Muscle glycogen concentration Table 1 As a result of the exercise and dietary manipulation, preexercise muscle glycogen concentration before both IExshort and IExlong was signi®cantly lo[r]
Trang 1High-intensity exercise and muscle glycogen availability in humans
P.D BALSOM, G.C GAITANOS, K SOÈDERLUND and B EKBLOM
Department of Physiology and Pharmacology, Karolinska Institute and University College of Physical Education and Sports, Stockholm, Sweden
ABSTRACT This study investigated the effects of muscle glycogen availability on performance and selected physiological and metabolic responses during high-intensity intermittent exercise Seven male subjects completed a regimen of exercise and dietary intake (48 h) to either lower and keep low (LOW-CHO) or lower and then increase (HIGH-CHO) muscle glycogen stores, on two separate occasions at least a week apart On each occasion the subjects completed a short-term (<10 min) and prolonged (>30 min) intermittent exercise (IEX) protocol, 24 h apart, which consisted of 6-s bouts of high-intensity exercise performed at 30-s intervals on a cycle ergometer Glycogen concentration (mean SEM) in m vastus lateralis before both IEx short and IEx long was signi®cantly lower following LOW-CHO [180 (14), 181 (17) mmol kg (dw) ±1 ] compared with HIGH-CHO [397 (35), 540 (25) mmol kg (dw) ±1 ] In both IEx short and IEx long , signi®cantly less work was performed following LOW-CHO compared with HIGH-CHO In IEx long , the number of exercise bouts that could be completed at a pre-determined target exercise intensity increased by 265% from 111 (14) following LOW-CHO to 294 (29) following HIGH-CHO (P < 0.05) At the point of fatigue in IEx long , glycogen concentration was signi®cantly lower with the LOW-CHO compared with HIGH-CHO [58 (25) vs 181 (46) mmol kg (dw) ±1 , respectively] The plasma concentrations of adrenaline and nor-adrenaline (in IEx short and IEx long ), and FFA and glycerol (in IEx long ), increased several-fold above resting values with both experimental conditions Oxygen uptake during the exercise periods in IEx long approached 70% of V o 2max These results suggest that muscle glycogen availability can affect performance during both short-term and more prolonged high-intensity intermit-tent exercise and that with repeated exercise periods as short as 6 s, there can be a relatively high aerobic contribution.
Keywords blood lactate, carbohydrates, catecholamines, diet, FFA, glycerol, glycogen,
hypoxanthine, oxygen uptake, performance.
Received 2 March 1998, accepted 17 November 1998
It is now well established that with prolonged continuous
exercise, time to fatigue at a moderate submaximal
ex-ercise intensity is related to pre-exex-ercise muscle glycogen
concentration (BergstroÈm et al 1967) With
high-inten-sity exercise the relation between the availability of
muscle glycogen and performance is less clear This is
due, at least in part, to differences in the type, intensity
and duration of the exercise tests which have been used
and differences in the exercise and dietary regimens
which have been implemented to manipulate
pre-exer-cise muscle glycogen concentrations (cf Maughan et al
1997) Furthermore, relatively few studies have included
muscle biopsies to determine muscle glycogen
concen-trations
During high-intensity exercise there is a rapid break-down of muscle glycogen Gaitanos et al (1993) reported that glycogen concentration in m vastus lateralis decreased
by 14% (43 mmol kg [dw]±1) after only one 6-s bout of
`all-out' exercise performed on a cycle ergometer Therefore, during repeated bouts of short-duration high-intensity exercise performed over a prolonged period of time, it could be expected that glycogen availability may become a limiting factor for the ability to sustain a high-power output Indeed, Bangsbo et al (1992b) reported that time to fatigue during an exercise protocol which included repeated 15-s bouts of high-intensity exercise, interspersed with 10-s recovery periods, was prolonged following a high carbohydrate intake
Correspondence: Karin SoÈderlund, Karolinska Institute, Department of Physiology and Pharmacology, Box 5626, 114 86 Stockholm, Sweden.
Trang 2MATERIALS AND METHODS
Subjects
Seven highly motivated physically active male physical
education students volunteered to participate in the
study The mean (and SD) age, body mass and VO 2 maxof
the group was 24.0 (3.7) years, 72.9 (10.8) kg and
4.2 (0.3) L min±1 The study was approved by the Ethics
Committee of the Karolinska Institute and the subjects
were informed of the test procedures prior to giving their
consent to participate
Procedures and exercise protocols
Subjects performed two high-intensity intermittent
ex-ercise protocols 24 h apart, on two separate occasions
separated by at least 1 week, on a specially adapted
Cardionics Wingate friction-loaded cycle ergometer with
rounded handle bars and toe-clips (Cardionics AB,
Stockholm, Sweden) The ®rst exercise protocol
(IExshort) consisted of ®fteen 6-s bouts of high-intensity
exercise, interspersed with 30-s rest periods During each
work period subjects were instructed to try to maintain a
pedalling frequency of 140 r.p.m This was followed 24 h
later by a prolonged intermittent exercise protocol
(IExlong) where subjects performed repeated 6-s bouts of
high-intensity exercise at a target pedalling frequency of
140 r.p.m., again interspersed with 30-s rest periods, until
a prede®ned point of fatigue Twenty-four hours before
completing IExshortsubjects cycled for 2 h (Exdeplete) to
lower glycogen stores, i.e
Exdeplete 24 h IExshort 24 h IExlong
During the 48 h between Exdepleteand IExlong, a diet low
in carbohydrate was consumed (LOW-CHO) on one
occasion to maintain low glycogen stores and on the
other occasion subjects consumed a high-carbohydrate
diet (HIGH-CHO) to increase glycogen stores The
or-der in which the two diets were administered was
ran-domized Before completing Exdepletefor the ®rst time
subjects visited the laboratory on at least six occasions to
become familiarized with performing high-intensity
in-termittent exercise On one of these visits, maximal
ox-ygen uptake and oxox-ygen uptake during four 4-min
submaximal work loads was measured using a graded
continuous exercise test on the cycle ergometer
IExshortand IExlongwere preceded by a standardized
15-min warm up which consisted of continuous
sub-3 h prior to performing each exercise protocol Glycogen-depleting exercise (Exdeplete)
Subjects performed continuous submaximal exercise on the cycle ergometer at a work load corresponding to 70%
of VO2maxfor 90 min This was followed by four 1-min exercise periods at »110% of VO2max, interspersed with 2-min rest periods, and ten 10-s bouts of high-intensity exercise interspersed with 50-s rest periods
Short-term intermittent exercise protocol (IExshort) This protocol consisted of ®fteen 6-s bouts of high-in-tensity exercise, interspersed with 30-s rest periods At the start of each work period, subjects began pedalling with no friction applied to the ¯ywheel When a pedalling frequency of 120 r.p.m was reached (this took less than
1 s) data collection was initiated and the resistance was instantaneously (and automatically via feedback from an on-line computer) added to the ¯ywheel by the lowering
of a mechanical lever arm which was held in a raised position with an electro-magnet With the resistance in place, subjects were instructed to try and maintain a pedal frequency (target speed) of 140 r.p.m for 6 s A visual feedback system, consisting of a series of lights on a graded scale, was used to guide subjects to maintain the target speed Pedalling frequency was measured via a photoelectric sensor which was connected on-line to a computer This method has been described in more detail previously (Balsom et al 1993, Balsom 1995) The work load was individually selected for each subject so that in the control condition »8 work periods could be com-pleted (i.e for the entire 6 s) at the target speed This was determined during the habituation visits The mean power output for the ®rst exercise period was »958 W which is more than three times greater than that which, during continuous exercise, would have elicited maximal oxygen uptake Muscle biopsy and venous blood samples were taken at rest and directly after the last work period
In addition, a venous blood sample was taken 15 min post-exercise to measure peak hypoxanthine accumula-tion (cf Hellsten 1993) Fingertip blood samples (25 lL) were taken at rest and 3 min post-exercise
Prolonged intermittent exercise protocol (IExlong) This exercise protocol is represented schematically in Fig 1 Subjects were instructed to complete as many 6-s bouts of high-intensity exercise as possible on the cycle ergometer, at the target speed of 140 r.p.m The exercise periods were performed as described for IExshortbut with less friction applied to the ¯ywheel During the ®rst
30 exercise periods, the load was 80% of that used in
Trang 3IExshort Thereafter, it alternated between 70 and 90%
(see Fig 1) The test was terminated at a point of fatigue,
de®ned as the point where either mean or end pedalling
frequency (mean of last 2 s) decreased to below
135 r.p.m for 2 out of any 3 consecutive work periods
Subjects were allowed to drink water ad libitum during this
exercise protocol Muscle biopsy samples were taken at
rest and directly after the last exercise period Venous
blood samples were taken at rest, after every 30th exercise
period, and 1 and 15 min post-exercise Fingertip blood
samples (25 lL) were taken at rest, after the 5th, 10th,
15th and 31st work periods, after every 30th work period
thereafter, and 3 min post-exercise Oxygen uptake was
measured continuously over six work and recovery
pe-riods, beginning at the start of the 20th, 50th and 70th
work period Three Douglas bags were used on each
occasion Bag 1 to collect expired air from all six work
periods, Bag 2 to collect expired air from 0 to 15 s of the
®rst three recovery periods and Bag 3 to collect expired
air from 15 to 30 s of the last three recovery periods
Diet
The low carbohydrate diet consisted of 4% CHO (% of
total energy intake) and »3000 kcal day±1 The mean fat
and protein content was 64 and 32%, respectively
Sub-jects were supplied with a `food box' which contained all
the food that they were to consume during the 48-h
pe-riod between Exdepleteand IExlong They were instructed
not to consume any additional food but encouraged to
drink water ad libitum During the high-carbohydrate diet,
CHO intake accounted for 67% of the total energy
in-take The total energy intake was also »3000 kcal day)1
and the mean fat and protein content was 20 and 13%,
respectively
Measurements
Muscle biopsies
Muscle biopsies were obtained from m vastus lateralis
using a Weil Blakesly chonchotome (Wisex, MoÈlndal,
Sweden) After local anaesthesia to desensitize the
sur-rounding tissue, a 5±7-mm long incision was made in the
skin and underlying muscle fascia Using the
concho-tome, a piece of muscle weighing »70 mg, was removed
(»1 cm under the fascia) It was immediately frozen in
liquid nitrogen, stored at ±70 °C and later freeze-dried,
powdered and analysed for glycogen using a method adapted from Bergmeyer (1970) as described by Harris
et al (1974) In each experiment, biopsies were taken from different sites in the same leg
Blood lactate Fingertip blood was haemolysed in a buffer solution (YSI 2357) of 1 : 2 dilution containing triton (5 g L±1) and stored at ±20 °C Whole blood lactate concentrations were measured enzymatically using a YSI 2300GL lactate analyser (Yellow Springs Instruments, Ohio, USA) as described by Foxdal et al (1992)
Plasma hypoxanthine, adrenaline, nor-adrenaline, glycerol and FFA
Venous blood samples (»5 mL) were drawn from a catheter, inserted in a super®cial forearm vein, using heparinized syringes EGTA (30 lL per 1.5 mL) was added to the samples which were to be analysed for FFA
to ensure precipitation with Ca2+ Blood was directly chilled and centrifuged Plasma was stored at ±20 °C Plasma hypoxanthine concentration was analysed using high-performance liquid chromatography (Bioanalytical Systems, Indiana) with a method modi®ed from Wung & Howell (1980), as described by Hellsten (1993) Adren-aline and nor-adrenAdren-aline were analysed by high-perfor-mance liquid chromatography (Bioanalytical Systems, Indiana) with a method modi®ed from Hjemdahl et al (1979) Glycerol was analysed using a ¯uorometric method as described by Lowry & Passonneau (1973) FFA concentrations were analysed using an enzymatic method modi®ed from Shimizu et al (1979) as described
by (Kiens et al 1993) Plasma hypoxanthine concentra-tion was measured from the blood samples taken 15 min post-exercise whereas the remaining measurements were made on the blood samples taken within 1 min after the cessation of exercise
Oxygen uptake Oxygen uptake was measured using the Douglas bag technique The volume of expired air in each Douglas bag was measured using a Tissot spirometer (WE Collins,
MA, USA) Fractions of oxygen and carbon dioxide were determined using a Beckman S-3A and LB-2 gas analyser, respectively (Beckman Instruments, Fullerton, USA) The gas analysers were calibrated with gases containing 16.04% oxygen and 3.85% carbon dioxide (as checked by AGA Gas AB, Sundbyberg, Sweden)
Figure 1 A schematic representation of the
prolonged intermittent exercise protocol IEx long
Trang 4accepted at the 0.05 level.
RESULTS
Muscle glycogen concentration (Table 1)
As a result of the exercise and dietary manipulation,
pre-exercise muscle glycogen concentration before both
IExshortand IExlongwas signi®cantly lower following the
exercise and diet regimen which included the low
car-bohydrate content (LOW-CHO) than following the
ex-ercise and diet regimen which included the high
carbohydrate content (HIGH-CHO) During the 15
sprints in IExshort, the glycogen breakdown was 53
(14) mmol kg (dw)±1(from 180 (14) to 127 (22) mmol
kg (dw)±1P < 0.05) The corresponding value following
HIGH-CHO was 78 (29) mmol kg (dw)±1 (from 397
(35) to 319 (15) mmol kg (dw)±1, P < 0.05) The
dif-ference in glycogen breakdown between the two
exper-imental conditions was not signi®cantly different In
both IExshort and IExlong, postexercise glycogen
con-centration was signi®cantly lower with LOW-CHO
compared with HIGH-CHO
Performance
IExshort (Fig 2)
The resistance applied to the ¯ywheel was individually
determined for each subject (see Methods) The mean of
the seven subjects was 6.8 (0.6) kg, which at the target
speed of 140 r.p.m corresponded to a power output of
958 (90) W For the ®rst 3 s of each of the 15 exercise
periods, there was no decrease in pedalling frequency in
either of the experimental conditions The mean
pedal-ling frequency for these 3 s in LOW- vs HIGH-CHO
was 139.4 (0.7) and 139.1 (1.3) r.p.m., respectively
(P > 0.05) As can be seen in Fig 2, subjects were not
able to maintain the target speed during the last 3 s over
all of the 15 exercise bouts The decline in pedalling
frequency was, however, signi®cantly greater over the last
four bouts in the LOW-CHO condition compared with
HIGH-CHO (P < 0.05)
IExlong(Fig 3)
The resistance applied to the ¯ywheel for each subject
was expressed as a percentage of the resistance used in
IExshortwhich, for the ®rst 30 exercise periods, was 80%
For the seven subjects, this represented a mean load of
5.6 kg (i.e 80% of 6.8 kg) which, at the target speed of
140 r.p.m corresponded to a power output of 784
(71) W For the remaining work periods until the point of
overall performance times (i.e work + recovery) were
67 and 178 min, respectively
Blood lactate and plasma hypoxanthine, adrenaline, nor-adrenaline, FFA and glycerol (Table 1) Values for both IExshort and IExlong are presented in Table 1 For each metabolite, the post- or peak-exercise concentrations were signi®cantly higher than the pre-exercise values (as the concentration of the catechol-amines in plasma has been shown to fall rapidly on cessation of exercise (cf Kjaer 1992) peak values are presented in preference to post-exercise values) As can
be seen in Table 1, no signi®cant differences were found
in post-exercise FFA and glycerol values, between LOW-and HIGH-CHO When comparisons were made from the measurements made at regular intervals during ex-ercise, however, it was observed that at any given time point, FFA and glycerol values were higher (for all seven subjects) in the LOW-CHO experimental condition Also as can be seen in Table 1 no signi®cant differences were found in post-exercise blood lactate concentrations When comparisons were made from the ®ngertip blood samples taken at regular intervals during exercise, indi-vidual but nonsigni®cant variations in blood lactate concentrations were observed
Oxygen uptake and RER Values measured during IExlongare shown in Table 2 It can be seen that oxygen uptake during the 6-s exercise
Table 1 Muscle glycogen and blood metabolites for the intermittent exercise protocols IEx short and IEx long for the two experimental conditions (values are means and SEM , n = 7)
Pre Post or Peak pk Pre Post or Peak pk IEx short
a Glycogen 180 (14) 127 (22) 397 (35)* 319 (15)*
b Lactate 1.3 (0.1) 9.7 (0.9) 1.5 (0.2) 10.5 (1.1)
c Hypoxanthine 5.8 (0.5) 25.4 (4.0) 4.9 (0.7) 18.9 (3.8)
d Adrenaline 0.36 (0.1) 2.38 (0.4) pk 0.22 (0.0) 1.59 (0.5) pk
d Nor-adrenaline 2.8 (0.6) 23.1 (5.3) pk 2.9 (0.6) 17.9 (4.1) pk IEx long
Glycogen 181 (17) 64 (22) 540 (25)* 151 (48)* Lactate 1.2 (0.2) 4.3 (0.6) 1.7 (0.2) 4.1 (0.4) Hypoxanthine 6.3 (0.8) 14.0 (1.4) 4.5 (0.6) 13.7 (1.2) Adrenaline 0.70 (0.3) 3.57 (0.5) pk 0.26 (0.0) 4.33 (0.5) pk Nor-adrenaline 3.1 (0.7) 21.8 (0.9) pk 2.5 (0.3) 20.9 (1.8) pk
e Glycerol 120 (17) 653 (69) 83 (12) 576 (58)
e FFA 604 (62) 1669 (251) 215 (60)* 1852 (68)
a mmol kg (dw) )1 b mmol L )1 ce lmol L )1 d nmol L )1
*HIGH-CHO signi®cantly different from LOW-CHO.
Trang 5periods was signi®cantly higher than during the
subse-quent recovery periods No signi®cant differences in any
of the measurements were found between the two
ex-perimental conditions (i.e LOW- and HIGH-CHO)
The mean maximal oxygen uptake of the group was 4.2
(0.1) L min±1 RER values measured at the ®xed time
points during both exercise and recovery were
signi®-cantly higher during HIGH-CHO compared with during
LOW-CHO
DISCUSSION
The main ®nding from the current study was that
fol-lowing the exercise and dietary regimen which included
the high carbohydrate content (HIGH-CHO) subjects
were, compared with the exercise and dietary regimen
which included the low carbohydrate content
(LOW-CHO), better able to maintain a high-power output over
the ®fteen 6-s bouts in IExshortand able to perform an
average of 183 more sprints in (IExlong) where standar-dized 6-s bouts of high-intensity exercise were repeated until fatigue
It is well established that the concentration of gly-cogen in skeletal muscle can be manipulated by changes
in the carbohydrate content of the diet and/or deplet-ing exercise (BergstroÈm et al 1967) In the current study, the standardized exercise protocol used to lower muscle glycogen stores included a 2-h period of
Figure 2 End pedalling frequency (mean and SEM of last 3 s,
n 7) for the LOW-CHO (j) and HIGH-CHO ( ) experimental
conditions * signi®cantly different from HIGH-CHO.
Figure 3 Relationship between pre-exercise muscle glycogen con-centrations (m vastus lateralis) and time to fatigue (sum of work and rest periods) for each of the seven subjects during the prolonged intermittent exercise protocol IEx long
Table 2 Oxygen uptake (L min ±1 ) and RER values measured during IEx long (see Methods for sampling procedures) for the two experimental conditions (means and SEM , n = 7)
V O2(bouts)
RER
20±25 0.89 (0.01) 0.80 (0.01) 0.77 (0.02) 1.05 (0.02) * 0.91 (0.02) * 0.89 (0.02) * 50±55 0.86 (0.02) 0.80 (0.01) 0.78 (0.02) 1.03 (0.02) * 0.92 (0.01) * 0.90 (0.02) * 80±85 à 0.87 (0.01) 0.80 (0.01) 0.75 (0.01) 1.01 (0.02) * 0.91 (0.01) * 0.89 (0.02) *
àn = 5.
*HIGH-CHO signi®cantly different from LOW-CHO.
signi®cantly different from 0 to 6 s.
Trang 6by, on one occasion, a diet with a low carbohydrate
content to maintain low glycogen concentration and on
the other occasion a high carbohydrate content to
in-crease muscle glycogen stores From the results
pre-sented in Table 1, it can be seen that the exercise and
dietary regimen used in the current study was successful
in both lowering and keeping low (LOW-CHO) and
lowering and then raising (HIGH-CHO) muscle
gly-cogen concentrations
When using exercise and dietary regimens to
ma-nipulate glycogen stores, it is important to avoid
po-tential non-glycogen related `side-effects' which may
impair subsequent performance during an exercise test
protocol (cf Grisdale et al 1990, Maughan et al 1997)
The seven subjects who participated in the current
study were physical education students who on an
av-erage trained more than three times per week
There-fore, it was not expected that the depleting exercise per
se would have affected performance during the
inter-mittent exercise protocols The two diets administered
in the current study were isoenergetic Subjects were
given food boxes and menus with clear instructions on
the type and quantity of food to eat Therefore, the
observed differences in performance could not be
at-tributed to differences in the total energy intake
be-tween the two diets In addition, previous studies have
shown that reducing the carbohydrate content in the
diet to less than 10% of the total energy intake results in
metabolic acidosis (Greenhaff et al 1987, 1988) In the
current study, the carbohydrate content with
LOW-CHO was restricted to 4% of the total energy intake
The fat and protein content of this diet was 64 and 32%
of the total energy intake, respectively It has been
suggested that when muscle glycogen concentrations
are manipulated with such a low carbohydrate content
and such high fat and protein contents, factors other
than pre-exercise muscle glycogen concentration, for
example, the body's acid±base status, can impair
per-formance (cf Maughan et al 1997) In a recent study by
Maughan et al (1997), a group of male subjects were
®rst administered a diet with a low carbohydrate
con-tent and then prior to performing a bout of
high-in-tensity exercise administered sodium bicarbonate or
sodium citrate to restore normal acid±base status
Compared with a control situation, the time to fatigue
did not change with the administration of the
bicar-bonate This led the authors to conclude that the
metabolic acidosis that accompanies a low carbohydrate
high-protein diet cannot be regarded as the primary
cause of fatigue during high-intensity exercise
moderate submaximal exercise intensity is affected by pre-exercise muscle glycogen concentration The
in-¯uence of the availability of muscle glycogen on the ability to sustain a high-power output during intense exercise, is however, not so well de®ned This may be explained, at least in part, by the numerous different exercise and dietary models that have been used in an attempt to investigate this issue (Jacobs et al 1981, Maughan & Poole 1981, Wootton & Williams 1984, Greenhaff et al 1987, Symons & Jacobs 1989, Bangsbo
et al 1992a, Snyder et al 1992, Jenkins et al 1993, Nevill
et al 1993) It should be noted that muscle glycogen concentrations were only directly measured in one (Symons & Jacobs 1989) of the above-mentioned studies
The exercise pattern used in the current study con-sisted of repeated 6-s bouts of high-intensity exercise This exercise pattern has previously been associated with a rapid utilization of muscle glycogen Gaitanos
et al (1993) reported that muscle glycogen concentra-tion decreased from 317 to 201 mmol kg±1dw after ten 6-s `all-out' sprints on a cycle ergometer It should
be emphasized, however, that the exercise intensity used in the current study, although several-fold greater than that which would have elicited VO2max, was not a maximal `all-out' effort from a stationary start as used in the study by Gaitanos and co-workers This difference
in exercise is re¯ected by the lower glycogen utilization rates found in the current study (Table 1)
In IExshort, the subjects were not able to maintain the target power output for the entire 6-s duration of each exercise period over the 15 trials with either of the dietary conditions They were however, compared with the LOW-CHO, able to maintain a higher power out-put following the HIGH-CHO diet This suggests, that under the conditions employed in this study, high-in-tensity exercise performance was affected by the availability of muscle glycogen The mean post-exercise glycogen concentration after the 15 sprints in IExshort
following LOW and HIGH-CHO were 128 (23) and
319 (15) mmol kg (dw)±1, respectively It is clear from these values that the muscle was not entirely depleted
of glycogen It should be pointed out, however, that no measurements were made at the single ®bre level in the current study and thus information about variations in glycogen concentration within the different ®bre types was not available Almost 25 years ago, Gollnick et al (1973, 1974) reported that at high-exercise intensities glycogen depletion occurred initially in the type II ®-bres Thus one may speculate, albeit tentatively and without direct evidence, that the earlier onset of fatigue
Trang 7observed in the LOW-CHO condition can be partly
explained by a selective glycogen depletion in type II
®bres (cf FrideÂn et al 1989) This hypothesis assumes,
however, that there was a signi®cant contribution from
anaerobic glycolysis throughout the 15 exercise periods
Gaitanos et al (1993) reported that during the last of
ten 6-s bouts of all-out exercise, glycolysis contributed
to only 16.1% of the total ATP production As
previ-ously mentioned, although the exercise and recovery
durations and the mode of exercise used in the current
study was the same as that used by Gaitanos and
co-workers there were differences in the exercise intensity
In IExlong, the subjects were able to complete 111
bouts in LOW-CHO and 294 bouts in HIGH-CHO The
resistance applied to the ¯ywheel in IExlongwas, on an
average only 20% less that used in IExshortwhere subjects
were not able to maintain the target pedalling frequency
over 15 exercise bouts in either of the experimental
conditions Thus, the effect of decreasing the resistance
by only 20% had a marked in¯uence on performance It
should be noted that the mean power output in IExlong
was 778 W, which is equivalent to »200% of VO 2max In
IExlong, the time to fatigue following the
high-carbohy-drate diet was a 2.5-fold greater than following
LOW-CHO This improvement in performance is in agreement
with that reported by Bangsbo et al (1992a) who found
that the time to fatigue during running on a motor-driven
treadmill with high-intensity intermittent exercise (15-s
work periods performed with 10-s intervals) was greater
following a high-carbohydrate diet compared with
fol-lowing a low carbohydrate diet
Previous reports have identi®ed an important role
for lipid oxidation during high-intensity intermittent
exercise (cf EsseÂn 1978) In the prolonged intermittent
exercise protocol performed in the current study there
was a gradual increase in the plasma concentration of
FFA and glycerol over time, suggesting that the
con-tribution from lipid oxidation to the total energy
de-mand was also increasing (cf Hagenfeldt & Wahren
1971) This seems to agree with the ®ndings of EsseÂn
et al (1977) who reported that during intermittent
ex-ercise regulatory mechanisms cause a gradual
down-regulation of glycolysis and an increase in fat oxidation
Although no signi®cant differences were found in
post-exercise FFA and glycerol values between LOW- and
HIGH-CHO, pre-exercise plasma FFA concentrations
were signi®cantly higher with LOW-CHO, and at all of
the sampling time points during exercise, FFA and
glycerol values were higher (for all seven subjects) in
LOW-CHO It has been previously reported that
in-creased plasma FFA concentrations can impair
glyco-gen utilization (Costill et al 1977) and glucose uptake
(Hargreaves et al 1991) Furthermore, as can be seen in
Table 2, RER values measured at the ®xed time points
during both exercise and recovery were signi®cantly
higher during HIGH-CHO compared with during LOW-CHO This is an indication that, as has been previously shown with moderate intensity continuous exercise (Christensen & Hansen 1939), during LOW-CHO there was a greater contribution to the total en-ergy expenditure from fat combustion than compared with during HIGH-CHO In apparent contrast to these
®ndings, however, was the fact that in IExlong no sig-ni®cant differences between the two experimental conditions could be found in glycogen breakdown per exercise bout This latter ®nding is, however, only based on two glycogen measurements (i.e pre- and post-) and therefore is not sensitive enough to account for possible changes over time
The mean oxygen uptake during the 6-s work peri-ods was »70% of maximum oxygen uptake (Table 2) This is in agreement with previous ®ndings from our laboratory where we reported that during repeated
40-m sprints, with 30-s recovery periods, oxygen uptake during the recovery periods increased to 66% of max-imum oxygen uptake These results suggest that during this type of exercise energy produced aerobically is important not only to fuel recovery processes, but also
to resynthesize ATP during contractile activity Indeed Gaitanos et al (1993) reported that over ten 6-s `all-out' sprints, performed at 30-s intervals, there was a de-crease in the contribution from anaerobic metabolism
to the total energy production and a subsequent shift towards aerobic metabolism
The increase in plasma concentration of catechol-amines observed in the current study is in agreement with the ®ndings of Brooks et al (1990) who also re-ported signi®cant increases during repeated 6-s running sprints In the current study no differences were found, between the two experimental conditions, in either peak adrenaline or peak nor-adrenaline concentrations
In IExlong, with LOW-CHO however, concentrations
of both adrenaline and nor-adrenaline were higher at the sampling time points, compared with HIGH-CHO
It is evident that during high-intensity intermittent ex-ercise the secretion of catecholamines plays an impor-tant role in both fat and carbohydrate metabolism (cf Kjaer 1992)
In conclusion, the results of this study have shown that muscle glycogen availability can affect performance during both short-term and more prolonged high-in-tensity intermittent exercise with very short-duration exercise periods Furthermore, even with exercise pe-riods as short as 6 s, there can be relatively high de-mands on aerobic energy turnover when this type of exercise is performed over a prolonged period of time This study was partly supported by a grant from the Karolinska Institute Research foundation Paul Balsom was supported by a grant from the University College of Physical Education and Sports, Stockholm and the Swedish National Centre for Research in Sports.
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