jUNIE 1974 F I S I O T E R A P I E 13 ENDURANCE TIME IN SLOW AND FAST CONTRACTING MUSCLE GROUPS by Svend M o lb ec h , M .Sc. and Sop hu s H . Johan sen, M .D .* fJoIbech, S- a n d J o h a n se n , S. H . W ork-environm .-hlth 10 (1973) 62— 64. E n d u ra n c e tim e in slo w a n d fa s t c o n ­ tracting m u sc le g ro u p s. T h e e n d u ra n c e tim e (th e m a x im u m d u r a tio n o f a su s ta in e d is o m e tric m u sc le c o n tra c tio n ) f a slow a n d fa s t c o n tra c tin g m u sc le g ro u p w as in v e s tig a te d in six su b je cts. T h e m u sc le g ro u p s e x a m in e d w ere the p la n ta r a n d th e d o rs a l flex o rs o f th e a n k le jo in t. T h e m e a s u re m e n ts w e re m a d e w ith a 90° flexion in th e k n e e joint. A fte r d e te rm in a tio n o f th e m a x im u m iso m e tric m u sc le stre n g th , th e e n d u ra n c e tim e w as m e a su re d a t 50 per c e n t o f th e m a x im u m stre n g th . T h e p l a n t a r flex o rs sh o w ed a s ta ti s t i c a l l y sig n ific a n t lo n g e r e n d u ra n c e tim e than did th e d o rs a l flexors. T h is in d ic a te s t h a t th e a b ility o f a m u sc le g ro u p to m a in ta in a su sta in e d iso m e tric c o n tra c tio n d e p en d s o n th e re la tiv e p r o p o r t io n o f slow a n d f a s t c o n tra c tin g m u sc le fibers. S tria te d m u sc le s a re c o m p o s e d o f slo w a n d fa s t c o n ­ tracting m uscle fib ers (2, 3). T h e slo w (re d ) fibers a re ■jch in m ito c h o n d ria a n d m y o g lo b in a n d a re re la tiv e ly " i n , w hereas f a s t (w h ite) fib ers c o n ta in a sm a lle r n u m b e r o f m ito c h o n d ria , less m y o g lo b in , h a v e a la rg e r d ia m e te r, a n d a r e ric h e r in g ly co g e n . S c h m a lb ru c k (7) an aly zed h u m a n m u sc le s f o r fiber ty p es a n d d e m o n ­ strated th a t th e so le u s m u sc le c o n ta in e d 89% slow fibers (type C ), w h e re a s t h e tib ia l m u sc le c o n ta in e d 46% only, th e re st b e in g c o m p o s e d o f B a n d A fibers, which a re p o o r e r in m ito c h o n d ria . R o h m ert (6 ) d e m o n s tra te d t h a t e n d u ra n c e tim e (th e m axim um d u ra tio n o f a su s ta in e d iso m e tric m u sc le c o n ­ traction) w as in d e p e n d e n t o f m u sc le g ro u p a n d su b je c t and only d e p e n d e n t o n th e re la tiv e lo a d . In an e a rlie r stu d y (5) w e fo u n d th a t e n d u ra n c e tim e in elbow flexion d u rin g p a r tia l c u ra riz a tio n w as significantly d e c re a se d w ith + tu b o c u r a r in e b u t show ed a tendency to in c re a s e w ith d e c a m e th o n iu m . T h e slow and fa st fibers h a v e a d iffere n t se n sitiv ity to w a rd s curarizing a g en ts. T h e m a rk e d r e d u c tio n in e n d u ra n c e time fo u n d w ith + tu b o c u r a r in e th u s w as e x p la in e d by the facts th a t th e slo w fibers a re b e st fit f o r su s ta in e d isometric c o n tra c tio n , a n d a t th e sa m e tim e m o re sensitive to w a rd s th e d ru g th a n th e f a s t fibers. T he D a n ish N a tio n a l A s s o c ia tio n f o r I n fa n tile Paralysis, a n d th e D e p a r tm e n t o f A n e sth e sia II, G e ntofte H o s p ita l, H e lle ru p , D e n m a rk . T h e s e re su lts suggest th a t e n d u ra n c e tim e , defined as th e m a x im u m tim e a n is o m e tric m u sc le c o n tra c tio n c an b e m a in ta in e d a t a c o n s ta n t te n s io n , c o u ld v a ry w ith th e c o n te n t o f slo w a n d fa s t fibers. T h e a im o f th is s tu d y , th e re fo re , w as to in v estig a te w h e th e r th e re is a differe n ce in th e e n d u ra n c e tim e s o f tw o m u sc le g ro u p s, w h ic h , a re k n o w n to b e slo w a n d fa s t, re sp ec tiv e ly . F o r th is p u rp o s e w e h a v e c h o se n to s tu d y m .s o le u s in a p la n ta r flexion u n d e r c o n d itio n s w h ic h re d u c e th e c o n trib u tio n o f th e g a stro c n e m iu s m u sc le to a m in im u m , a n d m .tib ia lis a n te r io r in d o rs a l flexion o f th e a n k le jo in t. T a b le 1. S u b je c ts’ d a ta , th e m e a n v a lu e s o f th re e m e a ­ s u re m e n ts o n e a c h su b je c t o f th e m a x im u m iso m e tric s tre n g th in p la n ta r a n d d o rs a l flexion, a n d th e c o r re ­ sp o n d in g a v e ra g e e n d u ra n c e tim e s a t a p p ro x . 50% o f th e m a x im u m iso m e tric stre n g th . M eth od s and P rocedures T h e su b je c ts w ere p la c e d in th e su p in e p o sitio n o n a ta b le w ith 90° flexion in th e h ip a n d k n e e jo in ts , th e th ig h b e in g fa s te n e d to a v e rtic a l b o a rd , th e lo w e r leg fa s te n e d o n a h o r iz o n ta l p la te . T h e su b je c ts w o re solid sh o e s o n w h ic h a m o d ifie d ice s k a te w as screw ed a n d s tra p p e d in th e c o n v e n tio n a l p o s itio n so t h a t th e fo rc e s e x e rte d w ere tra n s m itte d f r o m t h e tip o f th e sk a te . A s tra in g a u g e d y n a m o m e te r w as m o u n te d o n a w ire w h ic h c o n n e c te d th e tip o f th e s k a te w ith e ith e r a c o m p le te fix atio n — f o r th e m a x im u m iso m e tric s tre n g th m e a s u re m e n ts — o r a tru n d le , 15 cm d ia m e te r, o n b a ll b e a rin g s f o r th e e n d u ra n c e tim e m e a su re m e n ts. Subj. Sex A g e H e ig h t W eig h t M a x im u m iso m e tric E n d u ra n c e tim e y e ars cm k g s tre n g th p l a n ta r d o rsa l flex io n flexion in m in. p la n ta r d o rsa l flexion flexion KH BO AJ BS LJ AP Overall average 44 30 17 56 19 18 162,5 176.0 166.0 65.5 64.5 55,0 80,7 98,9 73,0 166,5 182,0 179,0 73.0 76.0 68.0 69,9 103.0 106.0 25.6 34,3 34.7 33.0 39,2 34.0 2,75 3,86 4,16 3,71 3,29 3,48 3,54 ± 0 ,5 6 1,63 1,25 1,60 1,98 1,17 1,88 1,58 + 0,34 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) 14 P H Y S I O T H E R A P Y JUNE, 1974 A ll m e a s u re m e n ts w e re p e rfo rm e d o n th e sa m e leg, w ith th e a n k le jo in t a t 9 0 °. T h e te n s io n s w e re re c o rd e d by m e a n s o f an in k w ritin g p o te n tio m e te r. T h e e x p e rim e n ts w e re p e rfo rm e d on e a c h su b je c t w ith se v e ra l d a y s ’ in te rm is sio n . A f te r s tra p p in g th e s u b je c t to th e te s t ta b le th r e e m a x im u m is o m e tric flexions w ere p e rfo rm e d w ith in te rv a ls o f a few se c o n d s. T h e b est o f th e th re e w as ta k e n as th e m a x im u m iso m e tric s tre n g th f o r t h a t te s t d a y . T h e su b je c t th e n re ste d fo r 20 m in. In th e m e a n tim e a lo a d c o rre s p o n d in g to 50% o f th e m a x im u m s tre n g th w as a tta c h e d to th e sk a te v ia th e w ire a n d h u n g o v e r th e tru n d le . A f te r th e re s t­ ing p e rio d , th e s u b je c t w as to ld to h o ld th is lo a d w ith th e a n k le a t 90° as lo n g as p o ssib le . T h e m a x im u m tim e th e f o o t c o u ld be k e p t in th e rig h t p o s itio n w as th e e n d u ra n c e tim e . E n d u ra n c e tim e w as in v e s tig a te d in th re e fe m a le a n d th r e e m a le s u b je c ts ra n g in g fro m 17 to 56 y e a rs o f age. R esu lts T h e c h a ra c te ris tic s o f th e su b je c ts a p p e a r in ta b le 1, as d o th e v a lu e s o f m a x im u m p la n ta r a n d d o rs a l flexion (a v e ra g e o f th e th re e b e st re su lts , o n e f o r e a c h te s t d a y ). I t a ls o sh o w s th e a v e ra g e o f th e c o rre s p o n d in g e n d u ra n c e tim e s m e a s u re d a t a p p ro x . 50% o f th e m a x im u m is o m e tric s tre n g th (50,0% + 0 ,5 f o r p la n t a r flexion, a n d 50,0% + 0 ,7 f o r d o rs a l flexion). TTie s tre n g th in p l a n ta r flexion is tw o to th r e e tim e s g re a te r th a n in d o rs a l flexion. T h e d iffere n ce in e n d u r a n c e tim e be tw ee n p l a n ta r a n d d o rs a l flex io n a t th e sa m e re la tiv e lo a d (50% o f m a x . stre n g th ) is s ta tis tic a lly sig n ific a n t (p < 0 ,0 0 5 , S tu d e n t’s t test, p a ire d o b s e rv a tio n s ). D iscu ssion T h e o b se rv e d d iffere n ce in e n d u ra n c e tim e be tw ee n th e tw o m u sc le g ro u p s c a n n o t be e x p la in e d by d iffer­ e nces in b lo o d flow. H y d r o s ta tic fa c to rs w ill d im in ish b lo o d flow to th e sa m e d e g re e in b o th m u sc le s in th e e x p e rim e n ta l p o s itio n , a n d it is m o s t lik e ly t h a t c ir­ c u la tio n th ro u g h th e m u sc le s is se v e re ly im p e d e d a t an iso m e tric c o n tr a c tio n o f 50% o f th e m a x im u m s tre n g th (1, 4). A t th e o n s e t o f s ta tic w o rk in a su b -m a x im a l c o n ­ tra c tio n p ro d u c in g 50% o f th e m a x im u m iso m e tric stre n g th , th e d e g re e o f e ffo rt w ill be m o d e r a te . A s tim e passes, th e e ffo rt w ill in c re a s e , a n d a t th e cessa ­ tio n o f th e e n d u r a n c e tim e p e rio d th e e ffo rt w ill be m a x im u m , a lth o u g h th e m e c h a n ic a l te n s io n is u n ­ c h a n g e d . T h e d is trib u tio n o f th e te n s io n -p ro d u c in g fibers w ill c h a n g e d u rin g th e w o rk p e rio d , a s fa tig u e g r a d u a lly a ffects m o re a n d m o re fibers. T h e fibers p o o re s t in m y o g lo b in a re th e first to be a ffe c te d d u rin g sta tic (iso m e tric ) w o rk o f su c h lo n g d u r a tio n t h a t a n a e ro b ic p ro c e sse s b e c o m e p r e d o m in a n t. T h e n u m b e r o f c a p illa rie s p e r m u sc le fib er in red m u sc le , a n d th u s th e la rg e r o x y g e n sto re , c a n n o t e x p la in th e o b se rv e d d iffe re n c e in e n d u ra n c e tim e . ( p < 0 ,0 0 5 , S tu d e n t’s t te s t, p a ire d o b s e rv a tio n s ). ( p < 0 ,0 0 5 , S tu d e n t’s t te s t, p a ire d o b s e rv a tio n s ). In is o m e tric p la n ta r flexion th e p rim e m u sc le w ill be so le u s, e sp e c ia lly w h e n th e lo w e r e x tre m ity is flexed in th e k n e e jo in t; th is flexion m in im iz e s th e c o n trib u tio n o f te n s io n f r o m th e g a s tro c n e m iu s m u sc le . I n d o rs a l flexion th e m o st im p o r ta n t m u sc le w ill b e th e anterior tib ia l m u sc le . D u e to th e differe n ce in d istrib u tio n of fiber ty p es in th e tw o m u sc le s, so le u s w ill b e better fit f o r sta tic w o rk th a n tib ia lis; th is e x p la in s th e differ­ ence in e n d u ra n c e tim e b e tw ee n th e tw o m u sc le groups fo u n d a t 50% o f th e m a x im u m iso m e tric stre n g th . The differe n ce c o u ld a c tu a lly be e x p la in e d by a n y decisive difference in th e p h y sio lo g ic a l fu n c tio n s , characteristic o f th e tw o fib er ty p e s. T h e m o s t o b v io u s explanation, h o w e v e r, seem s to be th e re la tiv e ly lo n g d u ra tio n of th e single c o n tra c tio n o f th e slow , red m u sc le fiber. In a s u b m a x im u m c o n tra c tio n , te ta n u s is a ch iev e d at a lo w e r fre q u e n c y th a n in a fa s t, w h ite fiber. T hus a given m e c h a n ic a l te n s io n can be p ro d u c e d by a rela­ tively sm a lle r n u m b e r o f fibers. T h e e n d u ra n c e time o f th e d o rs a l flexors is in g o o d a g re e m e n t w ith Roh- m c r t’s d a ta (6 ), w h e re a s t h a t fo u n d f o r p l a n ta r flexion is sig n ific a n tly lo n g e r. T h is m e a n s th a t th e c o m p o s itio n o f m uscles, as re­ g a rd s th e d is trib u tio n o f fib er ty p e s, w ill influence the.* te n s io n -tim e c u rv e in su c h a w ay th a t p re d o m in a n c y o f slow C fibers w ill p ro lo n g e n d u ra n c e tim e , a t lea sr a t 50% o f th e m a x im u m iso m e tric stre n g th . T h e finding seem s to in d ic a te th e im p o rta n c e of c h o o sin g m a in ly m u sc le g ro u p s w ith a re la tiv e ly high c o n te n t o f re d m u sc le fib ers f o r o c c u p a tio n a l work d o m in a te d by iso m e tric (sta tic ) w o rk a n d fo r work o f lo n g d u ra tio n w ith m o re o r less im p e d e d circulation. A lso th e fin d in g m ig h t e x p la in differe n ce s in fatigue in d iffe re n t m u sc le g ro u p s d u rin g o c c u p a tio n a l w ork. As e n d u ra n c e tim e seem s d e p e n d e n t n o t o n ly o n th e rela­ tiv e lo a d p u t o n th e m u sc le d u rin g s ta tic contraction, b u t a ls o on th e m u sc le c o m p o s itio n , th e m a x im u m iso­ m e tric stre n g th o f a m u sc le g ro u p d o e s n o t give a tr u e p ic tu re o f th e c a p a b ility to p e rfo rm sta tic work. W h e n tra in in g m u sc le stre n g th (e.g., p a tie n ts d u rin g and a f te r h o s p ita liz a tio n , re h a b ilite e s a f te r a p e rio d o f un­ e m p lo y m e n t) it is lik e ly t h a t th e ty p e o f tra in in g used in fluences th e m a x im u m iso m e tric s tre n g th a n d the en­ d u r a n c e tim e o f th e sa m e m u sc le g ro u p to different e x te n ts d e p e n d in g on th e te n d e n c y o f th e train in g to in c re a s e re d p ro p e rtie s o f th e in v o lv e d m u sc le fibers. R eferences 1 B a rc ro ft, H . a n d M ille n , J. L . E . T h e b lo o d f l o \ J ' th ro u g h m u sc le d u rin g su s ta in e d c o n tra c tio n . J > p h y sio l., (L o n d .) 97 (1939) 17— 31. 2 G u th , L . T r o p h ic in flu e n ce s o f n e rv e on muscle. P h y sio l, rev. 48 (1968) 645—<387. 3 H ess, A . V e r te b r a te slow m u sc le fibers. Physiol, rev. 50 (1970) 4 0 — 62. 1 H u m p h r e y s , P. W . a n d L in d , A . R . T h e b lo o d flow th r o u g h a c tiv e a n d in a c tiv e m u sc le s o f th e forearm d u rin g su s ta in e d h a n d g r ip c o n tra c tio n s . J. Physiol.. (L o n d .) 166 (1963) 120— 135. 5 M o lb e c h , S. a n d J o h a n se n , S. H . E n d u ra n c e time in s ta tic w o rk d u r in g p a r tia l c u ra riz a tio n . J. appl. p h y sio l. 27 (1969) 44— 48. “ R o h m e r t, W . E rm ittlu n g v o n E rh o lu n g s p a u s e n fur sta tisc h e A rb e it des M e n sc h e n . In t. Z . a n g ew . Physiol. 18 (1960) 123— 164. 7 S c h m a lb r u c k , H . D ie q u e rg e stre ife n M u s k e lfa se rn des M e n sc h e n . E r g e b n . A n a t. E n tw ic k l.-G e sc h . 43 (1970) 1— 73. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. )