas- ing aining in ensi y in INC signi ican ly (P = 0.031) inc eased he numbe o esponde s o 13 o 15 (87%). The ene gy equi alen highe aining in ensi ies inc eased he a e o esponde s mo e e ec i ely han con inued mode a e aining in ensi ies (P = 0.012). Conclusion High-in ensi y in e al aining inc eases he a e o esponse in VO2max o endu ance aining e en when he o al ene gy expendi u e is held cons an . Main aining mode a e endu ance aining in ensi ies migh no be he bes choice o op imize aining gains. T ial Regis a ion Ge man Clinical T ials Regis e , DRKS00031445, Regis e ed 08 Ma ch 2023—Re ospec i ely egis e ed, h ps:// www. d ks. de/ DRKS0 00314 45 *Co espondence: Ma cel Reu e [email p o ec ed] Full lis o au ho in o ma ion is a ailable a he end o he a icle Page 2 o 11 Reu e e al. Spo s Medicine - Open (2023) 9:35 Key poin s • P olonging an ae obic aining p og am a e 10weeks wi h cons an in ensi y o 55% HR ese e and cons an ene gy expendi u e o 16 mo e weeks will no inc ease he a e o aining esponde s. • Inc easing aining in ensi y o 70% HR ese e a cons an ene gy expendi u e is no su icien o u he inc ease he a e o esponde s a e a 10-week mode a e-in ensi y aining pe iod. • T aining in he high in ensi y domain inc eases he a e o esponde s in ene gy-ma ched aining in e en ions. Keywo ds Ae obic i ness, ˙ V O2max, HIIT, Ene gy expendi u e, Endu ance aining, Non esponde , Response In oduc ion Endu ance aining is an e ec i e way o imp o e ca di- o espi a o y i ness (CRF) and posi i ely impac on isk ac o s o ca dio ascula disease [1–3]. Ye , i is also known ha adap a ions o a s anda dized aining g ea ly di e be ween indi iduals. Pe sons showing only mino o no adap ions o aining s imuli a e equen ly e med non esponde s. The e o e, conside a ion o indi idual a iabili y in aining esponse has become inc easingly impo an in aining s udies in ecen yea s. The He - i age s udy was one o he i s examples o la ge-scale in e en ion s udies ha has epo ed a la ge he e ogene- i y in esponses o VO2max a e a s anda dized aining in e en ion [4]. The phenomenon o li le o no aining esponse has been in es iga ed by o he esea che s since hen [5–7]. Ross and colleagues compa ed eigh endu - ance aining s udies and ound a ange o adap ions om minus 33% o plus 118%, independen o exe cise du a- ion, in ensi y and ial sample size [8]. I is no clea whe he adjus ing he aining modali ies (e.g., aining olume o in ensi y) has a posi i e in lu- ence on he a e o esponde s. The dose– esponse ela- ionship o endu ance aining is in luenced by aining olume, in ensi y and aining equency [9–11]. Highe doses o aining ha e p o en o elici mo e p onounced aining adap ions [12, 13]. Fu he mo e, Bona iglia and colleagues ha e shown ha be ween-subjec a iabili y in aining esponse is due o he aining dose and ex e nal ac o s a he han in e indi idual di e ences in ain- abili y [14]. The ex en o which modi ica ions o ei he in ensi y o olume a e he leading ac o is a ques ion wi h la ge ele ance o exe cise aining p esc ip ions o achie e he in ended aining adap ions. The e is e i- dence ha aining p o ocols wi h highe in ensi ies such as high-in ensi y in e al aining (HIIT) a e mo e e ec- i e in elici ing inc eases in VO2max han mode a e in en- si y aining [10, 15, 16]. Due o he high a iabili y o adap ions o endu - ance aining, i has been sugges ed ha he e icacy o aining in e en ions has o be analysed beyond me e compa ison o main e ec s like VO2max [17, 18]. In his ega d, a common app oach is o compa e he a es o non esponde s be ween g oups [6, 19]. In e ms o in e - indi idual a ia ions in aining esponse, Bona iglia and colleagues [12] ound ha highe doses o aining p oduce highe a es o esponse. In a di ec compa i- son o mode a e in ensi y aining wi h ene gy-ma ched HIIT, Ma u ana and colleagues [7] ound a g ea e e ec on VO2max and a lowe non esponde a e o HIIT. Mon e o and Lundby [19] obse ed ha highe aining dose h ough inc eased olume is an e ec i e app oach o achie e a meaning ul esponse in VO2max o pa ici- pan s showing no esponse a e an ini ial aining in e - en ion. Howe e , his ine i ably leads o mo e ime consump ion. And since he s udy was lacking a con ol g oup, he ques ion o whe he ex ended exposu e o he same aining dose is su icien o elici a esponse emains unanswe ed. Fo a s anda dized EE, pa icipan s who pe o med aining wi h highe in ensi ies we e less likely o show a non esponse han pa icipan s aining wi h lowe in ensi ies [6]. Howe e , i should be no ed ha in his s udy he aining equency was 5 week−1 and he e o e p esumably highe han in mos aining beginne s. These indings indica e ha o non espond- e s who a e unable o pe o m mo e o longe ain- ing sessions, aining wi h highe in ensi ies migh be a p omising al e na i e o achie e g ea e and con inuous aining adap ions. A me hodological di icul y is he iden i ica ion o non esponde s, as he e is no uni o m es ablished de i- ni ion o high-, low- o non esponse. Indi iduals a e e med esponde s when hei indi idual esponse exceeds a ce ain h eshold. The h eshold o non e- sponse has o example been as se as a VO2max imp o e- men ≤ 0L min−1 [20], o less han 5% [21, 22]. Mo eo e , some s udies use a coe icien o a ia ion o 5.6% [5] de i ed om he li e a u e. Al hough hese h esholds a e a s aigh o wa d me hod o ca ego ize esponde s, hey do no conside he ue aining-induced esponse. In his con ex , mo e a en ion has been gi en o he neces- si y o dis inguishing he ue aining-induced esponse om wi hin-subjec a iabili y and measu emen e o s Page 3 o 11 Reu e e al. Spo s Medicine - Open (2023) 9:35 o he speci ic se ing [17]. Acco dingly, some au ho s ha e sugges ed o use speci ic s udy designs such as epea ed es ing o eliabili y ials o analyse ue ain- ing esponses [23, 24]. A mo e indi idualized app oach o ake he day- o-day biological e o and measu emen e o in o accoun when in e p e ing and ca ego izing he esponse is calcula ing he echnical e o o meas- u emen (TEM) and using i as h eshold o adap ions [24, 25]. I should be no ed ha a dicho omous app oach in which indi iduals a e labelled as (non-) esponde s on he basis o a single h eshold is no wi hou c i icism. A single esponse h eshold is mo e likely o dis inguish a “ ue” esponse om an unce ain esponse han a “ ue” non esponse [26, 27]. Mo eo e , classi ying sub- jec s as non esponde s on he basis o a single pa ame e neglec s he wide ange o possible biological aining adap a ions [28]. Ano he me hodological challenge in es ing he independen e ec o aining in ensi y lies in he necessi y o con ol o he o e all aining dose ( ol- ume × in ensi y). This can be achie ed by es ima ing and s anda dizing he o al ene gy expendi u e (EE) be ween di e en aining modali ies. In his s udy, we sough o compa e esponse a es be ween 26weeks o mode a e in ensi y aining (55% HR ese e) and aining wi h an inc ease in aining in en- si y ( o 70%HR ese e and o 95%HRmax) a e 10 and 18 weeks, espec i ely. The di e ences on he g oup le el a e subjec in ano he publica ion and no dis- cussed in de ail he e [29]. I was hypo hesized ha non- esponse a e can be educed o e en elimina ed h ough an inc ease in in ensi y wi h ene gy expendi u e held cons an . Ma e ial andMe hods All pa icipan s signed w i en in o med consen . The s udy p ocedu es we e in acco dance wi h he Decla a- ion o Helsinki and he s udy was app o ed by he E hics Commi ee o he Medical Associa ion o Saa land (iden- i ica ion numbe 219/19). S udy Design andRandomiza ion Two aining g oups o un ained indi iduals pe - o med 26 weeks o endu ance aining. The con ol g oup (CON) kep all aining a iables cons an o e he 26 weeks, whe eas he in e en ion g oup (INC) inc eased aining in ensi y a e 10 and again a e 18 weeks o aining ( o de ails see aining below). The exe cise EE was s anda dized on a wi hin-subjec basis by p opo ionally lowe ing he aining olume in INC when he aining in ensi y was inc eased. T ead- mill es s o olun a y exhaus ion we e pe o med h ee imes a baseline (T01, T02, T03), a e 10weeks (weeks 10), a e 18weeks (weeks 18) and a e 26weeks (weeks 26) o aining. Using a minimiza ion echnique a week 10, subjec s we e alloca ed o ei he CON o INC [30, 31]. Fac o s o balancing we e age, sex, baseline VO2max, VO2max esponse a week 10 (yes/no), and he magni ude o VO2max esponse a week 10. The objec i e o his p o- cedu e was o balance he amoun o esponde s in bo h g oups. Fo his eason he in aindi idual a iabili y in VO2max (mL min−1) was used as each pa icipan ’s h esh- old o esponse [5]. The in aindi idual a iabili y (iV) ep esen s he day- o-day a iabili y o VO2max be ween T02 and T03 and was calcula ed as ollows: Pa icipan s Fo y-eigh (48), un ained subjec s we e ec ui ed o pa icipa ion. Inclusion c i e ia we e, age 30–60 yea s, un ained s a us (las 6 mon hs: < 1 h week−1 endu - ance- ype physical ac i i y) and non-smoking ( o a oid e ec s om cessa ion/ educ ion). Exclusion c i- e ia we e BMI > 30 kg m−2, es ing blood p essu e (RR es ) ≥ 160/100mmHg, o al choles e ol ≥ 300mg dL−1, maximum oxygen up ake (VO2max) > 50mL kg−1 min−1 o men; > 45mL kg−1 min−1 o women, i on de iciency (Fe - i in ≤ 34ng mL−1), hy oid dys unc ion (TSH ≤ 0.34 mU L−1 ≥ 4.0ng mL−1), medica ions wi h po en ial in luence on a ge pa ame e s (e.g. be a-blocke s) and p egnancy. Thi y- wo pa icipan s (46 ± 8 yea s, 25.4 ± 3.3 kg m−2 and 34 ± 4mL min−1 kg−1) comple ed he 26weeks ain- ing p og amme o which one da a se could no be ana- lysed due o echnical issues. A pa icipan low cha is gi en in Fig.1. A baseline and a weeks 10, he e we e no be ween-g oup di e ences o CON and INC (P > 0.05). Pa icipan s’ cha ac e is ics a e displayed in Table1. T aining Pa icipan s pe o med 26weeks o walking o jogging on 3day week−1. Fo he i s 10weeks all pa icipan s ained a mode a e in ensi y (50min pe session a 55% HR ese e [%HRR]). A e week 10, INC inc eased in ensi y o 70% HRR and o a HIIT p o ocol a e week 18. HIIT was pe o med wi h a 10min wa m-up a 70% HRmax ol- lowed by 4 imes 4min a 95% HRmax in e spe sed wi h 3min a 70% HRmax and a cool-down a he same in en- si y. CON ained wi h mode a e in ensi y h oughou he en i e cou se o he s udy. The leng h o aining ses- sions we e adjus ed o main ain a cons an wi hin-subjec EE. Adhe ence o he p esc ibed aining in ensi y was eco ded wi h a hea a e moni o (Sigma R1 Duo + ID. F ee, Sigma-Elek o GmbH, Neus ad , Ge many). iV =|T02 −T03|/meanT02T03 Page 4 o 11 Reu e e al. Spo s Medicine - Open (2023) 9:35 Ene gy Expendi u e Oxygen up ake a he indi idual exe cise hea a es was measu ed du ing he eadmill es s and used o es ima e he EE o pa icipan s exe cise hea a es by use o an a e age calo ic equi alen (4.85kcal L−1 O2) [32]. The EE o a ce ain hea a e was hen mul iplied wi h he p esc ibed aining ime a ha hea a e. By adjus ing session leng h, he o e all wi hin-subjec EE was kep cons an in INC h oughou all in e en ion phases. Fo HIIT, his implied an indi idual adjus men o he cool- down leng h. Tes ing The i s baseline es (T01) se ed as habi ua ion o he maximal eadmill es and a medical examina ion wi h es ing and exe cise elec oca diog am (ECG). The ol- lowing wo baseline measu emen s (T02, T03) we e pe o med o de e mine he in aindi idual a iabili y and he TEM o ˙ V O2max. The baseline da a ep esen s he mean om bo h es s. Be o e each eadmill es , an h o- pome ic da a (heigh , weigh , BMI and body a ) as well as hemodynamic cha ac e is ics ( es ing hea a e and blood p essu e) we e aken. Body a pe cen age was es i- ma ed by a 10-si e skin old me hod wi h a Ha penden Fig. 1 Pa icipan low cha . Heal h issues we e espi a o y ac in ec ions o musculoskele al p oblems (e.g. o e use inju ies) Table 1 Pa icipan s cha ac e is ics a baseline Values a e means ± SD; BMI body mass index, HR hea a e, BP blood p essu e, ˙ V O2max maximum oxygen up ake, Vmax maximum speed, Lamax maximum lac a e, RERmax maximum espi a o y exchange a io, P = be ween-g oup di e ences a baseline Men Women P n13 18 An h opome ic da a Age (yea s) 47 ± 9 46 ± 8 0.65 Heigh (cm) 176.0 ± 6.8 167.2 ± 6.6 0.01 Weigh (kg) 83.9 ± 12.4 68.2 ± 11.1 0.01 BMI (kg m−2) 27.0 ± 2.9 24.3 ± 3.3 0.02 Body a (%) 21.3 ± 2.9 23.6 ± 3.9 0.09 Hemodynamic cha ac e is ics a es HR (bpm) 68 ± 7 69 ± 8 0.79 Sys olic BP (mmHg) 124 ± 8 118 ± 9 0.06 Dias olic BP (mmHg) 83 ± 6 78 ± 8 0.04 Peak exe cise pe o mance VO2max (mL min−1) 3019 ± 378 2231 ± 324 < 0.01 VO2max (mL min−1 kg−1) 36.3 ± 4.1 33.1 ± 4.2 0.04 Vmax (km h−1) 12.1 ± 1.1 10.4 ± 1.2 0.01 HRmax (b m−1) 186 ± 15 183 ± 11 0.47 Lamax (mmol L−1) 9.3 ± 2.0 8.7 ± 2.4 0.42 RERmax 1.2 ± 0.1 1.2 ± 0.1 0.89 Page 5 o 11 Reu e e al. Spo s Medicine - Open (2023) 9:35 calipe . Hemodynamic measu es a es we e aken a e a en-minu e- es in supine posi ion. T eadmill es s we e pe o med on a Woodway ELG 70 (Woodway GmbH, Weil am Rhein, Ge many) and gas exchange measu e- men s we e conduc ed con inuously using a b ea h-by- b ea h sys em (Me aLyze ® 3B, Co ex Biophysik GmbH, Leipzig, Ge many) which was calib a ed acco ding o manu ac u e ’s ins uc ion. T eadmill es P o ocol The eadmill was se o a cons an incline o 0.5%. All es s s a ed a 4.0 km h−1. E e y 3 min, speed was inc eased by 1.0 km h−1. The submaximal hea a e (HRsubmax) du ing he g aded exe cise es was aken a he end o las 3min s age. When he espi a- o y exchange a io (REF) exceeded 0.95 o a leas 30s his was de ined as las 3min s age. A e wa ds, he es s con inued using a ampwise p o ocol wi h a speed inc emen o 0.8km h−1 pe minu e un il ol- un a y exhaus ion [33]. Maximal pa ame e s we e only analysed i a leas wo o he ollowing c i e ia we e ul illed: (1) HRmax ≥ 220-age, (2) maximal blood lac a e concen a ion > 8mmol L−1, (3) maximal RER > 1.1 [34]. Responde Classi ica ion Responde s we e iden i ied a e he 26-week ain- ing in e en ion by de e mining he TEM, which is desc ibed as a conse a i e measu e o assesso e o and day- o-day a ia ion in conduc ing an exe cise es [6]. This applica ion is conside ed e y eliable in p o- iding an es ima e o he echnical e o which is una - ec ed by a change in he mean [35]. Values ≤ 1 × TEM we e conside ed a non esponse, > 1 × TEM a esponse. As some au ho s sugges a mul iple o he TEM (2 × TEM) as a mo e conse a i e measu e o esponse [23], hese h esholds a e gi en in Fig.2, oo. The TEM was calcula ed by di iding he s anda d de ia ion o he di e ence sco e (Sdi ) by √2 [35]. Fig. 2 Indi idual changes compa ed o baseline in VO2max [mL min−1 kg−1] a e 10, 18 and 26 weeks o aining o CON and INC. The h ough line dis inguishes ini ial esponde s om non esponde s (1 × TEM = single echnical e o , 2 × TEM = mul iple echnical e o ); + elimina ed non esponse; − esponse u ned in o non esponse. Each ba ep esen s one subjec . The o de o ba s is consis en in all igu es. The g oup mean is illus a ed h ough he ba in he backg ound Page 6 o 11 Reu e e al. Spo s Medicine - Open (2023) 9:35 S a is ical Analyses The sample size was es ima ed a p io i using an e ec size o compa ison be ween g oups om wo ou o ou a ms o a p e ious INC aining in e en ion s udy by Helge ud e al. (“4 × 4” s. “LSD”: dppc2 = 0.547) [10]. G*Powe ( e sion 3.1.9.4) was used o calcula e he sample size o an ANOVA (main and in e ac ion e ec in VO2max [mL min−1 kg−1]) wi h α = 0.05, 80% powe , 2 g oups, 2 es ime poin s. This led o a equi ed sample size o n = 29 in o al o 15 subjec s pe g oup, espec- i ely. Taking in o accoun an es ima ed d op-ou a e o 28% [36], he aim was o s a he s udy wi h n > 40 espec i ely n > 20 pe g oup. IBM’s S a is ical Package o he Social Science (SPSS 27; IBM; USA) was used o s a is ical analysis. Sex di - e ences a baseline we e examined using - es s o inde- penden samples. Mann–Whi ney-U-Tes s we e used o compa e baseline cha ac e is ics be ween esponde s and (unce ain)non esponde s. The pe cen age o espond- e s was calcula ed on he numbe o indi iduals who me less han 1 × TEM be ween week 10 and week 26 o each g oup. Fishe ’s Exac Tes was used o compa e he a e o esponde s be ween g oups a week 10, week 18 and week 26. The McNema Tes was used o compa e he a es o esponse be ween week 10 and week 26 wi hin- g oup. A P alue o < 0.05 o he α-e o was conside ed s a is ically signi ican . Resul s Compliance, Exe cise In e en ion andTes C i e ia Rega ding equency o aining, adhe ence o p esc ibed exe cise hea a e and EE, he e we e no s a is ically signi ican g oup di e ences (P > 0.05). CON comple ed 79.6 ± 6.95 and INC 79.8 ± 8.09 sessions. The a e age EE pe aining session es ima ed by indi ec calo ime y was 401 ± 105kcal wi h no g oup di e ence (P = 0.914). A e age exe cise HR up o week 10 was 100 ± 2% (CON) and 101 ± 2% (INC) o p esc ibed HR, om week 10 o week 18 i was 101 ± 4% (CON) and 99 ± 1% (INC) and om week 18 o week 26 i was 99 ± 1% (CON) and 97 ± 2% (INC). The d op-ou a e was 35% (Fig.1). HRmax (b min−1), Lamax (mmol L−1) and RERmax indica ed maxi- mal exhaus ion in bo h g oups om baseline (CON: 186 ± 12; 9.5 ± 2.2; 1.2 ± 0.1; INC: 182 ± 13; 8.4 ± 2.1; 1.2 ± 0.1) o weeks 26 (CON: 185 ± 11; 9.5 ± 2.2; 1.3 ± 0.1; INC: 181 ± 12; 9.4 ± 2.0; 1.3 ± 0.1). TE M = Sdi √2 Baseline Analysis The e we e no g oup-di e ences a baseline o an h o- pome ic da a o age (P = 0.73), BMI (P = 0.95), BF (P = 0.41), hemodynamic cha ac e is ics a es o HR (P = 0.65), BPsys (P = 0.44), BPdias (P = 0.29) and peak exe cise da a o ela i e VO2max (P = 0.57), absolu e VO2max (P = 0.71), Vmax (P = 0.50), HRmax (P = 0.44), Lamax (P = 0.17) and RERmax (P = 0.52). Baseline cha - ac e is ics a e shown in Table1. The e we e no di e - ences be ween esponde s and non esponde s a baseline o age (P = 0.06), BMI (P = 0.62), HR es (P = 0.65), BPsys (P = 0.32), BPdias (P = 0.38), ela i e VO2max (P = 0.98), absolu e VO2max (P = 0.86) and Vmax (P = 0.86). Mean Responses Baseline da a as well as changes o e ime o all pa am- e e s a e p esen ed in Table2. A e 26weeks o aining INC imp o ed ˙ V O2max sig- ni ican ly by 3.4 ± 2.7mL kg−1 min−1 (P = 0.002) as well as om week 18 o week 26 (P = 0.002). Changes in CON we e no signi ican (0.4 ± 2.9mL kg−1 min−1). Fo ela- i e VO2max, imp o emen s we e signi ican ly highe o INC han o CON (P = 0.02). The mean alues o ela- i e VO2max a baseline and a e weeks 10, weeks 18 and weeks 26 o CON we e 34.9 ± 3.6, 35.6 ± 4.5, 34.1 ± 3.9, 35.3 ± 4.0mL kg−1 min−1 and o INC we e 34.0 ± 5.2, 35.2 ± 5.5, 35.0 ± 5.1, 37.4 ± 4.9 mL kg−1 min−1, as desc ibed elsewhe e in mo e de ail [29]. Ra e o Responde s Changes in VO2max a e 10, 18 and 26weeks o aining o CON and INC a e shown in Table2. INC inc eased he a e o esponde s mo e e ec i ely han CON (P = 0.012). A e 10weeks o aining a an in ensi y o 55%HRR, he e we e no be ween-g oups di e ences in he a e o esponde s o ela i e VO2max (CON: 50%; INC: 53%; P = 0.569), HR es (CON: 56%; INC: 33%; P = 0.179) and HRsubmax (CON: 50%; INC: 47%; P = 0.569). A e 18weeks o aining he e we e also no be ween- g oups di e ences in he a e o esponde s o ela i e VO2max (CON: 25%; INC: 47%; P = 0.189), HR es (CON: 62%; INC: 40%; P = 0.186) and HRsubmax (CON: 50%; INC: 67%; P = 0.283). A e 26weeks o aining he di e ences in he a e o esponde s we e g ea e o INC han o CON o ela- i e VO2max (CON: 37%; INC: 87%; P = 0.009) and HR es (CON: 25%; INC: 87%; P < 0.001) bu no o HRsubmax (CON: 63%; INC: 67%; P = 0.553). Wi hin g oup analysis showed ha he a e o esponse was inc eased be ween week 10 and week 26 in INC o ela i e VO2max om 53 o 87% (P = 0.063) and HR es Page 7 o 11 Reu e e al. Spo s Medicine - Open (2023) 9:35 om 33 o 87% (P = 0.008). F om week 18 o week 26 INC inc eased he a e o VO2max esponde s om 47 o 87% (P = 0.031). In CON he e was a non-signi ican dec ease in he a e o esponse o ela i e VO2max om 50 o 37% (P = 0.754) and HR es om 44 o 25% (P = 0.25) om week 10 o week 26. Discussion This s udy in es iga ed he ole o an inc ease in ain- ing in ensi y while main aining EE o inc easing he esponde a e o endu ance aining in heal hy un ained subjec s. Wi h an a e age EE pe aining ses- sion o 400kcal, posi i e e ec s can be achie ed by bo h mode a e and in ensi e aining. As expec ed, a high in e indi idual a ia ion o he esponse was obse ed. Table 2 Baseline alues and changes a e 10, 18 and 26 weeks o aining o g oup and esponse Δ = changes a e 10, 18 and 26weeks o aining o CON and INC in VO2max[mL min−1 kg−1]; VO2max [mL min−1], Vmax [km h−1]; HR es [b min−1]; HRsubmax [b min−1]; alues a e p esen ed as means ± SD; = esponde , n = non esponde ; n (%) = numbe ( a e) o esponde s/non esponde s Baseline Δ weeks 0–10 n (%) Δ weeks 10–18 n (%) Δ weeks 18–26 n (%) VO2max [mL min−1 kg−1] CON 34.9 ± 3.5 2.5 ± 1.4 [8 (50)] 1.5 ± 3.5 [4 (25)] 2.0 ± 2.2 [6 (37)] n 34.9 ± 3.9 − 1.1 ± 1.1 [8 (50)] − 2.4 ± 2.3 [12 (75)] 0.7 ± 1.3 [10 (63)] INC 33.0 ± 4.7 2.6 ± 1.1 [8 (53)] 1.3 ± 2.5 [7 (47)] 2.4 ± 2.0 [13 (87)] n 35.0 ± 5.9 − 0.2 ± 0.8 [7 (47)] − 1.5 ± 2.3 [8 (53)] 2.1 ± 0.1 [2 (13)] VO2max [mL min−1] CON 2595 ± 601 167 ± 15 [6 (37)] 171.0 ± 39.6 [2 (12)] 139.3 ± 144.7 [4 (25)] n 2596 ± 542 − 54 ± 93 [10 (63)] − 151.4 ± 175.1 [14 (88)] 47.8 ± 120.5 [12 (75)] INC 2645 ± 531 145 ± 64 [9 (60)] 78.7 ± 124.4 [6 (40)] 123.0 ± 142.2 [11 (73)] n 2343 ± 473 − 3 ± 60 [6 (40)] − 162.1 ± 125.9 [9 (60)] 116.8 ± 129.9 [4 (27)] Vmax [km h−1] CON 11.2 ± 1.4 0.8 ± 0.3 [15 (94)] 0.2 ± 0.3 [16 (100)] 0.0 ± 0.3 [16 (100)] n 13.2 ± 0.0 0.4 ± 0.0 [1 (6)] / ± / [0] / ± / [0] INC 10.8 ± 1.5 0.8 ± 0.3 [14 (93)] 0.5 ± 0.4 [14 (93)] 0.5 ± 0.4 [14 (93)] n 12.2 ± 0.0 − 0.4 ± 0.0 [1 (7)] 0.2 ± 0.0 [1 (7)] − 0.6 ± 0.0 [1 (7)] HR es [b m−1] CON 74.4 ± 9.2 − 9.0 ± 6.8 [7 (44)] − 0.7 ± 5.8 [6 (38)] − 1.6 ± 5.2 [4 (25)] n 65.3 ± 6.6 − 1.0 ± 1.6 [9 (56)] 3.5 ± 3.6 [10 (62)] 1.2 ± 3.0 [12 (75)] INC 70.6 ± 4.2 − 8.2 ± 2.3 [5 (33)] − 7.2 ± 5.7 [6 (40)] − 3.0 ± 3.9 [13 (87)] n 66.7 ± 6.3 2.3 ± 4.1 [10 (67)] − 0.3 ± 5.7 [9 (60)] − 1.0 ± 2.8 [2(13)] HRsubmax [b m−1] CON 137.3 ± 7.7 − 7.4 ± 3.1 [8 (50)] − 1.5 ± 5.7 [8 (50)] − 0.5 ± 4.1 [10 (63)] n 134.6 ± 16.0 − 2.7 ± 2.3 [8 (50)] 2.1 ± 8.6 [8 (50)] 1.4 ± 4.9 [6 (37)] INC 132.2 ± 15.6 − 10.8 ± 3.3 [7 (47)] − 3.5 ± 5.5 [10 (67)] − 2.0 ± 7.6 [10 (67)] n 132.7 ± 8.8 4.2 ± 3.8 [8 (53)] − 4.4 ± 5.7 [5 (33)] 0.9 ± 7.5 [5 (33)] Page 8 o 11 Reu e e al. Spo s Medicine - Open (2023) 9:35 Responde s o VO2max we e obse ed in bo h g oups and by s a i ied andomiza ion equally dis ibu ed (CON = 50%; INC = 53%). The main indings o his esea ch a e (1) h ough an inc ease in aining in ensi y esponse a e o VO2max was inc eased om 53 o 87% despi e o al EE emaining he same, (2) gi en a cons an ene gy consump ion, a high in ensi y in e al aining e ec i ely inc eases he a e o esponde s as indica ed by an addi ional inc ease in esponse a e be ween week 18 and week 26 (i should be no ed ha his inc ease occu ed a e 18 weeks o aining had al eady been comple ed) and (3) he aining s imulus a an in ensi y o 70%HRR is no su icien o inc ease esponse a e 10weeks o aining a 55%HRR. We we e able o show ha o s anda dized isocalo ic aining in e en ions he choice o in ensi y a ec s he magni ude o ΔVO2max as well as he a e o esponse. When aining in ensi y was inc eased om 55%HRR o 70%HRR be ween week 10 and 18weeks no inc ease in esponse a e was obse ed a e 8weeks. Only when INC pe o med HIIT be ween week 18 and week26, he esponse a e was signi ican ly inc eased (+ 75%) in mos pa icipan s. This sugges s ha o isocalo ic in e en- ions he in ensi y mus be in he high in ensi y domain o p oduce mo e esponde s. P e ious s udies ha e al eady shown ha HIIT is mo e e ec i e han mode a e aining o imp o emen s o CRF when EE is con olled o [37, 38]. A no el inding o ou s udy is ha he e does no seem o be a linea ela ionship be ween in en- si y and esponde a es as aining wi h 70%HRR did no inc ease esponse a e e ec i ely. I is likely ha ce ain physiological pa hways leading o highe VO2max a e ac i a ed pa icula ly h ough a aining s imulus abo e a ce ain h eshold. The e is e idence ha HIIT yields g ea e imp o emen s in mi ochond ial con en han wo k-ma ched mode a e aining [39]. To ma and col- leagues ela ed he highe e ec i eness o HIIT o a la ge ac i a ion o as - wi ch ib es and an ad anced mi o- chond ial biogenesis [40]. Despi e hese wo ks i is s ill unclea wha he exac pa hways a e ha lead o highe VO2max. I seems unlikely ha e en o a s anda dized aining p og amme esponde s imp o e hei ΔVO2max h ough a uni o m physiological adap ion p ocess [41]. Al hough ou da a show ha HIIT is mo e likely o ig- ge a esponse in ma ke s o CRF, he e we e s ill 2 ou o 8 ini ial non esponde s in INC a e 26weeks. This ag ees wi h indings o Timmons and colleagues [42] ha 20% o indi iduals show no imp o emen in ae o- bic capaci y e en a e high in ensi y aining. Conside - ing Mon e o’s and Lundby’s [19] s udy which shows ha non esponse can be elimina ed h ough aining doses o up o 300min pe week, we conclude ha o some indi iduals an adjus men o HIIT o a gi en aining dose migh no be su icien and highe o e all aining doses, i.e. highe EE, a e necessa y in hese cases o elici a aining esponse inc ease esponse. The o e all e ec s on VO2max as well as he a e o esponde s a e he i s 10weeks o CON we e compa- able wi h o he s udies in es iga ing he e ec s o mod- e a e in ensi y aining [10, 43]. O e he nex 16weeks, 3 addi ional pa icipan s, om an ini ial 8, became esponde s. F om he ini ial 8 esponde s a baseline 7 we e ca ego ized as non esponde s ( espec i ely unce - ain esponde s) a weeks 18 and 5 a week 26. Ou da a he e o e sugges ha con inued aining a he same in ensi y does no au oma ically ha e a posi i e e ec on he a e o esponde s. Addi ionally, in acco dance wi h in e na ional guidelines [44], we ound ha mode - a e in ensi y aining migh no always p oduce a las ing aining esponse when he aining dose is no in ensi- ied o e ime. One a ionale o his inding is ha he i s 10weeks o aining igge ed p ima ily adap ions o he ca dio ascula sys em ha led o imp o ed mecha- nisms o oxygen anspo and consump ion and he e- o e an inc ease in VO2max. These adap ions pla eaued and, in some cases, possibly e en eg essed when he aining dose emained he same and submaximal adap- ions (e.g. me abolic sys em, unning economy) p esum- ably became mo e p onounced o e ime. In his con ex , i mus be emphasized ha he exe cise hea a e was no signi ican ly adjus ed in CON a any poin o e 26weeks and e ec i ely inc eased o e ime. This may ha e caused a lowe me abolic s ain o e ime o a gi en hea a e because o highe me abolic e iciency, which could cause a dec easing aining s imulus o e ime o hese indi iduals. This assump ion is suppo ed by indings by Wea he wax and colleagues who compa ed esponse a es o a aining p esc ip ion based on %HRR wi h an indi idualized aining based on en ila o y h esholds [11]. The esponse a es we e 60% o he s anda dized and 100% o he indi idualized aining. In line wi h hese indings and o he s [45, 46] he ques ion emains whe he esponse a es indica e di e ences in ain- abili y o a e a consequence o a ia ion in he be ween- subjec me abolic s ain o a ixed hea a e pe cen age. Al hough he li e a u e shows ha indi idualized ain- ing p esc ip ion can p oduce mo e homogenous aining esponses [47], i should no be o e looked ha in many p ac ical se ings he necessa y me abolic es ing o ha ype o p esc ip ion is no easible. Fo he ime being, he majo i y o indi iduals ha ake up an unsupe ised exe cise egimen will ely on he in e na ional guidelines and use hea a e as he only physiological measu e o moni o aining in ensi y. In his con ex , ou s udy has shown ha he implemen a ion o high in ensi y e o s inc eases he likelihood o imp o ed i ness. Page 9 o 11 Reu e e al. Spo s Medicine - Open (2023) 9:35 Al hough he magni ude o adap ion o Vmax be ween he wo g oups we e in a ou o INC, he e we e no di e ence in he esponse a es o his pa ame e . This highligh s ha mode a e as well as in ense ain- ing p o ocols yield imp o emen s in unc ional capac- i y. Vollaa d and colleagues ha e al eady shown ha o un ained indi iduals imp o emen s in maximal un- ning speed a e no ela ed o VO2max [48]. Ou s udy has con i med hese indings, as he e was no uni o m pa e n o esponse o Vmax and VO2max. Al hough he p ognos ic alue o VO2max o o e all heal h and all- cause mo ali y is widely accep ed, i is s ill unclea o which deg ee VO2max o he highe le els o physi- cal ac i i y h ough which i is achie ed a e he causal ac o o imp o ed heal h [49]. We he e o e sug- ges u u e esea ch o ocus mo e on he connec ion be ween non esponse and heal h bene i s, acknowledg- ing he me hodical challenges in such esea ch designs. In his con ex i should be no ed ha an adap a ion below a ce ain h eshold is no equal o a non e- sponse as he e is a ange o unce ain y be ween “ ue” esponse and non esponse. Limi a ions Ou da a poin ou ha a single pos - es migh no be su icien o ca ego ize indi iduals as esponde s wi h ce ain y. We ha e shown ha o a s anda dized aining in e en ion he e is a conside able wi hin-subjec luc- ua ion o esponse o e ime. This is in line wi h o he ecommenda ions o epea ed es ing du ing an exe cise in e en ion [23, 24]. Ne e heless, he s eng h o his s udy is ha he wi hin-subjec a iabili y o VO2max was analysed by wo baseline measu emen s. In ou s udy, he TEM was 4.2 ± 2.7% and was compa able in bo h g oups. Thus, i was simila o he o en used coe icien o a i- a ion o 5.6% epo ed in li e a u e [5]. In o he s udies, mos subjec s showed a esponse in a leas one pa am- e e , excep o VO2max [5]. This highligh s ha he e i- cacy o aining in e en ions should be assessed h ough se e al measu es. Since he le el o in aindi idual a iabili y may ha e an in luence on he esponse, i would be in e es ing o see how in aindi idual a iabili y changes in he aining p ocess. Fo his pu pose, Ross and colleagues [8] ecom- mend including a con ol g oup o measu e he in e in- di idual esponse a iabili y and o pe o m addi ional measu emen s a he p e- and pos - es s. Pa icipan s we e ins uc ed o main ain ea ing habi s as well as daily physical ac i i y. As we did no eco d he daily physi- cal ac i i y, we canno ule ou iny in luences om his sou ce and sugges u u e esea ch o include he in lu- ence o li es yle habi s on he aining esponse. Conclusion The cu en s udy demons a es ha aining in he high in ensi y domain inc eases he a e o esponde s o VO2max o mos indi iduals e en when he o al ene gy expendi u e emains he same. Thus, aining a in ensi- ies a ound VO2max such as in HIIT is well sui ed o e ec- i ely inc ease he a e o esponde s. I he speci ica ions o a s anda dized mode a e endu ance aining emain cons an a e he pe iod o 10weeks o aining, u he aining adap a ions a e unlikely. The e o e, aining in he high in ensi y domain is an e ec i e way o achie e and main ain a aining esponse wi hou he need o a highe o al aining olume. This is o pa icula el- e ance o se ings, whe e indi idualized ainings p e- sc ip ions a e no easible. Abb e ia ions BMI Body mass index BP Blood p essu e CON Con ol g oup (mode a e in ensi y) CRF Ca dio espi a o y i ness EE To al ene gy expendi u e HIIT High-in ensi y in e al aining HR es Res ing hea a e HR ese e Hea a e ese e HRsubmax Submaximal hea a e INC In e en ion g oup (inc easing in ensi y) iV In aindi idual a iabili y Lamax Maximum lac a e RER Respi a o y exchange a io T01 Habi ua ion o he maximal eadmill es and a medical examina ion T02 Baseline measu emen 1 T03 Baseline measu emen 2 TEM Technical e o o measu emen Vmax Maximum speed VO2max Maximum oxygen up ake wk10 10 Weeks o aining wk18 18 Weeks o aining wk26 26 Weeks o aining Acknowledgemen s No applicable. Au ho Con ibu ions MR w o e he i s d a o he manusc ip and analysed and in e p e ed he da a om he subjec s. Me hodological quali y assessmen s we e conduc ed by FR, AV and TM. AH con ibu ed o idea concep ion. AB and LB conduc ed he da a collec ion and supe ised he aining in e en ion. All au ho s ead and app o ed he inal manusc ip . Funding Open Access unding enabled and o ganized by P ojek DEAL. We did no ecei e any unding o ou wo k om any o ganiza ions. A ailabili y o Da a and Ma e ials The da ase s used and/o analysed du ing he cu en s udy a e a ailable om he co esponding au ho on easonable eques . Decla a ions E hics App o al and Consen o Pa icipa e All pa icipan s signed w i en in o med consen . The s udy p ocedu es we e in acco dance wi h he Decla a ion o Helsinki and he s udy was app o ed