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Basic Sciences: Original Investigations

The Impact of Metabolic Stress on Hormonal Responses and Muscular Adaptations


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Medicine & Science in Sports & Exercise: June 2005 - Volume 37 - Issue 6 - p 955-963
doi: 10.1249/01.mss.0000170470.98084.39
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It 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 physiologists and trainers. Muscular hypertrophy and strength gains following 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, endocrine and neural factors. Of these factors, training-induced muscular hypertrophy might be at least partially related 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 the magnitude of anabolic hormonal responses (4,7,23). Kraemer 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 “hypertrophy-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 catecholamine 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-pituitary (25,29). For example, Gordon et al. (6) have demonstrated the effects of alkalosis treatment by sodium bicarbonate (NaHCO3) ingestion on exercise-induced hormonal response. The NaHCO3 treatment resulted in a greater pH and attenuated GH response to 90 s of cycle ergometer sprinting than the placebo treatment. Moreover, exercise with vascular occlusion markedly enhances metabolite accumulation within the muscles and concomitant GH secretion (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 understood, it is possible that activation of the hypothalamus–pituitary axis by afferent signals from muscle metaboreceptors 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 resistance exercise.

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 compared 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 induced, 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 responses and long-term adaptations of muscles.

Therefore, the purpose of the present study was to examine 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 continuous 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 responses, and also smaller increases in muscular size and strength after the period of training.


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) participated in this study. They were undergraduate or graduate students at the University of Tsukuba, and all had experience 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 before 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 experiment, 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 performed three exercises for the upper and lower limbs (lat pulldown, shoulder press, and bilateral knee extension) using weight-stack machines. These three exercises were chosen 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-body exercise 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 the subjects 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 accumulation of metabolites in the muscles without changing the major variables of the exercise regimen (i.e., total contraction time, work volume, number of sets, rest period between sets). The acute experiment involving NR and WR regimens was separated by more than 5 d. The subjects were instructed 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 overnight 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 analysis. Serum GH concentration was measured by radioimmunoassay (RIA) using kits from Daiichi Radioisotope Lab, Japan. The sensitivity of the GH assay was 0.05 ng·mL−1, 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 DPC Corporation, Japan. The sensitivity of the TES assay was 5.0 ng·dL−1, 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-performance liquid chromatography (HPLC) using kits from Tosoh Corporation, Japan. The sensitivity of these assays, and inter- and intraassay CVs were 6.0 pg·mL−1, 2.7, and 2.0% for E; and 6.0 pg·mL−1, 2.4, and 1.3% for NE. Blood samples were also obtained from the fingertip to measure lactate concentration (LA) using an automatic lactate analyzer (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 regimens 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 training during the experiment period.

Physical characteristics of the subjects.

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 exercise. 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 progressively 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 between 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 unilateral knee extension exercise were measured using an isokinetic dynamometer (COMBIT, Minato Medical Science, Japan). 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° s−1 was then measured. Three to five repetitions were carried out to determine 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 extension 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 per minute 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 repetition without full range of movement was not counted. Following the test, the performed exercise volume was calculated (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 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 correlation 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 significant 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 measures 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 followed by a Tukey's HSD post hoc test. A selective bivariate relationship was investigated using a Pearson product–moment correlation coefficient. P < 0.05 was considered as significant.


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 significant changes except for LA and NE. Between the NR and WR regimens, significant differences were observed in the postexercise 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 relationship (GHauc), the value after the NR regimen (559.6 ± 200.9 ng·mL−1) was approximately threefold higher than that after the WR regimen (185.0 ± 86.7 ng·mL−1; P < 0.05). However, no significant difference was seen in TES concentrations between the two regimens, although the postexercise value (30 min) in the NR regimen was lower than its preexercise value (P < 0.05).

FIGURE 1—Acute changes in blood lactate, growth hormone, and testosterone concentrations after exercises with the NR and WR regimens before the period of training. Values are means ± SE (:
N = 9). * Significant difference from preexercise value ( P < 0.05); # significant difference between the regimens ( P < 0.05).
FIGURE 2—Acute changes in epinephrine and norepinephrine concentrations after exercises with the NR and WR regimens before the period of training. Values are means ± SE (:
N = 9). * Significant difference from preexercise value (P < 0.05); # significant difference between the regimens ( 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 significant difference was observed in the pretraining values between the groups. For the NR group only, body mass significantly increased, and the percentage of fat significantly 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 the MRI 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).

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 between the groups ( P < 0.01).

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 the pretraining 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 significantly larger increase in the NR group (66.4 ± 5.2%) than in the WR group (39.0 ± 3.7%, P < 0.01).

Changes in muscular strength after the training period.
FIGURE 4—Changes in one-repetition maximum (1RM) of shoulder press and bilateral knee extension exercises during and after a 12-wk training period. Absolute values (:
left ) and percent changes ( right ) are shown. Values are means ± SE; ** significant difference from pretraining 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).

The changes in force–velocity relationships after the training period are shown in Figure 5. All values of isometric and isokinetic torques were normalized to the pretraining values of isometric torque. The NR group showed significant increases in isometric and isokinetic strengths at almost all velocities examined, whereas no significant changes were observed in the WR and CON groups. When compared between 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 significantly change after the training period in both the NR (3.6 ± 0.2 vs 3.8 ± 0.2 N·m·cm−2, NS) and WR (3.6 ± 0.2 vs 3.7 ± 0.2 N·m·cm−2, 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.

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°s−1). Values are means ± SE; * significant difference from pretraining value (:
P < 0.05); ** significant difference from pretraining value ( P < 0.01).

Changes in muscular endurance.

Muscular endurance 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 endurance 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 effective in improving muscular endurance.

Changes in exercise volume during shoulder press and knee extension exercises at 70% of 1RM after the training period.
FIGURE 6—Percent changes in work volume during shoulder press and bilateral knee extension exercises at 70% of 1RM after the period of training in the NR, WR, and CON groups. Values are means ± SE; * significant difference between the groups (:
P < 0.05).


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 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 designed 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 muscles, 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 previous studies that have shown that alkalosis treatment attenuates exercise-induced GH secretion (6), and that ischemic 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 responses. However, activation of motor centers may also directly stimulate the hypothalamus–pituitary axis and sympathoadrenal secretion (11). A more fatiguing NR regimen might cause a stronger activation of the motor center and thereby larger hormonal responses. Moreover, the traditional 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 to acute exercise (19). Further studies are needed to examine changes in various GH isoforms after resistance exercise.

We hypothesized that a NR regimen with greater metabolic 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 suppressed 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 ng·dL−1 vs mean of postexercise: 687.9 ± 21.9 ng·dL−1, P < 0.01). These changes in acute TES responses might play a significant role in training-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 contractions are considerably greater than with a training regimen using shorter, intermittent contractions. The current and previous results clearly indicate that continuous repetition without pause is an important factor for strength gains following resistance training.

The present NR regimen caused an increase of approximately 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 hypertrophy (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 increase in muscle CSA, and that this effect is related in part to the increase in muscle fiber recruitment by acidic intramuscular environment. During exercise with marked metabolic 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 postexercise 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 increasing insulin-like growth factor-1 (IGF-1) mRNA in the muscle than do its chronic changes (9). In addition, lines of evidence 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 increases 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. Previous 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 endurance 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 regimen-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 endurance 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 increasing 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 exercise-induced metabolic stress was associated with acute GH, E, and NE responses and chronic muscular adaptations. Moreover, these indicated that reducing rest periods to increase 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 this study. We are also grateful to Dr. Kaneko for assisting with the MRI measurements, and Dr. Kraemer for his constructive comments.

The study was supported by grants from the Ministry of Education, Science, Sports and Culture of Japan, and from the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.


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