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P h y s io lo g i ca l e t e r m i n a n t s o f E n d u r a n c e E x e rc is e

P e r f o r m a n c e

Edw ar d F Coyle

Human Per for mance Labor a tor y

De pa r tm en t of Kinesiology and H eal th Educat ion

The University of Texas Austin USA

C oy le , E .F . 1999 ). P hys io log ic a l de t e r m ina n t s o f e ndu r a nc e e xe r c i s e pe r f o r m a n c e . Journal of

Science and Medicine in Sport

3): 181-189.

P e r f o r m a nc e in e ndu r a nc e e ve n t s i s t yp i c a l l y e va lua t e d by the pow e r o r ve loc ity t ha t c a n

b e m a i n t a i n e d f o r d u r a t i o n s o f 3 0 r a i n . t o f o u r h o u r s . T h e t w o m a i n b y - p r o d u c t s o f

i n t e n s e a n d p r o l on g e d o x id a t iv e m e t a b o l i s m t h a t c a n l im i t p e r f o r m a n c e a r e t h e

a c c um ula t ion o f hyd r oge n ion i .e . l a c t ic ac idos i s) a nd h e a t i .e . hype I ~ he r m ia ). A m ode l

f o r e ndu r a n c e pe r f o r m a nc e i s p r e se n t e d t ha t r e vo lve s a r ou nd ide n t i f ic a t i on o f t he l a c t a t e

th r e sho ld ve loci ty w h ic h i s p r e se n t e d a s a f unc t ion o f num e r ou s m or pho log ic a l

c om pon e n t s a s w e l l a s g r o s s m e c ha n ic a l e f fi ci enc y. W he n c yc l ing a t 80 R P M , ro s s

m ech anic a l e f f ic iency

i s

pos i t ive ly r e l a t e d t o Type I m usc l e f i be r c om p os i t i on , w h ic h h a s

g r e a t p o t e n t i a l t o i m p ro v e e n d u r a n c e p e r f o rm a n c e . E n d u r a n c e p e r f o r m a n c e c a n a l s o b e

in f lue nc e d by a l t e r ing t he a va i l a b i l it y o f oxyge n a nd b lood g luc ose du r ing e xe r ci s e . The

l a t t e r n e e d f o r m s t h e b a s i s f o r in g e s t in g c a r b o h y d r a t e a t 3 0 - 6 0 g r a m s p e r h o u r d u r i n g

e xe r c is e . I n l a bo r a to r y s im u la t i ons o f pe r f o r m a nc e , a th l e t e s f a ti gue due t o hyp e r the r m ia

w h e n e s o p h a g e a l is a p p r o x i m a t e l y 4 0 C, i n a s s o c i a ti o n w i t h n e a r m a x i m a l h e a r t r a t e a n d

pe r c e ive d e xe r t ion . I t i s l ike ly t ha t t he c e n t r a l ne r vo us s y s t e m i s i nvo lve d in t he a e t io logy

o f f at ig u e f r o m h y p e r t h e rm i a . D e h y d r a t i o n d u r i n g e x e rc i se p r o m o t e s h y p e l ~ l e r m i a b y

r e duc ing sk in b lood fl ow , sw e a t ing r a t e a nd thu s he a t d i s s ipa t ion . The c om bina t ion o f

d e h y d r a t i o n a n d h y p e r t h e r m i a d u r i n g e x e r c i se c a u s e s l a r g e r e d u c ti o n s i n c a r d i a c o u t p u t

a nd b lood fl ow to t he e xe r c i s ing m u sc u la tu r e , a n d thu s h a s a l a r ge po t e n t i a l t o im pa i r

e n d u r a n c e p e r fo r m a n c e . E n d u r a n c e p e r f o r m a n c e i s o p t im i z e d w h e n t r a i n i n g i s a i m e d

spe c i f ic a l l y a t de ve lop ing ind iv idua l c om pone n t s o f t he m ode l p r e se n t e d a n d nu t r i t i ona l

s u p p l e m e n t a t i o n p r e v e n t s h y p o g l y c em i a a n d a t t e n u a t e s d e h y d r a t i o n a n d h y p el C h er m ia .

I nde e d , t he c ha l l e nge a t t he t r a ns i t i on t o a ne w m i l l e nn ium i s t o syne r g i s t i c a ll y n t e g r a t e

the se phys io log i c a l f a c to r s i n t r a in ing a n d c om pe t i t ion .

n t r o d u c t i o n

E v e n b e f o r e P h e i d i p p i d e s i s a l l eg e d to h a v e r u n f r o m t h e p l a i n s o f M a r a t h o n t o

A t h e n s t o p r o c l a i m t h e G r e e k v i c t o ry ov e r t h e P e r s ia n s , m e n a n d w o m e n h a v e

a w e d a t t h e i r a b i l i t y t o e x e r c is e fo r l o n g d u r a t i o n s . T h e y h a v e a l s o b e c o m e b a f f l e d

b y t h e i r p h y s i o l o g i c a l l i m i t a t i o n s . T h e r e m a r k a b l e s c ie n t if ic a d v a n c e s o f t h e p a s t

c e n t u r y h a v e i d e n t i f ie d a p l e t h o r a o f i m p o r t a n t i n d i v i d u a l p h y s i o l o g i c a l f a c to r s

t h a t r e l a te t o a t h l e t i c p e r f o r m a n c e . T h e g o a l o f t h e p r e s e n t r e v i ew , w r i t t e n a t t h e

t r a n s i t i o n t o a n e w m i l l e n n i u m , i s to d i s c u s s o u r c u r r e n t u n d e r s t a n d i n g o f h o w

n u m e r o u s p h y s i o l o g i c a l f a c t o r s a n d s y s t e m s i n t e r a c t i n e s t a b l i s h i n g th e l i m i ts o f

h u m a n e n d u r a n c e p e r f o r m a n c e a b il it y .

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eterminants of Endurance Performance

-Overv iew and De f in i t ions

T h i s r e v ie w w i ll f o c u s o n d u r a t i o n s o f e x e r c i se t h a t c a n b e e n d u r e d f o r 3 0 r a i n . t o

f o u r h o u r s . P e r f o r m a n c e c a n b e e v a l u a t ed b y t h e a m o u n t o f t im e r e q u i r e d t o

c o m p l e t e a g i v e n a m o u n t o f w o r k (i.e. p o w er ) o r b y t h e l e n g t h o f t i m e t h a t a g i v e n

p o w e r o u t p u t c a n b e m a i n t a i n e d (i.e . c a p a c it y ). F ig u r e 1 p r e s e n t s a s i m p l e b u t

u s e f u l s c h e m e o f t h e e n e r g e t ic s o f e x e rc i se u n d e r t h e s e c o n d i ti o n s a n d t h e m a j o r

s i t e s o f f a t i g u e ( Co y le , 1 9 9 5 ) . A l m o s t a l l o f t h e en e r g y ( i.e . A T P ) u t i l i z ed u n d e r

a e r o b i c c o n d i t i o n s w ill b e r e s y n t h e s i z e d i n t h e m i t o c h o n d r i a v i a o x id a t iv e

m e t a b o l i s m o f c a r b o h y d r a t e a n d f a t. T h e r e f o r e , p e r f o r m a n c e c a n b e d i r e c t l y

i n f l u e n c e d b y a l t e r i n g a v ai la b i li ty o f o x y g e n , c a r b o h y d r a t e a n d f a t. F u r t h e r m o r e

c h a n g e s i n m u s c l e m i t o c h o n d r i a l d e n s it y , a s o c c u r s w i t h e n d u r a n c e - t r a i n i n g ,

s e r v e t o r e g u l a t e o x i d a t iv e m e t a b o l i s m ( H o ll o sz y , 1 9 6 7 ; H o l l o s z y & C o y l e , 1 9 8 4 ) .

T h e t w o m a i n b y - p r o d u c t s o f i n t e n s e a n d p r o l o n g e d o x id a ti ve m e t a b o l i s m t h a t

c a n l i m i t p e r f o r m a n c e a r e t h e a c c u m u l a t i o n o f h y d r o g e n i o n ( i.e . a c id o s is ) a n d

h e a t ( i.e . h y p e r t h e r m i a ) . H y d r o g e n io n a c c u m u l a t i o n i n m u s c l e , a n d t h u s

a c i d o si s , i s ty p i c a ll y r e f le c t e d b y m u s c l e a n d b l o o d l a c ti c a c i d a c c u m u l a t i o n . T h i s

c a n b e a s e ns it iv e m e a s u r e o f t h e d e g r e e to w h i c h t h e e x e rc i si n g m u s c u l a t u r e i s

s t r e s s e d r e g a r d i n g i t s a b il it y t o m a i n t a i n p r o l o n g e d a e r o b i c A T P p r o d u c t i o n . I n

p r a c t i c a l t e r m s , i d e n t i f ic a t i o n o f t h e e x e r c i s e i n t e n s i t y e l ic i ti n g t h e ' b lo o d l a c t a t e

t h r e s h o l d ' i s v e r y p r e d i ct i v e o f e n d u r a n c e p e r f o r m a n c e a b i l it y a n d i t w i ll b e t h e

c e n t r a l f o c u s o f t h e p h y s i o l o g i c a l m o d e l d e v e l o p e d b e l o w ( C o y le , 1 9 9 5 ).

H e a t i s p r o d u c e d b o t h i n t h e c h e m i c a l r e a c t i o n s n e e d e d t o r e s y n th e s i z e A T P a s

w e ll a s b y t h e p r o c e s s o f A T P h y d r o l y s i s ( K u s h m e r i c k , 1 9 83 ). I n f ac t, m o r e t h a n

7 5 o f t h e e n e r g y l i b e ra t e d i n t h e o x i d a t i o n o f c a r b o h y d r a t e a n d f a t d u r i n g

e x e r c i s e i s c o n v e r t e d t o h e a t , w i t h l e ss t h a n 2 5 b e i n g c o n v e r t e d t o a c t u a l p o w e r

a n d m o v e m e n t ( C o yle e t al ., 1 9 9 2 ). A s a r e s u l t , t h e c o n t r a c t i n g m u s c l e s i m p a r t a

h u g e h e a t l o a d t o t h e b o d y . If t h i s h e a t i s n o t t r a n s f e r r e d f r o m t h e b o d y t o t h e

e n v i r o n m e n t , l a r g e ly f r o m b l o o d f lo w i n g t o t h e s k i n t h a t i s c o o le d b y e v a p o r a t i n g

s w e a t , a p e r s o n c a n i n c r e a s e t h e i r c o r e t e m p e r a t u r e t o le t h a l le v e ls w i t h j u s t a f e w

m i n u t e s o f i n t e n s e e x e rc i se . N e e d l e s s t o sa y , h y p e r t h e r m i a h a s g r e a t p o t e n t i a l t o

l im i t e n d u r a n c e p e r f o r m a n c e a n d i m p a i r h e a l t h e s p e c ia l ly w h e n e n v i r o n m e n t a l

c o n d i t i o n s a r e h o t a n d / o r h u m i d ( Co sti ll, 1 9 7 2 ).

H i g h s w e a t i n g r a t e s (e .g .; 1 - 2 l i tr e s p e r h o u r ) d u r i n g p r o l o n g e d e x e r c i s e c a u s e

d e h y d r a t i o n f f f lu i d r e p l a c e m e n t i s n o t a d e q u a t e . I t a p p e a r s t h a t t h e m a j o r

m e c h a n i s m s b y w h i c h d e h y d r a t io n i m p a ir s e n d u r a n c e p e r f o rm a n c e a r e b y

c a u s i n g h y p e r t h e r m i a a n d i m p a i r i n g c a r d i o v a s c u l a r f u n c t i o n , w h i c h s e e m t o

2

FAT ~ ~ ~ E ~ A T P - - - ~ POWER

C A R B O H Y D R A T E ~ ~

- ~

H e a t

LacticAcid H2 D ehy dration

Figure 1: Sche meo f aerobic energy product ion and by product accumulat ion.

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eterminants of Endurance Performance

c a u s e a d d e d h o r m o n a l a n d m e t a b o l i c s t r e s s ( F e b b ra i o e t al ., 1 9 9 4 ; G o n z f i l e z -

A l o n s o e t a l. , 1 9 9 5 ; G o n z f i l e z - A l o n s o e t a l ., 1 9 9 7 ; G o n z f i l e z - A l o n s o e t a l ., 1 9 9 8 ;

G o n z ~ e z - A l o n s o e t a l., 1 9 99 ). I n a d d i t io n t o d r i n k i n g w a t e r d u r i n g p r o l o n g e d

e x e r c is e , e n d u r a n c e a t h l e t e s b e n e f i t b y i n g e s ti n g c a r b o h y d r a t e i n o r d e r t o

m a i n t a i n b l o o d g lu c o s e o x id a t io n i n a n a t t e m p t t o p re s e r v e th e s u b s t r a t e s u p p l y

(i.e . c a r b o h y d r a t e ) n e e d e d t o e x e r c i s e i n t e n s e l y ( C o g g a n & C o y le , 1 9 9 1 ) .

E n d u r a n c e p e r f o r m a n c e is m a x i m i z e d w h e n s c h e d u l e s f o r f lu id a n d c a r b o h y d r a t e

s u p p l e m e n t a t i o n a r e d e v e lo p e d a n d p r a c t i se d .

M o d e l f o r E n d u r a n c e P o w e r a n d V e l o c i ty

F i g u r e 2 p r e s e n t s a m o d e l o f t h e n u m e r o u s p h y s i o lo g i c a l f a c t o r s t h a t i n t e g r a t e to

d e t e r m i n e t h e P e r f o r m a n c e - V e l o c i ty t h a t c a n b e m a i n t a i n e d i n a m o t i v a t e d a t h l e t e

w h o i s n o t d e h y d r a t e d , h y p e r t h e r m i c o r c a r b o h y d r a t e d e p l e t e d ( Co y le , 1 9 95 ). T h i s

m o d e l r e v o lv e s a r o u n d t h e c o n c e p t t h a t t h e b l o o d l a c ta t e t h r e s h o l d (L T) p r o v i d e s

i n f o r m a t i o n w h i c h i s m o s t i n d i c a t iv e o f t h e s t r e s s e x p e r i e n c e d b y t h e e x e r c is i n g

p e r s o n a s i t r e l a te s t o P e r f o rm a n c e - V e l o c it y . S i n c e t h e e a r l y s t u d y o f M a r g a r ia ,

E d w a r d s a n d D ill (1 9 33 ), i t h a s b e e n r e c o g n i z e d t h a t t h e p r o d u c t i o n o f l a c ti c a c id

b y m u s c l e i s i n d ic a t iv e o f m u s c l e m e t a b o l i c s t re s s . I t a p p e a r s t h a t m a r a t h o n

r u n n e r s a n d o t h e r e n d u r a n c e a t h le t e s w h o c o m p e t e fo r d u r a t io n s o f

a p p r o x i m a t e l y 2 - 3 h o u r s , s e t a p a c e t h a t i s s l i g h tl y f a s t e r (e .g . 7 5 % V O 2 m a x ) t h a n

t h e i n t e n s i t y o f t h e l a c t a t e t h r e s h o l d (e .g . 7 0 % V O 2 m a x ; C o s t i ll & F ox , 1 9 7 2 ;

F a r r e ll e t a l. , 1 9 7 9) . T h e p a c e a p p e a r s t o b e s e l e c te d a c c o r d i n g t o t h e s e n s a t i o n o f

s t r e s s i n t h e e x e r c i s i n g m u s c u l a t u r e t h a t is p r o p o r t io n a l t o m u s c l e a n d b l o o d

l a c t a t e c o n c e n t r a t i o n a n d i n d i ca t iv e o f t h e r a t e o f m u s c l e g l y c og e n o l y si s. It s h o u l d

~ P E R F O R M A N C E L .

~ L ~ C I T Y ) ' ~ P E R F O R M A N C E

I [ * P e r f o r m a n c e ~ ~ A B IL IT IE S

- ~ - [ , , ; ; ~ , ~

:

F U ? E~ IO N A L

M O R P H O L O G I C A LO M P O N E N T S

Figure 2: Mo del of the interrelat ionships of the physiological actors determ ining endurance performance abi l ity

mo dified from Coyle, 1995).

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D e t e r m i n a n t s o f E n d u r a n c e P e r f o r m a n c e

b e n o t e d t h a t i n e v e n t s l a s t in g a p p r o x i m a t e l y o n e h o u r , t r a i n e d c y c l is t s m a i n t a i n

a n i n t e n s i t y o f 8 5 - 9 5 V O 2 m a x , w h i c h i s w e l l i n e x c e s s o f t h e i r b l o o d l a c ta t e

t h r e s h o l d (i.e. 7 3 - 8 4 V O 2 m a x ) a n d p r o d u c e s a c o n s t a n t b l o o d l a c t a te

c o n c e n t r a t i o n o f 7 m M o n a v e r a g e ( C o y le e t a l. , 1 9 9 1 ).

T h e m o d e l f u r t h e r i n d i c a t e s t h a t L a c t a t e T h r e s h o l d P o w e r is a f u n c t io n o f t h e

L a c t a t e T h r e s h o l d V O2 a n d G r o s s M e c h a n i c a l E f f ic ie n c y ( or E c o n o m y o f

M o v e m e n t ) . L a c t a te T h r e s h o l d V O 2 i s a p h y s i o l o g ic a l m e a s u r e i n t h a t i t i s t h e r a t e

o f o x y g e n c o n s u m p t i o n t h a t s t r e s s e s t h e e x e r c i si n g m u s c u l a t u r e t o a c ce l e ra t e

g ly c o ge n o ly s is . T h e n u m e r o u s m o r p h o l o g i c a l c o m p o n e n t s a n d s y s t e m s t h a t

e s t a b l i s h L a c t a t e T h r e s h o l d V O 2 a r e s h o w n i n F i g u r e 2 a n d a r e i n t e g r a t e d i n th e

d i s c u s s i o n b e l o w . H o w e v e r , i t i s u s e f u l t o f i r s t i l l u s t r a t e h o w L a c t a t e T h r e s h o l d

V O 2 a n d G r o s s M e c h a n i c a l E f f i c ie n c y c o m b i n e t o e s t a b l i s h L a c t a t e T h r e s h o l d

P o w e r a n d u l t i m a t e l y P e r f o r m a n c e P o w e r .

y lin g P e r f o r m a n c e a n d M e c h a n i c a l E f f ic i e n c y a r e R e l a t e d

t o t h e P e r c e n t a g e o f T yp e I

M U S C l e F i b r e s

G r o s s M e c h a n i c a l E f f i c ie n c y i s c a l c u l a te d s i m p l y a s t h e r a t i o o f w o r k e d

a c c o m p l i s h e d p e r m i n . r e l a ti v e t o e n e r g y e x p e n d i t u r e p e r m i n . A s s h o w n i n F i g u r e

3 , G r o s s M e c h a n i c a l E f f ic i en c y w h e n c y c li n g a t 8 0 R P M r a n g e d f r o m 1 8 . 3 to

2 2 . 6 i n a p o p u l a t i o n o f e n d u r a n c e t r a i n e d c y c l is t s a n d i t w a s s ig n i f ic a n t ly

c o r r e l a t e d w i t h T y p e I m u s c l e f i b r e s (r = 0 . 7 5 ; C o y l e e t a l. , 1 9 9 2 ). T h i s f i n d i n g

s h o u l d b e i n t e r p r e t e d i n l ig h t o f t h e v e l o c it ie s o f c o n t r a c t i o n e m p l o y e d . P e a k

m u s c u l a r e f fi c ie n c y g e n e r a l ly o c c u r s a t a v e l o c i ty o f a p p r o x i m a t e l y o n e - t h i r d o f t h e

m a x i m a l s h o r t e n i n g v e l o c it y i n b o t h t y p e I a n d t y p e I I f i b r e s ( K u s h m e r i c k , 1 9 83 ).

I t i s li k el y t h a t t y p e I fi b r e s w e r e f o u n d t o b e m o r e e f fi ci e nt w h e n c y c li n g a t 8 0 R P M

b e c a u s e t h i s v e l o c it y i s c l o s e r t o t h e i r v e l o c it y o f p e a k e f fi c ie n c y c o m p a r e d w i t h

t y p e I I f i b r e s .

T h e i n f lu e n c e o n P e r f o r m a n c e P o w e r o f G r o s s E f f i ci en c y w h e n c y c li n g i s

i l lu s t r a te d b y c o m p a r i s o n o f s u b j e c t s g r o u p e d a c c o r d i n g t o t h e i r ~ T p e I m u s c l e

f i b re s ( H o ro w it z, 1 9 93 ). T h e n o r m a l t y p e I g r o u p w a s in d e e d s i m i l a r t o t h e

2 4

2 2

20

6

r = 0 .75

n = 1 9

I I I . . . .

20 40 60 80 1O0

T y p e I M u s c l e F i b e r s

Figure 3: The relat ionship betwee n yp e I muscle f ibres an d gross mech anical eff ic iency when cyc l ing at 80 RPM

(modi fied f rom Coy le et at 1992).

1 8 4

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eterminants of End urance Performance

a v e r a g e c o m p o s i t i o n o f t h e g e n e r a l p o p u l a t i o n w i t h a m e a n o f 4 8 t y p e I w i t h a

d i s t ri b u t i o n o f 3 8 - 5 4 . T h e m e a n v a l u e o f 7 3 i n t h e h i g h t y p e I g r o u p a l s o

r e p r e s e n t s v a l u e s r e p o r t e d i n s o m e e x c e p t i o n a l e n d u r a n c e a t h l e t e s ( H or o w it z e t

a l. , 1 9 9 3 ) . D e s p i t e h a v i n g a l m o s t i d e n t i c a l L a c t a t e T h r e s h o l d V O 2 a s w e l l a s

P e r f o r m a n c e - V O 2 (i.e . 4 . 5 1 / m i n ) s u b j e c t s i n t h e h i g h t y p e I g r o u p g e n e r a t e d 9

m o r e P e r f o rm a n c e - P o w e r c o m p a r e d w i t h t h e lo w t y p e I g r o u p ( 3 4 2 v s 3 1 5

w a t t s ; H o r o w i t z e t a l ., 1 9 9 3 ) . A r e l a t iv e l y h i g h o f T y p e I f i b r e s a n d s u p e r i o r

G r o s s M e c h a n i c a l E f fi c ie n c y c a n a l lo w c y c l is t s w i t h a b e l o w a v e r a g e L a c t a t e

T h r e s h o l d V O 2 a n d P e r f o r m a n c e - V 0 2 , t o p e r f o r m a s w e l l a s a t h l e t e s w i t h m u c h

h i g h e r L a c t a t e T h r e s h o l d V O2 a n d P e r f o r m a n c e - V 0 2 , y e t o n l y n o r m a l T y p e I

f ib r es . W e h a v e s e e n t h a t t h e c o e ff ic i e nt o f v a r i a t io n ( S D / m e a n ) f o r E ff i ci e n c y a n d

G r o s s M e c h a n i c a l E f f ic i e n c y i s a p p r o x i m a t e l y 4 - 6 i n c o m p e t i ti v e c y c li st s.

H o w e v e r , t h e c o e f f i c ie n t o f v a r i a t i o n i n o t h e r f u n c t i o n a l a b i l i t ie s (i.e . L a c t a t e

T h r e s h o l d V O2 a n d P e r f o rm a n c e - V O 2 ) i s r e l a t iv e l y g r e a t e r a v e r a g i n g 8 - 1 0 . T h i s

g r e a t e r v a r ia b i l it y i n L a c t a t e T h r e s h o l d V O 2 a n d P e r f o r m a n c e - V 0 2 i s p r o b a b l y d u e

t o t h e f a c t t h a t i t i s c o m p o s e d o f m o r e m o r p h o lo g i c al c o m p o n e n t s c o m p a r e d w i t h

E c o n o m y ( F i g u re 2 ).

M o r p h o l o g i c a l C o m p o n e n t s o f L a c t a t e T h r e s h o l d V O

F i g u r e 2 i n d i c a t e s t h a t s e v e r a l m o r p h o l o g i c a l c o m p o n e n t s i n t e g r a t e i n t o L a c t a t e

T h r e s h o l d V O 2 t h r o u g h t h e i r i n f l u e n c e o n M a x i m a l O x y g e n C o n s u m p t i o n (i.e .

V O 2 m ax ). S t r o k e v o l u m e a n d h e m o g l o b i n c o n c e n t r a t i o n i n f l u e n c e o x y g e n d e li v e ry

t o m u s c l e a n d h a v e g r e a t p o t e n t i a l t o i m p r o v e e n d u r a n c e p e r f o r m a n c e a b i l i t y .

T h i s i s i ll u s t r a te d b y t h e f i n d i n g s t h a t e l e v a t io n o f h e m o g l o b i n c o n c e n t r a t i o n f r o m

1 5 .7 t o 1 6 . 7 g / 1 0 0 m l , v ia r e d b l o o d c e ll i n f u s io n , i n c r e a s e d V O 2 m a x a n d g r e a t ly

i n c r e a s e d r u n n i n g t i m e t o e x h a u s t i o n a t h i g h i n t e n s i ti e s ( i.e . 7 . 2 m i n v s . 9 . 6 5 m i n ;

B u i ck e t a l. , 1 9 8 0 ) .

T h e r e is m u c h e v i d e n c e s u p p o r t i n g t h e i d e a i n t r o d u c e d b y H o l lo s z y e t al . (1 9 67 )

t h a t m i t o c h o n d r i a l d e n s i t y o r A e r o b i c E n z y m e A c ti v it y i s a m a j o r d e t e r m i n a n t o f

t h e d e g r e e o f m u s c l e s t r e s s d u r i n g e x e r c i s e ( F i n k e t a l. , 1 9 7 7 , H o l l o s z y & C o y le ,

1 9 8 4 , I v y e t a l ., 1 9 8 0 ). Iv y e t a l. ( 1 9 8 0 ) f o u n d t h a t m u s c l e r e s p i r a t o r y c a p a c i t y ,

w h i c h r e f le c t s m i t o c h o n d r i a l e n z y m e a c ti v it y , is h i g h l y r e l a te d t o t h e L a c t a t e

T h r e s h o l d V O 2 w h e n e x a m i n i n g a h e t e r o g e n e o u s g r o u p o f p e o p l e. I n a d d i ti o n , t h e

t i m e c o u r s e w i t h w h i c h A e r o b i c E n z y m e A c t i v it y (i.e . c i t r a t e s y n t h a s e ) d e c l i n e s

o v e r t h e c o u r s e o f 8 4 d a y s o f d e t r a i n i n g i s c l o se l y p a r a l le l e d b y r e d u c t i o n s i n

L a c t a t e T h r e s h o l d V O 2 ( C o yl e e t a l ., 1 9 8 5 ). T h e r e f o r e , t h e r e i s s t r o n g t h e o r e t i c a l

a n d d i r e c t s u p p o r t f o r t h e i d e a t h a t m i t o c h o n d r i a l A e r o b ic E n z y m e A c ti v it y i s a

m a j o r d e t e r m i n a n t o f L a c t a te T h r e s h o l d V O 2 . T h e c o n c e p t i s t h a t a s e n e r g y

e x p e n d i t u r e i s i n c r e a s e d , t h e V O 2 e li ci ti ng t h e l a c t a t e t h r e s h o l d r e p r e s e n t s t h e

r a t e o f e n e r g y e x p e n d i t u r e a t w h i c h m u s c l e c e ll h o m e o s t a s i s i s d i s tu r b e d

s u f fi c ie n t ly t o m o r e m a r k e d l y s t i m u l a t e g l y c o g e no l y s is a n d l a c t a t e p r o d u c t i o n .

T h e m o r e m i t o c h o n d r i a s h a r i n g i n t h e p o w e r o u t p u t , t h e h i g h e r t h e a b s o l u te V O 2

n e e d e d t o e li c it l a c t a t e t h r e s h o l d . A s d i s c u s s e d b e lo w , m e t h o d s f o r in c r e a s i n g t h e

n u m b e r o f m i t o c h o n d r i a s h a r i n g i n t h e o x i d at iv e p o w e r o u t p u t i n c l u d e n o t o n l y

i n c r e a si n g t h e n u m b e r o f m i t o c h o n d r i a i n a g i v en m u s c l e m a s s , b u t a l so

i n c r e a si n g t h e a m o u n t o f m u s c l e m a s s s h a r i n g i n t h e p o w e r o u t p u t (e.g.

D i s t r ib u t i o n o f P o w e r O u t p u t ).

T h e a m o u n t o f m u s c l e m a s s a c ti v e w h e n c y cl in g c a n b e e s t i m a t e d a s t h e

q u o t i e n t o f t h e t o t a l m m o l e s o f m u s c l e g l y c o g e n o x i d iz e d b y t h e w h o l e b o d y a n d

t h e m m o l e s o f g l y c o g e n u s e d p e r k g o f m u s c l e , o b t a i n e d f r o m m u s c l e b i o p s i e s (i.e .

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eterminants of Endurance Performance

vastus lateralis), during 30 min of cycling at 79% VQmax (Coyle et al., 1988;

Coyle, 1995). We have estimated that a group of subjects with a high Lactate

Threshold VO2 are able to distribute the power output of cycling over an

approximately 22% greater amount of muscle mass (Coyle et al., 1988; Coyle,

1995). We think it likely that the high lactate threshold group has developed the

ability to better utilize their hip extensors muscles for powering the cycling down-

stroke, possibly as a result of their greater number of years spent training for

cycling (Coyle et al,, 1988; Coyle, 1995). It seems most likely to us that greater

muscle fibre recruitment would result in higher peak muscle force. Our pedal

torque data support this interpretation in that elite cyclists with a high Lactate

Threshold VO2 maintained an 11.2% higher power output during the lh

performance (i.e. Performance Power) test by generating more torque during the

cycling down-stroke (Coyle et al., 1988; Coyle, 1995). This was accomplished by

generat ing 22% more peak torque compared with other competitive cyclists, with

peak torque occurring slightly before the cycle crank was horizontal in both

groups (i.e. 86-90 degrees). The greater peak torque generated by elite cyclists is

derived by producing 24% higher vertical force directed down-ward and 14O/o

higher horizontal force directed forward . Simply put, they pushed down harder.

Elite cyclists display this pattern even when cycling at a given absolute power

ou tput (Coyle et al., 1988; Coyle, 1995).

Exercise time to fatigue is also related to Muscle Capillary Density (i.e.

capillaries per ram2), which interac ts with Lactate Threshold VO2. Together these

two factors accounted for more than 92% of the variance in time to fatigue when

cycling at intensities well above lactate threshold (Coyle et al., 1988). A high

Muscle Capillary Density has been postulated to increase muscle perfusion by

reducing diffusing distances and thus aid in removing lactate from muscle. We

interpret this relationship to indicate that time to fatigue during exercise, at an

intensity above the lactate threshold, is related to both lactate production (as

reflected by Lactate Threshold VO2 or % VO2max at LT) and removal from muscle

(as related to Muscle Capillary Density).

The model presented predicts endurance performance in highly motivated

athletes who are not carbohydrate depleted, hypoglycemic, hyperthermic or

dehydrated. The remaining portion of this review will discuss how these factors

impair performance. It should be noted that discussion of the psychological

factors determining endurance performance is beyond the scope of this review. It

is assumed that highly motivated athletes can exercise to the point where

physiological factors reliably predict performance. This is not to say, however, that

psychological factors can't extend physiological performance even in highly

motivated athletes.

arbohydrate Feeding During Exercise

People fatigue after one to three hours of continuous exercise at moderate

intensities (i.e. 60-80% VO2max) in a cool environment due largely to depletion of

carbohydrate as a substrate (Coggan & Coyle, 1991). Carbohydrate feedings (i.e.

glucose, maltodextrins or sucrose) during exercise will delay fatigue by 30-60

minutes (Coggan & Coyle, 1991). However, this improvement in performance is

not due to a sparing of muscle glycogen use dunng cycling, but instead it appears

that the exercising muscles rely mostly upon blood glucose for energy late in

exercise (Coyle et al., 1986; Coggan & Coyle, 1991). Substrate oxidation changes

during prolonged exercise at 65-75% VQma x in endurance trained subjects when

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D e t e r m i n a n ts o f E n d u r a n c e P e r f o r m a n c e

f a s t e d o v e r n i g h t. A p p r o x i m a t e l y 5 0 o f t h e e n e r g y f o r e x e r c i se a t 7 0 V O 2 m a x i s

d e ri v ed f ro m f a t, w i th e q u a l c o n t r i b u ti o n s f r o m p l a s m a F F A a n d i n t r a m u s c u l a r

t ri g ly c e r id e s d u r i n g t h e e a r l y p e r io d . T h e r e i s a s m a l l i n c r e a s e i n p l a s m a F F A

c o n t r i b u t i o n o v e r t im e . T h e r e m a i n i n g 5 0 o f t h e e n e r g y i s d e ri v e d f r o m

c a r b o h y d r a t e . D u r i n g t h e e a r l y p o r t i o n s o f e x e r c is e , t h e m a j o r i t y o f c a r b o h y d r a t e

e n e r g y i s f r o m m u s c l e g l y c og e n . A s e x e r c i s e p r o g r e s s e s, m u s c l e g l y c o g en is

r e d u c e d a n d c o n t r i b u t e s le s s to t h e c a r b o h y d r a t e r e q u i r e m e n t s o f e x e rc i se a n d

t h e r e i s i n c r e a s e d r e l ia n c e u p o n b l o o d g lu c o s e . T h e r e f o r e c a r b o h y d r a t e s h o u l d b e

i n g e s t e d to s u p p l y t h e b l o o d w i t h e x o g e n o u s g lu c o s e a t a p p r o x i m a t e l y 1 g / m i n ,

l a te i n e x e rc i se . S t u d i e s w h i c h h a v e o b s e r v e d c a r b o h y d r a t e i n g e s t i o n t h r o u g h o u t

e x e r c i se t o im p r o v e p e r f o r m a n c e h a v e f e d s u b j e c t s a t a r a t e o f 3 0 - 6 0 g / h ,

b e g i n n i n g e a r l y i n e x e r c i s e ( C o g g a n & C o y l e , 1 9 9 1 ).

T h e s h o r t e s t d u r a t i o n o f m o d e r a t e t o h i g h i n t e n s i ty e n d u r a n c e e x e rc i se t h a t

b e n e f i t s f r o m c a r b o h y d r a t e f e e d i n g is n o t e n t i r e l y c l ea r . B l o o d g l u c o s e

c o n c e n t r a t i o n d e c l i n e s d u r i n g t h e s e c o n d h o u r o f e x e r c i s e a n d t h e r e f o r e i t i s

g e n e r a l ly b e li e v e d t o b e b e n e f i c i al f or d u r a t i o n s l a s ti n g l o n g e r t h a n t w o h o u r s .

H o w e v e r , c a r b o h y d r a t e f e e d i n g s a l s o b e n e f i t i n t e n s e e x e r c i s e ( i.e . 8 0 - 9 0

V O 2 m a x ) l a s t i n g o n e h o u r i n d u r a t i o n ( B e lo w e t a l. , 1 9 9 5 ) .

H y p e r t h e r m i a

H e a t p r o d u c t i o n d u r i n g i n t e n se e x e r c i se c a n e l e v at e c o r e a n d m u s c l e t e m p e r a t u r e

r a p id l y a n d i t s e e m s t h a t h y p e r t h e r m i a i s a n i n d e p e n d e n t c a u s e o f f at ig u e . It h a s

l o n g b e e n r e c o g n i z e d t h a t n o s i n g le f a c t o r p o s e s a g r e a t e r t h r e a t t o a n e n d u r a n c e

a t h l e t e 's h e a l t h a n d p e r f o r m a n c e t h a n d o e s h y p e r t h e r m i a ( C os ti ll, 1 9 7 2) . A t h l e t es

i n l a b o r a t o r y s tu d i e s s e e m t o c e a s e e x e r c i se a t r el a ti v e ly h o m o g e n o u s e s o p h a g e a l

t e m p e r a t u r e s a n d m u s c l e t e m p e r a t u r e s (i.e . 4 0 -4 1 o oC ; G o n z & l e z -A l o n s o e t al .,

1 9 99 ). T h e s e p o i n t s w e r e r e c e n t l y e m p h a s i z e d i n a s t u d y t h a t e x e r c i s e d s u b j e c t s

i n a n e n v i r o n m e n t t h a t c a u s e d h y p e r t h e r m i a b y li m it in g h e a t d i s s ip a t io n a n d

t h u s p r e v e n t e d s u b j e c t s f r o m e x e r c i s i n g t h e l o n g d u r a t i o n s t y p i c a l i n c o o l er

41

.--, 40

0

mo

v 39

~

m ~ 3 8

7

W E

36

Precooling

- - - 3 - - Contro l

_- Preheating

3 2 7 o

o 1 o 2 o 3 o 4 o 5 o 6 o

P e r f or m a n c e t im e m i n )

Figure 4: Per formance t ime dur ing exerc ise beg un wi th d i ffering esophageal temperatures mid i fied f rom

Gonz alez-Alonso e t aL, 1999),

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eterminants of Endurance Performance

e n v i r o n m e n t s [ e .g . > 2 h o u r s ) . B o d y c o r e t e m p e r a t u r e w a s l o w e r e d o r e le v a te d

b e f o r e e x e r c i se ( G o n z ~ l e z -A l o n s o e t a l. , 1 9 9 9) . A s s h o w n i n F i g u r e 4 , e x e r c i s e t im e

t o e x h a u s t i o n w a s i n v e r s e l y r e l a t e d t o t h e in i ti a l b o d y t e m p e r a t u r e (i.e. 6 3 , 4 6 , a n d

2 8 m in ) . F u r t h e r m o r e , i n c r e a s e s i n h e a r t r a t e a n d r e d u c t i o n s i n s t r o k e v o l u m e

p a r a l l e l e d t h e r i s e i n c o r e t e m p e r a t u r e ( G o n zS _ le z -A l o n so e t a l ., 1 9 9 9 ) .

T h e r e a r e s e ve ra l m e c h a n i s m s b y w h i c h h y p e r t h e r m i a m i g h t c a u s e f a ti gu e . It

c o u ld a f fe c t a r e a s o f t h e c e n t r al n e r v o u s s y s t e m s u c h a s t h e h y p o t h a l a m u s , t h e

m o t o r c o r t e x o r m u s c l e e f f er e n t s t h a t c a u s e p e r c e iv e d e x e r t i o n t o i n c r e a s e a n d

m u s c l e fi b re r e c r u i t m e n t to b e c o m e i n c r e a s in g l y d i ff ic u lt ( B r u c k O l s c h e w s k i ,

1 9 8 7 ; F u l l e r e t al ., 1 9 9 8 ) .

Dehyd ra t i on

S w e a t i n g r a t e d u r i n g i n t e n s e e x e r ci s e is t y p i ca l ly 1 - 2 li tr e s p e r h o u r a n d t h u s

a t h l e te s c a n b e c o m e d e h y d r a t e d f f f lu i d r e p l a c e m e n t i s n o t s u f f ic i e n t (G o n z ~l ez -

A l o n s o e t a l. , 1 9 9 5 ; N o a k e s , 1 9 9 3 ). I t r a r e l y i s b e c a u s e a s u f f i c ie n t v o l u n t a r i l y

d r i n k i n g r a t e i s u s u a l l y o n l y a b o u t o n e - h a l f t h e s w e a t i n g r a t e ( N o ak e s, 1 9 93 ). T h e

m a j o r d e t r i m e n t a l e f f ec ts o f d e h y d r a t i o n a r e h y p e r t h e r m i a a n d i m p a i r e d

c a r d i o v a s c u l a r f u n c t i o n . D e h y d r a t i o n c a u s e s v e r y l a rg e r e d u c t i o n s i n s t r o k e

v o l u m e t h a t c a n n o t b e s u f f i c i en t ly c o m p e n s a t e d b y i n c re a s in g h e a r t r a t e a n d a s

a r e s u l t c a r d i a c o u t p u t d e c l in e s d u r i n g e x e r c i s e a t a c o n s t a n t p o w e r o u t p u t a n d

o x y g e n c o n s u m p t i o n ( S a w k a e t al , 1 9 7 9 ; G o n z a l e z- A l o n s o et al . , 1 9 9 5 ; M o n t a i n

C o y le , 1 9 9 2 ). F u r t h e r m o r e , b l o o d f lo w t o th e e x e r c i s i n g m u s c l e s d e c l i n e s

( G o n z a l e z - A l o n s o e t a l. , 1 9 9 8 ), t h u s s t r e s s i n g m e t a b o l i s m b y r e d u c i n g t h e d e l i v er y

o f o x y g e n a n d s u b s t r a t e s . I m p a i r e d c a r d i o v a s cu l a r f u n c t io n f r o m d e h y d r a t i o n a n d

h y p e r t h e r m i a , p o s s i b l y s e n s e d a s a r e d u c t i o n i n a r te r i a l b l o o d p r e s s u r e ( G o rl z~ le z-

A l o n s o e t a l. , 1 9 9 5 ) , t r i g g e r s a l a r g e c a t e c h o l a m i n e r e s p o n s e t o e x e r c i s e w h i c h

a p p e a r s t o el ic it a d d e d m e t a b o l i c s t r e s s ( F e b b r a i o e t a l, , 1 9 8 8 ) . N e e d l e s s t o s a y ,

d e h y d r a t i o n a n d t h e c o n c o m i t a n t h y p e r t h e r m i a h a v e g r e a t po t en t ia l t o i m p a i r

e n d u r a n c e p e r f o r m a n c e a n d e n d a n g e r h ea l t h .

Re fe rences

Below, P.R., Mo ra-Rodriguez, R., Gonz~lez-Alonso, J., Coyle, E.F . (1995) Flu id and carbohydrate

ingestion independently impro ve performance during 1 h of intense exercise. M e d i c i n e a n d

S c i e n c e i n S p o r t s a n d E x e r c i s e 2 7 : 200-210.

Buick, F.J . , G ledhfl l, N., Froese, A .B., Sprie r, L., M eyers, E. C. (1980) Eff ect of induced

erythrocythemia on aerobic work capacity. Jou rna l of Applied P h y s i o l o g y 4 8 : 636-642.

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