Flexibility is an important variable for both health and athletic performance (2,3) and is influenced by factors such as sex (12), age (3), and training (11). As in any other component of physical fitness, adaptations are affected by the type of training performed, and increases in flexibility can be influenced by the type of flexibility training performed (5-7). However, the literature related to the influence of strength training on flexibility is limited.
Studies on elderly men and women indicate strength training alone can increase flexibility. Fatouros et al. (8) investigated the influence of 16 weeks of strength training alone on flexibility in sedentary elderly men 65 to 78 years old. Strength training significantly increased flexibility in some, but not all of the joints measured. Barbosa et al. (4) assessed the effects of a 10-week strength training program on the flexibility of sedentary elderly women 62 to 73 years old. Flexibility was evaluated with the sit-and-reach test pre and post strength training and demonstrated a significant increase. More recently, Nóbrega et al. (13) investigated the interaction of strength and flexibility training in groups of young sedentary men and women with a mean age of 21 years. The results indicated that, after 12 weeks of training, strength training alone did not significantly change flexibility, but did not significantly affect increases in flexibility when strength training was performed in conjunction with flexibility training.
The results of the aforementioned studies indicate strength training alone can increase flexibility in sedentary elderly men and women and that strength training alone does not significantly affect flexibility in young active men and women. Information on the effect of strength training alone on the flexibility of sedentary middle-aged women appears to be lacking. Therefore, the goal of the current study was to investigate the effect of a 10-week, circuit-based strength training program on the flexibility of sedentary middle-aged women.
Experimental Approach to the Problem
Prior to a 10-week training program, subjects were randomly divided into 2 groups: an experimental group (EG) and a control group (CG). All subjects had performed no regular physical activity program in the 6 months preceding the project's training program. Subjects assigned to the EG performed a 1-week resistance training familiarization period consisting of 3 sessions per week, during which the same exercises used in the training program were performed in a circuit fashion for 3 sets of 8 to 12 repetitions, using an 8 to 12 repetitions maximum resistance (RM), except for the abdominal exercise, which was performed for 3 sets of 15 to 20 RM. The training program's duration was 10 weeks, and it consisted of circuit weight training with all training sessions supervised by a person with expertise in resistance training. The weight training program performed followed the recommendations of the American College of Sports Medicine (1) for novice weight trainers in the initial period of training.
Training frequency was 3 sessions per week, with at least 48 hours of rest between sessions. A total of 30 sessions were performed in the 10-week training period with sessions performed between 6 and 9 pm. Adherence to the program was 100% for all individuals in the EG. The circuit training program included 7 exercises. The exercise order for the circuit training was as follows: free-weight flat bench press (FBP), smith machine squat (SQ), anterior wide grip lat pull-down (LPD), 45° leg press (LP), 30° inclined bench press (IBP), hack squat machine (HM), and abdominal crunch (ABS).
When an individual could perform more than the prescribed number of repetitions for all 3 sets of an exercise, the resistance for that exercises was increased. Before each training session, the subjects went through a specific warm-up, including 15 repetitions with 50% of the weight being used in the first and second exercises of the training session. The rest interval between sets was a 1:3 work-to-rest ratio, and a 2-minute rest interval was allowed between successive circuits.
Twenty women participated in the project. Ten subjects were randomly assigned to the EG (age, 37 ± 1.7 years; body mass, 65.2 ± 10.7 kg; height, 157.7 ± 10.8 cm; and body mass index, 25.72 ± 3.3 kg·m−2) and to the CG (age, 36.9 ± 1.2 years; body mass, 64.54 ± 10.18 kg; height, 158.1 ± 8.9 cm; and body mass index, 26.07 ± 2.8 kg·m−2). Study inclusion criteria were the following characteristics: sedentary for at least 6 months prior to the study; not performing any type of regular physical activity for the duration of the study other than the prescribed strength training included in the study; and not having any medical condition that could influence the collection or interpretation of the data. After informing subjects of the testing and training procedures to be performed during the study, all subjects signed an informed consent form approved by a human subjects institutional review committee.
Repetition Maximum Testing
Pretesting to determine the RM resistances to be used as the initial training weights was performed 48 hours and 96 hours after the last training session of the 1-week familiarization period. Repetition maximum resistances (10 RM for all exercises except ABS, for which 15 RM was determined) determined at these 2 times demonstrated excellent day-to-day test reliability for all exercises (FBP, 0.90; SQ, 0.90; LPD, 0.94; LP, 0.92; IBP, 0.90; HM, 0.90; and ABS, 0.92). Additionally, paired Student's t-tests did not show significant differences (p < 0.05) between the RMs determined on the 2 separate occasions. The heaviest resistance of the 2 RMs determined for each exercise was used as the initial training resistance for each exercise.
After the 10-week training program, subjects performed the RM tests 48 hours and 96 hours after the last training session. The posttesting RMs determined at these 2 time points demonstrated excellent day-to-day test reliability for all exercises (FBP, 0.94; SQ, 0.92; LPD, 0.96; LP, 0.96; IBP, 0.94; HM, 0.92; and ABS, 0.92). Additionally, paired Student's t-tests did not show significant differences (p < 0.05) between the RMs determined on the 2 separate occasions. The heaviest resistance of the 2 posttraining RMs and the 2 pretraining RMs was used to determine RM strength gains due to training and for statistical analysis.
The mass of all weight plates and bars used for determining the RMs were measured with a precision scale. The actual mass of all plates and bars was used to calculate the RM for each exercise. The RM tests were performed in the following order: FBP, SQ, LPD, LP, IBP, HM, and ABS. Ten RMs were determined for all exercises except for ABS, for which a 15 RM was determined. All machine exercise testing and training was performed on the same equipment (Life Fitness, Franklin Park, IL). To minimize possible errors in the RM testing, the following procedures were used. All subjects received standard instructions on the general routine of data assessment and the exercise technique of each exercise before testing. The exercise technique of subjects was monitored and corrected as needed during all testing sessions. All subjects received verbal encouragement during testing. The RM testing was performed at the same time of day as the training sessions (i.e., between 6 and 9 pm).
During the RM tests, each subject had a maximum of 3 RM attempts of each exercise with 2- to 5-minute rest intervals between attempts. After the RM resistance in a specific exercise was determined, an interval not shorter than 10 minutes was allowed before the RM determination of the next exercise. Standard exercise techniques were followed for each exercise. No pause was allowed between the eccentric and concentric phases of a repetition or between repetitions. For a repetition to be successful, a complete range of motion, as is normally defined for the exercise, had to be completed.
Flexibility was measured after the weight training familiarization week. The weight training program included exercises involving movement at the shoulder and elbow joints (i.e., FBP, LPD, and IBP), knee and hip joints (i.e., SQ, LP, and HM) and lower back (i.e., ABS). Therefore, flexibility was determined for 10 articular movements at these joints pre and post training. The movements tested were shoulder flexion and extension, horizontal shoulder adduction and abduction, elbow flexion, hip flexion and extension, knee flexion and extension, and trunk flexion and extension. Except for trunk movements, all measurements were determined on the right side of the body. The trunk flexion and extension movements and shoulder adduction were performed in the standing position. The shoulder abduction movement was performed while the individual was sitting on a chair. All the other movements were performed with the subjects in a supine position.
When determining flexibility, the evaluator assisted in the test movement until the subject experienced pain or mechanical limitation to the movement was reached. The measurements were taken with a Leighton Flexometer, always at the same time of day, using procedures previously described (10). Pretraining flexibility measures were determined 48 and 96 hours after the last training session of the familiarization week. Posttraining flexibility was determined 48 and 96 hours after the last training session. Flexibility tests were always performed between 9 am and noon. All flexibility measures both pre and post training showed excellent day-to-day test reliability, with intraclass correlation coefficients ranging between 0.92 and 0.98. Additionally, paired Student's t-tests did not show significant differences (p < 0.05) between the 2 pretesting determinations of flexibility or the 2 posttesting determinations of flexibility. The flexibility testing was performed by using a double-blinded procedure in which data collected during a previous test was not made available to the tester or the subjects when performing successive tests. The greatest flexibility measurement of the 2 posttraining values and the 2 pretraining values were used to determine flexibility gains due to training and for statistical analysis.
The Shapiro-Wilk and Levene tests, respectively, were used to verify the data's normality and homogeneity of variances. The initial values in RM tests and flexibility between the EG and CG were compared by Student's t-test. An analysis of variance was used to compare the flexibility measures and the RM for each exercise between the CG and EG pre and post training with a Tukey post hoc test when indicated. The level of significance for all statistical analyses was p ≤ 0.05. The statistical software used for all analyses was version 6.0 of Statistica Software (Statsoft, Tulsa, OK).
Before training, no significance differences were shown between the EG and CG for body mass, height, age, body mass index, or any of the RM or flexibility tests. After 10 weeks of training, significant strength gains in RM resistances in all exercises tested were shown by the EG pre- to post- training (Table 1), whereas the CG showed no significant changes in RM strength. Strength gains in the EG from pre training to post training ranged from 52.6% in the FBP to 84.2% in ABS.
The results demonstrate several changes in measures of flexibility pre and post training. Of the 4 shoulder movements examined, only horizontal adduction demonstrated a significant increase with weight training (Table 2). Neither elbow nor knee flexion showed a significant change with weight training (Table 3), whereas both hip flexion and extension (Table 4) and both trunk flexion and extension showed significant increases with weight training (Table 5). The control group showed no significant change in any of the flexibility measures determined (Tables 2-5).
It is generally believed that strength training alone has little or no effect on a joint's range of motion or flexibility. However, the major finding of the current study demonstrates that strength training alone in sedentary middle-aged women significantly increases flexibility of the hip and trunk, but does not affect flexibility of the elbow or knee and has a minimal effect on shoulder flexibility; 1 of 4 flexibility measures showed a significant increase. This finding indicates that strength training may increase the flexibility of some but not other joints. It is also important to note that strength training did not decrease significantly range of motion of any of the joints examined.
The finding that strength training may increase a joint's range of motion in middle-aged women is in agreement with previous studies performed with older adults demonstrating that strength training does increase flexibility (4,8). Barbosa et al. (4) assessed the effects of a 10-week strength training program on the flexibility of sedentary elderly women 62 to 73 years old. Flexibility was evaluated with the sit-and-reach test, and after strength training, a significant increase was demonstrated. Fatouros et al. (8) investigated the influence of strength training alone on flexibility in sedentary elderly men 65 to 78 years old during 16 weeks of training. In the study by Fatouros et al. (8), strength training was performed for 16 weeks, and after training, significant increases in range of motion in the sit-and-reach test, elbow flexion, knee flexion, shoulder flexion, shoulder extension, hip flexion, and hip extension were shown, whereas no significant changes were shown for hip adduction, hip abduction, or shoulder adduction. Both the current study and the study by Fatouros et al. (8) indicate weight training can increase range of motion during hip flexion and extension, but disagree concerning increases in flexibility at the shoulder, elbow, and knee joints. However, both studies do show increases in flexibility in some movements.
Collectively, the results of the current study on middle-aged women (35 to 39 years old) and the 2 previous studies on older women (62 to 73 years old) and older men (65 to 78 years old) indicate weight training does increase flexibility in some movements. Additionally, weight training did not negatively affect flexibility in any of the movements examined in these studies. These results indicate that in previously sedentary individuals, weight training in both middle-aged and older men and women can increase flexibility at some joints and has no significant effect on flexibility at other joints. However, Girouard and Hurley (9) demonstrated that in men 50 to 74 years old, strength and flexibility training performed concurrently for 10 weeks significantly increased range of motion in shoulder abduction and shoulder flexion, while hip flexion was not significantly changed. However, none of the changes in range of motion were significantly different from an inactive age-matched control group, indicating strength training alone would also have resulted in range of motion changes that were nonsignificantly different from the inactive control group. Additionally, whether flexibility is increased with weight training may be dependent upon the articulations studied, the methods used to measure flexibility, and the morphological characteristics of the individuals (3).
Differences in training programs, such as training duration, number of sets, and repetitions performed, can affect the training outcome. In the current study, the training duration was 10 weeks, and subjects performed 7 exercises in a circuit fashion for 3 circuits of 8 to 12 RM sets except for the abdominal exercise, which was performed for sets of 15 to 20 RM. In the study by Fatouros et al. (8), the training duration was 16 weeks, with 8 exercises, including at least 1 exercise for all the major muscle groups, performed in an alternating exercise order (i.e., alternating between arm and leg exercises). During weeks 1 through 4, subjects exercised using 55% to 60% of 1RM for 2 sets (14 and 12 repetitions in the first and second sets, respectively). In the remaining 12 weeks of training, subjects performed 3 sets with intensity increasing from 60% to 70% of 1RM in weeks 5 through 8 (12 repetitions in the first 2 sets and 10 repetitions in the third set) to 80% of 1RM in weeks 13 through 16 (8 repetitions per set). In the study by Barbosa et al. (4), the subjects performed 8 total exercises for a training duration of 10 weeks. Exercises for the major muscle groups were performed for 5 sets of 6 to 10 RM. For the assistance exercises, 3 sets of 6 to10 RM were performed, and for the calf and abdominal exercises, 3 sets of 10 to 15 RM were performed. Although the training methodologies used in these 3 studies differed, all studies demonstrated increases in flexibility. The results of these 3 studies indicate that a short (i.e., 10-16 weeks), total body weight training program with multiple sets of between 6 and 12 repetitions per set can increase flexibility in previously sedentary middle-aged and older men and women.
In contrast to the current study and previous studies on older adults (4,8), Nóbrega et al. (13) demonstrated no change in flexibility of healthy young (mean age, 21 ± 4 years) men and women due to 12 weeks of weight training. In this study, training consisted of a total body program of 9 exercises performed for 3 sets of 8 to 12 RM. The results indicated that weight training may not improve flexibility in healthy young individuals. Thus, whether weight training increases flexibility may in part be dependent on age.
The physiological mechanisms that may be responsible for increased flexibility are not well understood, but several hypotheses are possible. It has been previously suggested that strength training enhances the tensile strength of tendons and ligaments and increases muscle mass and contractility, thus allowing a greater range of motion (14). In the current study, no data providing information about the physiological mechanisms related to increases in flexibility were obtained. However, all muscle groups increased in strength, and only the elbow and knee articulations showed small nonsignificant flexibility reductions, while the other articulations showed increases in flexibility, indicating that flexibility increases can occur simultaneously with strength increases in previously sedentary middle-aged women.
In conclusion, the results of the current study suggest that short-term strength training increases flexibility in healthy middle-aged women. The flexibility increases, however, appear to be apparent at some joints, but not others. Of the 4 shoulder movements examined, only horizontal adduction demonstrated a significant increase. While neither elbow nor knee flexion showed a significant change, both hip flexion and extension and both trunk flexion and extension showed significant increases. Presently, there are insufficient data to draw conclusions concerning at which joints or movements weight training by itself will or will not normally result in increased flexibility. The data from the current study, however, indicate that during the first weeks of weight training, it is not necessary to perform stretching exercises to obtain flexibility increases at some joints. The current study was short in duration (i.e., 10 weeks). Thus, further studies must be conducted to investigate the impact of long-term strength training on flexibility.
Short-term weight training programs can improve flexibility in middle-aged women at some, but not all joints and movements. Thus, it may be possible to increase flexibility at some joints by performing only weight training in this population. However, if flexibility in all movements is desired, it is necessary to perform specific flexibility training for the movements in which an increase in flexibility is desired.
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Keywords:© 2008 National Strength and Conditioning Association
resistance training; resistive exercises; circuit training; stretching; mobility