PHYSIOLOGICAL ADAPTATIONS TO STRENGTH CIRCUIT TRAINING IN POSTMENOPAUSAL WOMEN WITH BONE LOSS
AND
MICHEL A. BRENTANO, EDUARDO L. CADORE, EDUARDO M. DA SILVA, ANELISE B. AMBROSINI, M. COERTJENS, ROSEMARY PETKOWICZ, ITAMARA VIERO, AND LUIZ F. M. KRUEL Grupo de Pesquisa em Atividades Aqua´ticas e Terrestres (GPAT), Exercise Research Laboratory, Federal University of Rio Grande do Sul, Porto Alegre, Brazil
ABSTRACT Brentano, MA, Cadore, EL, Da Silva, EM, Ambrosini, AB, Coertjens, M, Petkowicz, R, Viero, I, Kruel, LFM. Physiological adaptations to strength and circuit training in postmenopausal women with bone loss. J Strength Cond Res 22(6): 1816–1825, 2008—Strength training (ST; high intensity/low volume/long rest) has been used in several populations, including children, young adults, and older adults. However, there is no information about circuit weight training (CWT; low intensity/high volume/short rest) in apparently healthy postmenopausal women. The purpose of the present study was to analyze the effects of high-intensity ST and circuit training on isometric strength (IS), upper limb dynamic strength (ULS) and lower limb dynamic strength (LLS), muscle activation of _ 2max), time quadriceps (EMGquad), maximal oxygen uptake (Vo to exhaustion (TE), and bone mineral density (BMD). Twentyeight postmenopausal women were divided into 3 groups: 1) ST group (STG, n = 9, 45–80% 1 repetition maximum (1RM), 2–4 sets, 20–6 reps), 2) circuit training group (CTG, n = 10, 45–60% 1RM, 2–3 sets, 20–10 reps), and 3) a control group (CON, n = 9, no exercise). Significance level was defined as p # 0.05 for all analyses. After 24 weeks of training, increases were observed in STG and CTG. However, whereas in the STG, the IS (32.7%), ULS (28.7%), LLS (39.4%), EMGquad _ 2max (22%), and TE (19.3%) increased, CTG (50.7%), Vo showed changes only in IS (17.7%), ULS (26.4%), LLS _ 2max (18.6%), and TE (16.8%). BMD did not (42.2%), Vo change in any experimental group. In the CON, there were no changes in the variables analyzed. Our results suggest that ST
Address correspondence to Michel Arias Brentano, michel.brentano@ terra.com.br. 22(6)/1816–1825 Journal of Strength and Conditioning Research Ó 2008 National Strength and Conditioning Association
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and circuit training positively affect postmenopausal women’s muscular strength, muscular activation, and cardiorespiratory fitness, with no changes in BMD. _ 2max, KEY WORDS resistance exercise, electromyography, Vo bone mineral density, postmenopausal women
INTRODUCTION
S
trength training (ST) has been used in several populations, including children (21), adults (22), older adults (12), and athletes (20). It has been suggested that, besides increasing muscular strength, free-weight exercises increase muscle activation through isometric (8) and dynamic (11,12,21) training. This effect was suggested based on the analysis of the electromyographic (EMG) signal amplitude of upper (21) and lower (11,12,22) limb muscles. Muscle mass seems to change, expressed by a cross-sectional area increase (hypertrophy) of trained muscles (19) and alterations in type II muscle fibers since myosin heavy-chain isoforms seem to change from type IIb to IIa in both young and older adults (28,29). Increases in total and local bone mineral density (BMD), mainly found in the lumbar spine and femoral neck, are common after prolonged periods of high-intensity ST (14,31). At last, some modifications in training seem to enhance cardiorespiratory fitness expressed by significant increases in maximal _ 2max) (32) and muscular endurance (4). oxygen uptake (Vo The above-mentioned adaptations are possible as a consequence of different training strategies. High-intensity/ low-volume training, for instance, is likely to promote alterations in muscle strength, muscle activation, and muscle cross-sectional area (4,11). On the other hand, lowintensity/high-volume training is likely to promote more _ 2max (9) and muscle endurance (4), relevant alterations in Vo expressed by maximal repetitions performed usually at 60% of the 1 repetition maximum (1RM). Few studies have analyzed the effects of the lowintensity/high-volume training on older people (5). However, some of these studies have shown results similar to those _ 2max, found for youths, i.e., increases in muscular strength, Vo
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Journal of Strength and Conditioning Research and muscle endurance. Although the low-intensity/ _ 2max in high-volume training results in an increased Vo older adults, previous protocols for this population differ from circuit weight training (CWT), characterized by low intensity and short intervals between sets and exercises, _ 2max which is suggested produce the greatest increases in Vo in young subjects (9,10). Only 1 study of CWT in older adults (18) was found, and it involved only men with heart failure. Other variables, such as isometric strength (IS), muscle activation, and BMD, have not been analyzed in CWT for young and older individuals. Therefore, the purpose of this study was to compare the effects of traditional high-intensity ST with those of low-intensity CWT on muscle activation, _ 2max, time to exhaustion in exercise muscle strength, Vo (TE), and BMD in postmenopausal women with bone loss. It was hypothesized that, compared with a high-intensity ST, CWT will promote lower increases in strength, muscle activation, and BMD, but a greater increase in oxygen consumption.
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TABLE 1. Mean and SD (Mean 6 SD) of the body mass (BM), fat-free mass (FFM), fat mass (FM), and skinfold sum (SK) variables in the strength training (STG), circuit training group (CTG), and the control group (CON) (p . 0.05). STG
CTG
CON
BM (kg) Pre 56.7 6 5.8 60.6 6 8.8 61.4 6 5.9 Post 56.3 6 4.5 59.4 6 7.6 62.8 6 5.3 FFM (kg) Pre 38.5 6 3.5 39.4 6 4.3 40.4 6 3.7 Post 39.2 6 3.3 40.4 6 4.3 40.8 6 2.8 FM (kg) Pre 18.3 6 3.2 21.1 6 5.7 21 6 4.8 Post 17.1 6 2.9 20.2 6 4.7 22 6 4.6 SK (mm) Pre 156.3 6 23.4 176.1 6 42.5 173.2 6 47.4 Post 143.7 6 27.5 164.9 6 33.7 179 6 43.6
METHODS Experimental Approach to the Problem
The subjects of this study were divided into 2 training groups (ST and circuit training) and 1 control group. Both training strategies were organized in a linear, periodized way for 24 weeks. The lower limb dynamic strength (LDS), upper limb dynamic strength (UDS), lower limb IS, and EMG variables were measured and compared every 8 weeks (weeks _ 2max, TE, BMD, and anthropometric 0, 8, 16, and 24). The Vo variables were measured and compared only at the beginning and at the end of a 24-week training period. Subjects
Twenty-eight postmenopausal women from the Jardim Botaˆnico neighborhood (Porto Alegre City) participated in this study. Descriptive data are presented in Table 1. None of the subjects had neuromuscular injury or was engaged in any type of competitive exercise and practiced sports occasionally at a recreational level. All subjects had bone loss, and half were taking hormone therapy (HT). After identification of the subjects taking HT, subjects were divided into 2 subgroups: taking HT (n = 14) and not taking HT (n = 14). Then, the subgroups were randomly divided into 3 experimental groups: circuit training group (CTG; n = 9), ST group (STG; n = 10), and control group (CON; n = 9). After this grouping, 4–5 subjects taking hormone therapy remained in each experimental group. Hormone therapy was assumed as a covariant in each group. Before participation, all subjects were informed about the procedures, risks, and benefits of the study and signed an informed consent form approved by the Ethics Committee of the Federal University of Rio Grande do Sul. Procedures
Body Composition. Body mass (BM), fat-free mass (FFM), fat mass (FM), and skinfolds sum (SF) of all subjects were
calculated before and after 24 weeks of training. Skinfolds were measured at seven body sites (anterior thigh, chest, subscapular, suprailiac, triceps, axillary, and abdomen) and were used to estimate body density (15). Body composition was estimated by using the Siri equation (13). _ 2max and Time to Exhaustion in Exercise Vo
_ 2max and TE were measured by following an The Vo incremental treadmill (INBRAMED, 10200ATL) exercise protocol, similar to that used by De Vito et al. (5) in older adults. After a 2-minute warm-up at 3 kmh21 and 0% inclination, the velocity or inclination was increased in 1-minute stages (Table 2) until voluntary exhaustion. Subjects were encouraged to continue to exercise as long as possible. During the exercise test and the preceding rest periods, the subjects breathed through a face mask. The expired gas was analyzed for O2 and CO2 content by an integrated computerized system (CPX/D, Medical Graphics Corporation, St. Paul, MN) breath by breath; however, the curve was smoothed with the mean values every 30 seconds (Cardiorespiratory Diagnostic Software Breeze Ex, 3.06 version). TE, in seconds, was assumed to be the period between the end of the warm-up until voluntary exhaustion and was obtained with a heart rate recorder (Polar, model S610). The test was considered valid if one of the following parameters was _ 2 met: 1) estimated maximal heart rate (220 2 age), 2) Vo plateau with the increase in intensity, and 3) respiratory exchange ratio .1.10 (4,5,27). Dynamic Strength (1RM)
All subjects attended 3 familiarization sessions with training exercises. UDS and LDS were measured during the arm curl and knee extension exercises, respectively, using the 1RM test. The initial load in each test was calculated based on the VOLUME 22 | NUMBER 6 | NOVEMBER 2008 |
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Adaptations Induced by Weight Training
_ 2max and time to exhaustion protocol. TABLE 2. Vo Stage
Velocity (kmh21)
Inclination (%)
Duration (min)
1 (warm-up) 2 3 4 5 6 7 8 9 10 11 12 13 11 (cool down)
3.0 4.0 4.0 4.0 4.0 5.0 5.0 5.0 5.0 6.0 6.0 7.0 8.0 3.0
0 2.5 5 7.5 10 10 12.5 15 17.5 17.5 20 20 20 0
2 1 1 1 1 1 1 1 1 1 1 1 1 2
electrode placement, the skin area was shaved, cleaned with isopropyl alcohol, and abraded to reduce skin impedance. The EMG signal was collected by an 8-channel unit (Model AMT-8 channel; Bortec, Calgary, Alberta, Canada) with a sample rate of 2000 Hz. The common mode rejection of the current system is 115 dB at 60 Hz with an input impedance of 10 GV. For the EMG analysis, the SAD32 data acquisition system (version 2.61.05 mp) was used to signal filtering (fifth order Butterworth band pass between 20 and 500 Hz) and windowing (1 second) in the maximal peak force of the higher MVC. In each window, 50-ms root mean square envelopes (Hamming) were used, and the mean of these envelopes was used to quantify the EMG signal of the muscles analyzed (25).
Bone Mineral Density
percentage of body mass (1). After the first trial, the load was corrected by the coefficients proposed by Lombardi (17) to estimate 1RM. After each adjustment, a new trial was carried out in a repetitive process until the 1RM was achieved. Three trials were conducted separated by a 2-minute rest. Then, if the 1RM was not reached, a new test was administered after 48 hours; this occurred in 2 subjects only in the pretraining test.
For the BMD analysis, dual-energy x-ray absorptiometry (QDR 4500A; Hologic, Bedford, MA) was used. Subjects were placed in a supine position or on their side while the x-ray scanner performed a series of transverse scans, moving from top to bottom of the region being measured at 1-cm intervals. All the scanning and analyses were done by the same experienced operator. Calibration of the densitometer was checked daily against a standard calibration block supplied by the manufacturer. Five separate scans were performed: lumbar spine (DMOL2L4), and different sites of the femur such as the trochanter of femur (DMOtroc), intertrochanter (DMOinter), femoral neck (DMOneck), and Ward’s triangle (DMOWard).
Isometric Strength and Electromyographic Signal
An isokinetic dynamometer (Cybex Norm; Lumex & Co., Ronkonkoma, NY) was used to measure the isometric torque (Nm) of the right knee extensors. Subjects were positioned with their hips and knees extended at approximately 110° and 107°, respectively (full extension = 180°), and their right knee was aligned with the dynamometer’s rotation axis (12). After a verbal command, each subject performed a maximal voluntary contraction (MVC) for 3 seconds. Three trials were conducted, with a 2-minute interval between each trial, and the highest value was used for later analysis. The torque signal was synchronized with the EMG signal and recorded in a desktop computer (Intel Pentium 133, 64 MB RAM). The EMG activity of the vastus lateralis (EMGVlat) and vastus medialis (EMGVmed) muscles was registered parallel to the isometric protocol. Bipolar (20-mm interelectrode distance) surface electrodes (Noraxon model 272) were placed longitudinally to the direction of the muscle fibers, following the recommendations of Pincivero et al. (25). The reference electrode was placed over the medial shaft of the tibia ;6–8 cm below the inferior pole of the patella. The positioning of the electrodes was marked on the skin, and these marks were remade periodically in order to ensure the same positioning for 24 weeks. The reliability of the EMG signal, using this methodology, was verified and a high level of intraclass correlation was observed (r = 0.972, p , 0.000). Before
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Training
Training sessions lasted for 1 hour, during which subjects first warmed up on either a cycloergometer or a treadmill for 5 minutes and then performed machine (with weight stacks and pulleys) and free-weight exercises. The STG and CTG trained for 24 weeks, with 3 training sessions per week. Training was periodized in a linear way (2) aiming at a gradual increase in intensity during the 24 weeks. Progression loads and corrections are presented in Table 3. The CTG trained with 20–10 repetitions and 45–60% 1RM, performing 2–3 sets for each exercise. The STG trained with 20–6 repetitions and 45–80% 1RM, performing 2–4 sets for each exercise. Every 8 weeks, a new 1RM test was conducted to verify changes in muscular strength. However, every 4 weeks higher percentages were prescribed based on the last 1RM test (Table 3). The STG performed each exercise separately, with a 2-minute rest between sets, whereas the CTG performed the same exercises with no rest between exercises (Table 4). The following exercises were selected: leg press, hip abduction, hip adduction, knee extension, chest fly, reverse fly, arm curl, triceps push-down, sit-ups, and back extension. Both groups, however, performed the exercises, alternating the upper and lower limbs. The CON was asked to keep the same activities during the period of 24 weeks.
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TABLE 3. Periodization of circuit training (CTG) and strength training (STG). Weeks 1–4 5–8 9–12 13–16 17–20 21–24
CTG 2 2 3 3 3 3
3 20–15 3 20–15 3 15–12 3 15–12 3 12–10 3 12–10
STG 45–50% 1RM 45–50% 1RM 50–55% 1RM 50–55% 1RM 55–60% 1RM 55–60% 1RM
Statistical Analyses
Statistical analysis were performed with software (Statistical Package for Social Sciences, version 11.0), and variables are expressed as mean 6 SD. The normal gaussian distribution of the data was verified by the Shapiro-Wilks test. All data from this study were analyzed using a repeated-measures, _ 2max, and TE variables) or 4-level (IS, 2-level (BMD, Vo UDS, LDS, and EMG variables) design, analysis of variance (ANOVA). A between-group analysis was carried out using 1-way ANOVA. We performed Bonferroni post hoc test to analyze the differences between weeks (0, 8, 16, and 24) and groups (STG, CTG, and CON). Pearson product moment correlation coefficients were also calculated to determine the relationships between variables, and the intraclass reliability
2 3 20–15 2 3 15–12 3 3 12–10 3 3 12–10 3 3 10–8 4 3 8–6
45–50% 50–60% 55–65% 60–70% 65–75% 70–80%
1RM 1RM 1RM 1RM 1RM 1RM
of the EMG data was verified (model alpha). The statistical power for the n size ranged from 0.80 to 0.90, and the significance level was defined as p # 0.05 for all analyses.
RESULTS The 1-way ANOVA showed no differences between groups in all variables at the beginning of the study. All variables are presented separately.
Body Composition
After 24 weeks of training, no within- or between-group differences were found in any anthropometric data (Table 1).
TABLE 4. Training session scheme of circuit training (CTG) and strength training (STG) using two sets of training. CTG Exercise Leg press Inverse fly Leg press Inverse fly Knee extension Chest fly Knee extension Chest fly Hip adduction Sit-up Hip adduction Sit-up Hip abduction Back extension Hip abduction Back extension Arm curl Triceps push-down Arm curl Triceps push-down
STG Set 1st 1st 2nd 2nd 1st 1st 2nd 2nd 1st 1st 2nd 2nd 1st 1st 2nd 2nd 1st 1st 2nd 2nd
)
Rest None
) None
) None
) None
) None
Exercise
Set
Leg press Leg press Inverse fly Inverse fly Knee extension Knee extension Chest fly Chest fly Hip adduction Hip adduction Sit-up Sit-up Hip abduction Hip abduction Back extension Back extension Arm curl Arm curl Triceps push-down Triceps push-down
1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd
Rest
2 min 2 min 2 min 2 min 2 min 2 min 2 min 2 min 2 min 2 min
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Adaptations Induced by Weight Training significantly after 24 weeks of training. Regarding lower limb muscle activation, considering just the pre- and post-tests, the STG showed significant increases in the EMGVlat, EMGVmed, and EMGQuad variables, while the CTG and CON showed no modifications. Regarding the intermediate measures of EMGVlat, EMGVmed, _ 2max and time of exhaustion (TE) in exercise Figure 1. Means, SDs, and intra- and intergroup comparisons of the Vo and EMGQuad muscle activavariables. (a) Different from the first test (p , 0.01); (e) Different from the control group (CON) (p , 0.01). tion, after the first 8 weeks of training, we observed significant alterations only in the STG, in which EMGVlat increased _Vo2max and Time to Exhaustion in Exercise significantly from the second to the last test, as well as All subjects performed valid tests. We observed that both EMGQuad, which increased during the same tests. _Vo max and TE increased significantly in both training 2
groups after 24 weeks of training (Figure 1). However, the CON showed no changes in these variables. At the end of the 24 weeks of training, the STG and CTG showed higher _ 2max and TE values than the CON (Figure 1). Vo Dynamic Strength (1RM)
After 24 weeks of training, LDS increased significantly in both the STG and CTG. The same occurred with UDS (Figure 2). The CON showed no changes in LDS and UDS during the training period. At the end of 24 weeks, the STG and CTG demonstrated greater UDS and LDS values than the CON (Figure 2). The intermediate tests indicated that the STG experienced significant increases in LDS from the second to the third test and from the third to the last test, whereas the CTG showed a significant alteration from just the second to the third test. The UDS in the STG increased from the second to the third and from the third to the last test. In the CTG, after the first 8 weeks of training, UDS increased significantly just at the end of the 24 weeks.
Bone Mineral Density
There were no alterations in the BMDL2L4, BMDneck, BMDinter, BMDtroc, and BMDWard variables in all groups after the 24-week period (Figure 4). However, it can be noted that the STG showed higher means (not significant) after training, while the CTG showed lower means (not significant). In the sample analyzed, HT did not affect the results since no interaction between heart rate and time was found (Table 5). Correlations
After 24 weeks of training, we found correlations between _ 2max (r = 0.73, p = 0.000), LDS, and TE (r = 0.72, LDS and Vo _ 2max (r = 0.59, p , 0.01), and IS and TE p = 0.000), IS, and Vo (r = 0.54, p , 0.01).
DISCUSSION
The major finding of our study is that weight training improves both strength and cardiovascular fitness of postmenopausal women to the same degree. However, as expected, circuit Isometric Strength and Electromyographic Signal weight training seems to be less effective at improving lower Data from IS and the EMG signal are shown in Figure 3. limb muscle activation and BMD. Our results fail to fully Isometric strength in the STG and CTG increased support our hypothesis and contradict results of other studies that show different adaptations in strength and cardiovascular fitness with different training strategies (4). Given this, we discuss all the variables separately. After 6 months of training, there was an increase in _ 2max and TE in both the Vo Figure 2. Means, SDs, and intra- and intergroups comparisons of the upper limb dynamic strength (UDS) and STG and CTG, whereas no lower limb dynamic strength (LDS). (a) Different from the first test (p , 0.05). (b) Different from the second test (p , differences were found in the 0.05). (c) Different from the third test (p , 0.05). (d) Different from the circuit training group (CTG) (p , 0.05). (e) CON. Most studies involving Different from the control group CON (p , 0.05). traditional ST (high intensity,
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Figure 3. Means, SDs, and intra- and intergroups comparisons between the isometric strength (IS), vastus lateralis muscle activation (EMGVlat), vastus medialis muscle activation (EMGVmed), and quadriceps muscle activation (EMGQuad) variables. (a) Different from the first test (p , 0.05). (b) Different from the second test (p , 0.05). (c) Different from the third test (p , 0.05). (d) Different from the circuit training (CTG) (p , 0.05). (e) Different from the control group (CON) (p , 0.05).
low volume, and long rest periods) have not reported any _ 2max in young adults (4). However, positive effects in Vo TE seems to be increased by a higher running economy as a result of ST (32). Older people may have a different response since some studies have revealed positive adapta_ 2max and TE in this population with hightions in Vo intensity ST (7,32). The increased muscular strength seems to affect positively these variables in older subjects (32) and may be one of the factors that promoted the increase in _ 2max and TE in our study. Obtaining the ‘‘real’’ measure Vo _ 2max in sedentary individuals, especially older adults, of Vo can be difficult due to low levels of the LLS. Maximal treadmill protocols are characterized by an increased intensity of effort produced by a progressive inclination that demands the production of high levels of lower limb force. As a result, sedentary individuals would not be able to reach the ‘‘real’’ maximum. Vincent et al. (32) demonstrated that older individuals with a greater force production in the knee extensor muscles are more resistant to fatigue than untrained individuals. Furthermore, we observed a significant correlation of posttraining LDS with TE (r = 0.72, p = 0.000) _ 2max (r = 0.73, p = 0.000). and Vo _ 2max and Other aspects may be related to the increase in Vo TE. There is practically a consensus, for instance, that, in young adults, ST promotes alterations in myosin heavy-chain isoforms, converting type IIb to type IIa isoforms (29), thus
suggesting the ability of a ‘‘pool’’ of type IIb fibers to transform into more metabolically oxidative fibers. These alterations have been attributed to high levels of lactate at the end of sets and recently were also observed in older women (28). Vascular and biochemical peripheral adaptations that would provide a greater supply and utilization of oxygen in the muscle may also explain the better endurance performance in older subjects after being submitted to ST. The findings by Frontera et al. (7) support this idea. They observed that older subjects after 12 weeks of high-intensity training (80% 1RM) showed _ 2max (38.6 6 1.2 vs. 40.5 6 1.6 a significant increase in Vo mlkgFFM21min21, p , 0.05), and this result was attributed to a concomitant increase in relative capillarization (15.1%, p = 0.042) and citrate synthase activity (38%, p = 0.018). Thus, we suggest that it is reasonable to assume that the increase in maximal oxygen uptake found in the STG is a result of peripheral adaptations. Regarding the CTG, the studies involving traditional weight circuit training for young adults have shown low to _ 2max (9,10). It must be noted that moderate increases in Vo the increments found in our study were considerably higher (STG, 22.6%; CTG, 18.6%) than those reported by Gettman et al. (10) and Frontera et al. (7), whose methodologies were similar to those used in the CTG and STG, respectively. The short duration (12 weeks) of these previous studies VOLUME 22 | NUMBER 6 | NOVEMBER 2008 |
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Figure 4. Means, SDs, and intra- and intergroups comparisons between L2-4 bone mineral density (BMDL2-L4), femoral neck bone mineral density (BMDneck), trochanter of femur bone mineral density (BMDtroc), Ward’s triangle bone mineral density (BMDWard), and intertrochanter bone mineral density (BMDinter).
_ 2max (CTG, 5%; may account for the modest increases in Vo STG, 12%). Gettman et al. (10) used only knee extension and knee flexion exercises in contrast to the present study that proposed at least 1 exercise for all major muscle groups. Our results shows that LDS, ULS, and EMG activity of the vastus lateralis muscle are modified in the first 8 weeks of training in both training groups in agreement with the results of other studies that showed adaptations in an early period in sedentary subjects (12). Significant increases in IS, LLS, and ULS were observed in the training groups; however, no differences were found between the STG and CTG. These results corroborate the findings of Vincent et al. (32), who divided subjects ages 60–83 years old into 2 training groups with different intensities (50 and 80% 1RM) for 6 months and observed significant increases in muscle strength at these 2
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intensities, but no differences between groups. Our results may be explained by the fact that subjects had no previous experience in ST. This kind of individual shows a great increase in muscle strength after training at different intensities (16). Peterson et al. (24) and Rhea et al. (26) suggest that untrained individuals have the most significant improvements in strength with loads of about 60% 1RM and support a high increment in the CTG. Moreover, older people frequently show selective atrophy of type II fibers (sarcopenia) and motor neuron loss (3,6), a fact that may impair a greater increase in strength with higher loads. After 24 weeks of training, an increase in EMG activity was observed only in the STG. This group showed greater EMG activity in both muscles analyzed compared with the CTG and CON. Our results support the hypothesis that
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TABLE 5. Time vs. hormone therapy: analysis of the lumbar bone mineral density (BMDL2L4), femoral neck bone mineral density (BMDneck), trochanter of femur bone mineral density (BMDtroc), intertrochanter bone mineral density (BMDinter), and Ward’s triangle bone mineral density (BMDWard) variables vs. hormone therapy during 24 weeks (p , 0.05). Variable
F test
p
BMDL2L4 BMDneck BMDtroc BMDinter BMDWard
3.543 0.709 1.859 0.050 0.061
0.072 0.408 0.185 0.824 0.807
high-intensity training promotes a greater activation of trained muscles. Strength training seems to emphasize an increase in motor unit recruitment and/or in the firing rate of the active motor units of trained muscles (11,12,21–23), and these adaptations probably explain our results because we found no changes in body composition. Although this difference was expected to cause greater force production in the members of the STG, this did not occur. This paradox might be accounted for by a phenomenon that has been associated with an increase in muscle strength: the decrease in the antagonist muscle coactivation (8,11). Garfinkel and Cafarelli (8) submitted young adults to 12 weeks of isometric training and found an increase in the knee extensor muscle IS, with no differences in EMG activity. The same results were observed in the CTG. Few studies have analyzed the effects of ST on the antagonist muscle co-activation in older adults; however, a decrease in co-activation seems to be possible in this population (11,12). In the circuit training group, although there were no differences in EMG activity after 24 weeks of training, our results indicate that the STG and CTG had a similar pattern in the first 8 weeks of training, when they showed a significant increase in strength and EMG activity. Several studies with circuit weight training have reported positive effects on muscle strength or endurance performance (9,18); however, none of them analyzed the EMG signal or had a training period longer than 12 weeks, except for Gettman et al. (9), with 20 weeks of training. Thus, it is difficult to compare the results from those studies to those obtained in our study. A possible explanation for the lack of differences in EMG activity in the circuit training group after the initial 8 weeks of training may be the low training intensity, which might not be enough to recruit the higher threshold motor units (30) responsible for an important increase in EMG amplitude (11,12,21). Tesch et al. (30) analyzed the glycogen depletion of type I, IIa, IIab, and IIb fibers of the vastus lateralis muscle in untrained individuals
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after performing 3 protocols at different intensities (30, 45, and 60% 1RM). All protocols consisted of 5 sets of 10 repetitions of the knee extension exercise. It was observed that type I and IIa fibers had a significant decrease in glycogen in all exercise intensities, whereas there was a decrease in glycogen in type IIab and IIb fibers only at 60% 1RM, thus suggesting that type IIab and IIb fibers are used only at intensities above 60% 1RM. In the present study, the CTG was submitted to intensities between 45 and 60% 1RM, justifying a limited recruitment of fast-twitch fibers and, consequently, the unaltered EMG activity after the training period. Although circuit weight training does not seem to increase muscle activation in postmenopausal women, it may be an important tool to maintain neuromuscular conditioning in this population. Our results confirm that, in the first period of ST, neuromuscular adaptations are important to the enhancement of muscle strength. This can be observed from the increase in EMG activity, which indicates an increase in the motor unit recruitment and firing rate (11). After 24 weeks of training, we found no difference in BMD in all the sites analyzed in the STG, CTG, and CON. These results corroborate with the findings of studies with similar training periods (14). However, other studies showed significant increases in specific regions with relatively shortterm training programs (4–6 months) in healthy subjects (19,27,31). Short-term training programs that show positive adaptations to BMD are characterized by the use of high loads throughout the training period. In the study from Vincent and Braith (31), although the training period was the same as in our study, loads of 80% 1RM were administered during the whole training period. Menkes et al. (19) and Ryan et al. (27) adopted exactly the same training methodology. From the first training sessions, sets of repetitions to exhaustion were employed, i.e., subjects performed 5RM (approximately 90% 3RM); then the loads were decreased until 15 repetitions could be performed. This methodology is similar to that followed by Vincent and Braith (31), because during the whole training period, the relative intensity was high and remained high. The training mentioned above might be classified as nonperiodized, characterized by constant intensity and volume throughout the training period (2). In the present study, the STG was submitted to high loads (70–80% 1RM) for only 2 months, a period that might be too short to produce significant differences in BMD. Comparing the results of our study with those of others that found increases in BMD, it can be suggested that nonperiodized highintensity ST may be a better choice to stimulate osteogenic activity than linear, periodized training in a short-term training program. In a linear, periodized training program, longer periods may be necessary in order to produce an effect on BMD in postmenopausal women with bone loss. The low intensities normally adopted in traditional circuit weight training may limit the possibility of increasing BMD (9,10). We used 45–60% 1RM intensities that seem to be VOLUME 22 | NUMBER 6 | NOVEMBER 2008 |
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Adaptations Induced by Weight Training insufficient to promote the appropriate mechanical stress to stimulate increases in BMD (31). Therefore, the circuit weight training probably can increase BMD if higher intensities are used in long-term training programs using a nonperiodized scheme. Besides training intensity, exercise type may influence BMD. The improvement of BMD is achieved by mechanical forces (33) like compression, tension, and torsion. Specifically in our study, leg press, hip adduction, and hip abduction exercises promote femur mechanical stress by gravity and/or muscle contraction. In exercises executed in a standing position such as an arm curl, the same effect is produced by the action of gravity, while lumbar spine stress was achieved in sit-ups and back extensions. These exercises were chosen not only to improve BMD, but to improve strength and the conditioning of the whole body. However, it seems that the lack of other important ‘‘compressive’’ exercises (squat, military press) may impair positive adaptations at the femur and lumbar spine BMD. In conclusion, we suggest that ST and circuit weight training can increase dynamic strength and IS in postmenopausal women. The increases in muscle strength are probably due to adjustments in the trained subjects’ neuromuscular system. Based on our EMG findings, ST seems to emphasize an increase in motor unit recruitment and/or in the firing rate of the active motor units of trained muscles, whereas we speculate that circuit weight training could result in inhibited antagonist muscles (coactivation inhibition), providing an increase in strength similar to that of ST. Both ST and circuit weight training can increase the _ 2max and TE of this population, effects that can be Vo mediated by the increased muscle strength of the lower limbs. The BMD of postmenopausal women does not seem to be affected by these training programs in a short period of time. However, we suggest that high-intensity training conducted from the first sessions could produce significant effects with shorter training periods.
PRACTICAL APPLICATIONS We compared the effects of traditional high-intensity weight training with those of low-intensity circuit weight training. According to our results, circuit weight training is an effective training strategy to improve neuromuscular and cardiorespiratory conditioning of postmenopausal women with no history of resistance training. However, if the main objective is to maintain or increase BMD, nonperiodized, high-intensity weight training may be the best choice.
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