MSSE 2005 GOTO - The Impaact of Metabolic Stress on Hormonal Responses and Muscular Adaptations

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The Impact of Metabolic Stress on HormonalResponses and Muscular Adaptations

KAZUSHIGE GOTO1, NAOKATA ISHII2, TOMOHIRO KIZUKA1, and KAORU TAKAMATSU1

1 Institute of Health and Sport Sciences, University of Tsukuba, Tsukuba, Ibaraki, JAPAN; and 2 Department of LifeSciences, Graduate School of Arts and Sciences, University of Tokyo, Komaba, Tokyo, JAPAN

ABSTRACT

GOTO, K., N. ISHII, T. KIZUKA, and K. TAKAMATSU. The Impact of Metabolic Stress on Hormonal Responses and Muscular

Adaptations. Med. Sci. Sports Exerc., Vol. 37, No. 6, pp. 955–963, 2005. Purpose: The purpose of this study was to examine the impact

of exercise-induced metabolic stress on hormonal responses and chronic muscular adaptations. Methods: We compared the acute and

long-term effects of an “NR regimen” (no-rest regimen) and those of a “WR regimen” (regimen with rest period within a set).

Twenty-six male subjects were assigned to either the NR ( N 9), WR ( N 9), or control (CON, N 8) groups. The NR regimen

consisted of 3–5 sets of 10 repetitions at 10-repetition maximum (RM) with an interset rest period of 1 min (lat pulldown, shoulder

press, and bilateral knee extension). In the WR regimen, subjects completed the same protocol as the NR regimen, but took a 30-s rest

period at the midpoint of each set of exercises in order to reduce exercise-induced metabolic stress. Acute hormonal responses to both

regimens were measured followed by a 12-wk period of resistance training. Results: Measurements of blood lactate and serum hormoneconcentrations after the NR and WR regimens showed that the NR regimen induced strong lactate, growth hormone (GH), epinephrine

(E), and norepinephrine (NE) responses, whereas the WR regimen did not. Both regimens failed to cause significant changes in

testosterone. After 12 wk of resistance training, the NR regimen caused greater increases in 1RM (P 0.01), maximal isometric

strength (P 0.05), and muscular endurance (P 0.05) with knee extension than the WR regimen. The NR group showed a marked

increase (P 0.01) in muscle cross-sectional area, whereas the WR and CON groups did not. Conclusion: These results suggest that

exercise-induced metabolic stress is associated with acute GH, E, and NE responses and chronic muscular adaptations following

resistance training. Key Words: MUSCULAR STRENGTH, MUSCULAR HYPERTROPHY, REST PERIOD, LACTATE

I

t has been shown that resistance training for several

weeks increases muscle cross-sectional area, strength,

and power (2,24). Development of optimal resistance

exercise regimens has been a major focus of physiologistsand trainers. Muscular hypertrophy and strength gains fol-

lowing resistance training are thought to be dependent on

the intensity of exercise, in such a way that an intensity of

more than 65% of 1-repetition maximum (1RM) is required

to achieve a substantial effect (16).

The mechanism(s) underlying these muscular adaptations

involves many factors, that is, mechanical, metabolic, en-

docrine and neural factors. Of these factors, training-in-

duced muscular hypertrophy might be at least partially re-

lated to the secretions of endogenous anabolic hormones

such as growth hormone (GH) and testosterone (TES) (5).

The type of exercise regimen has a significant affect on themagnitude of anabolic hormonal responses (4,7,23). Krae-

mer et al. (13) have shown that regimens using moderate

exercise intensity (10RM) and shorter rest periods (1 min)

considerably elevate GH and TES concentrations, whereas

those using higher intensity (5RM) and a longer rest period

(3 min) do not. The former protocol is known as a “hyper-trophy-type regimen”, and is used typically by bodybuilders.

It has been known that local accumulation of metabolic

subproducts such as lactate and hydrogen ions stimulates

sympathetic nerve activity and exercise-induced catechol-

amine secretion (3). The similar afferent pathway has been

shown recently to play an important role in the regulation of

anabolic hormone secretions from the hypothalamus-pitu-

itary (25,29). For example, Gordon et al. (6) have demon-

strated the effects of alkalosis treatment by sodium bicar-

bonate (NaHCO3) ingestion on exercise-induced hormonal

response. The NaHCO3

treatment resulted in a greater pH

and attenuated GH response to 90 s of cycle ergometersprinting than the placebo treatment. Moreover, exercise

with vascular occlusion markedly enhances metabolite ac-

cumulation within the muscles and concomitant GH secre-

tion (25,29), but not TES (29). Although the mechanism by

which anabolic hormone secretions, especially GH, are

stimulated by acid–base changes is yet to be fully under-

stood, it is possible that activation of the hypothalamus–

pituitary axis by afferent signals from muscle metaborecep-

tors has a role in this action (6,25,28). Hence, an exercise

regimen with greater metabolic stress should cause greater

anabolic hormone and catecholamine responses to resis-

tance exercise.

Address for correspondence: K. Takamatsu, Ph.D., Institute of Health and

Sport Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki,

305-8574, Japan; email: [email protected].

Submitted for publication August 2004.

Accepted for publication December 2004.

0195-9131/05/3706-0955/0

MEDICINE & SCIENCE IN SPORTS & EXERCISE®

Copyright © 2005 by the American College of Sports Medicine

DOI: 10.1249/01.mss.0000170470.98084.39

955



In addition, it has been suggested that metabolic stress has

a substantial role in stimulating muscular hypertrophy and

strength gains (20,21,22,26). Schott et al. (21) have reported

that an isometric training regimen resulting in a greater

decrease in intramuscular pH induced a greater muscle

hypertrophy compared with a regimen with only small

change in pH. Rooney et al. (20) revealed that improvement

in strength following 6 wk of dynamic resistance training

was significantly greater in the regimen without rest com-pared with a regimen with a 30-s rest period between each

repetition, although the magnitude of muscular hypertrophy

was not evaluated. The enhanced metabolic accumulation

within muscles caused by vascular occlusion resulted in

marked increases in muscular strength (22) and muscle

cross-sectional area (26) following resistance training.

These findings suggest that exercise-induced metabolic

changes may be associated with muscular adaptations to

resistance training. Moreover, these effects might be in-

duced, in part, by secretion of anabolic hormones, but no

previous studies have been conducted on the influences of

exercise-induced metabolic changes on acute hormonal re-

sponses and long-term adaptations of muscles.

Therefore, the purpose of the present study was to exam-

ine the impacts of exercise-induced metabolic stress on

acute hormonal responses and chronic muscular adaptations

following a 12-wk training period. For this purpose, we

compared the acute and long-term effects of two exercise

regimens with same relative intensity and volume, that is, a

regimen without rest period within a set (no-rest regimen)

and a regimen with a rest period (rest regimen).With con-

tinuous repetitions, the former exercise regimen would

cause the marked change in acid–base balance. Conversely,

brief rest periods inserted into the middle of each set would

reduce accumulation of metabolites, thereby resulting in an

attenuation of exercise-induced metabolic changes (21). We

hypothesized that a regimen with intraset rest period would

cause smaller anabolic hormone and catecholamine re-

sponses, and also smaller increases in muscular size and

strength after the period of training.

METHODS

General Procedure

Twenty-six healthy male subjects (age, 22.7 0.5 yr;height, 172.0 1.1 cm; body mass, 65.9 1.5 kg) partic-

ipated in this study. They were undergraduate or graduate

students at the University of Tsukuba, and all had experi-

ence with recreational resistance training. However, none of

the subjects were involved in any regular training program

at the beginning of the study. No medication (e.g., anabolic

steroids, creatine, sympathoadrenal drugs) was taken by the

subjects, which would have been expected to affect physical

performance. They were informed about the experimental

procedure as well as the purpose of the present study, and

their written informed consent was obtained in advance. The

study was approved by the ethics committee for human

experiments at the Institute of Health and Sport Sciences of

the University of Tsukuba.

The subjects were assigned to experimental ( N 18) and

untrained control (CON; N 8) groups on the basis of

muscular strength and physical characteristics measured be-

fore the training period. The experimental group was further

divided into a “no-rest regimen” (NR) group ( N 9) and

“regimen with rest period” (WR) group ( N 9). Subjects

were matched according to physical characteristics, training

history, and muscular strength, and there were no significant

differences for any parameter between the groups.

The present study consisted of two separate experiments.

In the first part of the experiment, acute hormonal responses

to the NR and WR regimens were investigated using the

subjects in the NR group. In the second part of the experi-

ment, the long-term effects of 12 wk of resistance training

with NR and WR regimens were investigated. Muscular

strength, endurance, and the cross-sectional area (CSA) of

the quadriceps femoris (QF) were measured before and after

the period of training.

Study of Acute Hormonal Response

Subjects and exercise protocol. Nine male subjects

in the NR group participated in this experiment. They per-

formed three exercises for the upper and lower limbs (lat

pulldown, shoulder press, and bilateral knee extension) us-

ing weight-stack machines. These three exercises were cho-

sen for this study because they represent upper- and lower-

limb exercises commonly used in exercise programs.

Although several studies have used whole-body programs

with higher work volume (13,15), our study included only

three upper- and lower-limb exercises, for the reason that

the subjects were unable to adhere strictly to a whole-bodyexercise regimen with a greater work volume and metabolic

stress at the beginning of the training period. Furthermore,

a low-volume regimen using four exercises (bench press, lat

pulldown, knee extension, leg curl) has been shown to

produce significant elevations of lactate and GH (14).

The NR regimen (regimen without rest) consisted of 10

repetitions 3–5 sets (three sets for the lat pulldown and

shoulder press, and five sets for the knee extension) at

10RM. The subjects were allowed to rest for 1 min between

all the sets and exercises. In the first set of exercise, the

exercise intensity was set at approximately 75% of 1RM

(10RM). Thereafter, the intensity was adjusted to allow thesubjects to complete 10 repetitions in each set. In the WR

regimen, the subjects completed the same protocol as the

NR regimen, but took a 30-s rest period at the midpoint of

each exercise set (between the fifth and sixth repetitions).

The subjects performed the exercise at the same relative

intensity, number of repetitions in each set, and interset rest

period as those for the NR regimen. The WR regimen with

the intraset rest period was designed to reduce the accumu-

lation of metabolites in the muscles without changing the

major variables of the exercise regimen (i.e., total contrac-

tion time, work volume, number of sets, rest period between

sets). The acute experiment involving NR and WR regimens

956 Official Journal of the American College of Sports Medicine http://www.acsm-msse.org



was separated by more than 5 d. The subjects were in-

structed to lift and lower the load at a constant velocity,

taking about 2 s for each concentric and eccentric action. All

exercise sessions were preceded by stretching of the major

muscle groups and a single set of warm-up exercises at 50%

of 1RM.

Blood sampling and analyses. Following an over-night fast, the subjects came to the laboratory at 9:30 am and

rested for 30 min before the first blood collection. Venous

blood samples were obtained from an indwelling cannula in

the antecubital vein before, and at 0 (immediately after the

exercise), 15, and 30 min after exercise. Serum samples for

hormone analyses were stored frozen at 85°C until anal-

ysis. Serum GH concentration was measured by radioim-

munoassay (RIA) using kits from Daiichi Radioisotope Lab,

Japan. The sensitivity of the GH assay was 0.05 ng mL1,

and the inter- and intraassay coefficients of variation (CV)

were 3.6 and 3.4%, respectively. Serum testosterone (TES)

concentration was measured by RIA using kits from DPCCorporation, Japan. The sensitivity of the TES assay was 5.0

ngdL1, and the inter- and intraassay CV were 8.4 and

5.9%, respectively. Plasma concentrations of epinephrine

(E) and norepinephrine (NE) were measured by high-per-

formance liquid chromatography (HPLC) using kits from

Tosoh Corporation, Japan. The sensitivity of these assays,

and inter- and intraassay CVs were 6.0 pgmL1, 2.7, and

2.0% for E; and 6.0 pgmL1, 2.4, and 1.3% for NE. Blood

samples were also obtained from the fingertip to measure

lactate concentration (LA) using an automatic lactate ana-

lyzer (YSI1500 Sport, Yellow Springs Instruments, U.S.).

Long-Term Effects of Resistance Training

Subjects and exercise protocol. Twenty-six male

subjects were assigned to the CON, NR, and WR groups.

The NR and WR groups performed the same exercise reg-

imens as those used in the experiments on acute hormone

responses (see above). The physical characteristics of the

subjects are shown in Table 1. Resistance training using lat

pulldown, shoulder press and bilateral knee extension was

performed for 12 wk (23 sessions in total). Exercise training

was performed only once in the first week, and thereafter

twice per week until the 12th week. The training sessions

involved one-on-one supervision by the same assistant

trainer throughout the experimental period. The subjects

were asked to refrain from performing other resistance train-

ing during the experiment period.

Measurements of muscular strength. Muscular

strength was evaluated as 1RM for the shoulder press and

knee extension exercises. Maximal isometric and isokinetic

strengths were also measured for the knee extension exer-

cise. Before measuring 1RM, a warm-up with 10 repetitions

at 50% of the perceived 1RM and stretching of the major

muscle groups subjected to the exercises were performed. In

the present study, 1RM was basically measured by progres-

sively increasing the load until the subjects were unable to

perform a lift. However, when the 1RM exceeded the

equipped weight of the machines, an estimation method

using submaximal load was applied. The relationship be-

tween RM and %RM was as follows: 2RM 95% of 1RM;

3RM 92.5% of 1RM; 4RM 90% of 1RM. Before this

study, the reliability of this estimation was confirmed by

comparing the actual and estimated 1RM in the same group

of subjects. This comparison showed that the intraclass

correlation coefficient was greater than 0.90, and confirmed

reliability was high.

Maximal isometric and isokinetic strengths of the unilat-

eral knee extension exercise were measured using an isoki-

netic dynamometer (COMBIT, Minato Medical Science, Ja-

pan). The subjects were familiarized with the test procedure on

several occasions before taking measurements. They sat on a

chair with their right leg (dominant side) attached firmly to the

lever of the dynamometer. Maximal isometric strength was

measured at a knee angle maintained at 100° (180° at full

extension). The subjects were instructed to exert maximal force

for 3 s. The highest value of 2–3 trials was adopted. Maximal

isokinetic strength at 60, 120, 180, 240, and 300° s1 was then

measured. Three to five repetitions were carried out to deter-mine the peak torque for joint angle ranging between 90 and

180°. A rest period of 2 min was allowed between each trial.

The order of angular velocities tested was randomized, and the

same order was used in each subject between before and after

training period.

Evaluations of muscular endurance. Before and

after the period of training, muscular endurance of the upper

and lower limbs was evaluated by the maximal number of

repetitions for the shoulder press and bilateral knee exten-

sion exercises with the same relative load, that is, 70% of

pre- or posttraining 1RM. Each subject was instructed to

perform the exercise at a frequency of 30 repetitions perminute until exhaustion. Muscular exhaustion was defined

as the moment when the weight ceased to move or the

subjects failed to maintain the prescribed pace. Any repeti-

tion without full range of movement was not counted. Fol-

lowing the test, the performed exercise volume was calcu-

lated (load repetition) and used as a measure of muscular

endurance.

Measurements of muscle cross-sectional area.

The CSA of the thigh at its midpoint was measured using

nuclear magnetic resonance imaging (MRI) with a body coil

(MRI; 1.0 T, Magnetom Impact, Siemens, U.S.). Fifteen

serial sections with a sectional thickness of 15 mm were

TABLE 1. Physical characteristics of the subjects.

VariableNR group(N 9)

WR group(N 9)

CON group(N 8)

Age (yr)Pre 23.1 0.9 21.9 0.7 23.3 0.9

Height (cm)Pre 171.7 2.3 170.3 1.7 174.3 1.2

Body mass (kg)Pre 65.2 3.0 66.4 2.4 66.3 2.2Post 66.4 2.8** 66.9 2.3 66.8 2.4

% fat

Pre 20.3 1.2 20.9 1.4 19.6 1.2Post 18.8 1.3* 20.2 1.4 20.1 1.3

Values are means SE. Pre and post indicate the values obtained before and after thetraining period, respectively. * Significant difference from pretraining value (P 0.05);** significant difference from pretraining value (P 0.01).

ROLE OF METABOLITES IN RESISTANCE TRAINING Medicine & Science in Sports & Exercise

957



acquired with the field of view, repetition time, and echo

time being 240 mm, 800 ms, and 20 ms, respectively, and

the scan matrix 256 256. Image acquisition was started

after the subject lied in the supine position with his legs

extended and relaxed. The scan time was approximately

13–15 min.

From the obtained serial sections, those for two portions

near the midpoint (halfway between the trochanter major

and head of the tibia) of the thigh were chosen for the

analysis of muscle CSA. On the selected cross-sectional

images, the outlines of quadriceps femoris (QF) were traced

by the same expert. Traced images were inputted into a

computer (Power Macintosh G4, Apple Computer), and the

CSA of the QF were measured using NIH image software

(version 1.61). The maximal isometric strength per unit of

CSA was then determined as an index of neuromuscular

function. The measurements were repeated twice for each

image, and the mean values were adopted. A strong corre-

lation between the first and second measurements (r 0.99)

indicated a high reliability of the measurements.

Statistical analysis. Data are expressed as means

SE. In the study on acute hormonal response to the NR and

WR regimens, a two-way analysis of variance (ANOVA)

with repeated measures was used. In the event of a signif-

icant F -ratio, a Tukey’s HSD post hoc test was used to

compare means. In the study on the long-term effects of

resistance training, a two-way ANOVA with repeated mea-

sures and a Tukey’s HSD post hoc were used. Differences

between the percent changes among the three (NR, WR,

CON) groups were examined by a one-way ANOVA fol-

lowed by a Tukey’s HSD post hoc test. A selective bivariate

relationship was investigated using a Pearson product–mo-

ment correlation coefficient. P 0.05 was considered as

significant.

RESULTS

Acute hormonal and lactate responses. Figures 1

and 2 show acute changes in blood LA, serum GH, and TES

(Fig. 1), and plasma E and NE (Fig. 2) before the period of

training. No significant difference was seen in resting LA

and hormone concentrations between the exercise regimens.

In the NR regimen, LA and hormone concentrations (except

for that of TES) showed significant elevations after exercise,

whereas those of the WR regimen showed no significantchanges except for LA and NE. Between the NR and WR

regimens, significant differences were observed in the post-

exercise concentrations of LA, GH, and NE, among which

GH showed the largest difference. When the GH response was

assessed by the area under the time-concentration relation-

ship (GHauc), the value after the NR regimen (559.6

200.9 ngmL1) was approximately threefold higher than that

after the WR regimen (185.0 86.7 ngmL1; P 0.05).

However, no significant difference was seen in TES concen-

trations between the two regimens, although the postexercise

value (30 min) in the NR regimen was lower than its preex-

ercise value (P 0.05).

Changes in muscle cross-sectional area and

body composition. Changes in physical characteristics

over the period of training are shown in Table 1. No sig-

nificant difference was observed in the pretraining values

between the groups. For the NR group only, body mass

FIGURE 1—Acute changes in blood lactate, growth hormone, andtestosterone concentrations after exercises with the NR and WR reg-

imens before the period of training. Values are means SE ( N 9).* Significant difference from preexercise value ( P< 0.05); # significantdifference between the regimens ( P < 0.05).




significantly increased, and the percentage of fat signifi-

cantly decreased after a 12-wk training period, indicating an

increase in lean body mass (LBM).

Figure 3 shows the percent changes in muscle CSA of QF

after the 12-wk training period. To reduce errors in the

measurement associated with a slight mismatch between the

sectional portions obtained before and after the training

period and incidental deformations of muscles during theMRI processes, two sections around the mid portion of the

thigh were selected, and the mean tissue CSA was obtained

from these sections. No significant difference was observed

in the pretraining values of CSA between the groups. The

CSA in both the NR (12.9 1.3%) and WR (4.0 1.2%)

groups significantly increased after the training period, and

the change in CSA was significantly larger in the NR group

than in the WR group (P 0.01). No significant change in

CSA was seen in the CON group after the training period

(0.3 1.3%, NS).

The percent change in CSA after the 12-wk training

period did not show clear correlations with acute exercise-

induced GH increase (difference from preexercise: r 0.66,

P 0.05) and GHauc (r 0.66, P 0.05). However, a

slightly clearer correlation was found between the peak

value of GH after acute exercise and the percent change in

CSA following the training period (r 0.67, P 0.05).

Hormones other than GH did not show a positive correlation

with the percent change in CSA.

Changes in muscular strength. Changes in 1RM

during and after the training period are shown in Table 2 and

Figure 4. No significant differences were observed in thepretraining values between the groups. In both the NR and

WR groups, 1RM of the shoulder press and knee extension

exercises significantly increased after the training period,

and posttraining increases were also significantly greater in

the NR and WR groups than in the CON group. However,

no significant difference in 1RM of the shoulder press was

seen between the effects of the NR and WR regimens. On

the other hand, 1RM of the knee extension showed a sig-

nificantly larger increase in the NR group (66.4 5.2%)

than in the WR group (39.0 3.7%, P 0.01).

TABLE 2. Changes in muscular strength after the training period.

Variable NR group WR group CON group

Maximal isometric strength (Nm)Pre 251.0 15.2 266.6 18.4 247.4 16.9Post 300.6 23.0** 282.7 16.8 251.1 17.1

1RM (kg)Shoulder press

Pre 60.3 3.9 60.3 3.6 55.3 3.9Post 77.9 4.6**†† 73.0 2.1**†† 54.0 3.3

Knee extensionPre 69.3 4.2 71.9 3.8 78.5 5.3Post 114.6 6.7**†† 99.3 4.5**† 79.3 5.1

Values are means SE. Pre and post indicate the values obtained before and after thetraining period, respectively. ** Significant difference from pretraining value (P 0.01);† significant difference from CON group (P 0.05); †† significant difference from CON

group (P 0.01).

FIGURE 2—Acute changes in epinephrine and norepinephrine con-centrations after exercises with the NR and WR regimens before theperiod of training. Values are means SE ( N 9). * Significant

difference from preexercise value ( P < 0.05); # significant differencebetween the regimens ( P < 0.05).

FIGURE 3—Percent changes in muscle cross-sectional area (CSA)after exercise training in the no-rest regimen (NR; N 9), regimen

with a rest period within a set (WR; N 9), and untrained (CON; N

8) groups. Values are means SE; ** significant difference betweenthe groups ( P < 0.01).


959



The changes in force–velocity relationships after the

training period are shown in Figure 5. All values of isomet-

ric and isokinetic torques were normalized to the pretraining

values of isometric torque. The NR group showed signifi-

cant increases in isometric and isokinetic strengths at almost

all velocities examined, whereas no significant changes

were observed in the WR and CON groups. When comparedbetween groups, isometric strength showed a significantly

greater increase in the NR group (19.1 3.1%) than in the

WR (7.2 3.2%) and CON (1.5 1.0%) groups. The

maximal isometric strength per unit of CSA did not signif-

icantly change after the training period in both the NR (3.6

0.2 vs 3.8 0.2 Nmcm2, NS) and WR (3.6 0.2 vs

3.7 0.2 Nmcm2, NS) groups. In addition, no significant

difference was seen in the percent changes of maximal

isometric strength per unit of CSA between the groups,

suggesting that the strength gain in NR group was caused

primarily by muscular hypertrophy.

Changes in muscular endurance. Muscular endur-ance was evaluated as the exercise volume performed at

70% of 1RM for the upper- and lower-limb muscles (Table

3 and Fig. 6). In the upper-limb muscles, no significant

changes were seen in the NR and WR groups after the

training period. In the lower-limb muscles, muscular endur-

ance in the NR and CON groups significantly improved

after training period, whereas that in the WR group did not.

In addition, the percent change in exercise volume was

significantly greater in the NR group (41.8 10.2%) than

in the WR (7.8 8.0%) and CON (5.9 1.7%) groups,

indicating that only the NR regimen was substantially ef-

fective in improving muscular endurance.

DISCUSSION

This study showed that a NR regimen caused larger

elevations of blood LA, GH, and NE concentrations than a

WR regimen. In addition, training with NR regimen caused

FIGURE 4—Changes in one-repetition maximum (1RM) of shoulderpress and bilateral knee extension exercises during and after a 12-wk

training period. Absolute values (left) and percent changes ( right) areshown. Values are means SE; ** significant difference from pre-training value ( P < 0.01); # significant difference from CON group

( P < 0.05); ## significant difference from CON group ( P < 0.01);†† significant difference between the groups ( P < 0.01).

FIGURE 5—Changes in force–velocity relationships after the period of training. All values of knee extension strength were normalized to the

pretraining values of isometric strength (0°s1). Values are means SE; * significant difference from pretraining value ( P < 0.05); ** sig-

nificant difference from pretraining value ( P < 0.01).




much larger increases in muscle CSA and strength than with

the WR regimen, although both regimens had the same

relative intensity and volume. As these regimens were de-

signed to induce different metabolic responses, the specific

adaptations in these regimens may be related to differences

in exercise-induced metabolic stress.

The effects of the NR regimen in inducing elevations of

GH, E, and NE concentrations are consistent with previous

studies (12,23). However, the secretions of GH and NE were

abolished when a short rest period (30 s) was inserted in the

middle of an exercise set (WR regimen). The attenuation of

hormone responses in the WR regimen may result from a

smaller amount of metabolite accumulation within the mus-

cles, because the blood lactate concentration after the WR

regimen was significantly lower than after the NR regimen

(Fig. 1). Several studies have shown that accumulation of

metabolic subproducts stimulates the secretions of GH and

catecholamine through afferent signals from intramuscular

metaboreceptors (6,25,28). This is consistent with the pre-

vious studies that have shown that alkalosis treatment at-

tenuates exercise-induced GH secretion (6), and that isch-emic exercise with increased metabolite accumulation

causes greater GH and NE responses (25,29). Therefore, it

appears that intraset rest in the WR regimen reduces acidity

via lactate production, causing smaller GH and NE re-

sponses. However, activation of motor centers may also

directly stimulate the hypothalamus–pituitary axis and sym-

pathoadrenal secretion (11). A more fatiguing NR regimen

might cause a stronger activation of the motor center and

thereby larger hormonal responses. Moreover, the tradi-

tional measurements of GH concentration have focused only

on the main 22 kDa isoform, whereas recent studies have

shown that other isoforms of GH have specific responses toacute exercise (19). Further studies are needed to examine

changes in various GH isoforms after resistance exercise.

We hypothesized that a NR regimen with greater meta-

bolic stress would produce larger hormonal responses than

a WR regimen. However, this hypothesis was not supported

for TES, because both regimens failed to evoke a significant

TES response, suggesting that TES responses are not greatly

affected by exercise-induced metabolic changes. This is

partially consistent with finding that muscle ischemia

caused by vascular occlusion did not enhance the TES

response to acute exercise (29). TES secretion is also sup-

pressed for several hours after a single bout of strenuous

resistance exercise (18). However, in the present study,

measurements after the period of training showed small but

significant elevations of postexercise TES concentration

(preexercise: 626.8 15.6 ngdL1 vs mean of postexer-

cise: 687.9 21.9 ngdL1

, P 0.01). These changes inacute TES responses might play a significant role in train-

ing-induced muscular adaptations (2).

The percent changes in 1RM and maximal isometric

strength were significantly greater in the NR group than in the

WR group (Figs. 4 and 5). In addition, maximal isokinetic

strength improved at almost all angular velocities in the NR

group, but not in the WR and CON groups (Fig. 5). Rooney et

al. (20) have shown a greater increase in dynamic strength in

a regimen with repeated six repetitions without rest compared

with a regimen with a 30-s rest period between each repetition.

Similarly, Schott et al. (21) have demonstrated that gains in

strength resulting from isometric training with long, fatiguing

FIGURE 6—Percent changes in work volume during shoulder press

and bilateral knee extension exercises at 70% of 1RM after the periodof training in the NR, WR, and CON groups. Values are means SE;* significant difference between the groups ( P < 0.05).

TABLE 3. Changes in exercise volume during shoulder press and knee extensionexercises at 70% of 1RM after the training period.

Variable NR group WR group CON group

Shoulder press (J)Pre 609.8 58.7 546.9 36.3 454.3 49.0Post 681.1 41.0†† 624.3 17.3†† 453.6 43.1

Knee extension (J)Pre 878.6 71.3 884.8 44.2 897.7 75.1Post 1219.5 86.5**# 931.8 45.1 955.6 92.6*

Values are means SE. Pre and post indicate the values obtained before and after thetraining period, respectively. Exercise volume was calculated as load number of

repetitions. * Significant difference from pretraining value (P 0.05); ** significantdifference from pretraining value (P 0.01); # significant difference from WR group (P

0.05); †† significant difference from CON group (P 0.01).


961



contractions are considerably greater than with a training reg-

imen using shorter, intermittent contractions. The current and

previous results clearly indicate that continuous repetition

without pause is an important factor for strength gains follow-

ing resistance training.

The present NR regimen caused an increase of approxi-

mately 13% in muscle CSA, whereas the WR regimen had

no such effect (Fig. 3). Recent studies have suggested that

enhanced metabolic stress within the muscles may strongly

stimulate protein synthesis and concomitant muscle hyper-

trophy (21,26,27). For example, Schott et al. (21) have

shown that an isometric training regimen with a greater

decrease in intramuscular pH could induce a larger degree of

muscle hypertrophy than that with a smaller pH change.

Takarada et al. (26) have also shown that resistance exercise

with moderate vascular occlusion provokes a marked in-

crease in muscle CSA, and that this effect is related in part

to the increase in muscle fiber recruitment by acidic intra-

muscular environment. During exercise with marked meta-

bolic changes, additional motor unit recruitment would be

induced to keep a given level of force, as shown by the

elevated electrical activity of the muscles 17). In the present

study, although electrical activity was not measured, we

speculate that greater metabolic stress in the NR regimen

would have affected muscle fiber recruitment. This might be

responsible for the larger muscular hypertrophy in the NR

regimen.

As shown in Figure 1, significant elevations of postexer-

cise GH concentration were seen only in the NR regimen.

Although the actual roles of circulating anabolic hormones

in muscle growth are still unclear, combinations of GH and

mechanical loading would activate anabolic processes in

skeletal muscle (15). The acute elevation of GH has been

suggested to play a more significant role in increasinginsulin-like growth factor-1 (IGF-1) mRNA in the muscle

than do its chronic changes (9). In addition, lines of evi-

dence have indicated that GH and IGF-1 play crucial roles

in the growth, development, and maintenance of skeletal

muscle. In the present study, the magnitude of the acute GH

responses showed a positive correlation with relative in-

creases in muscle CSA (P 0.04–0.05), implying that GH

might contribute to exercise-induced muscular hypertrophy.

However, actions of GH for muscular hypertrophy are not

direct, and might be mediated by locally produced growth

factors (5). More research should be conducted to determine

whether GH plays a substantial role in resistance training–induced muscular hypertrophy.

Elevations of resting hormone concentrations may also be

related to muscular adaptations to resistance training. Pre-

vious studies have shown positive correlations between

changes in resting TES concentration and both muscular

strength development (2,10) and hypertrophy (8). However,

the present resting concentrations of GH, TES, IGF-1, and

cortisol did not change after the period of exercise training

in the NR group (data not shown).

Interestingly, a different adaptation in muscular endur-

ance was observed between the upper and lower limbs. In

the lower limbs, improvement in muscular endurance was

significantly greater in the NR group than in the WR and

CON groups, whereas that in the upper limb was not regi-

men-dependent (Fig. 6). The reason for this is unclear, but

previous studies have suggested that upper-limb muscles

have a greater trainability than lower-limb muscles that are

more involved in daily physical activities (1). Therefore, a

considerable improvement in muscular endurance of the

upper limb might take place even after the WR regimen with

intraset rest period. The interpretation of muscular endur-

ance data for knee extension needs precaution, because the

work volume in the CON group increased after the period of

training, possibly because of some familiarization with the

testing. Despite this fact, however, the improvement of work

volume during knee extension was much larger in the NR

group than in the WR and CON groups.

In conclusion, the NR regimen, consisting of moderate

intensity and short interset rest periods, evoked strong LA,

GH, E, and NE responses, and was also effective in increas-

ing muscular size and strength after a period of training. In

contrast, both acute and long-term effects were markedly

diminished when a brief rest period was inserted into each

set of exercise. These results suggested that resistance ex-

ercise-induced metabolic stress was associated with acuteGH, E, and NE responses and chronic muscular adaptations.

Moreover, these indicated that reducing rest periods to in-

crease metabolic stress was an effective strategy for gaining

substantial effects of resistance exercise. Development of

resistance exercise regimens designed to produce muscular

hypertrophy should take this phenomenon into account.

The authors are grateful to the subjects who participated in thisstudy. We are also grateful to Dr. Kaneko for assisting with the MRImeasurements, and Dr. Kraemer for his constructive comments.

The study was supported by grants from the Ministry of Educa-tion, Science, Sports and Culture of Japan, and from the ResearchFellowships of the Japan Society for the Promotion of Science forYoung Scientists.

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4. DURAND, R. J., V. D. CASTRACANE, and D. B. HOLLANDER. Hor-

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MSSE 2005 GOTO - The Impaact of Metabolic Stress on Hormonal Responses and Muscular Adaptations