Muscle glycogenolysis during differing intensities of weight

Muscle glycogenolysis during differing intensities

of weight-resistance exercise

ROBERT A. ROBERGS, DAVID R. PEARSON, DAVID L. COSTILL,

WILLIAM J. FINK, DAVID D. PASCOE, MICHAEL A. BENEDICT,

CHARLES P. LAMBERT, AND JEFFREY J. ZACHWEIJA

Human Performance Laboratory, Ball State University, Muncie, Indiana

47306

ROBERGS, ROBERT A., DAVID R. PEARSON, DAVID L. Cos-

TILL, WILLIAM J. FINK, DAVID D. PASCOE, MICHAEL A. BENE-

DICT, CHARLES P. LAMBERT, AND JEFFREY J. ZACHWEIJA.

Muscle glycogenolysis during differing intensities

weight-resis-

tance exercise. J. Appl. Physiol. 70(4): 1700-1706,1991.-Skele-

tal muscle glycogen metabolism was investigated in eight male

subjects during and after six sets of 70% one repetition maxi-

mum (1 RM, I-70) and 35% 1 RM (I-35) intensity weight-resis-

tance leg extension exercise. Total force application to the ma-

chine lever arm was determined via a strain gauge and com-

puter interfaced system and was equated between trials.

Compared with the I-70 trial, the I-35 trial was characterized by

almost double the repetitions (13 t 1 vs. 6 t 0) and half the

peak concentric torque for each repetition (12.4 t 0.5 vs. 24.2 &

1.0 Nm). After the sixth set, muscle glycogen degradation was

similar between I-70 and I-35 trials (47.0 t 6.6 and 46.6 t 6.0

mmol/kg wet wt, respectively), as was muscle lactate accumu-

lation (13.8 t 0.7 and 16.7 t 4.2 mmol/kg wet wt, respectively).

After 2 h of passive recovery without caloric intake, muscle

glycogen increased by 22.2 t 6.8 and 14.2 t 2.5 mmol/kg wet wt

in the I-70 and I-35 trials, respectively. Optical absorbance mea-

surement of periodic acid-Schiff-stained muscle sections after

the 2 h of recovery revealed larger absorbance increases in fast-

twitch than in slow-twitch fibers (0.119 t 0.024 and 0.055 -+

0.024, I) - 0.02). Data indicated that when external work was

constant, the absolute amount of muscle glycogenolysis was the

same regardless of the intensity of resistance exercise. Never-

theless the rate of glycogenolysis during the I-70 trial was ap-

proximately double that of the I-35 trial.

glycogen; lactate; glycogenesis; glycolysis

PAST RESEARCH

evaluating muscle glycogenolysis and

energy metabolism has predominantly been restricted to

running or cycling (5,

7, 13, 19, 21, 24, 37).

Of these two

modes, cycling has been the preferred exercise because of

the ease in quantifying power output and the relative

subject accessibility for additional invasive and noninva-

sive data collection. This research has shown that as the

intensity of exercise increases, muscle glycogenolysis in-

creases and provides a greater supply of glucose residues

for glycolysis and ATP production

(G-17, 36).

During

high-intensity exercise, both muscle glycogenolysis and

an increased glycolytic flux occur almost immediately (7,

15, 18, 22, 24, 37, 41).

Compared with cycling and running, limited research

has been performed to evaluate muscle energy metabo-

lism during resistance-type exercises

(11, 27, 29, 31, 41).

Early research indicated that short-duration weight-re-

sistance exercise predominantly taxed the muscle’s

stores of ATP and creatine phosphate, with negligible

glycogenolysis and a minor glycolytic energy contribu-

tion

(31).

However, recent research by MacDougall et al.

(29) has indicated that muscle glycogen content can de-

crease by 25% (23 mmol/kg wet wt) in the biceps brachii

after three sets of

weight-resistance repetitions to

muscular failure. Also, Tesch et al.

(41)

reported that

heavy weight-resistance exercise of the legs reduced

muscle glycogen content of the vastus lateralis by

26%

(42

mmol/kg wet wt). Results from the Tesch investiga-

tion also revealed that muscle lactate and glycolytic in-

termediates increased, indicating the flux of glycogen-

derived glucose residues into the glycolytic pathway.

Despite this research, no data exist that compare mus-

cle glycogenolysis during differing intensities of weight-

resistance exercise. Therefore, it is unclear whether the

positive relationship between exercise intensity and the

magnitude of glycogenolysis is retained throughout a

wide range of weight-resistance exercise intensities.

Consequently, this study was designed to quantify the

relationship between skeletal muscle glycogenolysis and

exercise intensity when an indirect measure of force ap-

plication was equated between trials. Two regimens of

resistance exercise were used as follows:

1) 70%

one repe-

tition maximum

RM, I-70) intensity and low repeti-

tion vs. 2)

35% 1

RM (I-35) intensity and higher-repeti-

tion exercise. In addition, the influence of these exercise

regimens on specific fiber type glycogen content and the

rate and magnitude of glycogen synthesis without caloric

supplementation during 2 h of postexercise recovery

were evaluated.

METHODS

Eight males currently participating in weight training

and accustomed to leg extension resistance exercise

served as the subjects for this research. Before the study,

the procedures of the research were explained to each

subject and a written informed consent was read and

signed by each participant. All procedures were per-

formed after approval by the University Internal Review

Board.

Procedures. Each subject first reported to the labora-

tory for recording and measurement of standard physical

and descriptive characteristics, familiarization with the

leg extension machine and experimental protocol, and

determination of the

RM of each leg (Fig.

1).

To ensure

1700 OEl-7567/91 $1.50 Copyright, (c> 1991 the American Physiological Society

GLYCOGENOLYSIS DURING RESISTANCE EXERCISE

1701

bl bl bl

70% 1 RM 35% 1 RM

Familiarization

1 RM

I I

0 1 2 3

DAYS

Sets ’

Time (HE)

FIG 1. Research

blood sampling (bl).

protocol and timing of’

muscle biopsies (B) and

stable preexercise glycogen values in an optimal range of

100-120 mmol/kg wet wt, subjects refrained from exer-

cise for 2 days before testing and were advised of suitable

varieties and quantities of food to provide approximately

4 g carbohydrate per

kilogram body weight. The content

of each subject’s diet during the 2 days preceding the first

trial was monitored by dietary recall and analyzed with a

computer program (Nutricalc;

3.6 t 0.4 g carbohydrate/

kg body wt).

The leg extension exercise was performed on a model

4107 Eagle leg extension machine (Cybex). The machine

was modified by incorporating a force transducer (strain

gauge) in a shin pad attached to the leg extension lever

arm. In addition, a potentiometer was

attached to the

rotational axis of the lever. Both the force transducer

and potentiometer were electronically connected to an

analog-to-digital converter and then interfaced with a

computer

(Apple II) and software . The computer soft-

ware was programmed to calculate the angle of rotation,

the total force accumulation, and the eccentric and con-

centric components of contraction duration and the ac-

cumulated force for each repetition. During leg extension

exercise, the force applied to the strain gauge and lever

arm was read and accumulated at approximately 39

readings/s and continually displayed on the computer

screen. After each set of repetitions, values for each repe-

tition and the mean kinetic data of the set were printed.

Force output from the strain gauge was calibrated us-

ing a regression equation generated by the placement of

known weights on the strain gauge. The angle of rotation

was calibrated with the use of a regression equation de-

termined by lever movement through known angles via a

goniometer. To provide a more comprehensible unit of

work than force accumulation, external concentric work

was calculated as the product of the weight of the stack

and vertical distance lifted. The vertical distance was

calculated from a regression between angular displace-

ment and the vertical displacement of the stack. The

correlation coefficients (r) from each regression calibra-

tion were linear (r = LO, respectively).

Each subject performed two trials (Fig. 1). The first

trial (I-70) consisted of a high-intensity bout of one-

legged leg extension exercise involving six sets of six repe-

titions at I-70 of the weakest leg (left). A 2-min rest in-

terval separated each set. Based on preliminary testing,

the 1 RM leg strength of each subject’s dominant leg was

consi stently higher th .an the contralateral leg, yet these

differences were not significant according to a paired t

test analysis (52.1 t 1.6 vs. 54.6 t 1.5 kg; P = 0.23).

Neverthel ess, to ensure successful completion of the I-70

trial, each subject completed this t rial with the dominant

leg (right), with the use of the 1 RM for the left leg.

Because the subjects were familiar with the exercise, the

time for each repetition and the concentric and eccentric

components were not regimented. However, subjects

were instructed to lift as consistently as possible within

and between trials.

The second trial (I-35) was performed on the following

day with the contralateral leg and involved six sets of

low-intensity (variable repetitions) leg extension exer-

cise at I-35. The total number of repetitions per set could

not be predeterm .ined, because leg extension exercise

con tinued in eat h set until the force accumulation

matched that of the I-70 trial.

Muscle biopsies from the vastus lateralis were per-

formed before each trial, after sets 3 and 6 (final) of exer-

cise, and after 2 h of recovery (2, 14). To prevent sam-

pling of previously traumatized muscle, muscle biopsies

were performed from two incisions located at least 3 cm

apart, and repeated muscle biopsies from the same inci-

sion were angled proximally and distally (9). Muscle

biopsies after the sets 3 and 6 of exercise were performed

with the subject remaining in the leg extension seat,

whereas the biopsies at rest and after 2 h of recovery were

performed with the subject in a supine position. No calo-

ries were ingested during the 2-h recovery, and subjects

remained in a supine position throughout this period.

Blood samples from an antecubital vein were obtained

before exercise, 2 min after set 6, and 2 h postexercise.

Arterialized blood samples from a hyperemized ear lobe

were obtained before exercise, after sets 3 and 6, and 2 h

postexercise.

Analytical methods. Serum glucose concentrations

were analyzed with the use of a YSI glucose analyzer

(model 23A), and blood lactate concentrations were de-

termined enzymatically from whole blood perchloric acid

extracts (28). Changes in venous plasma volume were

determined after measurement of blood hematocrit and

hemoglobin concentrations with the use of the equations

of Dill and Costill (10).

One large portion of each muscle specimen was

frozen

in liquid nitrogen within 30 s after collection and stored

at -8OOC. The remaining portion was mounted in traga-

canth gum, frozen in isopentane cooled to the tempera-

ture of liquid nitrogen, and stored at -80°C for subse-

quent histological analyses. Muscle lactate was deter-

mined from wet weight perchloric acid extraction,

assayed enzymatically, and expressed as millimoles per

kilogram wet wt (28). Muscle glycogen content was de-

termined by HCl hydrolysis and glucose assay and re-

corded as millimoles of glucosyl units per kilogram wet

wt (28).

Muscle histochemistry was performed on serial sec-

tions of tissue (w 10 ,um) cut in a cryostat and consisted

of myosin adenosinetriphosphatase (ATPase) staining

(pH 4.3 and 4.6) and periodic acid-Schiff (PAS) staining.

Skeletal muscle fiber types were determined from the

myosin ATPase stain preincubated at pH 4.3, and each

fiber was designated as fast or slow twitch by a dark or

light stain intensity, as described by Peter et al. (35).

1702

800

700

2600

WV 500

g 400

300

200

y2000

100

-1000

' ' ' ' * ' = I ' 1 * I - I - I - 1 ' 1 * , .

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

GLYCOGENOLYSIS DURING RESISTANCE EXERCISE

-7000 n B

:SOOO a c

15000

m c

:4000

5 g

:3000 Q

TIME (s)

FIG. 2. Force application and force accumulation during concentric

and eccentric phases of l-leg extension repetition. Data are from sub-

ject 4 during a simulated repetition from I-70 trial.

Fiber type glycogen content was determined photometri-

cally from PAS-stained muscle sections

(42,43),

without

correction for mean fiber type areas. Because one sub-

ject’s muscle sections did not react with the PAS stain,

fiber type glycogen content was evaluated for seven sub-

jects.

Statistical design. Muscle glycogen and lactate and

blood lactate and glucose concentrations were analyzed

by two-way analysis of variance (ANOVA) using trial

and time as repeated factors

(25).

Comparisons between

means of two different trials were analyzed by one-way

ANOVA with the use of specific two-way ANOVA re-

peated factor error terms, and differences between the

multiple means within a trial were assessed by the Tukey

test

(25).

Fiber type differences in optical absorbance

after data were combined from both trials were analyzed

by two-way mixed-design ANOVA with fiber type as a

between-group factor and time as a repeated factor.

Muscle fiber type, additional fiber type PAS data, and

simple pairwise comparisons were analyzed by paired t

test. All data are expressed as means t SE, and results of

statistical tests were evaluated at the 0.05 confidence

level.

RESULTS

The leg extension exercise involved concentric contrac-

tions at the knee against resistance, followed by con-

trolled eccentric contractions of the knee extensors dur-

ing the lowering of the lever arm. Although a known

amount of weight was lifted, the resistance throughout

each contraction was variable because of the presence of

a nonuniformly arced cam connected to the cable and

rotational axis of the lever arm. Figure 2 presents data

from subject

during a simulated repetition from the I-70

trial. The cam permitted the largest muscle force develop-

ment midway (-50”) in the leg extension phase, after

which a progressive decrease in concentric force oc-

curred. Force application increased during the eccentric

phase to a value approximating 70% of peak concentric

force (at -35”) and then decreased. This profile was sim-

ilar for all subjects.

The physical characteristics of the subjects and de-

scriptive characteristics of the I-70 and I-35 trials are

presented in Table 1. The resistance load of the I-70 trial

was almost double that of the I-35 trial, resulting in a

similar relationship for peak concentric torque. The num-

ber of repetitions during the I-70 trial was approximately

half that of the I-35 trial, whereas the duration of each

repetition was similar between trials (2.9 t 0.3 and 3.1 t

0.4

s, respectively).

Because the cam was consistent between each trial and

for each subject, external work was calculated as the

product of the weight of the stack and the distance lifted.

External work was calculated for the concentric phase

only and was similar between I-70 and I-35 trials (Table

1).

Although these values are biased by the mechanical

advantage provided by the cam, they are presented to

provide an alternative more comprehensible unit of work

than force accumulation. Nevertheless, because of the

likelihood of muscle glycogenolysis continuing during

the eccentric phase of each contraction, the force accu-

mulation data would more accurately reflect the biochem-

ical work performed in each trial.

Muscle and blood biochemistry. The subjects’ muscle

glycogen and lactate concentrations at rest and during

exercise are presented in Table 2. The subjects began the

study with moderate muscle glycogen stores. However,

despite the dietary and exercise constraints, two sub-

jects’ resting muscle glycogen concentrations were rela-

tively high

(161.0

and 169.6 mmol/kg wet wt).

Six sets of weight-resistance exercise of different in-

tensities but similar force accumulation and external

work resulted in similar amounts of muscle glycogen deg-

radation (46.9 t 6.6 vs. 46.6 t 6.0 mmol/kg wet wt). Al-

though not significant (P = 0.07), the decline in glycogen

during the first three sets in the I-35 trial tended to be

larger than during the I-70 trial. The rate of glycogenoly-

sis during the I-70 trial (0.46 t 0.05 mmol. kg wet

wt-l l s-l) was almost double that of the I-35 trial (0.21 t

0.03 mmol l kg wet wtt’ l s-l or 1.3 t 0.03 vs. 0.61 t 0.01

mmol l kg -’ l rep-‘, respectively). The different glyco-

genolytic rates were to be expected because the I-35 trial

required almost twice as many repetitions as the I-70

trial to obtain equal force accumulation (Tables 1 and

2).

TABLE 1.

Descriptive characteristics of the subjects

and trials

Age, yr

23.2kO.4

Weight, kg 78.9k3.0

Body fat, % 13.42 1.5

Lean body mass, kg 68.O-tl.7

1 RM, kg

Left 52.1k1.6

Right 54.6+ 1.5

Resistance load, kg

r-70 38.2+: 1.1

I-35 9.5+1.1

Peak torque, Nm

I-70 24.221 .O

I-35

12.4-tO.5

Work, Nm

I-70

715.6k7.1

I-35

761.2k56.4

Repetitions per set

I-70 6.0-t0.0

I-35

12.7kl.l

Values are means + SE. Peak torque was for concentric phase only.

Work was calculated as sum of weight of stack and vertical height lifted

during concentric phase of exercise.

GLYCOGENOLYSIS DURING RESISTANCE EXERCISE

1703

TABLE

2. Muscle glycogen and lactate concentrations

and total accumulated force

[GlYlnl~

vdn9

mmol/kg wet wt mmol/kg wet wt

F, N

Rest

H 120.3+_10.8

l.Ok1.3 0.0

L 122.4k9.7

1.hO.l 0.0

Set 3

H 95.6k9.3

11.7k1.6 9,169+1,059

L 91.026.9

13.021.6 9,204+1,028

Set 6

H 73.4s 1

13.8+0.7 17,995&2,004

L 75.9k9.2

16.7k4.2 17,623&2,090

95.6kll.l 1.3kO.l

90.2k9.6 1.2kO.2

Values are means + SE. [Gly], and [La],, muscle glycogen and

lactate concn; F, total accumulated force; H and L, high- and low-in-

tensity leg extension exercise. No significant differences (P < 0.05)

existed between trials for either variable.

Muscle lactate concentrations increased to 13.8 t 0.7

and 16.7 t 4.2 mmol/kg wet wt after set 6 of the I-70

and

I-35 trials, respectively. Two minutes after set 6, muscle

lactate concentrations remained approximately twice

that of blood (Table 3).

Blood glucose concentrations were similar between

trials before exercise, after set 6, and 2 h postexercise

(Table 3). No differences were detected after correction

for plasma volume shifts. During the 2-h postexercise

recovery period, muscle glycogen synthesis after the I-70

and I-35 trials occurred at 11.1 t 3.4 and 7.2 t 1.3

mmol . kg wet wt-’ l h-l, respectively. These increases in

muscle glycogen during recovery approached signifi-

cance (P = 0.08; Table 2). No significant relationships

existed between muscle glycogen synthesis and muscle or

blood lactate concentrations after set 6 of exercise.

Muscle histology. Muscle fiber type data varied consid-

erably between biopsy specimens for any one subject

(range 37-65%). Consequently, each subject’s mean fiber

type proportions were determined from the combined to-

tal of the eight sections, from which a mean of 2,993

fibers per subject were counted.

The linear relationship (r = 0.89) existed between ab-

solute muscle glycogen concentrations and PAS stain

optical absorbance values. On the basis of this relation-

ship, changes in the content of glycogen between fiber

types and between trials were evaluated. As expected

from the muscle glycogen concentrations (Table 2), no

differences in total absorbance existed between I-70 and

I-35 trials at any time. In addition, individual fiber type

absorbance values were similar between trials (Table 4).

To evaluate further glycogen degradation and storage

between fiber types, data from both trials were combined

to increase the sample size (n = 14). Fiber type compari-

sons revealed that absorbance values in fast-twitch fibers

were significantly larger than those in slow-twitch fibers

at rest (Fig. 3). Different rates of glycogenolysis between

fiber types were implied by a significant interaction be-

tween fiber type absorbance and time (P = 0.038). These

differences were also demonstrated by the significantly

larger decreases in absorbance of fast-twitch compared

with slow-twitch

fibers after the third set (0.218 t 0.045

vs. 0.140 t 0.014, P

= 0.041) and between sets 3 and 6

(0.147 t 0.027 vs. 0.066 t 0.021, P = 0.014). A larger

increase in absorbance also occurred in fast-twitch fibers

during the 2-h recovery (0.119 t 0.024 vs. 0.055 t 0.024, P

= 0.02).

DISCUSSION

The methodology used in this research was unique in

that muscle force development during weight-resistance

exercise was quantified and equated between two trials of

differing intensity. Prior research of weight-resistance

exercise has been hampered by the inability to control for

variability in muscle force production or total work. This

method was shown to be reliable because comparisons

between trials for peak torque, force accumulation, and

repetition times provided an internal calibration consis-

tent with the proposed differences in exercise intensity

and external work.

Glycogenolysis, muscle lactate, and blood lactate. As in-

dicated in Tables 1 and 2, a greater rate of muscle glyco-

genolysis occurred in the I-70 trial; yet the longer dura-

tion of the I-35 trial resulted in similar glycogen degrada-

tion. These findings imply that the total amount of

muscle glycogenolysis was dependent on the magnitude

of muscle force development and that the rate of glyco-

genolysis was dependent on exercise intensity. These re-

lationships are not surprising because several investiga-

tors have demonstrated an exponential increase in mus-

cle glycogenolysis with increasing exercise intensity

(percent maximal 0, uptake) during cycling or running

(36). The interesting fact is that muscle glycogenolysis

appears to be dependent on exercise intensity during in-

tense weight-resistance exercise.

The finding of a near linear decrease in muscle glyco-

gen during six sets of leg extension exercise differs from

the reported decrease in glycogenolysis during maximal

intermittent cycle ergometry (32, 37). These studies re-

vealed that skeletal muscle glycogenolysis was high dur-

ing the initial bouts but then declined after the third bout

of intense exercise. The results were influenced by a de-

creasing power output during the successive bouts of ex-

ercise as well as an increased oxidative contribution and

the reversal of phosphorylase b-to-a activation (5, 6,

8, 37).

Muscle contraction during leg extension weight-resis-

tance exercise comprises both eccentric and concentric

TABLE

3. Changes of blood parameters

APV, 5%

LaI,, mM

KW,, mM

Rest

1.2kO.l 93.4k3.0

1.4kO.l 87.5k2.9

Set 6

H -7.3k1.6 6.1+0.4? 100.6+2.7”(‘

L -10.5+1.2* 7.0+0.6t 94.7t3.8-f

H 1.5-tl.l$ 1.01-0.1% 91.7+2.5$

L 2.1+1.6$ 1.2&0.0$ 8&O&2.1$

Values are means + SE. PV, plasma volume; [La-],, arterialized

lactate concn; [Glc],, venous glucose concn. * Significantly different

from I-70 trial, P < 0.05. t Significantly different from rest, P < 0.05.

-i: Significantlv different from set 6, P < 0.05.

1704

GLYCOGENOLYSIS DURING RESISTANCE EXERCISE

TABLE

4. Fiber type optical absorbance data

Total

I-70

Total

I-35

Pre

Set 3

Set 6

0.7220.09 0.77kO.09 0.67-+0.10 0.69+0.10 0.75+0.10

0.64+0.10

0.55+0.08 0.55-tO.08 0.55+0.08 0.51+0.05 0.53kO.05

0.49kO.06

0.42kO.05 0.37kO.04 0.46kO.07 0.42kO.06 0.42+0.05 0.41+0.07

0.51+0.06 0.52kO.06 0.50+0.06 0.48+0.07 0.50+0.06 0.4420.08

Values are means st SE and represent total absorbance adjusted for fiber type proportions (Total) and absorbance readings from fast- (FT) and

slow-twitch (ST) fibers. No significant differences existed between trials or fiber types.

muscle actions, and an elevated intramuscular pressure

remained throughout the duration of each repetition.

Muscle blood flow has been shown to be occluded during

isometric contractions in excess of 20% maximal volun-

tary contraction (l2), whereas muscle blood flow in-

creases linearly with exercise intensity during running

and cycling (26). These contrasts in blood flow and mus-

cle contraction patterns may result in energy metabolism

differences between weight-resistance exercise and in-

termittent intense cycling or running.

The average decrease in muscle glycogen during the

I-70 and I-35 trials (47 mmol/kg wet wt) was larger than

the 23.0 mmol/kg wet wt reported by MacDougall et al.

(29). The subjects of the MacDougall study performed

three sets of single arm bicep curls to muscle failure, with

3-min recovery between sets. Although the total number

of repetitions and mean resistance were not reported by

MacDougall, the number of repetitions and total exercise

time were likely to be less than the I-70 trial of this study.

The glycogenolysis of this study was similar to the 42.0

mmol/kg wet wt reported by Tesch et al. (41). Subjects

from the Tesch investigation performed a total of 20 sets

of five different leg exercises to contractile failure (6-12

repetitions), exercising for approximately 10 min with a

total duration of 30 min. The subjects from this study

exercised for approximately 2 and 4 min for the I-70 and

I-35 trials, respectively, with total durations equaling 12

0.85-t--

0.80-j

w 0.75

g 0.70 *

2 0.65

g 0.60

+ Fast Twitch

* Slow Twitch

0 1000 2000

2 Hr Recovery

FORCE ACCUMULATION

(Kg)

FIG. 3. Fiber type optical absorbance from periodic acid-schiff-

stained muscle sections. Absorbance data are combined from I-70 and

I-35 trials (n = 14) and are plotted relative to force accumulation from

leg extension machine. Data for 2-h recovery are also included. Before

exercise, fast-twitch fibers had a significantly higher absorbance (*P <

0.05). When an ordinal time scale incorporating the 4 data points of

each fiber type was used, a significant interaction existed between ab-

sorbance and time (P = 0.04).

and 14 min. Consequently, the glycogen degradation of

this study occurred over a shorter time period with more

stringent control over exercise and rest interval dura-

tions and muscle force development.

Assuming no change in muscle glycogen content dur-

ing the 2-min rest periods between sets, the glycogeno-

lytic rate in the I-70 and I-35 trials of this study were 0.46

and 0.21 mmol l kg wet wt-l l s-l, respectively. Spriet et

al. (39) measured the rate of glycogenolysis in the human

vastus lateralis during electrical stimulation. During the

first 16 contractions, muscle glycogenolysis occurred at a

rate of 0.41 mmol l kg wet wt-’ l s-l and decreased to 0.17

mmol . kg wet wt-’ l s-l during the remaining 32 contrac-

tions. In other studies, the calculated glycogenolytic rate

after 30 s of sprint running approximated 0.56 mmol l kg

wet wt? l s-l (7), and 30 s of maximal intermittent isoki-

netic cycling has yielded glycogenolytic rates between 0.4

and 0.65 mmol. kg wet w-t-’ l s-’ (32, 37). These latter

values are slightly higher than the rate of our I-70 trial

and indicate a similarity in the rate of skeletal muscle

glycogenolysis between intense cycle ergometry and

weight-resistance exercise.

The majority of muscle lactate accumulation occurred

during the first three sets in each trial. In fact, the muscle

lactate accumulation during the six sets of the I-70 trial

was attained after three sets in the I-35 trial. Conse-

quently, the pattern of lactate accumulation differed be-

tween trials, despite similar glycogen degradation (Table

2). Although the data prevent a thorough evaluation of

glycolysis, it appears that a greater glycolytic stimulus

may have occurred during the final three sets of the I-35

trial. This is understandable, given the lower intensity

and longer exercise duration. To account for the similar

glycogenolysis yet different lactate accumulation be-

tween trials, a larger accumulation of glycolytic interme-

diates would have had to occur during the latter three

sets of the I-70 trial. This interpretation requires further

research yet is supported by the increase in glycolytic

intermediates known to occur with fatigue during isomet-

ric muscle contractions, high-intensity running or cy-

cling, and weight-resistance exercise (7, 18, 20-22,

32, 39).

Lesmes et al. (27) reported greater glycogen degrada-

tion in fast-twitch fibers after “isokinetic” resistance ex-

ercise. Our results appear to support this finding and

imply that resistance exercise causes greater glycogenoly-

sis in fast-twitch than in slow-twitch fibers. However,

these results are not evidence of preferential fast-twitch

fiber recruitment. Fast-twitch fibers are known to have a

larger glycolytic capacity than slow-twitch fibers. In ad-

GLYCOGENOLYSIS DURING RESISTANCE EXERCISE

1705

dition, the significantly larger resting glycogen stores

within the fast-twitch fibers of this study (Fig. 3) would

have favored greater fast-twitch fiber glycogen degrada-

tion.

Glycogenesis. The rates of glycogenesis during the I-70

and I-35 trials occurred at 11.1 t 3.4 and 7.2 t 1.3

mmol 9 kg wet wt-’

h-l, respectively, and were similar to

the 5-9 mmol

kg wet wt-l

h-l values reported for gly-

cogen synthesis after submaximal exercise with carbohy-

drate feedings (23). The rate and total amount of postex-

ercise glycogen synthesis during the I-70 trial are also

comparable with the findings of Hultman (21) (16.2

mmol

kg wet wt-l

h-l). These comparisons indicate

that a large substrate supply for glycogenesis existed

without carbohydrate ingestion and/or significantly ele-

vated blood glucose concentrations (Table 3).

Previous explanations of high rates of glycogen synthe-

sis after intense exercise have been based on the poten-

tial for lactate to be an endogenous muscle glycogenic

precursor (1, 19, 21, 29, 30, 40). Direct evidence for this

process has been provided by in vitro studies with the use

of animal skeletal muscle. Enzyme activity of the gluco-

neogenic enzymes phosphoenolpyruvate carboxykinase

and fructose bisphosphatase and minor activity of pyru-

vate carboxykinase have been shown in white (fast-

twitch glycolytic) vertebrate muscle (1, 34). In addition,

perfusion or incubation of fast-twitch glycolytic muscle

fibers with solutions of high lactate concentrations (4-12

mmol/l) has resulted in high rates of muscle glycogen

synthesis and evidence of the incorporation of 14C from

[ 14C] lactate into glycogen (33).

Despite this evidence, doubt remains that lactate con-

version to glycogen exists under in vivo physiological

conditions (3,4). The data of Hermansen and Vaage (19)

and Hultman (21) indicated limited lactate removal from

exercised muscle; however, the results from the Herman-

sen study were estimations from blood flow measure-

ments of the lower leg rather than the thigh, and differ-

ent subject groups and exercise modes were used for dif-

ferent aspects of data collection. It is unreasonable to

assume that increases in blood lactate amounting to 20

mmol/l do not originate from the exercised muscle and/

or that lactate efflux ceases during the recovery process.

Data from Harris et al. (18) have shown that muscle lac-

tate declines appreciably during the first 4 min of recov-

ery from high-intensity exercise without detectable in-

creases in muscle glycogen and that this lactate removal

is blood flow dependent. Furthermore, short-term exer-

cise to exhaustion results in a large increase in glycolytic

intermediates above pyruvate. These increases often

amount to concentrations equal to or in excess of gluco-

syl unit equivalents of lactate (18,41). Although our data

do not allow a more definitive appraisal of this topic, the

contribution of muscle lactate or glycolytic intermediates

to glycogen synthesis after intense exercise remains un-

clear.

On the basis of the positive relationship between exer-

cise-induced glycogen degradation and subsequent gly-

cogen synthesis (44), it is understandable that fast-

twitch fibers also had a larger amount of postexercise

glycogen storage. Nevertheless, it must be emphasized

that considerable glycogen synthesis also occurred in

slow-twitch muscle fibers (Table 4). This latter fact fur-

ther decreases the likelihood of the conversion of intra-

muscular lactate to glycogen in this study, which

previous research has shown should occur preferentially

in fast-twitch glycolytic muscle (1, 33, 34).

Conclusions. The central finding of this investigation

was that skeletal muscle glycogenolysis was similar in

magnitude between 75 and 35% 1 RM intensity weight-

resistance exercise when an indirect indicator of muscle

force development was equated between trials. In addi-

tion, the results revealed that skeletal muscle glycogenol-

ysis occurred at comparable rates with those reported

during maximal isokinetic cycle ergometry (32,37). Nev-

ertheless, the contribution of glycogen-derived glucose

residues to muscle energetics during weight-resistance

exercise remains uncertain.

The postexercise synthesis of glycogen occurred at

rates higher than reported after exhaustive submaximal

exercise when carbohydrate feedings were ingested dur-

ing the first 6 h of recovery. The predominance of glyco-

genolysis and glycogenesis in fast-twitch fibers was in-

terpreted as a reflection of the greater glycogenolytic and

glycolytic capacities of this fiber type and the bias of the

significantly larger resting glycogen content of the fast-

twitch fibers.

We are grateful to Dr. Duane Eddy and Dr. Lee Engstrom for recom-

mendations during the preparation of the manuscript and to Pat Lam-

bert, Glen Beard, Shi Xiaochai, Jeff Widrick, and Dawn Anderson for

advice. The computer assistance provided by Geoff, Sue, and Laura

Camm is also appreciated.

Address for reprint requests: R. A. Robergs, Human Performance

Laboratory, Dept. of Health Promotion, Physical Education and Lei-

sure Programs, University of New Mexico, Albuquerque, NM 87131.

Received 22 March 1990; accepted in final form 14 November 1990.

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