Research related to health and fitness has sought to identify the benefits of exercise. According to the American College of Sports Medicine (1), physical fitness is related to health through 5 basic components: body composition, aerobic capacity, strength, muscular endurance, and flexibility. Among these, strength and flexibility are important physical fitness variables, and appropriate levels are necessary not only for the promotion and maintenance of health and functional autonomy but also for the safe and effective participation in sports (1).
Several studies (3,7,11,17,18) have found that isolated strength training (ST) promotes flexibility gains. Simão et al. (17) examined different volumes of ST and their effects on flexibility, where groups showed significant flexibility increases leading to the conclusion that ST performed without Flexibility Training (FLEX) promotes flexibility gains, regardless of the ST volume. Similar conclusions were found by a study conducted by Monteiro et al. (11). They examined the effect of ST, in a circuit fashion, on flexibility in sedentary adult women and found that ST increased flexibility. Simão et al. (18) examined the effects of ST and FLEX, isolated and simultaneously for 16 weeks and found that ST was able to generate gains in flexibility and strength, even when isolated, whereas strength gains occurred only in groups where there was specific ST. Fatouros et al. (7) verified the effects of different intensities of ST and its effects on the flexibility in the elderly. After 6 months of training, data suggested that ST (regardless of the intensity) generates gains in flexibility, and after 6 months of detraining, the higher training intensities were more efficient in maintaining the gains in strength and flexibility; Barbosa et al. (3) investigated the effects of 10 weeks of ST on flexibility behavior of sedentary elderly women, showing that the resistance training used caused a significant increase in flexibility.
It has been reported in the literature that stretching before strength testing may reduce acute strength performance (9) and isotonic (19), isometric (maximal voluntary contractions), (4) and isokinetic (6) strength. The methodological components of applied FLEX, such as the volume and intensity, seem to be very different from those applied in day-to-day practice of gyms, and this influence becomes more questionable when we consider the amount of FLEX previously applied to a strength test (18). The possible interactions between ST and FLEX, both acute and chronic, deserve further attention.
Many factors can influence the flexibility and strength gains, such as range of motion performed in training, the degree of individual's physical condition, age, specific training, and methodological variables of prescription (exercises order, number of exercises, sets, repetitions, rest intervals, and training method) (11,17,18). Despite the many studies cited, there are no studies in the literature that compared the chronic responses of different combinations of these trainings using dynamic stretching. Furthermore, most studies have a sedentary sample, in contrast to this study. Continuing to examine the influence of ST and FLEX is important to fully understand the synergistic or counteractive effects of concurrent strength and flexibility exercises among trained populations and using dynamic stretching routines. Thus, the aim of this study was to analyze the strength and flexibility gains after 12 weeks of combined or isolated strength and dynamic FLEX by experienced women.
Experimental Approach to the Problem
Before 12 weeks of training, 28 women experienced in ST and FLEX were randomly divided into 4 groups: ST (n = 7), FLEX (n = 7), combination of strength and flexibility (ST + FLEX) (n = 7), and combination of flexibility and strength (FLEX + ST) (n = 7). Before 10 repetition maximum (10RM) testing, all groups performed 2 familiarization sessions with the 10RM testing procedures. The flexibility measurements were performed 48–72 hours after the last 10RM test. After flexibility measurements, the groups began the 12 weeks of training under the supervision of experienced physical education professionals. After 12 weeks of training, strength and flexibility were tested again by the same procedures as the pre-tests.
To be included in the study, the volunteers had to show the following characteristics: (a) consistent participation in ST and FLEX for at least 36 months immediately before the study, (b) agree not to participate in any type of regular physical activity beyond the prescribed training, and (c) be free from any condition that could influence their participation in training or the collection or interpretation of data. Twenty-eight women were randomly divided into 4 different groups. The study details were explained verbally and in writing, and all participants signed an informed consent form for participation in the study, according to the Declaration of Helsinki. The study protocol was approved by the Institutional Ethics Committee of the University.
Ten Repetition Maximum Test
All participants performed 2 familiarization sessions of the 10RM testing protocol, with 48–72 hours between sessions. The 10RM test protocol was described previously (10). The 10RM tests were performed on 2 nonconsecutive days in the bench press (BP) and leg press (LP) (Rotech, Goiás, Brazil) using a counterbalanced order. On the first day, the first 10RM test was performed, and then, after 48–72 hours, the 10RM test was repeated to determine the reproducibility of the test. The highest load reached during test days was considered the 10RM load. No exercise was allowed in the 48 hours between the 10RM tests, so as not to interfere with the reproducibility of the test results. To minimize errors during the tests, the following strategies were adopted (10): (a) standardized instructions on test procedures were provided to participants before testing, (b) participants received instructions regarding the technique for each exercise, and (c) all individuals received verbal encouragement during testing. The 10RM was determined by a maximum of 5 attempts for each exercise, with an interval of 5 minutes between them. After 12 weeks of training, the 10RM test was conducted to the pre-training to observe the possible strength gains. All tests were conducted in the morning, between 8 and 10 AM
Sit and Reach Test
Flexibility was assessed before and after 12 weeks through the sit and reach test (1). Flexibility measurement was performed 48–72 hours after the last 10RM test. The score used was the best of 3 trials with 10-second rest periods between each trial (1). All flexibility tests were conducted at the same time of day (between 8 and 10 AM). The data collected during the initial assessment were not available for the examiner to prevent information bias during the posttraining measures. The same procedure was performed after training.
The American College of Sports Medicine (1) recommends that when applying the sit and reach test, an adequate warm-up should be performed. Therefore, before the test, a 5-minute aerobic warm-up (walking on treadmill), at mild-to-moderate intensity, was performed, according to the Borg scale (5), and then, 4 stretching exercises were performed for the muscle groups involved in the sit and reach test (hamstrings, hip flexor, quadriceps, and calf). Two sets of 10 seconds of each stretching exercise were performed with a 10-second rest period between the sets and exercises. Immediately after performing the static stretches, the sit and reach test was performed.
Below is a detailed description of the 4 stretching exercises performed:
- Hamstrings—in a seated position with both legs straight, bend the trunk forward, trying to hold both feet.
- Hip flexor—feet in the anteroposterior direction, 1 knee resting on the floor, and the front leg with foot on the floor and knees bent. Push your hips forward while maintaining the posture of the trunk.
- Quadriceps—standing, holding 1 foot behind, and bringing it into the buttocks.
- Calf—feet in anteroposterior direction, the front leg flexed and the other extended, hands resting on the floor, trying to put the back heel on the floor.
Flexibility, through goniometry, was evaluated in 10 joint movements: shoulder flexion, extension, abduction and horizontal adduction, elbow flexion, hip flexion and extension, knee flexion, and trunk flexion and extension. The shoulder flexion, abduction, horizontal adduction, elbow, and hip flexion were performed in the supine position; shoulder and hip extension and knee flexion were held in the prone position; and trunk flexion and extension were held in the upright position, avoiding the compensatory movement.
To examine flexibility, the examiner adjusted the individual's body to the point of mild discomfort or anatomical limitation. Measurements were made using the goniometer Lafayette (Sammons Preston Rolyan #7514, Lafayette, IN, USA), following the procedures described by Norkin and White (14). The collected data were not available for the examiner during subsequent assessments. The flexibility measurements (sit and reach test and goniometry) tests were performed on 2 consecutive days to determine the reproducibility. The highest measurement obtained during test days was considered the flexibility score.
The training protocol for all groups included training sessions on alternate days, totaling 48 sessions. The minimum adherence for inclusion of data was participation in at least 44 sessions. Experienced physical education professionals supervised all training sessions.
The ST consisted of 8 exercises performed in 3 sets per exercise with periodized training intensities. In the first month, 8–12 RM were used, in the second month, repetitions and load were then changed to 6–10 RM, and 10–15 RM in the last training month. When individuals exceeded the maximum number of prescribed repetitions, the loads were adjusted. The exercise order for ST was as follows: LP, leg extension, leg curl, BP, front lat pull-down, seated shoulder press, biceps curl, and triceps pulley. Before each training session, individuals performed a specific warm-up involving 15 repetitions with 50% of the load in the first and second exercises of the sequence. The rest interval was set at 1 minute between sets and exercises.
Flexibility training consisted of exercises involving upper and lower limbs, shoulders, hips, and trunk. Dynamic stretching was performed for a total duration of 60 minutes, with 3 sets of each exercise, and 30 repetitions in each set. All stretching exercises were performed until the point of mild discomfort, and they were very similar to the goniometry test exercises. Between sets and exercises, no rest interval was allowed; however, there were changes in muscle group (e.g., upper body alternating to lower body).
The combined training involved the completion of FLEX followed by ST, or in the opposite order. All tests and training sessions were conducted in the morning (between 8 AM and 12 PM). To prevent bias, the researchers who randomized the groups and did the training orientation did not participate in testing measures, and researchers involved in strength and flexibility tests were not involved in the group formation and orientation process.
The Shapiro-Wilk normality test and homoscedasticity test (Bartlett's criterion) were performed to assess the distribution of data. The intraclass correlation coefficient (ICC) was used to determine the test-retest reproducibility of the 10RM tests and flexibility measurements. The ICC method was used based on repeated measures of strength and flexibility. The Student's t-test was used to analyze the differences between 10RM test and retest, before and after training. The two-way repeated-measures analysis of variance was used to analyze the differences between groups in 10RM load and measures of flexibility over time. Where appropriate, further analyses were conducted using the Tukey's post hoc test. For all cases, the adopted statistical significance level was p ≤ 0.05. The software Statistica version 7.0 (Statsoft, Inc., Tulsa, OK, USA) was used for all statistical analyses. The calculation of the effect size (difference in scores between pre-test and post-test divided by the SD pre-test) and the scale proposed by Rhea (15) were used to examine the magnitude of any treatment effect.
The 10RM testing indicated favorable ICCs (r = 0.99) for all exercises. All groups significantly improved strength in the LP (FLEX: p = 0.0187; ST: p = 0.0001; FLEX + ST: p = 0.0034; ST + FLEX: p = 0.0021). All groups except the FLEX group significantly improved strength in the BP (FLEX: p = 0.1757; ST: p = 0.0001; FLEX + ST: p = 0.0017; ST + FLEX: p = 0.0035). When comparisons across groups were conducted, only the ST group differed significantly from the FLEX group in strength changes on the LP (p = 0.0056) (Table 1).
The interaction between training modalities and time was observed for BP (p = 0.00043) and LP (p = 0.00006). Training modality significantly influenced only LP (p = 0.0494). Time significantly influenced both BP (p = 0.0001) and LP (0.0001) results.
The effect size analysis showed moderate gains for the BP exercise in all groups except in FLEX group where there was loss of strength (effect size = −0.06). Small treatment effects were calculated in the LP for FLEX and ST + FLEX groups, with moderate gains for the FLEX + ST group and a large effect for the ST group (Table 1).
The sit and reach test and goniometry ICCs were between 0.97 and 0.99. There was no interaction between training modalities and time (p = 0.3959). For flexibility data, the sit and reach test showed no significant results for any group in any of the comparisons (p > 0.05), and to the effect size, all the results for all groups were trivial (Table 2). Furthermore, the analysis did not show significant results for any group in any of the comparisons (p > 0.05) for goniometry variables, with all groups demonstrating small treatment effects on flexibility.
The aim of this study was to analyze the strength and flexibility gains in different groups (isolated and combined) after 12 weeks of training. Overall, the main findings of this study were significant strength gains in all groups in the LP exercise, and in the BP, improvements in all groups except the flexibility group. Although there were nonsignificant differences between groups, an examination of the effect sizes for each treatment showed a difference in strength treatment effects for all groups including ST with small differences between groups. Flexibility training alone was not shown to result in sizeable treatment effects in strength. Each group showed a trivial treatment effect in flexibility measures with nonsignificant differences between groups; however, the FLEX and FLEX + ST groups had the highest effect sizes of the 4 groups. Based on these data, ST does not seem to have any negative impact on flexibility and may result in similar improvements in flexibility as compared with dynamic stretching alone. There may be a slight reduction in strength development with the addition of FLEX; however, the statistical evaluation showed that these differences may lack sufficient reproducibility. Although small, the reduction in treatment effects seen in those groups including FLEX is notable and deserves further research attention.
The greatest points of distinction in this study were the sample, composed of experienced women, different from all other studies that used sedentary subjects and the use of dynamic stretching rather than static stretching as used by all other studies published on this topic. In the literature, only 1 study compared the effects of ST and flexibility, isolated and combined. Using a similar protocol to this study, Simão et al. (18) examined the strength and flexibility gains in sedentary women after 16 weeks of ST and FLEX, isolated and combined. The combined training group consisted of FLEX preceding the ST, and the training frequency for all groups was 3 weekly sessions. At the end of the intervention, the results showed that all groups increased in flexibility and only the groups that trained strength had strength increases with no significant differences between groups.
Analyzing the methodological differences between studies, this study included trained women and lasted 12 weeks but used a higher frequency of training. The ST protocol adopted by Simão et al. (18) included a linear periodization, differing only in some exercises used; whereas for FLEX protocol, this study used the dynamic method, whereas Simão et al. (18) used the static method. Nóbrega et al. (13) used ST before FLEX, finding that this order did not interfere with flexibility gains; however, in the study by Simão et al. (18), which used the opposite order, that is, flexibility preceding ST, there was a greater magnitude in the gains found.
Our findings were the opposite, when compared with other studies (3,6,8,11,16–18), which found significant flexibility increases following both ST and FLEX. The treatment effects in our study were trivial to flexibility enhancement; however, ST did not result in decreased flexibility nor was the flexibility intervention shown to be more effective than the strength-only program.
Regarding strength, all groups in this study showed some gains in LP strength; however, the FLEX group showed the lowest treatment effect. The FLEX group was the only group that did not show a significant treatment effect in BP strength. It is possible that the dynamic stretching routine was sufficient to improve lower-body strength among this population of women, but little gains can be expected in strength following flexibility exercises alone. Adding FLEX to the strength program did not decrease the treatment effects compared with ST alone.
When comparing this study and others with the protocol used by Nóbrega et al. (13), the main methodological difference is in the method of flexibility evaluation, where Nóbrega et al. (13) used the flexitest as assessment method, and this test has the main feature being the most subjective test of all because it consists of a comparative analysis between the maximum range of motion obtained and the standardized assessment maps, rating it on a scale 0–4 (2), being therefore more susceptible to variations in the results than the other assessment methods that have measures.
In a recent study, Morton et al. (12) evaluated the ST isolated compared with static stretching isolated and their effects on flexibility and strength after 5 weeks of training. In both training groups, exercises were programmed to use the same muscle-joint complex, with similar range of motion and movements. The results showed that ST, carefully constructed, including the main joint movements that an individual perform during your daily activities, may produce flexibility increases in the same magnitude of typical programs of static stretching (in most joint movements evaluated), concluding that a joint was able to maintain or gain flexibility according to their use, regardless of the type of training performed. This study also used a ST program including main joint movements combined with dynamic stretching but did not see significant improvements in flexibility in any group. Morton et al. (12) also found that strength levels increased significantly only in the strength group vs. the flexibility group. Although statistical improvements were measured in the FLEX group in this study, they were only found in the LP, and the treatment effect was small. These findings together suggest that neither static nor dynamic flexibility exercises can be expected to improve strength similarly to resistance training.
In conclusion, it seems that the inclusion of dynamic stretching exercises in a training program including resistance training can have no negative impact on strength development. The order in which strength and flexibility exercises are performed seems to have no effect on strength or flexibility gains; however, the lack of improvements in flexibility among subject only following a flexibility routine is of concern regarding this study. Further research may need to examine the effects of dynamic flexibility in combination with ST to examine its impact on flexibility improvements in different populations and different assessment methods.
It seems from our data that strength and flexibility exercises can be performed concurrently, in any order, without a significant negative influence on either strength or flexibility development in this population evaluated. It does not seem that FLEX can enhance strength development compared with ST alone and may result in a slight decrease in overall strength development. Although our study did not show improvements in flexibility in any group, ST did not result in a decrease in flexibility, providing further evidence in the debate about strength and flexibility concurrent training. For populations desiring maximum strength development, strategic use of FLEX should be considered to secure the value but avoid the negative effect on strength adaptations.
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