Eu opean Jou nal o Spo Science
-
ORIGINAL PAPER
OPEN ACCESS
Validi y and P ac ical Applica ion o Muscle Oxygena ion
Moni o ing o Iden i ying Maximal Fa Oxida ion in
Cyclis s
Ande Roma a e
1,2
| Ai o Pinedo‐Jau egi
2,3,4
| And i Feldmann
5
| Ai o Vi ibay
6,7
| Jo dan San os‐Conceje o
2,8
1
Emen4spo , Leioa, Spain |
2
Depa men o Physical Educa ion and Spo , Uni e si y o he Basque Coun y UPV/EHU, Vi o ia‐Gas eiz, Spain |
3
Socie y,
Spo s, and Exe cise Resea ch G oup (GIKAFIT), Depa men o Physical Educa ion and Spo , Facul y o Educa ion and Spo , Uni e si y o he Basque
Coun y UPV/EHU, Vi o ia‐Gas eiz, Spain |
4
Resea ch G oup in Physical Ac i i y, Physical Exe cise and Spo (AKTIBOki), Depa men o Physical
Educa ion and Spo , Facul y o Educa ion and Spo , Uni e si y o he Basque Coun y UPV/EHU, Vi o ia‐Gas eiz, Spain |
5
Ins i u e o Spo Science,
Uni e si y o Be n, Be n, Swi ze land |
6
Glu 4Science, Physiology, Nu i ion and Spo , Vi o ia‐Gas eiz, Spain |
7
Ins i u e o Biomedicine (IBIOMED),
Uni e si y o León, León, Spain |
8
Vi ual Senso iza ion Resea ch G oup ‐ Visens (IT‐1381‐2019), Depa men o Physical Educa ion and Spo , Facul y o
Educa ion and Spo , Uni e si y o he Basque Coun y UPV/EHU, Vi o ia‐Gas eiz, Spain
Co espondence: Ai o Pinedo‐Jau egi ([email p o ec ed])
Recei ed: 15 Ap il 2024 | Re ised: 2 June 2025 | Accep ed: 15 July 2025
Funding: The au ho s ecei ed no speci ic unding o his wo k.
Keywo ds: inc emen al exe cise es | me abolic analysis | nea ‐in a ed spec oscopy | physiology | wea able
ABSTRACT
The accu a e de ec ion o se e al physiological miles ones, such as maximal a oxida ion (MFO), is an impo an ac o o
cycling pe o mance and o p og amming e ec i e and indi idualised aining. Howe e , he p ocedu e o iden i y he MFO is
o en oo complex and expensi e. Nea ‐in a ed spec oscopy (NIRS) echnology p o ides a nonin asi e measu emen ha can
be used o de ec di e en physiological a iables. The aim o his s udy was o assess he alidi y o u ilising he muscula
oxygen sa u a ion isualisa ion me hodology o he iden i ica ion o he MFO poin in ained cyclis s. Twen y‐ wo ec ea ional
endu ance‐ ained cyclis s (19 men and 3 women; age: 27.9 �5.4 yea s; body mass: 69.7 �7.1 kg and VO
2max
: 60.3 �7.0 mL/kg/
min) pe o med a submaximal and maximal exhaus ion es . All he da a we e collec ed on a single day. The alidi y o he
isualisa ion me hodology o he maximal a oxida ion poin was analysed agains a gas analyse . The de ec ion o maximal a
oxida ion (MFO) using he me hodology and de ice employed does no appea o accu a ely speci y he p ecise poin a which
MFO occu s (bias =90 �218 s and LOA =429 s). Howe e , ou esul s indica e ha i may be a alid echnique o iden i ying
he MFO zone; biases we e HR =4.7 �11.9 bpm, VO
2
=1.49 �5.7 mL/kg/min and powe =19.5 �31.2 W, whe eas he
conco dance coe icien s we e 0.783, 0.243 and 0.170, espec i ely. I is no possible o de ec MFO using NIRS de ice. Howe e ,
i is possible o de ec a gene al zone in which MFO occu s.
1
|
In oduc ion
T aining zones o in ensi y domains a e hemos used pa ame e s
o design exe cise aining aimed a pe o mance and heal h. To
de e mine he in ensi ies associa ed wi h each zone, di e en
echniques a e used. In mos cases, hey a e de e mined om
physiological changes in o med by lac a e concen a ion mea-
su emen , dynamics in espi a o y ou comes, hea a e beha -
iou and mos ecen ly, by assessing muscle oxygen sa u a ion
(Co in e al. 2007; Palla és e al. 2016; Kei e al. 2021; Send a‐
Pé ez e al. 2023). The accu a e de ec ion o hese changes is an
impo an ac o in unde s anding cyclis 's pe o mance and
This is an open access a icle unde he e ms o he C ea i e Commons A ibu ion License, which pe mi s use, dis ibu ion and ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly
ci ed.
© 2025 The Au ho (s). Eu opean Jou nal o Spo Science published by Wiley‐VCH GmbH on behal o Eu opean College o Spo Science.
Eu opean Jou nal o Spo Science, 2025; 25:e70025 1 o 9
h ps://doi.o g/10.1002/ejsc.70025
p og amming e ec i e and indi idualised aining. Fo example,
lac a e h eshold, in conjunc ion wi h maximal oxygen up ake
(VO
2max
) and e iciency, a e conside ed he main de e minan s o
cycling pe o mance (Joyne and Coyle 2008). Howe e , he
iden i ica ion o hese physiological a iables ypically equi es
cos ly equipmen , including de ices such as gas analyse s. These
p ocedu es a e beyond he scope o easonable applica ion o
mos coaches and cyclis s. In his ega d, e alua ion me hodolo-
gies applicable on a daily basis, such as hea a e, hea a e
a iabili y (Roge s e al. 2021; Ma eo‐Ma ch e al. 2022) o nea ‐
in a ed spec oscopy (NIRS) ( an de Zwaa d e al. 2016; Cayo
e al. 2021), ha e been p oposed o he de ec ion o bioma ke s
such as lac a e h eshold (Fa zam e al. 2018) and VO
2max
(Okushima e al. 2016; Feldmann e al. 2022). NIRS echnology
p o ides a nonin asi e measu emen o he balance be ween
oxygen deli e y and consump ion in ac i e issue (Fe a i
e al. 2011) and he eby, p o ides an in e es ing and p ac ical
echnique o de ec c i ical exe cise h esholds di ec ly in he
exe cising muscle ( an de Zwaa d e al. 2016). Despi e he
exis ing s udies, he e is s ill a need o u he esea ch o
es ablish associa ions be ween hese a iables and NIRS
echnology.
Me abolic ou comes, such as subs a e u ilisa ion, explain he
bioene ge ics o he wo king muscle and a e hea ily co ela ed
wi h he abo emen ioned physiological de e minan s. In his
ega d, i is well know ha ca bohyd a e (CHO) and a (FAT)
oxida ion impac on g oss e iciency and de e mines a igue and
du abili y (Ha g ea es and Sp ie 2020; MacDougall e al. 2022).
The assessmen o a oxida ion a es (FATox) se es as an in-
di ec measu e o an indi idual's mi ochond ial e iciency and
oxida i e capaci y unde exe cise s ess, which a ies ac oss
di e en demog aphic g oups (San‐Millán and B ooks 2018). In
his sense, an endu ance‐ ained popula ion will show g ea e
FATox capaci y and lac a e clea ance a highe absolu e exe cise
in ensi y, meaning a highe capaci y o p oduce ene gy h ough
oxida ion, when compa ed o seden a y people o pa ien s wi h
me abolic synd ome (San‐Millán and B ooks 2018). The e o e,
measu ing and moni o ing FATox as a ma ke o o al oxida i e
capaci y hin s he exe cising capaci y o a subjec . Such mea-
su emen s a e in eg al o de eloping ailo ed aining and
nu i ional s a egies aimed a op imising FATox o enhance
exe cise pe o mance and heal h ou comes. In ac , he a e o
maximal a oxida ion (MFO) and he speci ic in ensi y ange a
which i occu s a e c ucial o e ec i e weigh managemen ,
wi h implica ions o ca dio ascula and me abolic heal h
(Wang e al. 2015; Dandanell, Bosle P aes e al. 2017; Dan-
danell, Hus ed e al. 2017; Chá ez‐Gue a a e al. 2020), and hey
also co ela e wi h imp o ed endu ance pe o mance (F andsen
e al. 2017). T adi ionally, MFO is quan i ied using indi ec
calo ime y, a gold‐s anda d me hod o e alua ing me abolic
e iciency (Pu dom e al. 2018; Ama o‐Gahe e e al. 2019).
Howe e , he high day‐ o‐day a iabili y in MFO necessi a es
epea ed measu es o accu a e de e mina ion, which poses
challenges o i s p ac ical applica ion as a me abolic bioma ke
in clinical and a hle ic se ings (Ch zanowski‐Smi h e al. 2020).
Recen ad ances in me abolic es ing may p o ide mo e acces-
sible and eliable means o assessing MFO, sugges ing a need o
ongoing esea ch o e ine hese me hodologies o b oade use.
T aining in ensi y has been iden i ied as one o he mos
impo an ac o s in de e mining subs a e oxida ion, ha is,
MFO (Pu dom e al. 2018). NIRS has been shown o consis en ly
e lec change in exe cise in ensi y and shi s in exe cise in-
ensi y domains (Ki by e al. 2021). Speci ically, i has been
shown ha issue deoxygena ion is a unc ion o exe cise wo k
a e (Chuang e al. 2002) simila o he indings o MFO
(Thompson e al. 2006). In addi ion, NIRS de i ed exe cise
h esholds ag ee wi h s anda d exe cise h esholds indica e
shi ing me abolism such as he lac a e h eshold (G assi
e al. 1999) and en ila o y h eshold o espi a o y compensa-
ion poin (Yoge e al. 2022). I has been obse ed ha exe cise
in ensi y gene a es a clea pa e n o subs a e u ilisa ion as i
inc eases (Pu dom e al. 2018). Exe cise h esholds help iden i y
he c osso e poin whe e subs a e u ilisa ion ansi ions be-
ween a oxida ion and ca bohyd a e oxida ion (Bo el
e al. 2015; Pu dom e al. 2018). The gene al mechanism by
which NIRS could be use ul in de e mining subs a e u ilisa ion,
such as he c osso e poin o MFO, lies in he ac ha
oxygena ion measu ed by NIRS, in e ms o he balance be ween
oxygen supply and demand, is igh ly coupled o me abolic a e
(Bello i e al. 2013; Aze edo e al. 2020). The capabili y o
sus ain a ne ze o change (i.e., a a e o change equal o ze o)
implies insigh s in o me abolic a e conce ning subs a e
oxida ion e sus oxygen supply. As ATP and me abolic a e
inc ease, his a e becomes nega i e as oxygen demand exceeds
supply (Boye e and Manna 2022). Al hough a oxida ion has a
highe o e all ATP yield, he limi ed capaci y pe uni o ime o
gene a e ATP d i es he need o ca bohyd a e u ilisa ion
(Sp ie 2014; Pu dom e al. 2018). The e o e, as he me abolic
a e inc eases, a oxida ion is subs i u ed in a ou o ca bo-
hyd a e oxida ion (Maunde e al. 2018). De ec ing his bio-
ene ge ic shi acco ding o ex e nal in ensi y (pace, powe , e c.)
in a hle es o pa ien s ep esen s an oppo uni y o imp o e
hei pe o mance and me abolic heal h.
Measu ing muscle oxygen sa u a ion (SmO
2
) is a nonin asi e
me hodology ha can de ec changes in he balance be ween
oxygen deli e y and oxygen consump ion in he muscles du ing
exe cise (Vasquez‐Bonilla e al. 2021). As a esul , many s udies
ha e used NIRS o de ec di e en physiological b eakpoin s
(Rod igo‐Ca anza e al. 2021; Salas‐Mon o o e al. 2022). Fa -
zam e al. (2018) y o de ec lac a e h eshold using NIRS
echnology. They showed ha he Humon (Humon Be a,
Dynome ics Inc.) de ice is alid o de ec ing lac a e h eshold
powe (when a hle e’s lac a e blood concen a ion eaches
4 mmol/L) wi h a di e ence o �21.4 W and less han 3 min
compa ed o he in asi e me hod. In ano he s udy, he au ho s
Highligh s
�The de ec ion o he maximal a oxida ion poin using
muscle oxygen sa u a ion plo ed as a unc ion o ime
does no appea o accu a ely speci y he p ecise poin a
which maximal a oxida ion occu s.
�I is possible o de ec a gene al zone in which he
maximal a oxida ion occu s.
�Nea ‐in a ed spec oscopy echnology is use ul o
coaches wi hou ime o do complica ed analyses.
2 o 9 Eu opean Jou nal o Spo Science, 2025
15367290, 2025, 8, Downloaded om h ps://onlinelib a y.wiley.com/doi/10.1002/ejsc.70025 by Readcube (Lab i a Inc.), Wiley Online Lib a y on [22/07/2025]. See he Te ms and Condi ions (h ps://onlinelib a y.wiley.com/ e ms-and-condi ions) on Wiley Online Lib a y o ules o use; OA a icles a e go e ned by he applicable C ea i e Commons License
compa ed maximal lac a e s eady s a e (MLSS) wi h de ec ion
using NIRS echnology and concluded ha MLSS can be accu-
a ely de ec ed using NIRS echnology (Bello i e al. 2013).
Zu buchen e al. (2020) ound ha highe MFO alues a e
associa ed wi h a slowe slope o a e o muscle deoxygena ion
indica ing be e ma ching o O
2
deli e y and consump ion,
alongside a highe o e all muscle deoxygena ion and igh wa d
shi o a deoxygena ion b eakpoin . A g ea e a e o muscle
deoxygena ion is associa ed wi h a highe VO
2max
(Okushima
e al. 2016), u he unde pinning a po en ial applica ion o
NIRS o assess MFO.
The p ima y aim o his s udy was o assess he alidi y o u i-
lising he SmO
2
isualisa ion me hodology o he iden i ica ion
o he MFO poin in ained cyclis s. I was hypo hesised ha
he MFO poin could be de e mined by obse ing changes in
SmO
2
o e ime du ing a labo a o y‐based cycling es .
2
|
Ma e ial and Me hods
2.1
|
Pa icipan s
Twen y wo ec ea ional cyclis s (19 men and 3 women; age:
27.9 �5.4 yea s; VO
2max
: 60.3 �6.99 mL/kg/min; weigh :
69.65 �7.07 kg and 8 ∑Folds: 266.3 mm) pa icipa ed in his
s udy. All pa icipan s had mo e han 2 yea s o expe ience in
endu ance spo s and pe o med mo e han 10 h pe week o
endu ance aining. This s udy was app o ed by he Ins i u-
ional Resea ch E hics Commi ee (Cod. CEISH‐113/2019), and
all pa icipan s p o ided w i en in o med consen , as ou lined
in he Decla a ion o Helsinki (Holm 2013).
2.2
|
P ocedu es
All he da a we e collec ed on a single day in he labo a o y. The
pa icipan s ollowed hei usual die he days p io o he es .
48 h p io o he assessmen , he pa icipan s did no pe o m
mode a e o high in ensi y aining and he 24 h p io he
pa icipan s only pe o med low‐in ensi y aining o es ( hey
e ained om s eng h aining). Fou hou s be o e he s a o
he es , he pa icipan s did no inges any calo ic o s imulan
ood o d inks. Be o e he es session, all pa icipan s pe o med
a s anda dised wa m‐up p o ocol consis ing o 10 min a 50–75
w pedalling a a cadence o 80–90 pm.
2.2.1
|
MFO and VO
2max
Tes
Each pa icipan pe o med he es wi h hei pe sonal bike
ins alled on a s a iona y Tacx‐T aine de ice (Tacx Neo Sma
T2800, Wassenaa , Ne he lands) and associa ed so wa e (Tacx
T aine so wa e 4, Wassenaa , Ne he lands). The de ice was
calib a ed be o e each es as sugges ed by he manu ac u e .
A e he wa m‐up, pa icipan s pe o med an inc emen al es
s a ing a 75 w ( o women) and 100 w ( o men) wi h in-
c emen s o 25 w ( o women) and 35 w ( o men) e e y 4 min.
When he espi a o y exchange a io (RER) alue eached 1.0,
he pa icipan s s opped pedalling and es ed o 15 min. Du ing
he es pe iod, he pa icipan s consumed a d ink con aining
35 g CHO (Ca bolide , Fullgas Spo , As iga aga, Spain). Sub-
sequen ly, o he de ec ion o VO
2max
, pa icipan s pe o med
an inc emen al es o exhaus ion s a ing a he powe a which
RER =1.0 was eached, wi h in ensi y inc emen s o 25–35w/
min o exhaus ion. Du ing he es , he subjec s we e ins uc ed
o pedal a a cons an cadence be ween 80 pm and 90 pm. The
c i e ia sugges ed by Howley e al. (1995) we e used o con i m
VO
2max
. The hea a e was eco ded using Pola H10 ches s ap
(Pola Elec o Oy, Kempele, Finland).
2.3
|
Measu emen s
2.3.1
|
An h opome ic Measu emen s
Be o e he physical es , all pa icipan s unde wen an an h o-
pome ic assessmen . Body mass (kg), skin olds (mm), body
pe ime e s (cm) and body diame e s (cm) we e measu ed in
each pa icipan . Body mass was ob ained using an elec onic
scale (Seca Ins umen s L d., Hambu g, Ge many), and he six
skin olds ( iceps, subscapula , sup ailiac and abdominal) we e
measu ed wi h a callipe (Ha pende Lange, Camb idge, MA,
USA) ollowing he indica ions speci ied by Socie y o he
Ad ancemen o Kinan h opome y (ISAK). The sum o he six
olds was also calcula ed (∑ old).
2.3.2
|
Me abolic Analysis
Gas exchange was measu ed con inuously in he MFO and
VO
2max
es using an E goca d b ea h‐by‐b ea h gas analyse
(E goca d, Mediso , So innes, Belgium). B ea h‐by‐b ea h e-
co dings o VO
2
and VCO
2
we e ob ained h oughou he es ,
calib a ed o oom humidi y, low and O
2
/CO
2
concen a ion
p io o each es . The gas analyse s we e calib a ed using a
4.95% CO
2
–95.05% N
2
gas mix u e. P io o he calcula ion o
subs a e oxida ion, a 30 s olling a e age was applied o he
b ea h‐by‐b ea h VO
2
and VCO
2
da a o educe a iabili y and
imp o e in e p e abili y o he me abolic esponses. Hea a e
(HR) wi h a p os ap (Pola H10, Pola Elec o Oy, Kempele,
Finland) was eco ded h oughou he es . A he end o each
s ep, pa icipan s we e asked o he a io o pe cei ed exe ion
using he CR‐10 RPE scale, di e en ia ing espi a o y (RPE es)
and muscula (RPEmus) exe ion (I u icas illo e al. 2016).
2.3.3
|
Muscle Oxygen
The NIRS de ice was placed in he dominan as us la e alis
(VL), and SmO
2
(Moxy, Fo io i Design LLC, Minneapolis, MN,
USA) was measu ed du ing he es . The NIRS de ice was
placed a wo‐ hi ds be ween an e io supe io iliac spine and
he la e al side o he pa ella. The senso s we e ixed in place
using medical adhesi e ape and co e ed wi h a compa ible,
comme cially a ailable ligh shield o elimina e possible
ambien ligh in usion. The sampling a e was se a 2 Hz,
which sampled he ou wa eleng hs o e 20 cycles o an
a e aged ou pu e e y 0.5 s. An emi e ‐de ec o spacing o
25 mm p o ides a pene a ion dep h o 12.5 m. The issue
3 o 9
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hickness o he subjec s a he measu emen si e was
11.6 �4.2 mm.
2.4
|
Da a Analysis
The NIRS de ice and gas analyse we e synch onised and
con inuously collec ed da a h oughou he du a ion o he
p o ocols. The NIRS de ice was connec ed o a sma wa ch
(Ga min Fo e unne 935), om which SmO
2
da a we e ex ac-
ed om he o iginal ‘ i ’ ile eco ded by he sma wa ch. The
MFO poin was iden i ied using a gas analyse by obse ing he
dynamics o ca bohyd a e (CHOox) and a (FATox) oxida ion
o e ime. CHOox and FATox da a we e calcula ed in g ams
using he same equa ion employed by (San‐Millán and
B ooks 2018).
FATox (g·min−1)=1.67VO2(L·min−1)−1.67VCO2(L·min−1)
CHOox (g·min−1)=4.55VO2(L·min−1)– 3.21VCO2(L·min−1)
SmO
2
and subs a e oxida ion we e plo ed as a unc ion o ime
wi h a 30 s olling mean o isual de ec ion o he MFO poin .
The me hod o de ec he b eakpoin in SmO
2
elied on isually
iden i ying he wo kload ha led o a change he linea i y in
SmO
2
. The exe cise in ensi y a which he highes a oxida ion
a io was obse ed was es ablished as MFO (Ach en e al. 2002).
The MFO poin alues we e indi idually e i ied by wo e-
sea che s, wi h any disc epancies esol ed by consensus. The
me hod o de ec ing he MFO poin was blinded.
Also, segmen ed eg ession analysis using a ou h il e o de
was pe o med. The ime SmO
2
MFO poin de ec ed by isual
analysis was used as he s a ing poin o he segmen ed
eg ession. The in ensi y a he MFO poin de ec ed was indi-
ca ed in e ms o powe ou pu (PO), VO
2
and HR. The alue o
each a iable was calcula ed as he mean o 30 s, 15 s be o e and
a e he poin a which he MFO was de ec ed using each
me hodology.
2.5
|
S a is ical Analyses
Da a we e analysed using s a is ical en i onmen R (RS udio
Inc., Bos on, MA). Desc ip i e alues we e p esen ed as mean
and s anda d de ia ions (SDs). The alidi y o he isual SmO
2
me hod and segmen ed eg ession me hod was e alua ed using
Lin’s conco dance coe icien (LCC), mean absolu e pe cen
e o (MAPE) and ag eemen wi h he e e ence me hod (gas
analyse ). Ag eemen plo s wi h 95% limi s o ag eemen (LoA)
(mean di e ence �1.96 SD) we e gene a ed o isualise he
deg ee o ag eemen be ween each me hod and he e e ence
me hod (K ouwe 2008). He e oscedas ici y/homoscedas ici y
was assessed by calcula ing he Pea son co ela ion coe icien
be ween absolu e di e ences and e e ence alues. A me hod
was conside ed alid i he MAPE was less han 10% and he
LCC was g ea e han 0.7. This h eshold o MAPE (<10%) has
been commonly used in p e ious alida ion s udies in ol ing
physiological da a and wea able de ices and ep esen s an
accep able le el o ag eemen be ween me hods (BOUDREAUX
e al. 2018; Johns on e al. 2021; Ca ie e al. 2025). Signi icance
was se a p<0.05. The co ela ion was de ined as ollows:
negligible (0.00–0.30), weak (0.30–0.50), mode a e (0.50–0.70),
s ong (0.70–0.90) o e y s ong (0.90–1.00) (Mukaka 2012).
3
|
Resul s
Figu e 1illus a es he ag eemen be ween he e e ence
me hod and NIRS de ec ion. A deg ee o bias was obse ed
when compa ing ime poin s om each me hod
(bias =90 �218 s and LOA =429 s). Addi ionally, low
conco dance and high MAPE we e obse ed (LCC =0.23 and
MAPE =58.6%). Howe e , he bias o in ensi y alues was
HR =4.7 �11.9 bpm, VO
2
=1.49 �5.7 mL/kg/min and
PO =19.5 �31.2 W, whe eas he conco dance coe icien s we e
0.783, 0.243 and 0.170, espec i ely. The MAPE was 8.6% o
HR, 15.9% o VO
2
and 22.6% o PO. No he e oscedas ici y o
da a we e obse ed ( ime: = −0.53; HR: =0.15 and VO
2
:
= −0.44), which can be isually e alua ed in Figu e 1.
Table 1p esen s he ag eemen be ween he e e ence me hod
and segmen ed eg ession analysis. As can be obse ed, powe ,
VO
2
and ime we e no a alid due o high MAPE om 17.16%
o 61%. Howe e , HR was simila be ween bo h me hods. No
he e oscedas ici y was obse ed ( ime: = −0.79; HR: =0.17;
VO
2
: = −0.36 and PO: = −0.79).
4
|
Discussion
The aim o his s udy was o e alua e he alidi y o he SmO
2
isualisa ion me hodology o de ec ing he MFO poin in ained
cyclis s. Based on ou esul s, he de ec ion o MFO using he
me hodology and de ice employed does no appea o accu a ely
speci y he p ecise poin a which MFO occu s. Howe e , ou
esul s indica e ha i may be a alid echnique o iden i ying he
ange o exe cise in ensi y whe e MFO happens.
I is well‐es ablished ha exe cise es p o ocols play a c ucial
ole in he de ec ion o h esholds (Jamnick e al. 2018; Kou-
wijze e al. 2019). In ac , exe cise es p o ocols la gely de e -
mine exe cise h esholds and esul s, p esen ing a undamen al
challenge when a emp ing o b idge he gap be ween aining
and es ing. I has been documen ed ha sho s age o amp
p o ocols las ing no longe han 12 min a e ecommended o
achie ing a alid VO
2max
(Buch uh e e al. 1983), whe eas
longe s age es s a e used o de e mine lac a e h eshold and
s eady‐s a e esponses o bioene ge ics (Ben ley e al. 2007). The
du a ion and in ensi y o exe cise dic a e physiological e-
sponses, wi h measu able pa ame e s eac ing in a cascading
se ies o e en s (Bu on e al. 2004). Gi en he p oximi y o
exe cise h esholds esul ing om di e en physiological me -
ics, a complex in e ac ion o bioene ge ic pa hways is a play. I
we conside lac a e h eshold as a s anda d in exe cise h eshold
es ing, when compa ing he i s lac a e h eshold wi h MFO,
da a does no ully suppo hei in e changeabili y, e en hough
bo h h esholds ha e o en been used synonymously (Fe i
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Ma ini e al. 2022). This is despi e he clea in e ac ion be ween
lac a e and a oxida ion, wi h lac a e ac ing o ei he educe he
ci cula ion o ee a acids h ough he ac i a ion o HCAR‐1 o
educe be a oxida ion h ough down egula ion o β‐ke o hiolase
(B ooks 2020). The imp ecision wi h which exe cise es ed
h esholds a e de i ed, esul ing om highly sc ip ed exe cise
es p o ocols when compa ed o hei expec ed physiological
dependency, aises signi ican ques ions. Simila ques ions can
be aised when applying new echnologies, such as muscle
oxygena ion, de i ed om NIRS.
In he s udy, i is obse ed ha he de ec ion o he speci ic MFO
poin using NIRS does no coincide wi h he MFO poin
de ec ed h ough gas analysis (bias 42.07 �230.17 s). These
esul s a e consis en wi h s udies ha ha e de ec ed he onse
o blood lac a e accumula ion (OBLA) lac a e h eshold using
NIRS (Fa zam e al. 2018). In hei s udy, a 2 min and 32 s ime
di e ence was ob ained be ween es ima ions o blood lac a e
h eshold and NIRS echnology. Howe e , hese au ho s sug-
ges ed ha when conside ing powe da a, he e was a bias o
21.4 W in he de ec ion o he OBLA lac a e h eshold. In line
wi h a p e ious s udy, Ba e son e al. (2023) obse ed in a
sample o 10 eli e socce playe s ha he e we e no signi ican
di e ences be ween SmO
2
b eakpoin s and maximum de ia ion
(D
max
) lac a e h esholds 1 and 2. They sugges ed ha he i s
b eakpoin appea ed be ween he i s and second D
max
lac a e
h esholds. In his ega d, ou s udy ollows simila esul s.
Al hough he ime da a di e , i we compa e he measu ed in-
ensi y wi h HR and VO2 a which he change occu s, we ind
ha NIRS could de ec he ange o in ensi y a which MFO
occu s (HR bias: 1.22 �11.52 bpm and VO
2
bias:
0.52 �5.87 mL/kg/min). Ou objec i e has no been s udied in
he li e a u e, which makes i di icul o compa e wi h o he
s udies. Howe e , conside ing o he a emp s o de ec h esh-
olds, as in he p e ious s udy (Bello i e al. 2013), i was also
obse ed ha he maximal lac a e s eady s a e could be de ec ed
FIGURE 1
|Ag eemen plo s be ween Moxy de ice and e e ence me hod. Bias: mean bias be ween he Moxy and he e e ence me hod o e e y
pa icipan . Dashed ho izon al lines: limi s o ag eemen be ween he Moxy and he gas analyse MFO me hod. MFO: maximal a oxida ion.
TABLE 1 |Ag eemen be ween he e e ence me hod agains segmen ed eg ession analysis.
Va iable Bias Bias SD LOA LW LOA UP LOA LCC LW LCC UP LCC MAPE (%)
Time(s) 42.07 230.17 451.13 −409.06 493.19 −0.09 −0.45 0.30 61.53
HR (bpm) 1.22 11.52 22.57 −21.35 23.79 0.78 0.56 0.90 7.91
VO
2
(mL/kg/min) 0.52 5.87 11.51 −10.99 12.02 0.29 −0.08 0.60 17.16
Powe (w) 7.29 30.07 58.94 −51.65 66.23 0.24 −0.16 0.56 18.70
No e: Bias: mean bias be ween he Moxy and he Fa Max me hod o e e y pa icipan .
Abb e ia ions: LCC, Lin’s conco dance coe icien ; LOA, limi s o ag eemen ; LW LCC, lowe Lin’s conco dance coe icien ; LW LOA, low limi s o ag eemen ; MAPE,
mean absolu e pe cen e o ; SDs, s anda d de ia ions; UP LCC, uppe Lin’s conco dance coe icien ; UP LOA, uppe limi s o ag eemen .
5 o 9
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using NIRS echnology, as hey ound high co ela ions in HR
and VO
2
ma ke s de ec ed using NIRS and MLSS. The e o e, he
ini ial hypo hesis is no ully ul illed, as i is no possible o
de ec MFO poin by isualising he dynamics o SmO
2
o e
ime. The e o e, al hough he p ecise localisa ion o he MFO
poin was no possible using he applied NIRS me hodology, he
iden i ica ion o a gene al exe cise in ensi y zone whe e a
oxida ion is ele a ed may s ill hold some p ac ical alue. How-
e e , i s applica ion should be app oached wi h cau ion. As
shown by Ach en and Jeukend up (2003), e en small changes in
exe cise in ensi y can cause subs an ial al e a ions in a
oxida ion a es, complica ing he accu a e a ge ing o his
me abolic zone. Fu he mo e, ecen wo k has demons a ed
ha he in aindi idual a iabili y in Fa Max de e mina ion can
be la ge—e en unde con olled condi ions— hus ques ioning
i s eliabili y o indi idual exe cise p esc ip ion (Meye
e al. 2009). Combined wi h he limi ed ep oducibili y o NIRS‐
based me hods, his u he suppo s he need o in e p e he
NIRS‐de i ed MFO zone as an app oxima e indica o a he
han a p ecise p esc ip ion ool. Fu u e esea ch should aim o
e ine hese ools and es hei u ili y in applied con ex s wi h
igo ous con ol o e in luencing ac o s.
Al hough he use o isual inspec ion may limi objec i i y and
ep oducibili y, i e lec s he p ac ical app oach commonly
used by coaches and p ac i ione s when in e p e ing da a om
wea able de ices. Ou a ionale was o es he easibili y o
iden i ying he MFO zone wi h minimal da a p ocessing as
would be he case in applied se ings. Ne e heless, o add ess
he conce n ega ding subjec i i y, we also conduc ed
segmen ed eg ession analysis as p esen ed in Table 1. Al hough
he ag eemen wi h he e e ence me hod emained limi ed, his
compa ison highligh s he po en ial o in eg a ing mo e objec-
i e compu a ional me hods in u u e applica ions.
Iden i y MFO is no ewo hy due o i s implica ions in he
con ex o weigh managemen (Dandanell, Bosle P aes
e al. 2017; Dandanell, Hus ed e al. 2017), me abolic heal h and
a hle ic pe o mance (Wang e al. 2015; Chá ez‐Gue a a
e al. 2020). As such, i may se e as a aluable ma ke o
measu emen . T adi ionally, MFO has been de ec ed ia indi-
ec calo ime y (Pu dom e al. 2018; Ama o‐Gahe e e al. 2019),
a me hod ha is bo h cos ly and complex. Fu he mo e, i is
subjec o limi a ions, such as p o ocol dependence (Ama o‐
Gahe e e al. 2019) and high daily a iabili y (Ch zanowski‐
Smi h e al. 2020), ende ing i inapplicable o ou ine use by
a hle es’ popula ion. As i is well‐es ablished, MFO is in luenced
by se e al ac o s including die and physical exe cise (Maunde
e al. 2018). Consequen ly, changes in die o egula endu ance
exe cise can be expec ed o esul in imp o emen s in MFO
(Maunde e al. 2018) leading o equen misin e p e a ion o
he da a. In his ega d, i may be o g ea impo ance o pe i-
odically e alua e MFO in o de o adjus aining in ensi y and
ensu e ha a su icien aining s imulus and adap a ions a e
main ained. The indings o his s udy indica e ha i is easible
o de ec he in ensi y ange a which MFO happens u ilising
NIRS, educing he cos o measu emen compa ed o he
adi ional me hod and making pe iodic assessmen easible o
ca y ou . Gi en he ela i ely sho du a ion o he p o ocol
(app oxima ely 10 min o wa m‐up and 16–24 min o es ing),
i may be easible o e alua e MFO mo e equen ly du ing
aining phases whe e he objec i e is o imp o e MFO. This
would enable mo e equen adjus men s o aining in ensi y
(e.g., on a weekly basis) and nu i ional in e en ions.
5
|
Limi a ions
The use o NIRS o assess muscle oxygena ion is subjec o
inhe en limi a ions ha mus be conside ed when add essing
p ac ical ques ions. This is e iden in he p esen s udy and
sugges s ha u he esea ch is necessa y. The selec ion o
muscle si e and muscle he e ogenei y a e conce ns when
assessing a global esponse using a single NIRS p obe (Koga
e al. 2007; Okushima e al. 2015). The disco dance be ween
global exe cise h esholds and local h esholds has been well‐
documen ed in ecen yea s and mus be accoun ed o
(Okushima e al. 2015; Caen and Boone 2023). Al hough lac a e
measu emen s we e no included in his s udy, as he p ima y
objec i e was o de ec he MFO, indi ec calo ime y se es as a
mo e di ec means o assessmen (Bello i e al. 2013; Ba e son
e al. 2023). Addi ionally, al hough accep able il e ing and
modelling p inciples ha e been p oposed o assessing SmO
2
b eakpoin s, which should ela e o measu es, such as MFO,
hese p inciples ha e no been s anda dised. The app op ia e
NIRS pa ame e s and calib a ion me hods emain o be de e -
mined. The measu emen p o ocol employed in his s udy was
conduc ed in a labo a o y se ing, which may limi i s applica-
bili y o home use. I would be o in e es o in es iga e he
po en ial o using a ield p o ocol o de ec he MFO zone
u ilising NIRS echnology. Fu he mo e, NIRS is o en
compa ed o al e na i e me ics ha hemsel es ca y inhe en
limi a ions, esul ing in a compa ison be ween di e en
app oxima ing sys ems. Ano he impo an conside a ion is he
du a ion o he as ing pe iod p io o es ing. Al hough pa ic-
ipan s e ained om calo ic o s imulan in ake o ou hou s
be o e he assessmen , i is possible ha a longe as ing window
(e.g., six hou s) may ha e p o ided g ea e con ol o e baseline
me abolic condi ions such as blood glucose and iglyce ide
le els. Howe e , he selec ed du a ion was chosen o e lec
common p ac ice in exe cise es ing p o ocols and o balance
me abolic con ol wi h pa icipan com o and adhe ence.
Mo eo e , he use o isual inspec ion o iden i y he SmO
2
b eakpoin , al hough e lec ing eal‐wo ld applica ion, in-
oduces a deg ee o subjec i i y ha may limi ep oducibili y
compa ed o compu a ional app oaches. Fu u e s udies may
conside ex ending he as ing du a ion o minimise po en ial
con ounding e ec s on subs a e oxida ion. Finally, al hough
ou p o ocol aimed o e lec a ealis ic es ing en i onmen
wi hin a single labo a o y isi , he absence o epea ed es ing
sessions limi s he assessmen o in aindi idual a iabili y.
Gi en he known day‐ o‐day luc ua ions in a oxida ion a es,
u u e s udies should inco po a e mul iday es ing designs o
be e accoun o a iabili y and enhance he obus ness o
NIRS‐based MFO zone de ec ion.
We would like o highligh ha , despi e all he limi a ions we
ha e men ioned and o he s ha may be conside ed, he wo k
we p esen is inno a i e and p oposes a new di ec ion o u u e
esea ch.
6 o 9 Eu opean Jou nal o Spo Science, 2025
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6
|
Conclusion
In conclusion, i is no possible o de ec he MFO poin using
he NIRS de ice wi h he p o ocols and me hods employed in
his s udy. Disag eemen s we e obse ed be ween he me hods.
Howe e , i is possible o de ec a gene al zone in which he
MFO occu s. As such, coaches may use ou p oposed me hod o
assess he MFO zone o he i s ime. Based on his gene al
idea, a speci ic es should be conduc ed o assess he MFO.
Fu he esea ch is ecommended o iden i y app op ia e p o-
ocols ha can enable he de ec ion o he MFO poin and o
ex apola e o he ield.
Con lic s o In e es
D . Feldmann is a mino i y sha eholde o , and p o essional con ibu o
o Fo io i Designs LLC, he company ha manu ac u es and dis ibu es
he NIRS de ice used in his s udy.
Da a A ailabili y S a emen
The da ase s gene a ed and analysed du ing he cu en s udy a e
a ailable om he co esponding au ho on easonable eques .
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